Institute of Polymer Injection Moulding and Process Automation Thesis Supervisor
Univ.-Prof. DI. Dr. Georg Steinbichler Assistant Thesis Supervisor DI Clemens Kastner July 2019 JOHANNES KEPLER UNIVERSITY LINZ Altenbergerstraße 69 4040 Linz, Österreich www.jku.at DVR 0093696
Influence of selected
process parameters
on the surface quality of
physically foamed
polypropylene parts
Master’s Thesis
to confer the academic degree of Diplom-Ingenieur
in the Master’s Program
Sworn declaration
I hereby declare under oath that the submitted Master’s Thesis has been written solely by me without any third-party assistance, information other than provided sources or aids have not been used and those used have been fully documented. Sources for literal, paraphrased and cited quotes have been accurately credited.
The submitted document here present is identical to the electronically submitted text document.
Abstract
In this master thesis, the surfaces of microcellular injection foam molded polypropylene parts were investigated. The two main goals were to develop novel methods to qualitatively and quantitatively qualify the exterior, as currently no standard exists to do so, and to find the ideal parameters for producing a specimen without any surface defects. The experimental design was set up to see the influence of injection volume flow rate, melt temperature, mold temperature, super critical fluid content, and the inclusion of talc. For the mold heating, conventional and variotherm heating cycle were also deployed to verify the benefits of the latter. To analyze the surface, a photobox was set up to digitize the parts, and through calculating the overall average grayscale values and homogeneity for each run, comparisons were made to see how the different settings of different parameters influenced surface defects. Through these novel methods, the entire exterior could be analyzed quickly and cheaply by the usage of quantified data. The most influential parameter turned out to be the mold temperature, which contrary to contemporary knowledge, produced the worst surfaces under high values. The existence of swirl marks, silver streaks, post-blow effect, and tiger-stripes were also verified, identified, and suggestion were made on why they appear and how to eliminate them. The methods used to analyze the surface in this master thesis could be standardized, and suggestions were made on how to do so.
Acknowledgments
During the writing of this master thesis, I received a huge amount of support and assistance. First, I would like to express my gratitude towards Univ.-Prof. DI Dr. Georg Steinbichler, who gave me the opportunity and support to conduct my research under his institute. I would like to thank my supervisor, DI Clemens Kastner for his invaluable inputs and assistance that I received from him throughout my thesis.
I would like to acknowledge Borealis AG and Engel Austria GmbH who supported this writing financially and technologically. I am especially indebted to Daniela Mileva, Thomas Lummerstorfer, Georg Grestenberger, Wolfgang Kienzl, and the technicians at Engel, who made this thesis a reality. I would like to also express my gratitude towards Ing. Alfred Mayr who helped me with the construction of the photobox.
I would also want to thank my friends and colleagues who encouraged and supported me during the thesis. I am especially grateful to Mátyás Vass, who assisted me with the writing of the Matlab scripts.
And finally, I am grateful to my family, whose love and emotional support gave me strength during the pursuit of this master thesis. Without them, I would not have been able to make it this far.
Sworn declaration i
Acknowledgments iii
Symbols and indices v
Abbreviations vi 1 Introduction 1 2 Literature research 4 2.1 Foams . . . 4 2.1.1 Main properties . . . 4 2.1.2 Blowing agents . . . 5 2.1.3 Process steps . . . 6 2.2 Related technologies . . . 9 2.2.1 Structural-foam molding . . . 9 2.2.2 Variotherm . . . 14 2.3 Surface defects . . . 16
2.4 Evaluation methods and process parameters for surface quality . . . 18
2.4.1 Evaluation methods . . . 19
2.4.2 Process parameters . . . 20
2.5 Conclusion of literature research and personal assessment . . . 22
3 Experimental 24 3.1 Used equipment and materials . . . 24
3.2 Design of Experiment . . . 25
3.3 Methodology of Evaluation . . . 26
4 Results and discussion 30 4.1 Verification of measurements . . . 30
4.2 Injection volume flow rate . . . 32
4.2.1 Overall average grayscale . . . 32
4.2.2 Homogeneity . . . 34
4.3 Melt Temperature . . . 37
4.3.1 Overall average grayscale . . . 37
4.3.2 Homogeneity . . . 38
4.4 Mold temperature . . . 40
4.4.1 Overall average grayscale . . . 40
4.4.2 Homogeneity . . . 43
4.5 Super Critical Fluid content . . . 46
4.5.1 Overall average grayscale . . . 46
4.5.2 Homogeneity . . . 47
4.6 Influence of talc . . . 49
4.7 Conclusion of results . . . 51
5 Conclusion and outlook 55
Literature 58
Symbols and indices
A Surface area of bubble
c Concentration
D Diffusivity coefficient
D0 Pre-exponential factor
Eα Activation energy for diffusion
m Amount of mass
Mt Mass uptake at a certain time
M∞ Mass uptake at infinity
p Saturation pressure
S(ϕ) Wetting angle
S Solubility coefficient
T Temperature
γbp Interfacial energy between gas bubble and the polymer
Abbreviations
ABS Acrylonitrile butadiene styrene
CBA Chemical Blowing Agent
GIMP GNU Image Manipulation Program HDPE High-density polyethylene
LDPE Low-density polyethylene
PC Polycarbonate
PE Polyethylene
PO Polyolefin
POM Polyoxymethylene
PP Polypropylene
PPO Polyphenylene Oxide
PS Polystyrene
PVC Polyvinyl Chloride
SCF Super Critical Fluid
Tg Glass transition temperature
1
Introduction
In the industry, there is an increased incentive to reduce weight. This is true in particular in the automotive and the transportation sector, where the reduced consumption of fuel is beneficial both economically and ecologically. Viewing it differently, it also allows to put more features in the vehicle, improve certain features of it, or regarding transportation, it is possible to move more people and goods over the world. Among the many possibilities, one way to achieve this weight reduction is to substitute a compact polymer product with one that is a foam with a microcellular structure. These are injection molded parts, where the outer layer is compact, while the inner core is foamed. With this, it is possible to make a part that has similar mechanical properties as its compact counterpart, while also reducing weight. [KSS97, p. 101] Using less material for the same product not only benefits the end-user, but also the producer, who can make the same part at a lower expanse.
One area where it is possible to make this exchange is the exterior and interior applications for automotive parts. These are usually made from polymers, mainly, polypropylene. Its characteristics already helps the industry with good mechanical properties, coupled with weight reduction, and using structural foams, it would be possible to even further lower the material usage while keeping the compact part’s properties. The biggest obstacle for this to happen however is the fact that this technology, mainly the MuCell®-technology, causes surface defects on the exterior of the parts. These are flow marks, which due to the difference of index of reflection, have a different color, usually a silverish one, compared to the original one. This limits the applications where it is possible to use polypropylene structural foams, as it makes it hard to market a product that has these discolorations all over.
It is known that with certain materials, mainly polycarbonate, it is possible to overcome these defects with the help of the variotherm technology. [HLZ17] Using this, the mold heating system is replaced from one that uses constant heating to one that changes its temperature rapidly. The idea behind it, is that by heating up the mold and quickly cooling it down during and after injection of the polymer, the bubbles that make up the inner core of the part will not attach and freeze on the surface of the mold, and then by decreasing the temperature, further bubbles will not come out, because the outer layer will be frozen, thus achieving a defectless, homogeneous product. The company Borealis aims to use polypropylene with the MuCell® technology in combination of variotherm in order to achieve both weight reduction and a perfect surface. So far, very little research has been done that deals with these technologies when using polypropylene, therefore, this thesis was born to conduct a proper research on this topic, to try to achieve the best possible surface quality, and to prove this by using analytical methods.
