INVESTIGATIONS ON INJECTION MOLDED, LONG …injection molded, long-glass-fiber reinforced integral...

INVESTIGATIONS ON INJECTION MOLDED, LONG-GLASS-FIBER REINFORCED INTEGRAL FOAMS USING BREATHING MOLD

TECHNOLOGY

A. Roch, L. Kehret, T. Huber, F. Henning, P. Elsner Department of Polymer Engineering, Fraunhofer-Institute for Chemical Technology ICT, Germany

Fraunhofer Project Centre for Composite Research at the Western University, Canada

Abstract

This paper analyses the lightweight potential for automotive applications obtained by injection molded, long-glass-fiber reinforced integral foams using breathing mold technology. Extensive investigations of 2 material systems, PP-LGF and PA6-LGF, were carried out using both, chemical blowing agents and a physical blowing agent. During injection of a gas-loaded melt into a cold mold, integral foam occurs consisting of a compact skin and a foamed core over entire cavity, regardless of the complexity of the molded component. The integral foam design, which can be conceived as a sandwich structure, helps to save material in the neutral axis area and maintains a distance between load-bearing, compact (unfoamed) skin layers. This sandwich structure leads to a high bending stiffness at a low surface weight, which is required by many automotive applications, especially for large-area covering elements (such as door panels, spare wheel wells, seat shells and backrests, underbody assemblies or instrument panel supports).

The experiments showed that, at a constant surface weight, long-glass-fiber reinforced integral foams have a significantly higher bending stiffness than compact components, due to their higher area moment of inertia after expansion achieved through precision mold opening. The area moment of inertia and with this the bending stiffness is increased with the third power of the wall thickness. For that reason, a small increase in wall thickness leads to a significant higher flexural rigidity. Compared to the compact reference, an increase of the flexural rigidity for all investigated combinations of material and blowing agent could be realized. At a constant surface weight, the bending stiffness in these experiments could be increased by up to 600 %. An instrumented impact penetration test, applied for PA6-LGF50, showed growing energy absorption and a Charpy impact bending test, applied for PP-LGF30, showed nearly constant behavior with increasing density reduction. The delay time before mold opening influences the thickness of the solid skin. As seen with PP-LGF30, through thicker skins the bending and the impact strength seemed to increase, while the tensile properties seemed to slightly decrease.

Introduction

By combining injection molding of fiber-reinforced thermoplastics with foam injection molding (FIM), fiber-reinforced integral foams with compact skin layers and a foamed core can be produced in-situ in the mold as a single material system [Roc13]. A blowing agent is added to the polymer melt during plasticizing, causing the material to expand due to a pressure drop during injection. Several processing advantages, such as reduced clamping force, shorter cycle times, lower melt viscosity, as well as advantages for the molded part, like density reduction, higher dimensional stability, high specific bending stiffness, can be achieved using foam injection molding [Alt10]. These advantages are especially important when fiber-reinforced materials are used, where the reduced melt viscosity of gas-loaded thermoplastics leads to a

fiber preserving processing [Zha05].

Initially, the two phases blowing agent (gas or supercritical fluid) and the polymer melt are mixed to achieve a single-phase gas-polymer mixture in front of the screw. Single phase mixture is realized due to mechanical mixing of the rotating screw and additionally due to diffusion and dissolving of the blowing agent inside of the polymer melt during the plasticizing stage. In order to hold the blowing agent in solution, the compound has to be kept under a permanent pressure (back-pressure / screw position control necessary). Finally, after injection of the gas-loaded polymer, a pressure drop occurs resulting in the expansion of the material inside the cavity.

Figure 1: Physical phenomena during foam injection molding

During expansion inside of the cooling mold, integral foam is achieved consisting of a compact (unfoamed) skin and a foamed core over entire cavity, regardless of the complexity of the molded component. The integral foam design, which can be conceived as a sandwich or an I-beam, helps to save material in the neutral axis area and maintains a distance between load-bearing, compact skin layers, as shown in Figure 2. This sandwich structure leads to a high bending stiffness at a low surface weight, which is required by many automotive applications, especially by large-area covering elements (such as door panels, spare wheel wells, seat shells and backrests, underbody assemblies or instrument panel supports).

