7.3.1 Final degree of impregnation after production
After processing the differently impregnated tapes with various dwell times, micro-graphs served to determine the final degree of impregnation (DOIf) of the produced test panels. The micrographs were prepared by following the same procedure as de-scribed in subsection 2.3.4. Representative micrographs of the manufactured test panels are compared in Figure 7-4.
100% DOIi
80% DOIi 90% DOIi
90s 300s 90s + TF 600s 1200s Laminate Production - DOIf
Initial state - DOIi
Figure 7-4 Micrographs of laminates produced with varying dwell times based on differently impregnated tapes with highlighted non-impregnated areas if applicable.
The micrographs and the analyzed DOIf reveal complete impregnation after pro-cessing tapes with different DOIi after a dwell time of 90 s with additional thermo-forming or when the press time exceeds 600 s.
Tapes with a DOIi of 80 % show non-impregnated fiber bundles after a dwell time of 300 s while complete impregnation is observed when these tapes were thermo-formed. In general, the dwell time of the polymer above Tm and under pressure
is longer for laminates pressed with 300 s compared to laminates pressed with 90 s that were additionally thermoformed. However, additional impregnation progress during thermoforming is assumed to occur in the heating phase by infra-red sources.
Besides the laminate, the surrounding aluminum mold has to be heated. Compared to the use of aluminum foils as tooling for press forming, the heating phase is longer during thermoforming due to the use of a tool. The effect of the low pressure (ap-proximately 0.004 bar) applied to the laminate by the upper mold half during thermoforming on the impregnation progress is negligible.
With exception for the dwell time of 90 s, the processing of tapes with a DOIi of 90 % with prolonged press times or thermoforming resulted in completely impreg-nated fiber bundles. Table 7-3 summarizes the different dwell times used for the production of test panels in a static press and via thermoforming from differently impregnated tapes along with the resulting DOIf.
Table 7-3 Dwell times used to process differently impregnated tapes in a static press and via thermoforming along with the final DOIf of all test panels.
Designation of test panels
Dwell time DOIf analyzed from micrographs [%]
static press [s] thermoforming [s]
80 %_90 s 90 - 94.51±0.01
90 %_90 s 90 - 97.29±0.01
100 %_90 s 90 - 100.00±0.00
80 %_90 s+TF 90 8 100.00±0.00
90 %_90 s+TF 90 9 99.40±0.01
100 %_90 s+TF 90 8 99.81±0.00
80 %_300 s 300 - 95.66±0.01
90 %_300 s 300 - 100.00±0.00
100 %_300 s 300 - 100.00±0.00
80 %_600 s 600 - 100.00±0.00
90 %_600 s 600 - 100.00±0.00
100 %_600 s 600 - 100.00±0.00
80 %_1200 s 120 - 100.00±0.00
90 %_1200 s 1200 - 100.00±0.00
100 %_1200 s 1200 - 100.00±0.00
The results from FVC and thickness measurements of all test panels can be found in section A.3 of the appendix. Averaging the measured FVC over all test panels, a mean of 45.23±1.08 % is obtained. The thickness of test panels pressed for 600 s and 1200 s is increased as another ply ([012]) was added compared to the remaining laminates. The consolidated tape thickness was found to decrease with increasing press time in earlier studies. By adding another ply, an almost constant FVC is achieved for all test panels enabling comparability.
7.3.2 Flexural properties
Longitudinal flexural properties
The effects of the DOIi of tapes on the longitudinal flexural strengthσf1 and mod-ulus Ef1 of test panels produced with different dwell times in a static press and thermoforming are shown in Figure 7-5.
80%_1200s
Figure 7-5 a) Longitudinal flexural strengthσf1and b) longitudinal flexural modulusEf1of test panels made from CF-TP/B3S tapes with different DOIi that were processed with varying dwell times in a static press or thermoformed (+TF).
As discussed in Chapter 3, the significantly different cooling rates during processing in a static press (up to 20 °C/min) and during thermoforming (up to 380 °C/min) were found to have negligible influence on the crystallization and hence on the mechanical properties of CF-TP/B3S. Therefore, the mechanical test results from laminates produced in a static press can be compared to thermoformed test panels without limitation.
