I just discovered this website today, I already love it
Ok, here comes my question :
We wanted to produce by injection a plastic part with a Pa30% GF mix.
Unfortunately, it shrunk a lot and since we already have the mold, we need to find another material (mold modifications are too limited).
We tried with Polypropylene but in that case, it is not stiff enough.
So would you have any suggestions for other material? Knowing that it’s a box quite big (80x70 cm) with one opened face . Also it should resist to temperatures up to 70 - 80°C(at least).
Glass filled PP is pretty solid and heat resistant. It feels pretty comparable to glass filled PA, but
PP has way more shrinkage than nylon, so I’m surprised this wasn’t a problem for you. And with glass fibers, the shrinkage should be even less. Sounds like you have either geometry issues or processing problems. Temp & pressure may be something you need to consider first. Gate location, thinner walls at end of flow front and thicker walls around the gate may help (but not too much variation or warping can result.
Typically choosing the correct resin is based on many factors, most importantly where, what and how is this plastic component being utilized. The specific properties of resins (tensile strength, impact, etc…) usually dictates what type of resins need to be looked at.
There are many sites to look at for the resin guidelines…most notably from the resin manufacturer, along with many articles that support additional information.
Apparently the PA 30% GF Nylon resin was chosen for its strength properties, but I could be wrong. ABS & PC-ABS roughly have the same shrink rate, which may be why they were chosen. However, Polypropylene is in another category all together. I am going to concur with what MGNT8 said above, It’s usually the part geometry or the actual molding process. Although gate location, proper venting and cooling are critical.
Pa30% GF: Polyamide or nylon, reinforced with glass fibers. Strong & rigid, it kind of feels like steel only lighter. It even pings like steel when you drop it. Used a lot for metal replacement like a lot of under the hood components.
First, we didn’t choose ABS because of its density which was too high apparently (I joined the project after this decision so I’m just telling you what I was told…).
As for the fact PP should shrink more than PA, we have less problems on our side because PP has a homogeneous shrinkage while PA has a shrinkage which goes perpendicularly with the flow.
Anyway, we ended up with a mix of PP + Mica which is already stiffer and we’re gonna keep this one. I believe this project was not well handled and the mold was not conceived as it should have but we now don’t have the choice…
I’m no engineer nor designer, just in charge of quality and production so all the answers I gave you here are what I saw or what I was told. Also, I’m not a native speaker so if you add everything, there might be some mistranslations from me… Please feel free to correct me if I’m wrong