Acceptible Types of Plastic? 2

[sethro5hc]

I'm working on a few new products that will be used as automotive accessories, think canoe racks and things of that sort.

The designs we have are good, but the material suggested by our factory is entirely too expensive. They've suggested PC (didn't specify yet what specific type). We need a plastic material that will have good UV resistance and color fastness, and also needs to mate well with neoprene (our padding). With SolidWorks, I used a force of 1.5x larger than what we'll list as the maximum allowable on the product, and got the largest Von Mises stress to be 1001.7psi. The PC that I think they're using lists a yield strength of well over that.

Can anybody suggest a plastic that will have good UV resistance/color fastness, and will be strong enough for this application? I can find the strength properties for plastics relatively easy, but the UV properties are harder to come by. Many of these plastics have low resistance to UV in their raw form, but can be treated to attain acceptable UV resistance. The problem is I'm having trouble finding products that can and can't be treated for UV to an acceptable level.

PC is 2, 3, even 4x more expensive than many other commonly used plastics, and we've got to cut the price on the material. Any suggestions for plastics that can be treated for UV resistance and have good color fastness would really help out. Thanks guys

 

[IRstuff]

Have you looked at what your competitors use?

I would also caution against relying on your "max load" limit as your design point; did you account for g-shocks?

No one reads the cautions, and when your rack breaks because the car was only slightly overloaded, but flew off a 8-in curb and experienced triple the design g-load from the shock, they're simply not going to buy another from your company and will probably rag about your product to their friends.

TTFN

FAQ731-376

 

[patprimmer]

25% Glass filled nylon 6 is the industry standard and has been used successfully in similar applications for at least 25 years as a survey of leading products will confirm.

The material should have a copper iodide heat stabiliser to provide adequate surface protection from UV and about 0.5% fine particle carbon black to protect the matrix.

Glass fibres considerably improve properties after some exposure to UV by bridging surface cracks and stopping propagation.

Regards
Pat
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[sethro5hc]

Yea I've gotten the exact material that a competitor uses. "polycarbonate plus poly(styrene acrylonitrile butadiene)"

Problem is, PC is too expensive of a material. Others that I've found that may work (need to be UV resistant or have a UV treatment) are Acetal Homopolymer or Copolymer, ASA, HDPE, and rigid PVC. Now most of those are only about 1/2 to 3/4 as strong as PC as far as listed yield strengths go, but some of them have similar flex moduluses and hardness factors. And considering they are anywhere from half, to 1/4 the price of PC, I need to consider them.

The strength part I feel pretty confident that I can calculate accurately, it's just difficult to find info on UV treatments and color fastness. So if anybody has any suggestions other than the ones I listed above, it would help out.

 

[sethro5hc]

Hey thanks pat, I actually was just looking over some properties of the glass filled varieties of Nylon 6.

According to IDES, it looks like Nylon 6 with 25% glass is the most expensive Nylon 6. Nylon 6 with 15% glass is substantially cheaper, are its UV resistance properties a significant downgrade compared to the 25% glass material?

 

[patprimmer]

ABS is not suitable for use on automotive roof due to poor UV and fatigue and stress crack.

PC is marginal due to fatigue and solvent stress crack giving unexpected occasional catastrophic failures after exposure to some very common chemicals when loaded.

Acetal has poor UV resistance but is otherwise OK. It does not accept glass reinforcment as well as nylon.

ASA is a chance, but still wont match nylon for fatigue and overall toughness on a car roof under load and vibration.

HDPE, well I can't even take that as a serious suggestion. tensile and creep will both fall far short of requirements.

PVC will be lacking tensile and creep and fatigue.



Regards
Pat
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[sethro5hc]

Is Nylon 6/6 a comparable material? I see that it's cheaper than Nylon 6 and has the same glass mixture materials. I will check the properties myself but just wondering if you had any experience with the two materials. We're actually using nylon as part of a hitch adapter on a bike rack, but once again we're still in the early stages of product development and the factory has yet to specify which specific type of nylon (did the same when they just said "PC").

 

[sethro5hc]

Thanks again pat. Yea my boss and I shot down ABS early on in the design process due to its poor UV resistance.

I mentioned Acetal copolymer and homopolymer because I saw that there are UV-resistant grades of it that are acceptable for outdoor use.

ASA also lists several outdoor and automotive applications.

I guess I need to get down in the nitty gritty with some safety factors and see the absolute toughness and properties this material needs to have, then I can better forecast what will and won't work.