BlowingAgent

Polymer

mixing / solving

gas sorption and diffusion

single-phase gas-polymer mixture

nucleation cell growing andstabilization

injection unit mold cavity

Figure 2: Cross-section of a PP and PP-LGF integral foam and their density / young’s modulus distribution.

Integral foams can be considered as a sandwich or an I-beam.

In order to realize long-fiber-reinforced integral foams, LFT granules containing impregnated fibers in combination with chemical (CBA) or physical blowing agents (PBA) can be used. CBAs are substances (powder or granules added as a masterbatch) that decompose during plasticizing and release the foaming gases. For this processing configuration no additional machine equipment is necessary. In terms of PBAs the MuCell® process is the most common process technology, which uses N2 or CO2 as the foaming gas. For both processing options an injection unit with back-pressure / screw position control and a shut-off nozzle is necessary to keep the gas-polymer mixture under pressure, as shown in Figure 3. For LFT processing a fiber preserving screw geometry in both foaming processes is strongly recommended [Kni07; Sch03].

Figure 3: Processing of LFT using CBA (left) and MuCell® with LGF-Screw (right)

PP integral foam PP-LGF30 integral foam

LFT granulates +

Chemical Blowing

Agent (CBA)

non-return valve

shut-off

nozzle

screw position /

back pressure control

gas dosing station

(SCF unit)

CO2

N2

injector

LFT granulates

shut-off

nozzle

fiber-preserving

gas-mixing-zone

non-return valvesscrew position /

back pressure control

In general, two different foaming techniques can be chosen:

Low-pressure process: partial filling of the cavity ( low cavity pressure)

High-pressure process: complete filling of the cavity and decompression of the mold ( high cavity pressure)

In the first case, the residual flow path in the cavity and the shrinking of the material are compensated by the occurring blowing pressure, therefore there is no need to apply a packing pressure. For that reason a relatively low cavity pressure is occurring and significantly lower clamping forces compared to conventional injection molding are needed. Using this molding technique, only small density reduction and hence only small weight savings in the range of roughly 5-15% (depending on wall thickness / flow length ratio) can be achieved. Unfortunately, due to streaks that occur during foaming, in many cases the surface quality is not sufficient for visible part applications.

Figure 4: Low pressure (left) and high pressure process (right)

For the high-pressure process, the cavity needs to be filled completely. For this purpose, often a short packing pressure is applied. With a short delay time, before opening the mold, the skin layer thickness of the integral foam can be adjusted very precisely. After increasing the cavity volume the occurring pressure drop causes the building of the integral foam. In contrast to the low-pressure process, a quite homogenous foam design along the flow path, a comparatively good surface quality, and significantly higher density reductions up to over 50 % can be achieved. The mold cavity can be increased by precision mold opening of a shear-edge mold or by core-back technology. Figure 5 shows some examples of possible mold designs. In literature many synonyms for this foaming technique can be found including breathing mold, core-back, negative embossing, foaming with decompression or precision mold opening. [Eck81; Mül06; Spö10]

partial filling of the cavity

Complete filling by blowing pressure

precis ion mold opening

complete filling of the cavity

short delay time skin layers

Low-pressure process High-pressure process

Figure 5: Examples of mold technologies for high-pressure processes A) Shear-edge mold, B) Short-stroke tool frame, C) Multi-core-back

Experimental

The investigations with PP-LGF30 (Dow Automotive, initial granule length 11 mm) and PA6-LGF50 (BASF, initial granule length 12 mm) have been carried out using both, a chemical and a physical blowing agent. The production of all specimens in these experiments was carried out using the same injection molding machine: Engel duo 700 pico combi M. For CBA endothermic, commercial grades from Clariant Masterbatches GmbH were chosen, and for PBA nitrogen (N2) was selected. For the investigations with CBA a standard injection unit with a screw diameter of 105 mm and long-fiber optimized screw geometry was used. For the incorporation of the PBA into the polymer melt a second injection unit, within the same injection molding machine, equipped with MuCell®, a screw diameter of 80 mm and a special fiber-preserving screw geometry was applied.