Referring to short dwell times of 90 s, the longitudinal flexural strength σf1 slightly decreases with decreasing DOIi. This goes along with incomplete impregnation
(95-97 %) found after processing tapes with a DOIi of less than 100 % for 90 s.
An overall low strength level is found for test panels pressed for 90 s compared to laminates with extended press times. This indicates that a dwell time of 90 s is not only insufficient to achieve complete impregnation in the case of 80 %- and 90 %-impregnated tapes but also deficient for consolidation between plies referring to 100 %-impregnated tapes. Extending the dwell time or with additional thermo-forming leads to similar σf1 for completely impregnated tapes and tapes with a DOIi of 90 %. For all test series, the tapes with a DOIi of 80 % yielded the lowest σf1. The longitudinal stiffness (see Figure 7-5b) is not affected by the DOIi of the used tapes, with exception of tapes with a DOIi of 80 % press formed for 90 s.
Transverse flexural properties
The transverse flexural properties for test panels produced from CF-TP/B3S tapes with different DOIi and with different process settings are reported in Figure 7-6.
0
Figure 7-6 a) Transverse flexural strength σf2 and b) transverse flexural modulusEf2 of lam-inates made of tapes with different DOIi that were processed with varying dwell times in a static press or thermoformed (+TF).
A press time of 90 s yielded a comparable transverse flexural strength σf2 within standard deviation independent of the DOIi. As observed for the longitudinal flex-ural strength, σf2 of laminates pressed for 90 s remains at a lower level than test panels that were pressed for prolonged press times or thermoformed. However, the level of σf2 obtained from laminates based on completely impregnated tapes is not reached when tapes with DOIi of 80 % and 90 % were pressed for 300 s or 90 s with additional thermoforming.
When pressed for 600 s, test panels made from partially impregnated tapes approx-imate the σf2 of laminates made from completely impregnated tapes. Tapes with a DOIi of 80 % and 100 % show even comparable values for σf2. When test panels were pressed for 1200 s σf2 becomes independent of the DOIi.
In contrast to the measured longitudinal properties, σf2 of test panels produced from tapes with a DOIi of 90 % is found to be slightly lower than of laminates made from tapes with a DOIi of 80 %. Slight increases in FVC for the laminates made from 90 % impregnated tapes may cause decreased values for σf2. Since the micrographs document complete impregnation for tapes with a DOIi of 90 % upon pressing for 300 s or thermoforming, the lower σf2 cannot be attributed to insuffi-cient impregnation. In addition, insuffiinsuffi-cient air evacuation and insuffiinsuffi-cient time for relaxation of the fiber bed during manufacture is assumed to cause lower σf2. After pressing for 90 s, the transverse flexural modulus Ef2 (Figure 7-6b) of 80 % impregnated tapes yields significantly lower values than laminates produced from tapes with a DOIi of 90 % or 100 %. As soon as the press time exceeds 300 s the transverse modulus becomes independent of the DOIi of tapes.
Obviously, the transverse flexural strength is more dependent on the DOIi than the longitudinal flexural strength. In general, tapes with a DOIi of 90 % faster converge or exceed reference values from completely impregnated tapes than tapes with a DOIi of 80 % upon processing.
7.4 Cost analysis
For process costing, the consumption of resources during processing is determined and evaluated monetarily. So, the arising expenses, divided into cost types, are cap-tured for each process step. The cost types comprise of material costs, labor costs, overheads, direct expenses and other recurring costs [7]. Recently, process costing has been favored over parametric, empirical-based and performance-oriented mod-els for cost accounting of components made of CFRP. This is mainly attributed to the low amount of data required for the estimation of process-based costs and the high transparency of the used cost accounting type.
7.4.1 Procedure
In the following, the theory is presented to calculate process-based costs by using the machine-hour-rate approach. The data collection was conducted by an expert consultation to establish the basis for process costing. In dependance of varying operating speeds in a double-belt press, the machine hour rate and the throughput is calculated.
