In this presentation, Karl Hoppe of RTP Company, discusses using the impressive performance benefits of "stiff and tough" very long fiber reinforced plastics as a replacement for metal or other engineered materials. Long fiber composites are easily processed via injection molding and can be customized to meet demanding application requirements.
Topics covered in easy-to-understand terms include:
Unique benefits of long fiber materials
Long fiber performance advantages
Cost advantages over other engineered materials
Metal replacement selection criteria
Design and processing considerations
Real world case studies of successful applications
About Karl Hoppe
Karl is the managing engineer for structural materials development, including long fiber composites, at RTP Company. Hoppe has been with RTP Company for over for 10 years and holds a Bachelor of Engineering degree from Winona State University’s Miller School of Engineering.
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.
Share This Page
•
• Free Access to 80,000 Datasheets