The influence of injection molding parameters on the surface quality of the part is imperative. Whether to use, for example, a relatively low or high mold temperature affects the surface, as different articles have shown. [Che08][Guo07][BP13] Finding which process parameters have the biggest influence in more detail helps to optimize the process, and hopefully, to annihilate the structural errors on the exterior of the part. During the literature research on this topic, many conflicting assumptions came around. Nearly all the articles selected a few certain parameters, and by setting a low and high value for them experiments were conducted on how much influence they have on the surface. Whether to use a relatively high or low setting for the parameters were not always clear, and their morphological reasoning behind them were sometimes in a disagreement with other papers. Nevertheless, in order to conduct the experiment for the polypropylene parts, a proper DoE had to be set up, in order to get the most amount of information in the shortest time possible. Therefore, the first main objective of this thesis was to find these parameters, and through analyzing the surface, compare the different process settings with each other.
exists to do so. In the literature, numerous attempts have been made to categorize surfaces: in one example, the surface roughness is measured, where a lower value means a better surface. The problem found with these is that neither of them sufficiently analyze the surface quantitatively and qualitatively at the same time. Most of these measurements are slow and restricted to a small area, which in the case of a long plaque with 800×400 mm dimensions, can take days to come to a proper conclusion. Also, it is not only important to give a number for a surface and compare it to another one, but to also see where the defects are, which areas of the part are more sensitive to it, and to see if there are any major damages done on the product. For example, in a few cases during this thesis’s experiment, large amount of gas was concentrated on certain areas of the specimen. Although this was barely detected by the measurements, looking at the digitized photo show us that the process could be unstable.
Thus, the second major objective of this thesis was to come up with an idea that can quickly and effectively analyze the surface, and through quantitative and qualitative analysis can show which process parameter setting is better. Exploiting the phenomena that the surface gets discolored with silver streaks due to defects, one can measure the grayscale level of the part. Using a photobox, a simple picture was taken of the many parts produced, and through a image analysis program, the measurement of the average grayscale level and homogeneity of the surface was conducted. This allowed for a quick quantitative measurement of the surface, that is used to compare the plaques with each other. On top of this, because the part is digitized, showing the actual grayscale levels pixel-by-pixel is also possible, which helps to see where the defects are. Showcasing these grayscale pictures also helps a layman to understand the intensity and the position of these structural errors.
Figure 1.1: A digitalized part, converted into grayscale levels
In the first step of this thesis therefore, research has been done on the current understanding of microcellular foams in relation to process parameter settings, and also on the possible methods to analyze the surface. It was important to see which parameters have been deemed by other researchers as the most important ones, because as mentioned before, due to time constraints, it was not possible to change every setting. Therefore a two-level full factorial DoE with four process parameters was done.
The parts produced in Engel were 800×400 millimeters, in black color to more easily see the defects, and had four different grains. These grains ranged from highly polished to very rough. This helps to compare how different types of surfaces react to the injection molded product. All in all, a total of 56 runs were concluded. The difference between them was not only process setting values, but also whether they were compact or foamed, if the mold was conventionally or variotherm heated, and also if using talc affects the surface or not.
the digitization of the parts under constant lighting settings and positions, to ensure that a proper comparison could be done. With GNU Image Manipulation Program (GIMP), the pictures were divided into four parts, corresponding to the four different surfaces, and an average grayscale level was calculated by using Matlab. With manipulation of the data, the homogeneity of the surfaces was also assessed. Finally, alongside the picture of the plaque, it was possible to make a conclusion whether the changing of the parameters improved or not the surface quality of the part.
In the following section, the literature research conducted for this thesis will be done. Next, the equipment and materials used will be explained in a more detail. After it, the results and their discussion will be written, and then, a final conclusion and outlook will finish this Master Thesis.
2
Literature research
The general aim of the literature research was to gain knowledge regarding the topics that this thesis deals with. Technology-wise, a closer look was made into foaming, its process steps, the MuCell® and the variotherm technology. Then, numerous articles were read and assessed in the topic of surface quality of structural foams. Finally, the most important process parameters were found for the DoE that was later conducted.
2.1 Foams
Polymeric foams are cellular structures that are made by the addition of either physical or chemical nucleating agents.[Oss06, p. 407] These additives are needed in order to create gas bubbles inside the polymer which make up the porous structure. Aside from the cell structure, the density of the bulk material and the polymer used are the most important factors in the final properties of the foam product. [Ste]
2.1.1 Main properties
Depending on the process, the cells are either in an open or closed form. In the case of the former, air and other substances can flow through more easily, because the cell faces are fractured. With closed-cell foams however, these faces are mostly intact. [Mil07, p. 2] In the case of injection molding, the cells produced are closed. [Ste] The density of the foam, or more precisely, the relative density determines not only the mechanical properties, but also the volume fraction of the polymer. Due to the bubbles that exist inside of the material, the density, thus the weight is also reduced compared to a compact part, which is a major advantage.[Mil07, p. 3][Eav04, p. 2] The polymeric material used for foaming can be either thermoplastic or thermoset. Depending on the reaction and the used material, they can be further divided into flexible or rigid products. [Ash07, p. 2]
One of the most important features of a foam is its thermal conductivity. The governing four factors for this are the conduction of heat through the polymer and the gas, the convection through the cells and radiation across the walls of the cells and the void. The contribution of the conduction of the gas and radiation makes up nearly 40 % of the total thermal conduction in the case of a closed foam. In relation to the three main attributes of foams mentioned before, it is visible that they have a huge contribution to conductivity; the cell structure influences the general behavior of the conduction (the amount of cells, cell faces etc.), the relative density affects the amount of air (or other substance) inside the cells and thermal properties of the bulk material are key when the heat is conducted through the bulk material. These factors contribute to the fact that compared to compact polymers, foamed products have an edge in terms of heat sealing capability. [Eav04, p. 6-7]
Another significant difference between a compact and a foamed part is energy absorption. The latter performs much better than the former, because foams have the ability to keep the maximum force affecting them below a level that would otherwise damage the structure. This is due to the fact that cells first are elastically buckled, and with increasing force, are subject to plastic yielding and finally, brittle crushing. Depending on the material used for the foaming process, and whether the cells are open or closed affects the ability of the foam to recover from the impact. Foamed products therefore are also used in packaging. [Eav04, p. 5]
Lastly, the compression behavior of foams also differ from solid polymers. At first, the cells are bent and stretched by the inner gas pressure in the case of closed cells. Then, these cells will collapse with increasing force, and finally, densification happens, as seen at figure 2.1. As with thermal conduction and energy absorption, cell structure, density and the material that makes up the foam plays a huge role here. Closed cell structures have inner gas pressure, which contributes
to compression largely in region 2 and 3. A higher foam density leads to a reduced strain where densification starts, and the material which makes up the solid part of the foam inherently affects the Young’s modulus. [Eav04, p. 3-4]
Figure 2.1: Compression stress-strain curve of a foam [Eav04, p. 4]
2.1.2 Blowing agents
Blowing agents are the key players in foaming. These additives make sure that bubbles are nucleated inside of the heated barrel, thus a cellular structure is achieved in the end of the process. There are several ways to do this, but two main categories of blowing agents are chemical and physical. [Pri98, p. 143]
Chemical blowing agents are supplied in powder, pellet, or liquid form and is added to the hopper. When it gets mixed with the polymer melt, due to the heat supplied by the heating of the barrel and internal frictions, it decomposes thermally into gases, that will initiate the foaming process. This reaction is either exothermic or endothermic: in the case of the former, energy is released during reaction, while in the latter, energy must be supplied continuously. Blowing agents can be organic or inorganic. Inorganics are mainly used in the rubber industry, while organics on the other hand are widely used, and usually release nitrogen during decomposition. [Pri98, p. 145][Jt83, p. 112]
The most common additive is Azodicarbonate, an organic exothermic blowing agent that de-composes between 170 °C and 200 °C. Nowadays, Hydrocerol® by Clariant is used commercially as an endothermic foaming agent, and is added to the hopper between 1 % and 4 % weight percent (wt%) . How to select from the available many blowing agents depends on the polymer and the process. [Pri98, p. 145][Jt83, p. 114][Alt18, p. 123]
Physical blowing agents are additives that are added to the plasticating zone of the extruder, and this means that a separate equipment is needed to supply them. Usually, carbon dioxide or nitrogen are used as inert gasses, which are dissolved under high pressure. As such, this reaction is not chemically, but physically induced. The gas can be generated by the vaporization of a low boiling point liquid, or when the pressure gets lower during the process, the gas through nucleation forms bubbles, and make up the cellular structure of the polymer. This is because the gas gets less soluble as pressure decreases, and by that, phase separation happens. [Oss06, p. 407][Pri98, p. 143] Inert gasses offer many advantages. Although carbon dioxide is one of the main perpetrator in global warming, it is more environmentally compatible, as it does not cause ozone-depletion.
