Recorded October 8th, 2013
Presented by RTP Company
Have you ever asked yourself, “How do I pick the right plastic for my application?” Then, join Steve Maki, Vice President Technology at custom compounder RTP Company, as he provides valuable insight into the thought process plastic engineers use to answer this fundamental question.
Steve will use example applications to test your knowledge during this informative presentation. It is a must see for those not familiar with plastic material selection and a valuable refresher for those who think they have mastered the topic.
Questions and Answers
Q: I have heard the homopolymer is more prone to centerline cracking. Is that true?
A: Centerline or weldline failure is usually affected most by part design and molding conditions. If you are comparing homopolymer PP to HDPE or copolymer PP and the failure is due to impact, I would agree that homopolymer PP will be worse but this is only because it has less impact resistance than HDPE or copolymer PP.
Q: could you comment on advantages of homopolymer vs. copolymer acetals?
A: Homopolymer acetal has slightly better physical properties while copolymer acetal is slightly more stable for processing (releases less formaldahyde).
Q: What is the highest performance virgin PAEK you can provide?
A: It depends on what you mean by performance, but the highest combination of strength, modulus, and impact in a PAEK (also commonly referred to as PEEK) would probably be RTP Company’s 60% long glass reinforced PEEK. Q: Was lots of talk on mech properties, what can be done to improve UV resistance?
A: Either choose a polymer that has inherent UV stability such as PMMA or add a UV stabilization package to the polymer of your choice.
Q: Resin bought by weight but used by volume. Important to take into account density differences (price per cc).
A: Yes most accurate way to compare part cost will be to use cost per volume. Formula to do this is $/cubic inch = Density X $/lb X 0.0361.
Q: How severely does glass fiber filled resin reduce tool and barrel screw life?
A: Equipment for molding reinforced plastics should use hardened tool steel (ie A2 or D2 steel) and then you should not experience any significant wear.
Q: I am being asked to supply a Fire retardant PP for the Aeronautics applications. What do you suggest? Can’t find an extrusion grade fire retardant pellet.
A: RTP company makes numerous FR compounds including PP that can meet the governments FAR requirements for aircraft interior applications.
Q: When molding VLF, we struggle quite often with discoloring of natural resins due to heat degradation. Could heat profile impact that? what type of profile would you recommend?
A: you are likely dealing with a VLF nylon compound as nylons (especially heat stabilized nylons) are prone to oxidizing and turning brown during any melt process. Some things that may improve this are lower barrel temperatures and less residence time in the injection barrel, change the heat stabilization package in the nylon, or run inert gas such a nitrogen into the hopper of the molding machine to prevent oxygen from getting into the process.
Q: We have problems with moisture in polycarbonate parts from our molder, are there compounds with similar characteristics that would be less affected by moisture?
A: Polycarbonate has unique properties so it may be difficult for you to find a drop in replacement. I would work with the molder of your parts to ensure that they have a properly functioning dessicant dryer and that the PC is dried to specifications prior to molding.
Q: I have a gps type application and am not familiar and concerned about the finish appearance; (i.e. piano black, vs matte micro textured surface) is this a function of the die/mold?
A: This is a function of the polymer type, colorant, mold surface and mold conditions. For instance if you want a piano black effect you need a polymer that is transparent combined with a transluscent black dye so that you get depth from the light that is reflected from surface. It should be molded into a highly polished tool using mold conditions for high gloss. For a matte finish we have found that the best way to control it is by tool texture and using consistent molding techniques.
Q: Even though they provide high strength, do the long fiber compounds not contribute to high warpage?
A: All fibers have a tendency to contribute to warpage but long fibers actually have less warp than a similar loading of short fibers in the same resin. I believe this is true because the long fibers bend in more directions than short fiber and provide more uniform shrinkage in all 3 dimensions.
Q: Can you comment on the upcoming “Ultra Performance” material’s use in thin-walled complex parts?
A: Three of the resins for Ultra Performance Compounds are semi-crystalline (PPS, PPA, and PEEK) so they have good flow into thin wall sections and PPS is probably one of the best resins in the industry for this other than maybe LCP. The Ultra Performing Compounds will have high loadings of either chopped glass, chopped carbon fiber, or long glass fiber. If the parts have extremely thin walls it might be best to go with the chopped glass or carbon fiber in a PPS resin.
Q: How does VLF affect flow rates?
