EKO Mold Shares with You: Some Basic Knowledge About Extrusion Molds

Since extruders can produce a wide variety of plastic products in different shapes, each shape and specification requires a dedicated mold that can form a specific profile or dimension during extrusion. Therefore, the types of molds used in extrusion are quite diverse — for example, there are specialized molds for producing pipes, rods, profiles, sheets, films, filaments, nets, and flat tapes, each with different structures and specifications.

The design and manufacturing quality of the forming mold have a major impact on the quality of plastic extrusion products. If the mold structure or dimensions are poorly designed or selected, the product may even be impossible to manufacture. The requirements for mold manufacturing and operating conditions are as follows:

1) Requirements for mold steel materials① Good heat resistance, with minimal deformation at high working temperatures.

② High wear resistance.
③ Corrosion resistance, to withstand long-term exposure to corrosive gases and molten plastic without rusting or oxidizing easily.
④ Good machinability.
⑤ Minimal oxidation and deformation during heat treatment.
⑥ Alloy steel used for mold lips and flow adjustment components should, in addition to the above properties, have good elasticity.

Common steels used for mold components include 45, 40Cr, 65Mn, T8A, T10A, 2Cd3, 3Cr13, and 38CrMoAlA.

2) Choosing the right mold structure for different raw materials

Different raw materials require mold structures suited to their forming characteristics.

• For PVC resin pipe extrusion, a straight-through type mold structure should be used.

• For PE or PP resin film extrusion, a mold with a spiral groove mandrel and a right-angle melt flow pathshould be selected.

3) Flow channel design inside the mold

The internal flow passage of the mold should be streamlined, with transitions between different diameters connected by smooth curves, so that there are no dead corners or stagnation zones in the melt flow path.

4) Compression ratio design

From the flow channel to the mold outlet, the cross-sectional area should gradually decrease to create a certain compression ratio.
This helps eliminate weld lines caused by the spider supports of the flow divider cone, ensuring product density and consistency.

The compression ratio refers to the ratio between the flow area at the exit of the flow divider cone and the flow area at the mold outlet.
Depending on the type of raw material, the compression ratio generally ranges from 3 to 10.

5) Mold component structure

The mold should have as few parts as possible, with a compact and symmetrical structure, to facilitate installation, disassembly, and the arrangement of heaters.

6) Strength and hardness of mold parts

All mold components must have sufficient working strength.
Key parts should be quenched and tempered, and the working surface of the die lip should have a hardness of no less than 40–45 HRC.

That’s all for this brief introduction to extrusion mold design and manufacturing, brought to you by EKO Mold.