Compared to chemical foaming agents, less gas is needed to achieve the same degree of foaming. These gasses are also cheaper, making them more cost-effective. They are also toxic and non-flammable. [Pra05, p. 2-3]
In this master thesis, the specimen were made with the MuCell®-technology. Its core component is the usage of a gas, usually nitrogen or carbon dioxide, in a supercritical fluid state. In this state, the gas is pushed beyond its critical point, and it is no longer possible to distinguish the matter between liquid or gas. [Sam] For carbon dioxide, the supercritical point is at 73.84 bar and at 37°C, while nitrogen has it at 33.9 bar and -147 °C. The supercritical fluid has lower viscosity, low surface tension, and high diffusion capabilities, which means that the additive will dissolve at a higher degree compared to other methods. [Alt18, p. 122]
Figure 2.2: The supercritical point and region for CO2[Alt18, p. 122]
2.1.3 Process steps
After the gas gets introduced into the polymer melt, the two-phase product needs to achieve a number of steps in order for it to become a one-phase product. First, the gas gets solved and diffused in the polymer, and through nucleation, cell growth, and cell stabilization, it is possible to injection mold foams.2.3
Figure 2.3: The foaming process [Li08, p. 5]
Solubility Solubility is the maximum amount of gas that can dissolve in a polymer. [FSUb] In
order to define the mass uptakes in a certain time, the following equation can be employed:
Mt M∞ = 1 − 8 π2 ∞ X m=0 1 (2m + 1)2exp " −4D(2m + 1) 2 π2t L2 # (2.1) where Mt and M∞ are the mass uptakes, t the defined time, D is the diffusivity coefficient, and L
[Neo96, p. 174] An easier way to estimate it is to use Henry’s law:
C1 = Sp (2.2)
Here, S is the solubility coefficient, and p is saturation pressure. [KN09, p. 658] These equations show, that solubility depends strongly on the diffusivity coefficient, but also on the size of the membrane and pressure.
Henry’s law for solubility, and connected to this, permeability is an idealized model, and is regarded as a Type I sorption mechanism. This can only be seen when pressure levels are moderate, and only permanent gases are observed. In a Type II situation, isothermic conditions according to Langmuir is considered. Here, a saturation capacity is reached when gases are sorbed at certain areas where the pressure is higher. This is usually found in cases where an amorphous polymer is used with pre-existant voids. Type III is also isothermic, but here, the solubility coefficient S is not permanent and increases with higher pressure values. This usually happens when a hydrophobic polymer is in contact with water. Type IV is the combination of the previous types, where different laws are applied at different pressures. [KN09, p. 681]
Diffusivity Diffusivity is the amount of matter that is passing through an area in a certain amount
of time. This phenomena can be explained with Fick’s law, which is:
dm
dt = −DA
dC
ax (2.3)
where D is the diffusivity coefficient, m is the amount of matter, and c is the concentration.This is an idealized model, because it is assumed that the diffusivity only depends on the temperature. However, simple gases even in polymers behave this way, as such, this law can be applied in the context of foams. [KN09, p. 663-665] An other way to describe diffusivity is to regard the process as a thermally activated one, according to the Arrhenius equation:
D= D0exp −E α RT (2.4)
D0 is the pre-exponential factor, while Eα is the activation energy needed for the diffusion to happen. This theory explains that in order for the gas to penetrate the polymer matrix it needs to jump from one cavity to another, and is described in the form of activation energy. Fickian diffusion however only works if the temperatures are above the glass transition of the polymer. In the case of temperatures below the glass transition temperature (Tg), a non-fickian diffusion theory
has to be used. Here, the diffusion coefficient is time and concentration dependent, because in the amorphous regions of the polymer, the sorption mechanism obeys Henry’s law, while the regions with microvoids follows the Langmuir Type II sorption model. [Gen04, p. 22-24] However, in the case of this master thesis, this mechanism shall not be considered, because the temperatures inside the injection molding machine’s barrel is way higher than the Tg of the material.
Nucleation Nucleation is the process when a new formation of phase begins. [Kas00, p. ix] In
case of foaming, this happens when solubility of the gas is reduced to a certain temperature and pressure. This leads to saturation, and the excess gas start to form new bubbles. [Oss06, p. 709]. The Gibbs free energy concept is used for classical nucleation theory, which states that this free energy is needed to create a void in the liquid. Once it reaches a critical level, the bubbles that are larger grow further, and meanwhile, the smaller ones dissolve. This critical bubble size is based on mechanical and thermodynamic equilibrium. [FB04, p. 440] There are three types of nucleation: homogeneous, heterogeneous, and mixed. [CS87]
Homogeneous nucleation happens very rarely. In this case, the formation of a new phase depends on the fluctuations of the old, and when a critical amount of secondary component is dissolved in the original phase then by this, a new, stable, secondary phase is created. [Wyp16, p. 3][CS87] This type of nucleation happens completely inside the liquid. Using Gibbs free energy equation, the nucleation of a critical nucleus can be given by the following equation:
∆G∗ = 16π
3∆p2γ 3
bp (2.5)
The γ3
bp part explains the interfacial energy between the gas bubble and the polymer. [CS87] The work required to make bubbles can be given by the following equation:
W = σA − W1− W2 (2.6)
where σ is the surface tension, A the surface area of the bubble, and W1 and W2 are the work
done on the environment, more specifically, the creation of new surfaces and the work to transfer molecules from the old phase to the new one. [BK75]
Heterogeneous on the other hand, happens at an interface between the volatile liquid and another phase that comes in contact with it. [BK75] The reason this happens more frequently than homogeneous nucleation, is that it needs a lower energy to overcome the phase transition. [Wyp16, p. 3] This can be seen in the equation,
∆G∗= 16π
3∆p2γ 3
bpS(ϕ) (2.7)
where S (ϕ) is a wetting angle, a geometrical factor, which represents the interfacial areas of the polymer-bubble and additive particle-bubble. This value is less than, or equal to 1, which proves that indeed, a lower or equal energy is needed for heterogeneous nucleation. [CS87]
Figure 2.4: Gibbs free energy required for homogeneous and heterogeneous nucleation [CS87, p. 489]
The work required to generate more bubbles are very similar to equation 2.6, but surface energies of the interfaces and the various areas for liquid-gas, solid-gas and solid-liquid is also taken into account. [BK75]
The two nucleation types are not mutually exclusive, which means the two can happen at the same time, and this is called mixed mode nucleation. Even though less energy is needed for hetero-geneous nucleation, homohetero-geneous can happen elsewhere, if the required energy is given. In the case of the former, nucleation will happen quicker and more often, because it is more thermodynamically favored.