Using the introduced machine equipment flat sheets with an area of 500 x 500 mm2 were produced for each combination of material and blowing agent using an shear-edge mold. A needle shut-off nozzle with an opening diameter of 5 mm was used as injection gate. Since the initial mold gap (PP-LGF30 3.6 mm / PA6-LGF50 2.5 mm) and therefore the injected amount of material was kept constant for each shot, a constant surface weight of each test specimen was obtained. Using the breathing mold technology (decompression of the mold) the final wall thickness and thereby the final volume of the plaques was adjustable. After injection of the gas-loaded melt, the mold was opened to a defined gap causing a pressure drop which initiated the foaming process. All other molding conditions were kept constant for each respective material system. Unfoamed references plaques as well as plaques with different density reductions (Table 1) were manufactured for each material combined with each blowing agent.

Table 1: Targeted wall thickness H and density reduction Δρ

PA6-LGF50 H [mm] 2.5 2.8 3.6 5.0

Δρ [%] 0 10 30 50

PP-LGF30 H [mm] 3.6 4.0 4.4 4.9 5.35 5.8

Δρ [%] 0 10 17 25 31 37

Test specimens for impact testing, 3-point-bending test and tensile test were cut out of the plaques in the main fiber direction using water jet cutting. For the PA6, the test specimens were dried in a circulating air oven before testing.

Equal initial wall thickness Different final wall thickness

A B C

Results and Discussion

In order to compare the mechanical test results of compact specimens with foamed specimens all strength values are shown in a density specific presentation. Additionally the values are related to the compact (unfoamed) reference processed with the MuCell® injection unit and plotted against the density reduction. This presentation facilitates the comparison between different density reductions and foaming processes used in these experiments. Thus influences of the blowing agent type and respectively the foaming process on the mechanical properties can be better estimated.

Results for PP-LGF30

Figure 6 (left) shows the relative, density specific bending stiffness that is described as the product of the bending modulus and the second moment of inertia I:

IES BB * (I)

12

* 3HBI (II)

According to this relationship, the second moment of inertia I and in turn the bending stiffness SB are increased with the third power of the wall thickness H. For that reason, a small increase in the wall thickness leads to a significant higher bending stiffness. Compared to the compact specimens an obvious increase of the flexural rigidity for both investigated foaming processes could be realized. For instance, at the highest density reduction an increase in bending stiffness of roughly 330 % using CBA and 275 % for MuCell® could be measured.

Figure 6: Relative, density specific bending stiffness (left) and skin layer thickness (right) over density reduction Δρ / wall thickness H

It can clearly be seen, that the CBA curve is representing superior bending behavior. This obvious gap in bending behavior can be attributed to different skin layer thicknesses, as shown in Figure 6 (right): in these experiments thicker skin layers could be qualitatively observed when CBA as the blowing agent was applied. Especially if exposed to bending loads, mainly the skin layers are strained. It is assumed that the specific type of the applied foaming gas play a decisive role in terms of occurring skin layer thickness. During decomposition of the endothermic CBA mainly CO2 was produced, while nitrogen (N2) was chosen for MuCell® as the

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blowing agent. Nitrogen is said to deliver a higher blowing pressure in combination with the most resins compared to CO2 at the same concentration. The hypothesis can be made that probably this behavior leads to a thicker foamed core for PBA, since a lower blowing pressure might not be sufficient for bubbles nucleation in the high-viscosity area (near the cooling cavity wall) of the cross section.

Figure 7: Relative, density specific Charpy impact strength over density reduction Δρ

Figure 7 shows the relative, density specific Charpy impact strength that was measured flatwise on unnotched specimens (Charpy ISO 179-1/1fU). During impact bending test, it is highly recommended to test the specimens flatwise (comp. Figure 8 right), since in reality a foamed part is always loaded flatwise. Furthermore, if loaded edgewise, foamed parts would be noticeable disadvantaged at higher density reductions due to the notch effect of the cells, like illustrated in Figure 8 left for PP-LGF30.

