Realistic Tolerances for Molded Plastic Parts

[pdizon]

Hello,

I am checking tolerances on plastic and elastomeric parts. Is +/-.004 inches a REALISTIC tolerance for molded silicone? The mold can easily be machined to those tolerances, but what can I expect for the final product?

Also, can I also hold those tolerances for molded thermoplastic and thermoset parts?

What can I typically expect from industry?

Thanks in advance!!!

 

[yanish]

I can't talk specificaly on the molded silicone as I have never run it. In regards to your other question, a lot depends on the size of the part. All plastics have a shrink factor that is fairly constant. Once you get the process dialed in, you can generally hold some pretty tight tolerences. The shrink factors are called out based on shrink rate per inch of plastic. If the part is 1 inch long, it may shrink .005" after molding, if it is 10" long, the shrink increases .050" after molding. Since the shrink rate can be changed by different factors in the plastic, ie molecular weight, moisture, flow rate, a larger part is harder to hold tighter tolerences. There are changes that can be made to the process to off-set some of the differences in the material. There are a number of good molding shops that can hold some pretty tight tolerences, once a part has been prototyped and they know what kind of problems they may run into in production. If you have the time to develope the process, you would have a good chance of success. Just expect to spend time and money getting the mold built and developed in order to hold the tighter tolerences.

 

[pdizon]

Thanks yanish,

My part is a thin frame about 1" x .6" with a square cross sectional thickness of about 0.75". It sounds like I may fall within the .005" shrinkage. We just need to get the part prototyped before production. Thanks again.

 

[yanish]

One thing I didn't mention earlier is that the shrink rates can very from material to material. The more crystalin the material is the more it will shrink. But based on the size of the part you are talking about, you should be able to dial it in pretty easily.

 

[pdizon]

Thanks again yanish for the plastic info.

Does anyone have experience with molding silicone and expected tolerances for the a thin frame about 1" x .6" with a square cross sectional thickness of about 0.75"?