The data collection and cost analysis has been conducted within the frame of the student project from Miriam Ernst and Patrick Consul [202].
Process costing
Within the frame of process costing, the production costs are calculated by adding material costs and manufacturing costs. These were further divided into direct costs and overheads as presented in Equation 7-1. In addition, special direct costs that cannot be attributed to material or manufacture are taken into account.
ΣP roduction costs=M aterial costs+M anuf acturing costs
=Direct material costs+Overheads
+Direct manuf acturing costs+Overheads +Special direct costs
(7-1)
With the presented cost analysis, the effect of partially impregnated tapes on pro-duction costs shall be evaluated. The material costs for carbon fibers, matrix as well as the manufacturing costs for spreading the fibers, grinding the polymer pellets and powder-coating prior to tape consolidation in a double-belt press are indepen-dent of the operating speed. Thus, the costs for powder-coated tows are excluded from the cost analysis. The focus lies on the direct costs and overheads for manu-facturing in a double-belt press.
To begin with, the production volume was estimated to set the basis for the cost analysis. The assumptions made to calculate a realistic production volume in 2020 are summarized in Table 7-4 and are based on the market report by the AVK Fed-eration of Reinforced Plastics and Carbon Composites e.V. in 2015 [3].
Table 7-4Assumptions made for cost analysis based on forecast for 2020.
Forecast 2020 Quantity
Sales quantity of carbon fibers in 2020 100,000 t/a Carbon fibers used with thermoplastics (2014) 15 % Proportion of prepreg layup processes (2014) 45 %
Target market share in 2020 1 %
Based on the data presented in Table 7-4, the production volume of carbon fiber reinforced UD tapes in tons per year for 2020 was calculated as follows:
P roduction volume (2020) =F orecast (2020)·CF RT P share·
P repreg layup share·T arget market share
= 67.5t/a
(7-2)
Subsequently, the manufacturing costs are determined by using the machine hour rate approach. The overheads for manufacturing is also known as factory overhead and comprise fixed maintenance costs, tooling costs, costs for auxiliary material, costs for power and occupancy. Since the required personnel to operate a double-belt press can be directly allocated to the equipment, the labor costs are also attributed to the direct manufacturing costs.
The running time or occupation time of the double-belt press is determined by subtracting the non-productive time from the productive time as Equation 7-3 shows. The non-productive time consists of down time and test time whereas main time, setup time, secondary time and additional time account for the productive time.
Running time=P roductive time−N on-productive time (7-3) Related to the double-belt press, the non-productive time is composed of the setup time, cleaning time, heating-up period and servicing period. In combination with the shift length, the actual working time is calculated that corresponds to the running time of the double-belt press. Based on the running time and the factory overhead, the machine hour rate can be determined:
M achine hour rate [e/h] = F actory overhead Running time
=Labor costs [e/h]· W orking time Running time +M aintenance costs[e/h]
+Auxiliary costs [e/h]
+Equipment costs [e/h]
+Occupancy costs [e/h]
+Depreciation[e/h]
+Downtime [e/h]
(7-4)
The depreciation of the double-belt press can be calculated as follows:
Depreciation[e/a] = Initial costs [e]
Recovery period [a]·Imputed interest (7-5)
The downtime of the equipment is evaluated by the failure probability of the util-ities, expressed as costs:
Downtime costs[e] =F ailure probability [−]·U tilities [e] (7-6) The total costs are further divided into fixed and variable costs. The fixed costs comprise of the imputed depreciation and interest, the occupancy costs and the fixed maintenance costs. These costs incur time-dependent, are independent of the capacity utilization level as well as of the operating rate. Costs for tooling, variable maintenance, auxiliary and equipment represent the variable costs.
7.4.2 Data collection
The feasibility of the selected operating speeds with regard to the production of partially impregnated tapes was proven by the previous study on the impregnation progress during processing. Aside from this information, written questionnaires were prepared to conduct interviews of experts to gather information about the fixed and variable costs of double-belt press equipment. The questionnaires were sent to the selected experts and complemented by telephone interviews.