Figure 8: Different impact behavior of flatwise and edgewise loaded integral foam specimens

Compared to non-reinforced foams that mostly show significant embrittlement during foaming [Spö10], the energy absorption capacity (impact strength), for both foaming processes, remained almost constant over the adjusted density reductions, comp. Figure 7. Similar to the bending behavior, a distinctive difference between CBA and MuCell® could be observed. In this case, the various skin layer thicknesses might influence the impact behavior as well, but

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furthermore, especially if exposed to dynamic loads, the average fiber length in the final molded part has to be taken into account. The average fiber length seems to have a bigger influence since already both compact references (not foamed) in Figure 7 shows a significant difference between MuCell and CBA processing. For that reason, it might be assumed that the smaller screw diameter and the additional non-return valve in combination with the gas-mixing zone of the MuCell® injection unit led to higher fiber length degradation than the screw applied for CBA. In addition, at the CBA injection unit the LFT material has to pass only a single non-return valve and the screw diameter is slightly larger. Generally, in order to achieve good impact and creep properties in long-glass-fiber-reinforced polypropylene the average fiber length should be as high as possible [Tho97; Tho96; Sch08].

Results for PA6-LGF50

Figure 9: Relative, density specific bending stiffness (left) and skin layer thickness (right) over density reduction Δρ

Figure 9 left shows the relative, density specific bending stiffness obtained by the PA6-LGF50 specimens. For instance at the highest density reduction an increase in bending stiffness of roughly 190 % using CBA and 630 % for MuCell® could be measured. Similar to the results from PP-LGF30, the CBA foamed specimens show a better behavior in terms of bending stiffness than the MuCell® specimens. This can also be explained by the different skin layer thicknesses as shown in Figure 9 right. As well in the experiments with PA6 qualitatively thicker skin layers could be observed if CBA as blowing agent was applied.

Unfortunately the targeted density reduction of roughly 50 % could not be realized when CBA was applied as the blowing agent. With a higher mold opening stroke, the molded parts showed huge sink marks after decompression. It is assumed that the amount of the blowing gas and respectively the foaming pressure obtained by the CBA was insufficient to realize higher density reductions. In comparison to PP-LGF30 processing, in the experiments with PA6-LGF50 higher melting temperature had to be used. This could have enabled the blowing agent to leak out of the injection barrel at the feeding zone area. Furthermore, it can be assumed that the PA6 material showed a higher elongation viscosity during foaming hindering the bubble growth. This fact can be attributed to the higher fiber content and the very different recrystallization behavior of PA6 in comparison to PP. Additionally, it has to be taken into account that mainly CO2 was produced during decomposition of the CBA, while nitrogen was used for MuCell® as the blowing agent. Both gases show a very different dissolving, diffusion and foaming behavior as already described above.

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Figure 10 shows the total penetration energy as a result of the instrumented impact penetration test. At a constant weight per unit area the foaming process showed a positive influence on the energy absorption capacity for both foaming processes and thus blowing agents. It is assumed that this can be attributed to the higher second moment of inertia after decompression of the mold.

Figure 10: Relative, total penetration energy over density reduction Δρ

Influence of the skin layer thickness on mechanical behavior

In the introduction of this paper, it was mentioned that the skin layer thickness can be adjusted by the delay time before opening the mold. During that time, the melt near the cooling cavity wall solidifies . The longer the waiting time, the more material becomes solid, but, at the same time, the less molten material remains in the cross section to build the foamed core. On one hand, the skin’s strength and stiffness is increased, but on the other hand the core gets weaker with rising delay time.

In order to investigate the influence of the delay time on the mechanical behavior of integral foams, an additional experiment was carried out using PP-LGF30 as the material system. As the delay time 5 and 10 s were selected. Table 2 shows the experimental plan in detail:

Table 2: Targeted wall thickness H and density reduction Δρ using a delay time of 5 s and 10 s

H [mm] 2.5 2.9 3.57

Δρ [%] 0 15 30

After molding of the parts and cutting out specimens, the following overall skin layer thicknesses (sum of top and bottom) could be measured:

after 5 s delay: tskin ≈ 0.75 mm

after 10 s delay: tskin ≈ 1.4 mm

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Figure 12: Relative, density specific tensile modulus (left) and strength (right) over density reduction Δρ

Figure 12 shows the tensile behavior of the test specimens obtained by this additional experiment. Both, elastic modulus (Young’s modulus) and tensile strength are reduced during foaming. Under uniaxial loading the whole cross section is uniformly elongated. While the solid skin shows similar behavior as the compact material, the foamed core reduces both, the stiffness and the strength of the whole profile. The latter can be attributed to the notch effect of the foam cells inside of the core that reduce the maximum bearable loads. The loss in strength and stiffness is more expressive when a delay time of 10 s (tskin ≈ 1.4 mm) was applied.

