Very Long Fiber Composites

Does the idea of replacing metal parts with light-weight and easy to fabricate reinforced plastics seem appealing but you are not sure how to get started? The impressive performance benefits of "stiff and tough" very long fiber reinforced composites will be explained by Karl Hoppe, senior product development engineer for structural products at custom compounder RTP Company. Mr. Hoppe reviews various aspects of using these unique materials; benefits of very long fiber materials, very long fiber performance advantages, metal replacement selection criteria, design and processing considerations for reinforced plastics, and real world case studies of successful applications.

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Questions & Answers from the Webinar

Q: two questions: 1) possibility for thin wall applications; 2) the surface quality of the molding parts.
A: The typical minimum wall thickness that is generally suggested is about 2mm thick. This, of course, depends on flow length and other design features, but thinner walls than that do risk damage to the fibers. However, we have also seen parts molded with much thinner walls (around 1mm or even less) that were able to maintain good fiber length. As far as "surface quality", you can get manufacture parts that, aesthetically, are very pleasing. That will depend on the resin system you're working with (semicrystalline materials are often times better), the level of fiber reinforcement (the lower the better for surface finish). There probably are not many combinations that would get you a true "Class A" finish, but each material and situation needs to be evaluated separately.

Q: Could this a good material for instep for trucks, instead of steel construction. What is the maximum pressure for this material?
A: Max stress for materials like this are dependent on the fiber orientation inside the part. The performance of the part will vary with the design of the part itself as well as design of the tool, the location of the gate, type of gating system, etc. VLF materials have been used as fairing brackets, running boards, bumpers, grab bars, front end modules, door modules, and many other structural automotive parts.

Q: What is the effect of VLF on wear and friction? Would VLF be a benefit to a polymer bearing?
A: VLF glass will have similar effects to wear and friction as a short glass fiber. It can reduce the wear of a material in a friction situation but can be abrasive to the mating part. It is generally no worse in regards to abrasiveness than a short fiber reinforced material, and in some cases less so because the abrasive portion of a glass fiber is the end. If you have longer fibers, you also have fewer of them (comparing with the same fiber loading), and thus you have fewer abrasive fiber ends. If you had a polymer bearing that already contained short glass and you wanted some higher mechanical properties (assuming the wear and friction properties were adequate), you could switch to a similar formula in a VLF product to get higher strength, stiffness, and impact resistance. VLF may not always bring a benefit to a polymer bearing - it depends on the function of the part you are making.

Q: In my experience I have seen applications start with long glass fiber and then over time get replaced with short glass. This may be due to fiber damage during molding. Any comments?
A: In general we have seen the same, though it is doubtful it's due to poor molding conditions. Part of this phenomenon is due to cautiousness on the part of OEMs. Think of it this way: you have a metal part and someone wants you to now make it out of plastic. The metal works fine, but there are some advantages that plastic could bring. What plastic do you choose? You're likely going to go to the highest performing material that you can affordably utilize. Once you prove successful, you have tools built and you're more willing to try other materials as further cost reduction. That's the way of materials - over time companies move to the lowest cost material that will work in the application. Sometimes it starts in VLF and stays in VLF. Sometimes it gradually changes to short fiber. But there are always metal replacement opportunities to start in VLF.

Q: Can you replace short fiber application with long glass fiber. What are shrinkage differences between the short and long fibers?
A: In most cases we would say that the shrinkage would be very similar. If you are molding a short fiber reinforced material then we generally would say you are ready to try a VLF product. Shrinkage is always very dependent on the design and molding conditions as it is on the material, so it's usually worth evaluating and determining how close you are. You can usually get enough information about the performance for you to determine whether slight tool modifications are of value.

Q: Please comment on Weldline strength for long glass fiber products. Generally like LCPs the weldline strength of long glass is poor. How do you overcome this weakness?
A: Weldline performance is generally a weakness when it comes to VLF materials. If you are designing a part that will inevitably have a number of weldlines (i.e., a part with numerous molded-in holes), then we usually suggest you at least try to gate the part in such a way so that you keep the weldlines away from high-stress areas. RTP Company does have some proprietary technologies that can be used to try to address the weldline strength issue if it can't be overcome by design.

Q: Does HDT for the same base resin, comparing short fiber vs. VLF is increased as well?
A: HDT is essentially a measure of stiffness at temperature. If you increase your flexural modulus, you should see at least slight increases in HDT as well. Within a resin system, we may see increases from 3 to 8°C at 1.8 MPa.

