Polyvinyl Chloride（PVC）

PVC (Polyvinyl Chloride) is an amorphous polymer with no distinct melting point. It begins to soften at 60°C, enters a viscoelastic state between 100–150°C, and becomes viscous at 150°C—releasing hydrogen chloride gas in the process. Degradation starts around 180°C, and rapid decomposition occurs at 200°C.

As a result, PVC has a relatively narrow processing window, typically between 160–190°C. Even within this range, prolonged exposure to heat can lead to degradation. Precise temperature control is essential during molding.

The Role of Additives

The type and amount of additives significantly influence PVC injection molding:

• Low-plasticizer rigid PVC has poor melt flow (with a flow length-to-thickness ratio L/T ≈ 130), making it difficult to fill molds. Higher injection pressure is often required.

• Stabilizers play a key role: using high-quality or higher amounts of stabilizers allows for slightly higher processing temperatures and longer residence time without decomposition.

Two Feeding Methods

PVC can be processed in two forms:

1.Powder Molding

Pre-mixed PVC powder can be used directly without drying. This method is more suitable for rigid PVC.

2.Pellet Molding

The pre-blended powder is first pelletized using an extruder or mixer before injection molding. This approach is better for flexible PVC.

Although PVC is low in hygroscopicity, pellets should be dried if moisture is present.

Molding Tips

• PVC has a low heat deflection temperature. To prevent deformation during demolding, use lower mold temperatures.

• A cooler mold also shortens cycle time, boosts productivity, and reduces melt residence time (lowering decomposition risk).

• Decomposition releases trace amounts of vinyl chloride monomer, HCl, and other volatiles. These can corrode the injection machine and mold.

• Use corrosion-resistant materials for the screw, nozzle, and mold.

• To avoid melt stagnation:

        1）Use larger nozzle diameters

        2）Design short and thick gates

        3）Keep mold surfaces smooth (or chrome-plated)

Machine Recommendation

A general-purpose screw injection machine is suitable for PVC molding.