A: Other than if you had a very small restrictive gate, the VLF would not affect flow rates any different than a similar loading of a short fiber. Also VLF products are typically manufactured with higher flow base resins so they typically flow better in the mold that short fiber (unless there is a small gate that causes fiber bridging).
Q: When adding a glass fiber will they align with the direction of flow?
A: Yes, glass fibers will be like logs in a river and line up in the direction of flow. Because the flow in an injection molded part is very complex it however is very difficult to always know what is the actual flow direction at any given section of the part. RTP Company has begun researching this with a process called x-ray computerized micro-tomography in which we can see every fiber in a part and map their directions. This should help us better predict compound performance when doing structural analysis via CAE.
Q: what can fillers be used to reduce the cost of high performance amorphous materials, without drastically affecting properties?
A: Amorphous polymers tend to loose considerable performance (especially impact) when a filler such as a mineral is added. If you are looking to cost reduce, I would suggest alloying with a lower cost polymer (ie reduce cost of PC by alloying with ABS).
Q: Any downsides to long fiber?
A: Maybe a slight increase in cost versus short fiber and they are usually not used in very thin walled parts (ie 0.060 inches or less).
Q: Do you need special molding machines or mold design for long glass fiber technology?
A: No, long fiber can be molded in conventional injection molding equipment with preferences to a low compression ratio screw and free flow screw tip. Molds should have larger gates, 0.090″ in diameter or larger, and corners with proper radius. A molding guide describing all of this can be found on RTP Company’s website at http://www.rtpcompany.com/info/molding/long/index.htm
Q: Would talc filled PA 6/6 reduce the price without loosing physical properties much?
A: Yes, I should have defined better in the webinar that one of the benefits of a filler such as talc in semi-crystalline resins such as nylon is to reduce overall cost while providing similar performance.
Q: Does fiber additive affect bonding with over molded materials? Either fiber added to the base plastic or added to the over molding material.
A: Yes, I would expect fiber will effect the bond strength when overmolding since it will reduce the amount of polymer that will be availble to try and bond to. Now this can be minimized if the plastic with fiber in the base part can be molded in such a way that you acheive a very resin rich surface. This can usually be accomplished by using hot mold & barrel temperatures and a fast injection speed.
Q: Any comments on the effect of part geometry on material selection?
A: Typically thin wall sections (<0.050) are best suited for semi-crystalline morphology.
Q: Question: From your experience, what is best technique to evaluate the moisture effect on mechanical properties for nylon grades …say over a period of time?
A: Best way is to use an accurate humidity chamber, put bars in selected humidity for enough time to come to equilibrium (may take a few weeks). Test bars as soon as they are removed from chamber.
Q: What do you consider tight tolerances?
A: This would really depend on part geometry. In my presentation amorphous morphology materials will have a shrink rate around 0.005 in/in with a wall thickness of 0.125 in. While a semi-crystalline resin will shrink at over 0.010 in/in. Thus much easier to control dimensions with the amorphous geometry.
Q: As amorphous biomaterials:, maybe some PLAs with high content of d-lactide monomer (PLDLA)
A: Yes, PLA is a polymer that can be molded in either the amorphous state or semi-crystalline state and I understand that high d-lactide content will tend to keep it amorphous.
Q: Can I use PA6 + 30%GF for an application with good wear resistance and good strength?
A: Yes, Nylon 6 + 30% GF will have good wear and abrasion resistance. It could be improved even further by adding PTFE.-
Q: Can the long fiber technology be used in thin wall components?
A: Yes, with careful molding conditions you can use long fiber in wall sections down to 0.060 inches
Q: Can you recommend resources for making these decisions like you outline in this webinar?
A: RTP Company Product Development Dept
Q: Does CPVC needs to dried before processing.If so, what temperature and for how long?
A: I am sorry but RTP does not work with CPVC
Q: Does RTP have a flexible solution with low permeability (isopropanol)?
A: I would assume that the ethylene polymers should be very good, HDPE, LDPE. LLDPE. If you need more flexibility maybe Surlyn?
Q: For calculations, when do I use flexural modulus instead of tensile modulus?
A: Use the modulus that best equates to how your part will be stressed in real life, tensile
Q: What’s the Tg for PP in Celsius?
A: around -10 C
Q: How does colorant affect the mechanical properties of materials?
A: Yes, colorant (especially pigment type) will always act as a contaminant and lower properties, especially impact resistance. You must be very careful to ues properpigments and at as low % as possible.
