Selecting the right plastic

[guimba]

Hello, our company is now getting into custom molded parts and I am looking for help in selecting the right materials for the applications. I tried the Thomas Regional directory for plastic consulting services and got a bevy of suppliers that seemed like overkill. I have a part modeled and an idea of what it wants to be made out of and some requirements. What is the typical process for arriving at the right polymer and if it wants to be a blend, etc.? I would appreciate any help in the right direction. Thanks!

Mechanical Engineer
Field Forensics, Inc.

Engineering isn't about perfect solutions; it's about doing the best you can with limited resources. - Randy Pausch

 

[Demon3]

Selecting the right plastic is not easy. There are many things to consider. Also, there will probably be several plastics that will work but the secret to making money is to find the cheapest one that works. My advice would be to ask a professional like Pud or Pat. The right advice from the start will save you a lot of time and money.

The first step is to identify your three main criteria, e.g. modulus, impact resistance and chemical resistance.

Chris DeArmitt PhD FRSC CChem

http://www.phantomplastics.com

Consultant to the plastics industry

 

[guimba]

Thanks Chris, I believe I can come up with those criteria easy enough. We currently have some samples of a part that was molded locally, but the cost per part is too high. My boss has asked if it can be sourced overseas and I'm looking to specify the material on the print. On the advice of a polymer supplier's rep it is made from a blend of 95% Flint Hills Resources RCPP, no. P5M6K-048 and 5% Exxon Vistamaxx 2120. The Vistamaxx was needed to make the part more pliable. I thought it could be made from a single resin but maybe not. This is the sort of thing I need help on. Thanks, all.

Mechanical Engineer
Field Forensics, Inc.

Engineering isn't about perfect solutions; it's about doing the best you can with limited resources. - Randy Pausch

 

[HDS]

http://www.ides.com/

http://matweb.com/

This will give you some more overwhelming options.

There maybe more options to lower the cost than just going overseas. If you are inexperienced you will want the best vendor either place to support you.

 

[guimba]

Thanks HDS, those are some good resources. I will check them out. BTW, is it possible to calculate the modulus of the final resin given the moduli of the two components and the blend ratio?

Mechanical Engineer
Field Forensics, Inc.

Engineering isn't about perfect solutions; it's about doing the best you can with limited resources. - Randy Pausch

 

[Demon3]

Yes, the modulus follows the rule of mixtures so it's just the average of two moduli weighted for the volume % of each in the mixture. Note, not the weight %. In your case for two polymers the weight % will be close enough though as the densities of the polymers are close.

Chris DeArmitt PhD FRSC CChem

http://www.phantomplastics.com

Consultant to the plastics industry

 

[patprimmer]

I take it the Flint Hills Resources RCPP, no. P5M6K-048 is random co-polymer polypropylene but I have no idea what the Exxon Vistamaxx 2120 is.

You need to state the application, the environment of use, the nature of the load, the properties of the existing material and any modes of failure or end of life.

From there we can progress.

Regards
Pat
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[HDS]

SIP and many other publish good guides to designing with plastics that cover material selection.

http://www.sabic-ip.com/gepapp/Plastics/global/jsp/Literature/LiteratureAndTools.jsp

Look for product selection guides.

Flint Hills Resources or basell will have some for PP.

Blending is probably adding some cost to your part. Finding a grade that doesn't need blending might provide some savings.

 

[patprimmer]

In the absence of almost all required data, I will take a guess at MDPE

Regards
Pat
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[IQIeduardo]

well, all depend of fluids and chemical substances that you will manage
may be this help you

http://www.harringtonplastics.com/chemGuide.cfm

Regards

 

[patprimmer]

Who mentioned chemicals, or UV light for that matter or temperature, or time, or colour, or flamability, or food contact or repetition of load or, or, or?

Regards
Pat
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[Demon3]

I fear that, as the OP is not a materials specialist, he does not grasp the complexity of specifying an appropriate material.

He describes himself as a "mechanical engineer"

To correctly specify material with confidence you will need professional advice.

Chris DeArmitt PhD FRSC CChem

http://www.phantomplastics.com

Consultant to the plastics industry

 

[guimba]

I appreciate the responses. The Vistamaxx is a propylene-based olefinic elastomer that the polymer rep suggested to make the random copolymer PP more pliable. I've calculated a desired modulus of 123 ksi for the material, based on the sample parts we received and a subjective evaluation. Part of the component is a hollow round tube about .25 in diameter which contains a crushable glass ampule. It releases its contents when squeezed through the tube walls (.020 in thick) and causes a colorimetric reaction when a suspected agent is present. So the two main requirements are that the material be pliable enough to allow easy squeezing of the ampule area, and good clarity to allow viewing of the color change. The expected environment will be -20°C to +40°C, it is a disposable device so no life issues other than shelf life (2 years), only failure mode will be if the tube walls are too thin and allow the glass to protrude (hasn't happened in testing), chemically needs to withstand a 50% solution of sulfuric acid, no UV exposure as it is packaged in a light-proof pouch, no flammability issues or food contact. I will research the links supplied to see if I can find a suitable single resin that can do this. Thanks again for the help. Cheers...

Mechanical Engineer
Field Forensics, Inc.

Engineering isn't about perfect solutions; it's about doing the best you can with limited resources. - Randy Pausch

 

[patprimmer]

I stick with PE. Match the density to the flex mod you require. If LDPE is still to stiff add a little EVA.

Regards
Pat
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