Figure 13: Relative, density specific bending stiffness (left) and flatwise Charpy impact strength (right)

In Figure 13 the bending stiffness (left) and the flatwise Charpy impact strength (right) are demonstrated: as might be expected, the thicker skin achieved after 10 s of delay time leads to a higher bending stiffness. The difference between 5 s and 10 s delay time is even more pronounced, the higher the density reduction respectively the final wall thickness was adjusted. At the maximum density reduction of 30 %, a 0.75 mm thick skin (after 5 s) leads to an increase of about 200 %, while a 1.4 mm skin (after 10 s) shows an increase of almost 300 %, compared to the compact reference.

The impact strength seems to be influenced by the skin thickness as well: with a 10 s delay time the impact strength is slightly increasing with rising density reduction. The impact strength level, obtained by the thinner skin, remains well below.

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Lightweight potential

Many automotive parts are mainly exposed to bending loads and therefore their corresponding mechanical flexural behavior is the most critical issue for the part. By using the introduced foaming technology, with injection into a small initial mold gap and mold opening to a specific dimension, the bending stiffness can be tailored to the needs of the application.

The achieveable weight reduction in comparison to the compact manufacturing is mainly influenced by the initial mold gap and by the mold opening stroke. Naturally there is a limitation for these two parameters set by the process: the initial mold gap cannot be chosen infinitely small since a complete filling of the cavity must be guaranteed. At the same time higher fiber degradation during injection due to higher shear rates must be taken into account. Additionally the mold cannot be opened to every dimension as sink marks and delamination between the solid skin and the foamed core may occur when a too high mold opening stroke is conducted. The necessary cooling time can become a critical issue at higher wall thicknesses and therefore should be considered. Beside of this, the delay time is another key to influence and adapt the mechanical behavior to the desired properties.

Figure 11: Lightweight potential of PP-LGF30 + CBA

The available potential for lightweighting obtained from the investigated PP-LGF30 material is demonstrated in Figure 11: in this diagram the (absolute) bending stiffness of the foamed samples (measured) and of theoretical compact references (calculated) is illustrated against their surface weight ΔwSF, related to the compact reference with a thickness H = 3.6 mm (value 1.0). The foamed samples show almost the same surface weight since the amount of injected material was kept constant. With an increasing surface weight the bending stiffness of the compact reference is growing parabolically, according to the formulae I and II.

For two foamed samples the corresponding values for the same wall thickness are indicated by the dashed orange lines. The additional surface weight of the compact part necessary for the same bending stiffness is highlighted by the green arrows. For instance, in order to increase the bending stiffness of a compact part about 175%, compared to the reference sample with H = 3.6 mm, an additional weight of 45% has to be accepted. The lightweight potential can be increased on one hand by a smaller initial mold gap or on the other hand by a larger mold opening stroke

smaller

init

ial

mo

ld g

ap

larger opening stroke /longer delay time

Δw SF = + 45 %

Δw SF = + 22 %

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surface weight ΔwSF [kg/m2]

PP-LGF30 compact (calculated)

PP-LGF30 CBA (measured)

and / or a longer delay time. The lightweight potential for PA6-LGF50 + MuCell® is illustrated in the same way in Figure 12.

Figure 12: Lightweight potential of PA6-LGF50 + MuCell®

Summary

Through the combination of foam injection molding of LFTs and breathing mold technology long-fiber-reinforced sandwich components with a high lightweight potential can be produced in one shot and on a large scale. Attention was paid in particular to the bending and impact behavior of the investigated foams. Furthermore, the influence of the delay time on occurring skin thickness and the corresponding mechanical behavior of LFT integral foams were investigated.

The experiments showed that, at a constant surface weight, long-glass-fiber-reinforced integral foams have a significantly higher bending stiffness than compact components, due to their higher area moment of inertia after expansion through precision mold opening. The area moment of inertia and with this the bending stiffness is increased with the third power of the wall thickness. For that reason, a small increase in wall thickness leads to a significant higher flexural rigidity.