Q: How does glass fiber length effect the dimensional stability (swelling) of Nylon 66 due to moisture.
A: Yes, glass fiber will reduce the swelling caused by moisture
Q: If you have to reduce cost in a ABS/PC Aesthetical part, could you use a PPLGF too?
A: I have had better luck using a mineral filled copolymer PP as it is more cost effective
Q: Impact modified PMMA resins are available with high optical transparency and only a modest decrease in HDT and modest increase in cost over unmodified PMMA. These were not mentioned.
A: I consider these alloys and did not have time to cover all the alloys out there
Q: A wide variety of biobased amorphous starch blends is commercially available.
A: Yes, these again are blends or alloys and I did not get to them.
Q: Interested in using a material that is commonly used in under the hood applications such as tractors or automobiles. Have requested quotes from various molders, but have only specified Nylon 6/6 30% G.F. Would this be too generic of a material callout? How is glass length perceived by the molder?
A: You could use this call out but if it is under the hood I would also suggest you specify heat stabilized. When using this generic call out, you should also list approved materials specifically by supplier and trade name
Q: Is there a correlation between high thermal performance and good cold cracking properties? Is a low temp Gardner test the best method for measuring cold cracking?
A: Plastics typically are brittle below their Tg and Tg also is a measure of high temperature performance. So if a plastics has a high Tg and is good for high thermal performance it is poor for cold temperature impact resistance. Not always that case though so suggest you review the data
Q: Moisture up take in PA readily done by measuring weight gain
A: Yes, with an accurate balance you can measure moisture uptake in nylons by weight change.-
Q: Please provide an example to show how $/lb vs $/sq in can be deceiving
A: PVC = $0.85/lb @ Sp Gravity = 1.40, PP = $1.00/lb @ Sp Gravity = 0.91. Appears that PVC is cheaper than PP. Using the formula that $/in3 = $/lb X SpGr X .0361 we find that PVC = $0.043/in3 and PP = $0.032/in3. PP is actually cheaper to produce your parts!
Q: Question: Are nucleators the same as clarifying agents? (In terms of making PP/PE transparent)
A: Clarifying agents are nucleators however all nucleators are not clarifiers (some nucleators like talc are too large and will block light)
Q: Question: Does a glass filler negatively affect wear resistance?
A: Glass fiber usually protects that part that contains it from wear and abrasion but it usually causes that mating part to suffer more wear and abrasion.
Q: Can you address the impact of fillers on material recyclability? Thanks!
A: The higher the aspect ratio of the filler the more it will be affected by recycling. Glass fiber can be damaged during recycling, glass beads can not.
Q: Some mention made of bio based semi crystalline material. Any commonly available amorphous materials. ( currently using a water dispersed acyrlic material ).
A: As mentioned above, PLA can be molded in its amorphous state and will be transparent.
Q: Hi, would increase in GF content increase the wear performance of the gear made from such a resin..?
A: As mention above, glass fiber and increasing it may protect the part that contains the glass but will be more abrasive to the mating part.
Q: What do you mean by dimensionally stable ?
A: My meaning for this presentation is low shrink rates and low warpage which leads to more accurate part dimensions.
Q: What is reinforcement ?
A: generally this is glass fiber, but could be mineral or carbon fiber as well. Anything that increases the tensile and flexural strength values
Q: Melt flow index – does it change with every recyclement ? if so by what % for every cycle
A: For polymers that undergo chain sission when they degrade, the MFI will always go up when recycled. How much depends on the polymer and many factors (For Polycarbonate we might see the MFI double for each heat history polymer sees. For polymer that crosslink on degradation, MFI will go down for each heat history.
Q: What is the difference between filled and reinforced?
A: Fillers are anything that can be added to a polymer, ie beads, mineral, fibers. Reinforcements are fillers but must have high enough aspect ration to increase tensile or flexural strength, ie glass or carbon fibers.
Q: What materials should be used for food and water certification?
A: Typically for most all polymers and additives you can find an FDA compliant grade. RTP products that contain a Z suffix in the nomenclature are manufacture with FDA compliant raw materials.
Q: YOU DID NOT MENTION APPLICATIONS WITH PPO
A: PPO as polymer by itself not readily usable in our industry because it is too viscous to be injection molded. The PPO’s that are out there, ie Noryl by Sabic, is actually a blend of PPO and HIPS or nylon, and I did not have adequate time to cover blends and alloys in this presentation.
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