Q: Have you established links between properties and actual fiber lengths realized in products?
A: It is very difficult to measure actual fiber lengths, or stated more correctly, it is very difficult to record an average fiber length for your composite. If you download the slides or watch the webinar again, take a look at some of the burn-off pictures. In order to adequately measurer fiber length, you need to separate the fibers without breaking them apart and then analyze the length of all of them. If you cut a section out, you need to disregard the fibers that you cut. There are some analytical methods that are being trialed at US National Laboratories, but the results are still difficult to ascertain.

Q: What is the role of fiber sizing in your products? By sizing I mean the material/chemical/treatment that ensures the level of bonding between fiber and polymer.
A: RTP Company generally doesn't add sizing to the fibers we use. Our glass fiber suppliers develop sizings that are intended to increase performance in certain resin systems. In some cases (like PP) we can include additives that enhance that bonding, but in most cases we rely on the sizing technology at our fiber suppliers.

Q: Do you use shorter molecular chain of the neat resin to improve the fluidity of the material during molding?
A: It is common to utilize materials that flow very well in order to aid in wetting out the fibers during the manufacturing process. As such, we would generally prefer lower molecular weights (i.e., higher melt flow) rather than trying to wet out a strand with something like an extrusion grade resin. In our evaluations we have seen little if any difference between the static mechanical performance by utilizing higher flow resins. In fact in some published studies that we have seen the results show improved performance with higher flow resins.

Q: So, Maybe I missed this, but is this a true pultrusion process or is it more of a wire coating process to make the pellets?
A: Thanks for your question - that's one thing that perhaps wasn't very clearly stated during the presentation. RTP Company's VLF process is a pultrusion process, or more correctly, we call it a melt impregnation process. It differs from a wire coating process in that we are making an effort to get a very high level of wet-out of the fibers. A wire coating process will not have very high levels of wet-out and may not be as easy to work with from a handling point of view.

Q: So on the impact testing, what were the energy levels used to demonstrate the differences?
A: This was a Gardner Impact unit that was set up according to ASTM D5240. Weights according to how it was listed on the unit were 134 lbs, 184 lbs, and 216 lbs.

Q: Can you combine VLF in connection of any resin, basically, but most interestingly with PBT, for instance?
A: Not all resins have the thermal stability to handle the VLF manufacturing process, nor do all resins flow readily enough to properly wet out the fibers. However, we have manufactured VLF PBT products. They are not as popular commercially as polyamides because of a combination of cost/performance, but there are certain situations where they may be of interest.

Q: Is there an optimum or minimum needed amount of fiber needed to have to positive performance of the long fibers?
A: You generally won't see less than 20% long fiber in PP and 30% in other resins, as it is difficult to manufacture those in line. Also, you seem to get more benefit from the longer fibers with higher loadings. That said, some of the lower loadings are used at high volumes in the automotive market, so there is value for VLF fiber loadings at different levels.

Q: How does VLF PP or PA react with water contact during lifetime?
A: VLF won't change the polymer's reaction to water. PP does not really absorb water nor is it affected by moisture. Polyamides in general absorb moisture and their mechanical properties change with higher levels of absorption. In polyamides moisture acts as a plasticizer and decreases strength and stiffness while increasing impact resistance.

Q: Is it possible to coat the glass fiber with e.g copper so you get the strength and conductivity at the same time?
A: The drawback to something like this is that the metal coating often interferes with the bonding of the fiber to the polymer, so you don't realize nearly the level of mechanical properties that you would without the metal coating. We would likely look to blend existing technologies as discussed towards the end of the presentation in order to achieve a VLF product that also has conductivity.

Q: Hi, any limitation in term of part design using LGF?
A: You can see some general recommendations for design and processing at our website.

Q: Any comments on processing VLF resins in hot runners?
A: The web link above will lead to this processing page which indicates that open channel type hot runner systems are acceptable.

Q: Are VLF used in compression molding processes?
A: Not pure compression molding (where you put the pellets into a mold), but parts are produced by extrusion-compression processes. In this type of process molten reinforced polymer is extruded onto a mold and then the mold is closed to spread the reinforced polymer across the form. In some cases we will offer pellets of 25mm or more at the request of some customers in order to help them achieve even longer fiber lengths.

Q: We already did a couple of mold-fill simulations. Is there VLF-material data available to support further simulations?
A: You can work with our CAE Support Services group to get the appropriate information to help you model VLF products in your part.

Q: Is this process comparable to BASFs Ultramid production?
A: "Ultramid" is simply BASF's trade name for their polyamide products and contains a wide variety of products, from unfilled resins to filled materials.