The experts were carefully selected from suitable companies and institutes. For the tape production process, experts from SGL Carbon GmbH and the research institute Thüringisches Institut für Textil- und Kunststoffforschung (TITK) were interviewed.
Experts from Sandvik TPS, Hymmen GmbH and Held Technologie GmbH were interviewed for data collection about double-belt press equipment. Sandvik TPS is the market leader for isochoric double-belt press, Held Technologie GmbH for isobaric double-belt press. Hymmen GmbH produces isobaric as well as isochoric double-belt press equipment [7]. Consulting experts from all three companies for double-belt press equipment enables the generation of a reliable information basis.
Due to the use of questionnaires, the data collection was carried out in a systematic manner that allows to compare the responses from the interviewed experts.
The collected data from expert consultation and inquiry were allocated to the categories labor costs and manufacturing costs as stated in Table 7-5. It is assumed that the operation of a double-belt press requires operatives only and no engineers or office workers.
The heating period is divided by the number of shifts as another heating at the beginning of the second shift is not applicable. Considering edge trim and scrap during the production, the actual produced volume becomes smaller. The required
Table 7-5Collected data used for the cost analysis.
Cost type Quantity
Labor costs
Required staff [persons] 2
Personnel costs [e/h] 45
Number of shifts [-] 2
Shift length [h] 8
Working time [h/week] 35
Working days per year (2016) [d/a] 253
Non-productive time [h] 2.28
Occupancy costs [e/(m2month)] 2.20
Power costs [e/kWh] 0.15
Initial costs [e] 4,000,000
Floor space required (equipment, periphery, logistics) [m2]
275
Work width [m] 1.5
Amount of tapes fed into double-belt press [-] 3
Tape spool length [m] 250
Imputed interest [%] 5
Recovery period [a] 10
Additional costs (e.g. reconstruction) [e] 0
Power input [kW] 500
Power requirement [%] 30
Operating performance (Power input * Power requirement) [kW]
150 Auxiliary materials (release agent) based on
2 m/min [e/h] 25
Cleaning time [h] 0
Setup time [min per spools/shift] 5
Scrap [%] 2
Edge trim [%] 5
Heating-up period [h] 2
Failure probability of equipment [%] 0
Utilities (steel strip) [e] 60,000
Service life (steel strip) [h] 10,833
production volume has to incorporate edge trim and scrap to produce the desired production volume as Equation 7-7 presents:
Required production volume[t/a] =P roduction volume +
=Edge trim+Scrap
= 72.23t/a
(7-7)
Based on the required production volume the number of identical double-belt press equipment is determined to manufacture the necessary amount of UD tapes.
Required equipment=Round up
Required production volume T hroughput per year
(7-8)
7.4.3 Monetary effect of partially impregnated tapes
Based on the collected data, the labor and manufacturing costs are determined for the varying double-belt press speeds as used during the study on gradual impreg-nation. Thus, the machine hour rate and finally the manufacturing costs for the produced partially impregnated tapes can be calculated as presented in Table 7-6.
Table 7-6 Calculation of the machine hour rate for varying operating speeds of a double-belt press.
Cost type Quantity
Fixed costs
Number of required equipment 1
Labor costs [e/h] 125.87
Occupancy costs [e/h] 2.51
Depreciation [e/h] 145.11
Power costs [e/h] 77.39
Variable costs
Operating speed [m/min] 2 4 8
Throughput per shift [kg/shift] 196.03 392.06 784.12 Throughput of kg UD Tape per hour [kg/h] 34.27 68.54 137.08 Edge trim, scrap costs [e/h] 57.39 114.78 229.57
Maintenance costs [e/h] 3.87 7.75 15.49
Auxiliary materials and utilities [e/h] 34.97 69.93 139.86
Downtime costs [e/h] 0
Machine hour rate [e/h] 447.11 543.34 735.80
Manufacturing costs tape [e/kg] 13.04 7.93 5.36
Reductions in manufacturing costs [%] Reference -39.18 -58.90
Obviously, the machine hour rate increases with increasing operating speed since more auxiliary materials are required and the incurring edge trim as well as scrap increase. At the same time the throughput increases proportionately to the oper-ating speed. The manufacturing rate thereby decreases with increasing operoper-ating speed. Producing tapes with an operating speed of 4 m/min, the manufacturing costs for UD tapes decrease by 39 % compared to tape production with 2 m/min.
