Custom PVC fittings with copper insert

Material and Heat Stress

First up, let's talk about the material and heat stress. PVC (especially rigid PVC) has a much higher shrinkage rate—around 5%—than metal inserts. When they cool, they don't shrink at the same rate. This mismatch creates massive internal stress, which can cause the fitting to crack or break later on. Then there's the fact that PVC is super sensitive to heat. If it gets too hot, it breaks down and releases hydrogen chloride gas. That not only corrodes your equipment but also leaves ugly silver streaks, bubbles, or burn marks on the product, so nailing the temperature is critical. On top of that, PVC melt is thick and doesn't flow easily. When you're trying to fill a complex mold around an insert, the material faces a lot of resistance, which easily leads to short shots or visible weld lines.

Mold Design

Next is mold design. You have to position the insert perfectly, and it needs to withstand the blast of molten plastic. If it shifts or the mold gaps are off, you'll get a displaced insert, warping, or flash. Also, these fittings have tricky shapes with lots of dead corners. Bad venting will trap air and cause burn marks or incomplete filling. And since the melt has to flow around the insert, poor venting or gate design will create obvious weld lines, which ruin the seal's strength. Finally, because the walls are usually uneven, cooling can be a nightmare, leading to warping. The joint between the fitting and the insert can also create a vacuum. If the draft angles aren't right or the venting is poor, getting the part out of the mold becomes a real struggle.