Cell growth and stabilization Finally, cell growth and stabilization is the last step in foaming.
This continues, as the pressure and gas concentration inside the bubble decreases until an equilibrium is reached. [SF97] It is necessary to have sufficient blowing agent to support the growth of the cells: as this is affected by the pressure, solubility and diffusivity of the blowing agent also affects the growing mechanisms. [Gen04, p. 41] Further influencing this process are the number and position of the nucleation sites, the time the cell is allowed to grow, and cell coarsening, which means that bubbles next to each other can influence each other, which causes uneven cell sizes. [Mil07, p. 13] In his work, Han et al. concluded, that the growth rate also depends on the processing parameters and the viscosity of the melt. [HY81] Amon et al. came up with the so-called cell model, which is the basis of many current studies on cell growth mechanisms. [AD84]
2.2 Related technologies
In the industry, there are many possibilities to make foams. Foam extrusion, thermoset reactive foaming, calendaring, compression molding, free foaming, rotational molding, and most importantly, structural-foam injection molding, are some of the ways to make foamed products. [Wyp17, p. 103-115][LPR07, p. 73-80] The parts made for the thesis were made with a kind of structural-foam molding, therefore, in the following, this process technology will be explored in more detail.
2.2.1 Structural-foam molding
The general idea of making foams in the injection molding process was to solve some of the problems that one encounters, like the occurrence of sinkmarks and warpage. [Hei16, p. 53] Next to reactive injection molding and gas-assisted injection molding, one answer to this is structural foam molding. [LPR07, p. 80-81] A structural foam is a component that has a cellular core that is sandwiched by a solid, compact skin. [Oss06, p.349-350] This happens, because the polymer melt at the mold wall gets cooled down, and the low viscosity prevents the blowing agents from expansion, which results in the creation of a compact layer. [Hei16, p. 54] The process allows to make large, thick parts that have good strength-to-weight ratio, with no sink marks and warpage. [Oss06, p.349-350] The main reason for the possibility to make larger specimen is because viscosity is lower as the gas gets released in the polymer melt, and this results in longer flow paths.[Hei16, p. 53] By foaming in this way, the bulk density of the part can be reduced by 50-90 %. The disadvantage of this type of technology, is that cycle time increases by the square of the part’s thickness, and the main topic of this thesis, surface defects can appear in various forms. The most common materials used are PE (Polyethylene), PP (Poly Polypropylene), PVC (Polyvinyl Chloride), PS (Polystyrene) and ABS (Acrylonitrile butadiene styrene).[RRR00, p. 1226] The five main processes to make structural foams are[Shu86][Xu10]:
• Low pressure process • High pressure process
• Gas-counter pressure process
• Two Component process (co-injection) • Microcellular Injection molding
Low pressure process Low pressure injection molding involves using pressure levels from 0.5 to
10 MPa and short shots into the mold, around 65-80 % of the volume of the total cavity volume. [Shu86, p. 47] The cavity is then packed by the expansion of the blowing agent, which in this case is due to the decomposition of the chemical blowing agent. Because the cavity is not fully filled at the beginning, and the employment of low molding pressures ensures that the pressure generated inside
is only generated by the blowing agent. [RRR00, p. 1230] In order to make this a possibility, one must use high injection speeds, high injection pressures, materials that can withstand the pressure generated by the gas, a shut-off nuzzle to prevent drooling, and the recommended wall-thickness parts made by this is 7-10 mm. [Shu86, p. 47][RRR00, p. 1230] The disadvantage with this type of technology, alongside with surface defects, is that the skin density is not uniform, as it is higher next to the gate due to the pressure being the highest in this region. This results in anisotropic properties over the length and non-uniformity when dyed, as the densities are also not constant. However, due to the low pressures employed, this technology causes the least amount of stress in the part. [Shu86, p. 48]
Figure 2.5: The low pressure structural foam molding process [KIL09, p. 274]
In the industry, the most widely known commercial process that uses low pressures is the UCC process (Union Carbide Corporation). Here, nitrogen is employed as a blowing agent, and mostly, PS, PC (Polycarbonate), and PPO (Polyphenylene Oxide) are the materials used to make structural foams. The densities of the parts produced are in the range of 250-950 kg/m3, and the thinnest
plaques possible are around 5 mm. Unfortunately, many of the disadvantages connected to the low pressure process also shows here. The surface roughness is relatively high, and the melt flow can be seen on the finished product. It is said that increasing the mold temperature and cycle time can help the surface. [Shu86, p. 49]
High Pressure Process The high pressure process uses 150 MPa, and in contrast to low pressure
applications, it uses full-shots. [Shu86, p. 59] Foaming is done by either letting the excess material flow back to the runner system, or more commonly, moving the plates so that the cavity expands. [BB91, p. 582] As the pressure inside gets lower than the saturation pressure due to the opening of the mold, the core gets foamed, and the skin of the material is formed by the cooled mold. By controlling the plates, it is also possible to control and vary the density values across the part. [RRR00, p. 1231-1232][Shu86, p. 61] By controlling the density, it is possible to make parts where either high strength is needed, or if acoustic dampening is required. Because the cavity is filled completely, gas splay marks disappear, and it is also easier to reproduce parts, as the cells are more uniform. Further advantages include the opportunity to use multi-cavity molds, shorter cycle times, and the possibility to produce larger parts. [BB91, p. 582] However, when using this technology, the cost of the mold is higher, as it has to withstand high pressures, and certain shapes are not possible to make. [Mac79]
The USM process, developed by USM Corporation, USA, is the most popular high pressure process.