 

[sethro5hc]

Nylon 6 (15% glass) seems to have a tensile strength almost exactly the same as PC according to one source online. The specific material was EMS Grivory Grilon BG-15/2. From IDES, 15% glass Nylon 6 is significantly cheaper than PC, if it has the same strength capabilities this could really save us some $ and help us approach our real target cost.

 

[Wrenchbender]

FWIW, we successfully used GFR Nylon 6/6 and 6/12 for applications needing high tensile & impact strengths, over -20F to 140F, plus UV resistance. Production was in the thousands; it was an injection molded part which weighed a little over a pound. I don’t remember the volume frax, but I want to say it was ~10% glass fiber. Colors were in either in black for best UV resistance, or orange for best visualization.

I’m not sure how you might joint nylon to neoprene, but your other requirements, including cost, can be probably be met with this type of mat'l.

 

[patprimmer]

Nylon 6 is somewhat easier to mould than 6.6 and has marginally better UV. 6.6 has better tensile and flex mod, but for a roof rack application, both have more than good enough properties.

Nylons will cycle smoking fast compared to most other materials on your list.

The properties change dramatically with moisture absorption, so be careful to asses properties both dry and conditioned to the hottest most humid environment you will encounter.

Regards
Pat
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[sethro5hc]

Speaking of all these materials, I had a post recently about which books were good for real world engineering applications, not just theory like many school books are.

In regard to these material questions, what's the best way to go about trying to check prices for materials? I recently graduated and have only been a working engineer for about 6 months, so I don't have the "off the top of my head" knowledge yet of things like which materials are significantly cheaper than others. Material properties are pretty easy to locate, and I have found average, high, and low prices for plastics on IDES. My question is what's the best way to go about finding real prices for these things? I work in GA, and our factory is in China. Obviously whatever local prices I find are pretty worthless to what it may cost our factory. I don't necesarrily need a way of knowing exact prices (our factory will tell us that) but what's the best way to go about at least finding prices relative to other materials? For instance, if a certain material is cheaper or more expensive than another. Prices for rubber are a little more difficult to locate than plastic/steel prices. I'm looking for different materials with similar elastic and hardness properties to neoprene.

How do you guys find this sort of pricing data? Vendor catalogs, websites, phone calls? What's the best way you've found?

 

[patprimmer]

Off the top of my head

Quite Cheap

Polystyrene
Polyethylene
Polypropylene
Some PET

Reasonably Cheap

PVC
ABS
SAN

Moderately Expensive

Cross linkable PE
Nylons 6 and 6.6
ASA
Acetal
Polyesters
Polyurethanes.
Santoprene
PEN

Quite Expensive

Other Nylons
Some Polyesters
Cellulosics
SMA
Noryl
PC
PTFE

Very expensive

Polysulphones
Exotic Nylons
LCP
PEEK
PEK
PEI
PPS
Polyarylate
TPX
Exotic fluoro polymers
PVdF

Rankings vary considerably over time, from place to place and by grade.

Often moulding costs can be more than material costs and different materials can vary substantially in moulding time and ancillaries like dryers and mould temperature control devices needed.

Specific gravity is also directly related to costs as you use material by shot volume, but buy it buy the Kg.

Some important properties are not evident on data sheets.

Some properties that seriously impact on product life or failures are not apparent at first glance. eg, the part crack at a certain load. Increasing elongation at break rather than fixing izod impact fixes the problem as it shares or distributes the load and reduces crack propagation.

Different plastics respond quite differently to reinforcements and additives.

Different plastics respond differently to environmental factors.

Data sheets quote optimised mouldings in laboratory conditions, and mostly only short term results.

When assessing life under load, fatigue and creep both need to be considered.

Education helps, but unfortunately there is no substitute for experience.

Regards
Pat
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[metman]

patprimmer (Publican)

Gave you an LPS for that neat rundown. This needs to go into a FAQ.

What is this (Publican) thing?

 

[patprimmer]

The management here started a social forum like a virtual pub and asked me to moderate it. We decided to call it Pats Pub.

Later as a bit of an in joke, they changed my whatever that particular feature is called to publican.

Regards
Pat
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[btrueblood]

Vote #2 for turning that post into a FAQ, Pat.

 

[patprimmer]

I might have to check it off against some check lists before I publish as an FAQ. That post was from the hip.

Regards
Pat
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