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Ambient (Room) Temp in Plastic Injection Molding

06/05/2017 12:44 PM

I am evaluating risks at a vendor of plastic injection molded parts (poly and nylon predominately) due to variances in received parts.

The vendor in question leaves the doors open in the summer rather than using air conditioning.

Could that be a source of variance in dimensionality of parts?

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#1

Re: Ambient (Room) Temp in Plastic Injection Molding

06/05/2017 1:16 PM

Yes nylon is susceptible to moisture absorption...

http://www.toray.jp/plastics/en/amilan/technical/tec_011.html

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#2

Re: Ambient (Room) Temp in Plastic Injection Molding

06/05/2017 2:04 PM

As SE indicated nylon is hygroscopic to an extent that will throw off dimensional measurements to Off-spec.

Controlled humidity in nylon processing, storage is of the essence.

Ambient room temperature matters only to the extent of the bulk expansion coefficient for the plastics involved, and also has an effect on the rate of water uptake, or loss.

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#3

Re: Ambient (Room) Temp in Plastic Injection Molding

06/05/2017 2:22 PM

Yes, Nylon is hygroscopic and the feedstock (typically pellets) can absorb moisture before moulding, as well the moulded part absorbing moisture afterwards. Absorbed moisture in the feedstock can also result in defects in the moulded part in addition to dimensional variances. An experienced vendor would know this and make every effort to regulate the process environment adequately. Leaving exterior doors open is not the way to do it.

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Re: Ambient (Room) Temp in Plastic Injection Molding

06/05/2017 6:53 PM

Any competent molder will dry the material before molding, if needed. Usually, Nylon needs it.

Dimensional qualities are affected by molding parameters, pre and post molding conditioning and storage of the parts. The mold temperature can also affect the dimensions. This assumes that the mold shrinkages were determined for your specific part/material combination and the material you chose is the proper one for the application.

The temperature of the customer inspection facility should be specified if that is significantly different than the inspection room temperature of the parts at the vendor.

These things should all be considered PRIOR to the start of any dimension critica project.

Talk to, and work with your vendor. You selected them for a reason, now you are partners and should both work toward a successful solution.

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#5

Re: Ambient (Room) Temp in Plastic Injection Molding

06/06/2017 5:12 AM

From years of experience, the ambient temperature has very little to do with finished product size from the moulding process. The plastic processing temperatures are far above any possible ambient, so the moulding process and tooling "coolant" will be the determining factors. We moulded parts from ambient 4 Deg C to 45 Deg C across multiple material types. We used coolants from 4 Deg C up to 135 Deg C. One factor that is important though is that the part temperature must be allowed to stabilise before measurement. Many injection moulded parts are ejected from tool when the material is solid (That is some tens of degrees below the HDT) and may still be larger since they have not cooled and shrunk to standard size. If size is that critical, there are (usually optical) measuring tools that temperature compensate for ambient air temp, but would still need parts to be normalised to ambient. Nylon "normalises" to ambient humidity over around 7 days per mm thickness, but this can be artificially accelerated. This process (artificial or accelerated) causes minor dimensional changes but has significant impact on flexibility/brittleness.

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Re: Ambient (Room) Temp in Plastic Injection Molding

06/06/2017 5:30 PM

Having worked for the last 17 years on injection molded nylon parts, there are several parameters that strongly affect the final part geometry. Initial design of the part, selection of wall thicknesses and appropriate mold cooling drive some of the greatest initial fails. I found that pack and hold time have strong influences in controlling parts fill, residual stresses and void size/location. The parts coming out are typically still quite warm and will "adjust" shape quite a bit over the first 24 hours of life. For critical geometry we frequently used cooling fixtures for the first several minutes out of the die as well as water baths to get the part down to room temperature as quickly as possible. Also important was how the parts were stacked and stored with the first 48 hours being the most critical. Often parts will still change shape if they are stacked and stored in a manner that deforms or stresses the parts while in storage. Some parts need to be restrained to keep them from crawling over time and with changes in humidity for nylon parts.

Other than part embrittlement if stored for long periods of time in low humidity, ambient temperature has little effect.

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