The obtained part’s density range from 200 to 800 kg/m3 with 250-500 mm dimensions. Compared
to UCC, the cycle time can be shorter by 20-25 %. A variation of this is the Hoover process, which combines low and high pressure structural foam molding. Here, the injection pressure is high (around 140-210 MPa), but the cavity’s is low (2.1-3.4 MPa). The materials typically used are PVC, PO (Polyolefin) and PS, and it is possible to lower the part thickness down to 1.1 mm. [Shu86, p. 61]
Gas-counter pressure process With the gas counterpressure technology, the mold is pressurized
with an inert gas, typically nitrogen, up to around 13 MPa. [BB91, p. 582][Car] This value should be higher than the gas used for foaming, but not too high, to prevent surface defects. [Shu86, p. 71] This is done so the bubbles inside the polymer will not break through to the surface. As the core expands, the skin solidifies, and with this, surface defects are lessened. [BB91, p. 582] Towards the end of the process, the cavity is completely filled, and the density reduction is achieved through the shrinkage of the material. [Shu86, p. 71] Major advantages of this type of foaming include reduced tension in the material, uniform surface and cell structure, and already existing machinery can be easily modified to create foams with gas counterpressure. [Pla][Car][BB91, p. 582] However, the sealing of the mold has to be solved with O-rings, the auxiliary equipment raises investment costs, and visible flow lines appear behind the core. [BB91, p. 582][Pla] The cycle time is also increased a bit, as the mold cavity has to be pressurized at every cycle. [Car]
Figure 2.7: The gas counter pressure technology [Car]
The company called Allied Chemical Corporation uses the so-called "Allied Process" to
manufac-ture foams with gas counterpressure. Typical density values range from 400 to 800 kg/m3 with
thicknesses around 3 mm. The melt is met with a heated mold surface (using the variotherm tech-nology), and because of the inner pressure of the mold, foaming is prevented, and the skin layer is formed first. An inert nitrogen gas at 8 MPa is employed, and is discharged via a bleed value after the skin has been formed, thus allowing the core to be foamed. The resulting parts have smooth, high-quality surfaces. [Shu86, p. 71-73]
Two Component process (co-injection) The co-injection technique was developed to combine
the good surface quality of conventionally molded parts with a foamed core. To do this, two injection molding machine is setup, and by the usage of a two-channel nozzle, the two materials are simultaneously injected into the mold cavity. [EAA81] The combination of two materials offers a few possibilities[EAA81][Shu86, p. 82]:
• Using an engineering polymer for the skin, and a less expensive for the foamed part
• Using a reinforced (e.g. with glass fibers) polymer as the core, and an unfilled for the compact part
• Simply using a tougher material for the skin, and a polymer with high Young’s modulus as the core
It is important to note however, that the resins should show a good adhesion between them, and the shrinkage should also be similar, to prevent separation of the core from the skin. [EAA81] Main combinations used for co-injection are PO-PS and PVC-PO systems, for example. [Shu86, p. 82] The advantages of this technology therefore, is the versatility, the many possibilities of combining different materials with different properties. The surface quality is also expected to be much better, if the process parameters are properly set. The cavity pressure is a bit higher than what is used by the other processes, but still lower than conventional injection molding. Cycle times are also better by 10-30 %, and the density of the part is also around the same value as with the other structural foaming techniques. [BB91, p. 584] The amount of blowing agent affects the ratio of the core/skin, the density, and the cooling time, as such, an optimum amount of foaming additive is desired, to achieve a balance between part weight and cycle speed. [Shu86, p. 84] The main drawback is the employment of two injection molding units, which means that the initial price can double compared to other techniques. [Des]
One of the other technical issues is that the blowing agent has the chance to get to the skin material, which is not desirable. The ICI (Imperial Chemical Industries) process solves this by using a switching valve at the mixing point of the two flows. The Battenfeld-Bayer process improves upon this by permitting simultaneous transition between the two materials. A typical density for the parts produced with the co-injection process ranges from 200 to 500 kg/m3 and the thickness can
be between 3 and 14 mm. [Shu86, p. 84-86]
Microcellular Injection molding Microcellular foam injection molding was first born at the
Massachusetts Institute of Technology, where a supercritical carbon dioxide blowing agent was
employed for foaming. The resulting cell sizes were under 100µm, and in one cm3, 100 million
bubbles were seen. [LPR07, p. 5][KSS97, p. 101]. The idea behind the technology, is that if sufficient number of bubbles smaller than the already existing critical material flaws exist, it is possible to reduce the density of the part, and yet not losing the material’s main mechanical properties. [KSS97, p. 101] With this, it would be possible to save material cost of mass produced items without sacrificing other properties. [Eav04, p. 146]
As mentioned before in chapter [2.1.2], one of the basic requirement for microcellular foaming is the employment of a super critical fluid (SCF), usually carbon dioxide or nitrogen. In addition, homogeneous nucleation is required to create small and a large number of bubbles. This will occur during thermodynamic instability, which happens when the gas solubility decreases. A sudden drop in pressure helps to facilitate this phenomena during the process. [Eav04, p. 146]
This process technology holds many advantages compared to other foaming techniques. First, as the SCF gets dissolved in the polymer melt, the glass temperature of said material will be lower. The direct consequence of this is that the melt viscosity also gets lower, which makes the polymer flow better. As such, a lower amount of injection pressure is needed, which could mean that the same part could be produced with a machine that has lower parameters. [GY12, p. 177] This also means that a lower clamping force is needed. [Trea] With that, mold wearing and mold costs are also lower, as maintenance costs are reduced. [Xu10, p. 7] And finally, cycle time can also reduced by 20-50 %. The main reasons for this are the following[GY12, p. 177]:
• The gas bubbles supply additional packing pressure, which means the holding phase can be disregarded.
• The bubbles that are growing are endothermic, which saves cooling time. • Less weight equals less time needed for cooling.
• The lower viscosity allows for faster filling speeds.
Figure 2.8: Cycle time between traditional and microcellular foam injection molding[GY12, p. 178]
One consequence of eliminating the pack and holding phase is improved dimensional stability, which correlates to no sink marks, no warpage, and no residual stress in the final part. [Xu10, p. 9] Due to the number of cells, part weight can also be reduced. On top of this, nearly all kinds of polymer can be foamed in a microcellular way. The thickness of the part can also be as low as 0.5 mm. [GY12, p. 179] This also means that this technology opens up opportunities to parts which otherwise would be difficult to make in full mold filling due to flow restrictions, as either the clamp force would be too low or there would be injection pressure limit. [Xu10, p. 8] Regarding mechanical properties, it can be summed up, that the flexural strength at 10 % weight reduction is around the same, but it gets lower with decreasing weight. It gets worse for tensile strength: more weight reduction equals to a lower strength. The impact properties vary however, and largely depend on the polymer. [GY12, p. 179]
One of the main disadvantages of this type of technology, and the topic of this thesis, is the surface quality of the finished product. Swirl marks, silver streaks, surface blistering and post-blow can appear on the skin of the part, which reduces the application scope. [GY12, p. 180] The non-transparency of the produced parts are also a limiting factor.[Xu10, p. 9] The process itself is more complicated to achieve than its counterparts, and the increased equipment investment, which comes from buying the machine needed to produce the SCF, and the special screw, that is necessary in the case of MuCell® is also a disadvantage [KIL09, p. 283]
Axiomatics Corp., later renamed as Trexel Inc., focused on commercializing this technology, and held patents rights from 1996 until very recently. [LPR07, p. 5][BOA01] Since then, different companies also started to develop their own microcellular injection molding technology. Optifoam® is made by IKV, and uses a special nozzle sleeve to dose SCF with a regular reciprocating screw. Sumitomo-Demag’s Ergocell® employs a dynamic mixer for the dosing of SCF, and a plunger for the injection, with a modified reciprocating screw. With ProFoam®, the SCF is dosed in the hopper with a special equipment, and also uses a regular screw. An extruder was also developed by Trexel and Engel, where the melt is plunged for injection, but it is not available on the market. The most popular one, and the one developed by Trexel is called MuCell®. [Xu10, p. 3]
In the MuCell® process, during plasticization, the super critical fluid is introduced to the melt. Then, it gets homogeneously mixed and disturbed in the polymer, creating a single phase solution of SCF and polymer melt. The cells start to nucleate when the melt is injected into the low pressurized cavity, and start to grow and stabilize uniformly due to the same pressure levels inside the mold cavity. [Trec] For this whole process however, a special equipment is required.