Increasing the operating speed further to 8 m/min the manufacturing costs reduce by 59 % compared to UD tapes that are produced with 2 m/min.
7.5 Correlation of mechanical properties and manufacturing costs
The influence of the different DOIiof tapes on flexural properties of test panels upon processing with different dwell times was previously determined during the study on gradual impregnation. Thus, the effects on mechanical performance resulting from the use of partially impregnated tapes can be correlated to the incurring manufacturing costs estimated by the cost analysis in Figure 7-7. The reference is represented by completely impregnated tapes that were pressed for 1200 s and is shown in the center of each of the diagrams below.
-60 -40 -20 0 20 40 60
compared to reference values obtained from completely impregnated tapes pressed for 1200 s.
Considering the strength in fiber direction as potential design criteria, the manu-facturing costs of UD tapes can be decreased by 39 % when a loss of about 11 % is acceptable after thermoforming or a press time of 300 s. Increasing the dwell time
to 600 s and 1200 s, only a minor loss of 5 % and 2 % is to be expected. Less than 10 % reduction in σf1 associated with cost savings in tape manufacturing of 59 % are achieved when tapes with a DOIi of 80 % are pressed for 1200 s.
As previously mentioned, the transverse flexural strength is affected to a greater extent than the longitudinal flexural strength. A loss in mechanical performance of about 10 % may be acceptable when considerable savings in manufacturing costs are achieved at the same time. When pressing tapes with a DOIi of 80 % and 90 % for 1200 s, less than 10 % loss in σf2 can be expected while reducing tape costs by 39 % to 56 %.
If stiffness in fiber direction is meant to be the dominating design allowable re-ductions of less than 5 % have to be approved resulting from the use of partially impregnated tapes. Transverse to the fiber direction, a moderate loss in stiffness can be expected when partially impregnated tapes are pressed for at least 600 s.
7.6 Conclusion and implications
In a comprehensive study, the influence of varying DOIion flexural composite prop-erties was tested. UD tapes with a DOIi of 80 %, 90 % and 100 % were produced by varying the operating speed and pressure in a double-belt press. Test panels made of these tapes were produced in a static press with press times from 90 s up to 1200 s. Some laminates were also thermoformed to account for a typical production process for CFRTP components.
The impregnation state before (DOIi) and after processing the partially impreg-nated tapes (DOIf) was analyzed from micrographs. The micrographs as well as the obtained DOIf document a further impregnation progress during CFRTP pro-duction.
To enable the evaluation of effects of DOIi on composite properties, four-point bend tests were conducted for all laminates. A dwell time of 300 s or 90 s with additional thermoforming was sufficient to complete impregnation of tapes with a DOIi of 90 % confirmed by the results for the longitudinal flexural strength σf1. Tapes with a DOIi of 80 % appeared to be completely impregnated after press-ing for 90 s with additional thermoformpress-ing. Exceedpress-ing the press time of 600 s, the longitudinal strength became independent of the DOIi. Stiffness in fiber direction was found to be less affected by the DOIi of the used tapes and shows comparable values for all test panels upon a dwell time of 300 s or thermoforming.
The flexural properties transverse to the fiber direction are affected by the DOIi
to a stronger extent indicating that dwell times need to be increased to enable fiber bed relaxation and sufficient consolidation of tape plies with a DOIi of less than 100 %. σf2 became independent of the DOIi upon pressing for 1200 s.
How-ever, 80 % and 100 % impregnated tapes yielded comparable values for transverse strength when pressed for 600 s. Stiffness in transverse direction was found to fur-ther increase with extending dwell time while it became independent of DOIi as soon as the press time exceeds 300 s.
The loss in mechanical performance was opposed to the monetary impact by
The loss in mechanical performance was opposed to the monetary impact by