Element in Resin Vs Molded Part

Injection molding is a process where a thermoplastic material, usually in pellet form is heated, melted and pushed into a mold and allowed to cool, taking on the shape of the interior of the mold. The material does not undergo any real change in composition, before or after that process.

Many thermoplastic injection-molding pellets are obtained by melting, extruding and physical chopping. Because most injection-molding resins are partly crystalline, the density of the pellet will depend on the rate of cooling after chopping. Cooling rate is often quite rapid, especially if water aided, so the pellet resin will be somewhat "trapped" in an amorphous state, with lower density than if it had been slowly cooled.

The injection molding process involves re-melting the pellets, injecting the melt often through fine orifices, and relatively slow cooling.

Beyond your observation about humidity changes, you may expect the partly crystalline molded article to exhibit
1) higher density than the pellet, due to slower cooling,
2) anisotropic mechanical properties due to fine orifice (possibly slit orifice) flow, and
3) a changed molecular weight distribution due to the additional thermal history in the molding process.

Mark Bingham
Relativity PL

Sorry, you are totally wrong and are misleading the OP with your incorrect information.

1. "There are many different types of plastics" But, we are discussing only thermoplastics.

2. "temperatures may vary wildly from machine to machine, operator to operator" Never, in any kind of professional, controlled injection molding process, never!

I'll concede that thermal degradation does occur, over time, on in uncontrolled processes. But the rest is bunk! That's why the use of re-ground materials are not allowed in some shops, for some products.

Sorry Lyn. I asked the CR4 admin to remove the post. While the unfortunate truth is that the company I work for, out of necessity, actually does the things I mentioned I didn't think it would be misleading or detrimental. I guess you haven't been in that kind of poorly managed and undisciplined molding shop. I have been stuck in one for too long. I defer to your wisdom sir.

That's OK. As usual, I went overboard with my response.

No, I've never worked in a shop like that.

I ran a captive molding shop with 6 molders (Arburgs mostly) and two thermoformers for 5 years.

We made small, mostly, parts for the government. Our operators never turned knobs if they weren't instructed to by the process control document.

My response to OP was brief because they did not appear to know much about the molding process, nor thermoplastic resins.

Your responses are very well informed and informative and I would not want to inhibit your activity by being overly critical, which I often am.

Please accept my apology for being a jerk.