First, the injection molding machine has to have a bit higher drive execution. Engel recommends using either the evc 440/120, vc 2550/500 with a screw diameter of 30 mm/35 mm or 60 mm/70 mm respectively, or the newer duo machines, including the duo 2500H with 70 mm/90 mm and the duo 11050 with 120 mm, just to name some examples. These usually also come with accumulators, which help during injection. [Kieb]
Next, the gas has to be supplied into the mixing zone of the plasticizing unit by a different equipment. Trexel has two series of products that can do it. The SII series supplies a continuous flow of gas and a valve opens when it is necessary. The T series on the other hand only provides the gas in the required amount discontinuously cycle by cycle, saving energy in the long run. [Kieb] And finally, the plasticizing unit is also different, as it can be seen at [2.9]. Usually, an L/D ratio of 24 is used. The screw has the standard 3-zone plasticizing part, but right after it, a rear shut-off nozzle is needed. The reason for this is because the SCF is metered in the next section, and this nozzle prevents the premature foaming of the material in the plasticizing zone. The mixing zone, where the gas gets introduced, has static mixers that help with the creation of the one-phase solution. At the end of the screw, a front shut-off nozzle is employed, because as previously mentioned, the viscosity gets lower with the reduction of the glass temperature of the polymer melt, thus this prevents the material to flow out of the plasticizing unit. [Kieb]
Figure 2.9: The MuCell screw [Kieb]
In the future, companies might not use microcellular foams, as nanocellular foaming has started to gain academic interest in the world. With 1015/cm3 cell density, it could be a promising new
technology. Reports show that parts with nano cell sizes show the Knudsen effect, which means that thermal conductivity is reduced by 2 or 3 orders of magnitude compared to conventionally foamed products. Some mechanical properties also show better results compared to microcellular foams, and due to the size of the bubbles, it could be used as battery separator, or a filter. [Cos14] However, nuclei coarsening, achieving a stable cell wall, and dissolving 25 wt % CO2 at lower pressures is
still a challenge in the industry. [Noa] If these problems could be solved, it is said that this type of technology will surpass the current state-of-the-art applications for insulation. [LDV15]
2.2.2 Variotherm
The variotherm technology is a type of mold heating system, where the temperature of the tool is quickly heated and cooled down, cycle by cycle. [Hei16, p. 94] The temperature is risen before the injection of the molten polymer, which then will have more flowability. This way, the melt can reach smaller details, down to the sub-micrometers of the mold more easily. Then, the tool is cooled down rapidly to give the part more stiffness and hopefully, a defect-free structure. [KIL09, p. 344] The figure 2.10 shows how the mold temperature changes in one cycle.
Figure 2.10: The mold temperatures during a cycle in (1) variotherm heating system (2) conventional
heating system [Wan09b]
This technology is relevant to this thesis, because numerous research paper have shown that using this type of mold heating system could bring benefits to the overall surface quality of foam injection molded parts. Cha and Yoon et al. showed that the moment when swirl marks appear is at the crystallization (for the crystalline parts) or at the glass transition temperature (for the amorphous regions). If the mold is cooled down below these temperatures, the no slip condition occurs, and these swirls appear, as there is no movement at this state. However, if the melt does not solidify, the gas that has not escaped from the polymer will be forced to move with the fountain flow, and will exit through a gas vent, if there is one. [Che08][Wan09b] A low mold temperature also produces parts with weld lines, and non uniform optical properties. [Wan09b] Maintaining a high enough mold temperature thus prevents the formulation of a frozen layer, keeping the polymer-gas mixture in a molten state on the surface of the mold. The resulting product should have a homogeneous skin without any collapsed bubbles, as these cannot attach and freeze at the surface. After a certain time, the mold is cooled down to give the part a compact layer. [Hei16, p. 54-55]
On top of this, a number of advantages can be attributed to variotherm. Liou et al. showed that putting an insulation layer that raises the temperature of the cavity reduces the injection pressure and the residual stress of the completed part by 35 %. [LS89] This is due to molecular relaxation from the flow-induced shear and elongated oriented state for temperatures above the glass transition temperature. [LS89][SZG16] Numerous study also show that weldlines disappear, and the gloss of the part also improves greatly with this technology. [CJC06][Wan09b][Wan09a][Hua11] The main reason for this is because high enough temperatures ensure that the molecule chains diffuse where the melt fronts meet. [CJC06] Hopmann et al. used a PC/ABS material to see whether using a conventional mold or variotherm mold heating system improved surface quality. By measuring the surface roughness, they showed that for variotherm, the roughness decreases, thus, the surface quality improves. They also concluded, that the temperature risen above the glass transition does ot show any more improvement in quality, and reaches a plateau. They also prove that the mechanical properties of the foamed polymer are diminished. [HLZ17] Interestingly, a different research paper presents the opposite regarding mechanical qualities. Bociaga et al. used a HDPE (high-density polyethylene) foam to investigate the influence of injection molding parameters of the part. They argue, that by increasing the mold temperature, the proportion of the crystalline phase increases, and with that, the weight, density, mechanical properties and gloss of the specimen increased. [BP13] However, variotherm was not used in this case, but this paper could still prove that high mold temperatures are necessary, which can be easily achieved by variotherm. The company GWK, a producer of the variotherm equipment, also promises 50 % less clamping force, excellent surfaces, 40 % thinner wall thickness, and a ROI of 13 months. [Kieb]
On the negative side of variotherm, cycle time is increased. This is due to the constant heating and cooling cycle of the mold heating system, because it takes time to properly achieve the set temperatures. [KIL09, p. 343] Energy-wise, it is also not highly efficient. The constant heating and cooling, next to excessive times required to achieve the needed mold condition increases energy
consumption. [EP, p. 29] GWK promises a return of investment of 13 months, but still, higher initial capital is needed to buy the proper equipment. [Kieb] Personal experience also shows that setting the temperature is not easy and requires trails. Although temperature sensors provide feedback to the equipment, the value of the mold temperature had to be set 20°C more than desired for the heating cycle.
