Nylon 66 plastic..gas entrapment 1

[henks580]

Our injected molded parts use use Nylon66 for insulator covers. We have always experienced "bubbles" build-up, but not too significant. Most of the gas entrapment has been eliminated by simply keeping surfaces (mold, runners, and cavities)clean. Lately, a significant increase in bubbles has been noticed. I've read that it is virtually impossible to get rid of these bubbles completely, but can be controlled. I've also heard of pre-heating methods that can reduce this effect. Are there any other suggestions as to what can be done to reduce/eliminate the gas entrapment? We use two types of insulators, one is nylon 66(pink) and the other is nylon 12 (transparent color). The bubbles are far more noticeable on nylon 12...but we attribute the bubbles to the color primarily and not the properties.

 

[patprimmer]

Do not presume the bubbles are trapped air. they may be voids or out gassing.

It is impossible to tell from what you say.

The most common causes of bubbles are moisture in the nylon granules or the colour masterebatch if used. This moisture out gasses at processing temperatures. The problem can be avoided by using nylon that is packed dry in sealed bags, and handled carefully, or by thoroughly drying the granules. Starting with dry granules is better as some damage is done during drying.

Another common cause is shrinkage voids, where there is insufficient plastic packed into the mould to compensate for shrinkage on cooling. The solutions are many, depending on many processing and mould design factors, but basically, more injection speed, pressure and time can be used in combination or individually. Increased mould temp can help, bigger gates, runners, sprue or nozzle may help, positioning the gate into the thickest section may help.

Trapped gas will also cause a burn as well as a bubble. Nylon at say 300 deg C with air at say 10,000 psi does cause significant reaction.

Regards

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[unclesyd]

patprimer is on the right track pertaining to the possibility of outgassing. Nylon 66 held above 258°C will start cross linking and liberating NH3 gas creating voids with the rate increasing with temperature. It is also not a good idea to restrain Nylon 66 polymer above 283°C for any length of time as there can be a very rapid decomposition take place.
This will also happen with Nylon 6-12.

As stated watch your moisture levels. Most Nylon 66 is made by SSP and the moisture is closely controlled. One thing to look out for is Nylon 66 flake is slightly hygroscopic.

 

[unclesyd]

Here is an accident report that illustrates and defines the potential for decomposition of Nylons. Though this report illustrates a rapid mass decomposition, the reaction is the same anytime Nylon is held at or above 258°C and as stated the rate changes rapidly with time and temperature above the melt temperature.

Read the final investigation report.

http://www.csb.gov/index.cfm?folder=completed_investigations&page=info&INV_ID=2