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Custom PVC fittings with copper insert
Custom PVC Fittings with Copper Insert
When you make PVC fittings with the insert molding process, you have some real technical headaches. In actual production, the main challenges usually come down to the following things: the material itself, heat stress, mold design, etc.
1. 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.
2. 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.
3. Injection Molding Process
The process begins by preheating the brass inserts to 110–130°C to prevent thermal shock and ensure good adhesion with the PVC melt. The heated inserts are then loaded into the mold cavity. For PVC, the melt temperature is typically maintained around 170–190°C, and the mold temperature is set to approximately 40°C. Injection should be performed at a slow speed (screw surface velocity not exceeding 0.15–0.2 m/s) to avoid excessive shear that could degrade the PVC material. A multi-stage injection profile is recommended, with packing pressure applied for 3–4 seconds to compensate for shrinkage and ensure dimensional stability.
If you have such items, please contact us: contact@yxplasticmolding.com
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