An instrumented impact penetration test, applied for PA6-LGF50, showed growing energy absorption and a Charpy impact bending test, applied for PP-LGF30, indicated nearly constant behavior with increasing density reduction. For PP-LGF30 the foaming using CBA delivered quiet higher impact strength: it is assumed that the average fiber length remain higher when processing with CBA machine equipment compared to MuCell®. For PA6-LGF50 both foaming processes showed a very similar behavior.

The delay time obviously influences the thickness of the solid skin: processing of PP-LGF30 showed that thicker skins are increasing the bending and the impact strength, while the tensile properties of integral foams are slightly decreasing. During impact bending test, it is highly recommended to test the specimens flatwise, since this load corresponds much more to the real situation. If loaded edgewise, foamed parts would be noticeable disadvantaged at higher density reductions due to the notch effect of the cells.

sma

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Δw SF = + 49 %

Δw SF = + 33 %

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With this knowledge a wide range of automotive parts that are mainly exposed to bending can be made lighter. By injection into a small initial mold gap and mold opening to a specific dimension, the bending stiffness can be tailored to the needs of the application. Beside the opening stroke and the initial mold gap, the delay time is another key in order to reduce the part weight, to save material and finally to influence and adapt the mechanical behavior to the desired properties.

Acknowledgements

The authors would like to thank the Fraunhofer-Gesellschaft for the support of this project and BASF for providing the test materials.

Bibliography

[Alt10] Altstädt, Volker; Mantey, Axel (2010): Thermoplast-Schaumspritzgießen. 1. Aufl. München: Hanser, Carl

[Eck81] Eckardt, Helmut; Alex, Kurt (1981): Structural and coinjection foam molding. In: Adv. Polym. Technol. 1 (2), S. 40–49. DOI: 10.1002/adv.1981.060010207

[Kni07] Knights, M. (2007): New screw design for long-glass microcellular foam molding. In: Plastics Technology (07), zuletzt geprüft am 25.01.2014

[Mül06] Müller, Norbert (2006): Spritzgegossene Integralschaumstrukturen mit ausgeprägter Dichtereduktion. Erlangen-Tennenlohe: Univ., Lehrstuhl für Kunststofftechnik (Technisch-wissenschaftlicher Bericht / Lehrstuhl für Kunststofftechnik, Universität Erlangen-Nürnberg)

[Roc13] Roch, A.; Huber, T.; Henning, F.; Elsner, P. (2013): LFT foams - lightweight potential for semi-structural components through the use of long-glass-fiber-reinforced thermoplastic foams. In: 29th International Conference of the Polymer Processing Society

[Sch03] Schemme, M. (2003): Langfaserverstärkte Thermoplaste. In: Kunststoffe (8), S. 106–109

[Sch08] Schemme, M. (2008): LFT – development status and perspectives. In: Plastics Additives & Compounding, S. 38–43

[Spö10] Spörrer, Andreas (2010): Leichte Integralschäume durch Schaumspritzgießen mit optimierten Werkstoffen und variothermen Werkzeugen. Dissertation. Universität Bayreuth, Bayreuth. Fakultät für angewandte Naturwissenschaften

[Tho96] Thomason, J. L.; Vlug, M. A. (1996): The influence of fibre length and concentration on the properties of glass fibre reinforced polypropylene: 2. Thermal properties. In: Elsevier Science Ltd. 27, S. 555–565

[Tho97] Thomason, J. L.; Vlug, M. A. (1997): Influence of fibre length and concentration on the properties of glass fibre-reinforced polypropylene: 4. Impact properties. In: Elsevier Science Ltd. 28, S. 277–288

[Zha05] Zhang, G.; Thompson, M. R. (2005): Reduced fibre breakage in a glass-fibre reinforced thermoplastic through foaming. In: Composites Science and Technology 65 (14), S. 2240–2249. DOI: 10.1016/j.compscitech.2005.04.050

INVESTIGATIONS ON INJECTION MOLDED, LONG …injection molded, long-glass-fiber reinforced integral...

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