2.3 Surface defects
During the literature research of different structural foam molding technologies, it became clear that one of the main issues is the surface quality of the parts. A bad looking part has its marketing value decreased, thus this limits the application scope of structural foams. [Xu10, p. 530][Hei16, p. 72] All in all, it is possible to differentiate between six surface defects [HH][Cen][HSK07]:
• Post-blow effect • Surface blistering • Swirl marks • Tiger stripes • Jetting • Silver streaks
Figure 2.11: Different surface defects on microcellular foam injection molded parts. a: post-blow, b: surface
blistering, c: swirl marks, d: silver streaks [HH][GY12, p. 180]
Post-blow effect Post-blow effect is also known as "continued foaming". [Ell18] It has been
reported, that these mainly appear at hotspots. [GY12, p. 181] These are areas of the part, where the cooling is not sufficient, and the temperature in these regions are near the glass transition temperature of the material. These areas are usually thick sections, hot spots in the mold, and uncooled core pins. [Treb] Another reason they could appear is when the SCF concentration is too high, and too much gas enters a number of bubbles, creating large cells on the surface. The overflow of gas creates higher pressures inside the bubbles, and if this is larger than the outside pressure, the cells will grow until they reach an equilibrium. [GY12, p. 181] Thus, it is recommended to enhance cooling, so that the mold temperature is uniform as possible, to make sure the cooling time is long enough, and finally, adjusting the SCF concentration also helps. [HH][Treb][Xu10, p. 301]
Surface blistering Surface blistering is a bit similar to the post-blow effect. It happens when a lot of tiny bubbles converge to the surface of a thin-walled part and creates a separate layer from the original product. [HH] It has also been suggested that moisture might be another cause for blisters. [Xu10, p. 301] Moreover, a high melt temperature might increase the cell sizes in the core, and during mold opening, surface blisters appear. [Hei16, p. 85] POM (Polyoxymethylene) without any fillers is the most susceptible crystalline material to this type of surface defect. [GY12, p. 181] To solve this and post-blow, adjusting of several injection molding parameters have been suggested, such as increasing the back pressure to keep the gas inside the core of the part, raising the screw speed to cause high shear rates for good mixture of the polymer-gas solution, and to reduce the energy of the blowout by lowering the injection speed, the mold and melt temperature. [Xu10, p. 300-301]
Swirl marks Swirl marks are basically grooves on the surface. [HH] If fountain flow is assumed
inside the cavity during injection, it can be concluded, that the temperature profile of the melt shows a parabolic pattern, which means that the center of flow front is around the melt temperature, while the outer regions should be around the mold temperature. If this is lower than the glass transition temperature for amorphous or melt temperature for semi-crystalline polymers, than the melt on the mold surface will not flow. [CY05] The marks appear, because the gas gets trapped between the mold and the unmoving polymer melt. [GY12, p. 180] These usually are shaped curly and in the direction of the flow, and appear near the injection gate. [HH] Therefore, the suggestion of eliminating these defects is to keep the mold temperature high, so that the melt will not solidify on the surface of the mold. It is important to note, that as mentioned before, when gas gets mixed with a polymer, it lowers the glass transition temperature and the viscosity of the melt, which means that a bit lower mold heat is also enough. [CY05] However, a too high temperature is also not desirable, as Yue W. et al. showed that the reason swirl marks appear near the gate, is because the temperatures are higher in this region, and this causes a lower viscosity which in turn results in a low melt strength, the gas by this has an easier time diffusing and the bubbles break up more easily near the surface next to the gate. [HH]
Tiger stripes Tiger stripes are characterized by their striped pattern in the direction of the flow
path, with a repeating glossy and cloudy parts, which is also visible on the opposite side of the part, but here, the stripes are reversed. One explanation for this phenomena is that the melt in the cavity gets unstable. The position of the symmetry center of the flow front gradually varies along the flowpath, which results in a "snake-like" flow, as seen at [2.12]. [HSK07] Another reason for this defect could be the stick and slip behavior of the melt, which is caused by the solidification of the polymer on the mold wall. [Guo07] Heuzey et al. believes that three factors are playing a role in this behavior, mainly, the surface cohesive strength of the not yet solidified melt, the adhesion between the solid layer and mold wall, and the high shear stresses in contact with the mold surface. [Heu97] A number of studies have also shown different views on the formation of tiger stripes. Examples include that this type of defect becomes more severe if the material has larger filler particles, higher melt viscosity, shear modulus and molecular weight. [Guo07] Among lowering these values, Hirano et al. propose that a narrow molecular weight distribution helps. [HSK07]
Jetting Jetting happens when a polymer melt is pushed through restrictive area, like nozzles, at high velocities, without contacting the mold wall. The resulting formulation can be seen as "small-scale welds". Changing the gate design and altering the melt temperature usually helps to solve the problem. [Cen] This type of defect is not a common occurrence, because proper mold design prevents jetting from happening.
Silver streaks On the other hand, silver streaks are one of the most common problems in
mi-crocellular injection molding. They come in two forms: silver threads and silver strip. The former is caused by the rupture of bubbles, while the latter is called a strip, as it looks like a strip which is in the same direction as the flowpath. [HH][Hei16, p. 72] It has been proposed by Michaeli and Cramer that silver streaks are caused by macroscopic bubbles that are ruptured and sheared along the mold wall during filling. These ruptured bubbles can also appear in front of the melt front when premature cell formation happens in the runner system, and if the adhesion is poor and the pressure is low, the cells can move along the interface, leaving a trail behind, as seen at figure [2.13] [HH][Xu10, p. 287] The silverish coloring comes from the difference in index of refraction caused by these ruptured bubbles. [Che08] By this, it can be concluded that the surface roughness also becomes higher, and upon this basis, many papers evaluating surfaces exploit this, and measure the roughness. [GY12] Compared to swirl marks, the bubbles that are ruptured are smaller and oriented in the flow direction. [HH]
Figure 2.13: The creation of silver streaks. [Kiea]
To defeat silver streaks, the two main approaches are different mold textures and process parameters. A textured mold could have several advantages. First, the gas that slides between the interface of the melt and the mold could vent out between the grooves of the mold surface. Secondly, it could prevent the sliding of the bubbles on the skin of the surface, which means that less bubble shearing would occur. And lastly, because the part would anyway have a very rough surface, it could hide the increase of surface roughness caused by the ruptured cells. [Xu10, p. 557] As for the process parameters, this was the topic of a number of research papers, that will be more closely looked at and summarized in the next section.
In conclusion, currently, three types of approaches are known to solve surface defects. The first one deals with the forming mechanisms of roughness (broken bubble at flowfront, sheared bubble at the interface of mold and melt), where it was proposed that coinjection and gas-counterpressure technologies helps to lower the surface roughness of the part. The second way is with a hot or coated mold, and the third is adjusting processing parameters. [Xu10, p. 288]
2.4 Evaluation methods and process parameters for surface quality
The main objectives of this master thesis were to find the best possible evaluations methods and the most important process parameters for a surface. Unfortunately, currently, no standards exist to characterize a surface. A number of research papers nevertheless conceptualized different methods that are based on the phenomena when a surface defect appears. For the processing parameters,
due to limited time, it was important to look at the current literature and see from the results which process conditions were the best, and which ones affected surfaces the most. Therefore, a critical review had been done to prepare for the part production at Engel, and certain considerations were made for the evaluation methods.
2.4.1 Evaluation methods
The most common method researchers used to analyze the surface is based on the surface roughness of the part. [HH][HLZ17][Lee11][Che08][GY12] As mentioned before in chapter 2.3, the main reason for the visible defects on a surface is mainly caused by the rupture of minuscule bubbles on the sur-face. This means that by measuring the roughness of the surface, one could evaluate quantitatively the surface of a part. A relatively high value equals a bad surface, and a lower one means that the quality is better.
The two values that can be calculated for roughness are Ra and Rz. [HH] Ra is the average
length between the valleys and peaks of the surface and the deviation from the mean value over the whole surface, while with Rz, an average of the five largest distances between the highest and
lowest peaks are calculated. [Fre]
Figure 2.14: The surface roughness model [HH]
Different papers measured roughness differently. Lee et al. measured the average surface roughness at three different locations, and based on the average of these three concluded a surface roughness value for the whole part. [Lee11] Guanghong et al. set the sampling length at 0.8 mm and an evaluation length of 4 mm, and calculated the average of the five values measured at three different points. [HH] Chen et al. set 9 different points on a 100×100 mm plaque, and averaged the roughness values of these points. [Che08] Hopmann et al. also used only eight points for measurements on a 170×145 mm part.