Q: What resin systems can be used to manufacture VLF compounds?
A: RTP Company can make VLF Compounds via many different resin systems. PP and PA-based products are most common. PPA and RTPU are also commonly manufactured. As a general rule, semi-crystalline materials respond more favorably to reinforcement, but some products based on amorphous resins (especially PC) are also available.

Q: How does VLF compare to D-LFT (inline compounded) as far as properties and material cost?
A: First of all, in case you are not familiar with D-LFT, it stands for “Direct-Long Fiber Thermoplastic”. It is a process that starts with the same ingredients that RTP Company would start with (resin, glass fiber roving, additives), but the end product is a finished part rather than an intermediate pellet (like RTP Company makes). It is difficult in most cases to justify a D-LFT line due to the start-up and equipment costs unless the product is a very large part and/or requires very high volumes, so it’s difficult to compare cost. Properties will also depend on where on the part you are testing, and that in turn depends on how well you have retained your fiber length and what the fiber orientation is in the area(s) of high stress. One note is that material specifications become more troublesome with D-LFT, because you don’t have that intermediate pellet that you can mold ASTM or ISO bars and test mechanical properties. That may put material certification on the shoulders of the part manufacturer rather than a materials supplier.

Q: What pellet/fiber length is typically used for injection molded VLF products, and can you offer some suggestions for maintaining the fiber length through the injection molding process?
A: VLF pellets (and therefore fiber) lengths of ½ inch (~12 mm) are most common for injection molded applications. In general, you want to reduce shear through the molding process in order to maintain fiber length. This can be done by reducing the compression ratio of the screw, reversing barrel profiles, and lowering injection speeds and back pressure. If you burn off the resin with a muffle furnace, it is not uncommon to see ¼ inch long fibers in a properly molded part compared with an estimated 0.030-0.040 inch lengths from short fiber. These and other more specific guidelines can be found here.

Q: How does surface appearance compare with short fiber compounds?
A: This will be very dependent on the part being molded. In many cases, long fiber materials disperse very well and can offer very fine surface aesthetics (though very few filled materials will achieve a “Class A” surface finish). In some cases the part size, part design, or tool design has a negative effect on the surface finish. Most of the same steps used to improve surface finish in short fiber reinforced compounds (higher melt temperatures, mold temperatures, faster fill speeds) can be used with long fiber materials, but you will want to ensure that you are maintaining fiber length with any process changes.

Q: How does cost compare between VLF and short fiber compounds?
A: There is certainly a premium that is required for VLF compounds vs. short fiber compounds, so if a short fiber product fulfills the requirements, then short fiber should be used rather than a VLF product in the same resin system. Long fiber is typically used when a short fiber product fails to meet some requirements. A different place where you can consider VLF vs. short fiber is in replacing more costly short fiber resin compounds with VLF products based on lower cost resin systems. Example: replacing short glass reinforced Nylon with VLF PP.

Q: What long fiber property myths need to be dispelled?
A:

• Long fiber does not improve anything that is inherent to the resin system it is reinforcing, so chemical resistance, maximum operating temperatures, and similar resin-dependent properties will essentially be unchanged.
• Long fiber reinforced materials are anisotropic, meaning that they will have different properties in different directions. Short glass reinforced resins behave similarly. In some cases with long fiber you can have less uniform orientation (or, more accurately, you can have more fibers curling and bending transverse to the flow), which may provide improved warp resistance in long fiber molded parts (compared to short fiber) and more consistent directional properties.

Q: How developed are modeling techniques for long fiber materials in flow analysis and structural analysis simulations?
A: Analytical tools for long fiber reinforced compounds are still being developed. In order to predict things like orientation and fiber length retention (which is more important than in short fiber reinforced materials), you have to be able to accurately measure both. Methods for doing this are still being developed, but in short it is difficult to measure fiber lengths without destroying the fiber. RTP Company has successfully used existing short fiber models to help customers predict properties related to orientation. However, because of fiber entanglement these tools may not always accurately predict shrink and warp. RTP Company’s CAE experts can help you determine how to use appropriate data and techniques in the specific analysis that you perform.

Q: Can VLF products be machined easily?
A: VLF products can be machined as easily as short fiber reinforced materials. Glass fiber will be abrasive on the tools used for cutting, but also helps improve the “machinability” of thermoplastics by reducing burring and providing a good plane with which the blade can cut through the material.

Q: What design suggestions can you make when considering converting a part from metal to plastic?
A: The most general comments are: radius all corners and edges to appropriate levels and to try to keep a nominal wall thickness throughout the part if possible. In order to compensate for different mechanical properties between metal and plastic, it may be necessary to add ribs or other design features that would not be necessary in a metal part. More specific comments on designing for thermoplastic compounds.