Metal guy trying to spec plastic (HELP)

[JohnHH]

I am overwhelmed by the number of resins and alphabet soup of plastic materials. How do you guys keep it straight?

I am designing a solenoid that has some plastic and rubber parts. I have talked to the GE plastics guys and they told me that I need PBT for one part and Polyurethane for another part.

My question is, what do I put on a drawing to generate RFQs for the molders? Do I need to find a resin and supplier that I like and spec that resin, or can I spec a resin family? My goal is to make the RFQ general enough that the molders have some flexibility of their resin suppliers.

John

 

[patprimmer]

I would have the parts moulded and tested before I made any spec permanent.

I would talk with several major engineering plastics suppliers before I specified a material type, as once the mould is made for one material type, there will be limits on what else you can try.

Some material suppliers that might be of interest are BASF, DuPont, Ticona, whatever Bayer call themselves now, DSM, RTP company, Mitsubishi, Sumitomo, Mitsui, Teijin, Asahi, whatever Atochem call themselves now.

Regards

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[Pud]

As Pat stated - you need to ensure that you have the correct material choice in the first place.

After that, I would suggest that you specify as a generic material: e.g. PBT 30%GF, or PA6 15%GF etc, rather than say GE Valox 230X, or BASF PU Grade 123xyz, which will increase the material cost as you will leave the moulder no choice of supply. Of course, there may be a case when you need to specify the material manufacturer, but imho it's best to avoid it if you can.

Cheers

Harry

 

[dwightdixon]

No two suppliers make the same plastic material. The long term performance of the product may not be the same from material supplier to material supplier because they all specify their own package of stabilizers and antioxidants. The short term performance of the product may not tell you the whole story about the product's long term performance. I would not leave myself open to long term problems just to allow the molders a little latitude in who they buy from and a very small cost savings on materials. Material suppliers are very competitive and prices are not that much different from supplier to supplier. Qualify a material supplier only after extensive testing and accelerated aging tests.

 

[patprimmer]

It really depends on the application.

Nylon for the bush in the wheel of a lawn mower can be specified nylon as even a low grade won't fail.

Nylon for a radiator header tank must be tightly specified as it requires a very special grade with regard to base polymer, additives and reinforcements.

As we know nothing of your application we cannot really advise.

Open or generic specs does lead to material cost savings, but tight specification does lead to consistent and predictable results.

Regards

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[JohnHH]

Thanks everyone,

I have a better understanding of plastic after reading your responses and doing some research.

I think that what I am going to do is (tell me if I am all wet):

1) Send out RFQs with generic material specs to get the potential short term cost benefit.
2) Qualify a small number of parts (200 - 300)
3) If the parts perform as required, revise the drawings to specify the material.

 

[patprimmer]

Get enough parts to test in several materials from several manufacturers as then if one sells out or an ingredient is changed, you have an alternate already tested.

I would expect it does not cost much extra to qualify a few at the same time.

This also prevents suppliers giving you the "locked in" price rather than the "free market" price.

As a raw materials supplier, I can tell you that premiums of about 20% are applied to locked in spec materials.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.