A lower cell size should also mean a better structure and surface, because the smaller the bubbles, the less visible it is on the exterior if it’s ruptured. A number of papers therefore also used SEM or other methods to measure the sizes of the bubbles. SEM was also used to digitize the inner morphology of the foam. [GY12][BR09][Lee11][HH][Guo07]
Guanghong et al. used the SEM images not only to see the morphological structure, but also calculated the cell sizes at the middle of a dumbbell specimen. The point of the injection is not clear in this paper. [HH] Barzegari et al. used a stereomicroscope and SEM to calculate cell sizes, densities and skin thickness of both sides of the part using Image-Pro Plus 4.5. The shape of the specimen, the location and number of measurement points is not discussed in this paper. [BR09] Lee et al. observed the cross-section of the part using SEM and the UTHSCSA Image Tool to check the cell diameters and densities. It is interesting to note that they chose "representative images" to prove their conclusions. At which positions they were created remains a mystery. [Lee11] For their surface roughness and cell size measurements, Guanghong and Yue set the characteristic point near the gate. [GY12] Guo et al. demonstrated two pictures, side by side, to show SEM images 90 mm away from the injection point to demonstrate how the cells were formed at different injection speeds. In the following figure, it can be seen that in this case, a much higher injection speed produced more uniform and unsheared cells. [Guo07]
Figure 2.15: SEM images showcasing cell structures at a., 10 mm/s b., 200 mm/s [Guo07]
And finally, the last two research papers found on this topic exploited the fact that on a black surface, a defect causes discoloring on the exterior of the part. Bociaga et al. used a glossmeter at a 60° light angle to measure the gloss of the surface and a colorimeter to see how much discoloring happened to the surface. These values then were compared to each other to see which process parameter had the biggest effect on the surface. [BP13] Guo et al. calculated the average grayscale levels of the part. The grayscale values were obtained by scanning the parts for digitized images, which were used in SigmaScan Pro to convert the pictures into black and white. The values of each pixels can be between 0, corresponding to a pure black surface, and 255, a pure white one. Here, they called these averaged values "light intensity", which were calculated across the sample length. It was proposed that if the value is below 20, than the quality was considered "good". [Guo07]
2.4.2 Process parameters
As previously mentioned, a way to achieve a better surface is to find the optimal process parameters for a given material. Guo et al. did a thorough research on the effect of injection molding parameters of TPO (Thermoplastic olefin) foams. They noticed that at low injection speeds, premature cell nucleation happened at the flow front, causing many defects on the surface, and employing a much higher speeds help, as seen in figure 2.15 However, bubble stretching still occurred under quick injections. For shot sizes, they conclude that an optimal level is desirable, as low amount of material propagate cell growth, and even though a high value of shot size helped, the void fraction decreased, thus reducing the foamy properties of the material. The melt temperature was also chosen to an optimal level, because they have seen improvements at lower levels, but the CBA (chemical blowing agent) used had a decomposition temperature higher than the "low setting", therefore, a middle value was chosen. For mold temperature, if they injected at a low level, jetting disappeared, but the stretching of bubbles were suppressed only if the cooling time was shortened. Lastly, an increase of back pressure helped only slightly, and was not regarded as an important factor. [Guo07] As they have used CBA, its results are not listed here. For the reason that they made their parts with chemical foaming agents, the suggestion they made for the parameters might not be fully relevant, but can give an indication on which parameters are important.
Bociaga et al. used HDPE with CBA, where they used a 33 DoE with injection velocity, mold
temperature, and melt temperature. They conclude, that a high mold tempering increases gloss, but a high foaming agent content makes the surface much worse, therefore, a middle-level value is recommended for the foaming content. [BP13] Again, due to the CBA, it is not recommended to make conclusions for the case of structural foams, but the suggestion that mold temperature is the most important factor can be used.
A research paper similar to this master thesis is also available by Hopmann et al. They used a PC/ABS blend with MuCell®, and injection molded plaques with four different surface grains which were a mirror finished, a leather structured, a housing structured, and a brush polished one. They also employed variotherm in the hopes of improving the surface quality. A three level DoE with
melt temperature, injection speed, mold temperature, and blowing agent was used. They conclude that variotherm greatly helped, thus mold temperature was regarded as the most important factor, next to melt temperature. For the mold tempering, it should be set close to the glass transition temperature of the material, because raising it more reaches a plateau, and more heating does not help. However, they had also seen that if the flow-path is too long, the surface quality decreases, because as they say, foaming is a flow-path dependent behavior. This means that the weight distribution also changes as foaming occurs during the movement of the melt. They also note, that the pressure at the end of the cavity is lower by a factor of four, which will not be able to "heal" the defects on the surface by variotherm. [HLZ17]
Chen et al. also used a PC resin for microcellular foaming, and used multiple settings for injection speed, mold and melt temperature. They proved that raising all of these values, especially mold temperature, help the surface, as flow marks disappeared, and roughness decreased. They
also reach the conclusion that heating the mold beyond the Tg does not help anymore, and high
injection speeds reduce not only the contact time between the mold and the material, but also the melt temperature drop during injection. Regarding tempering, it is recommended to use induction heating instead water heating. [Che08]
The next research paper written by Hu et al. set four levels for melt temperature, injec-tion speed, injecinjec-tion pressure, pre-injecinjec-tion locainjec-tion and SCF injecinjec-tion time. They employed a polypropylene microcellular foam, the same type of material used in this thesis. By measuring the surface roughness, they argue that the SCF injection time was the most important factor. Although a higher gas content means a faster consumption of foaming agent, which should result in smaller cells, the excessive SCF causes cell coalescence, and this results in a small number of large bubbles, which increases roughness. They also conclude that a high injection velocity will prevent prema-ture cell formation in the runner system, and a high injection pressure will make sure that bubble formation will start in the cavity, and as the cell will require more energy to grow, the size of it will be lower. A high melt temperature will cause a greater amount of SCF diffusion into the melt, but also reduces melt strength and surface tension of the mixture, which means the larger cells will split into smaller ones. With the pre-injection location parameter, they saw that with increasing flow-path, the roughness also increases. [HH] As also noted by Hopmann et al., the cavity pressure has an effect on this, and this research paper also verifies that increasing the pressure inside the cavity will have a positive effect. [HLZ17][HH]
Lee et al. microcellular foamed LDPE (low-density polyethylene) and PP parts, and employed different settings for the speed, dosage time, and the amount of gas. They propose that by reducing the degree of super saturation, the activation energy required for bubble nucleation will be higher, therefore, it will reduce cell nucleation. By doing this, a similar effect when using high injection speeds will happen, namely, it prevents premature cell nucleation in the runner system. If the amount of gas reaches a certain limit, it will supersaturate the melt, thus cell nucleation will increase, and will create swirl marks on the surface. It should be noted however, that the sizes of the bubbles where they noticed no swirl marks were more than 100 µm for PP, and 600 µm for LDPE, which is much higher than the usual bubble sizes for microcellular injection molding, as referred previously. [Lee11]
Barzegari et al. studied the relationship between molding conditions and morphology for LDPE structural foams. The DoE they had set up were at 4 levels, and had blowing agent content, mold, melt temperature, injection pressure, and back pressure as important parameters. They saw that the important factors were injection pressure and the blowing agent content for cell sizes and densities. Mold temperature and back pressure in this case only affected the thickness of the skin significantly, while mold and melt temperature had no effect on the cell density. However, this research paper only dealt with inner morphology, and not surface quality, but nevertheless, gave ideas which parameters should be considered for this thesis. [BR09]
