What are the structural characteristics of plastic-steel profile molds?

① Plastic-metal co-extruded profiles are produced by co-extruding and bonding two materials within a mold. The co-extrusion die contains separate channels for the metal liner and the plastic melt, as shown in the figure below. After being extruded from the extruder, the melt enters the die via a connecting head, undergoes redirection and redistribution through the flow channel and turning zone of the distributor plate, is further subdivided and refined in the die's distribution section, compressed and shaped in the compression zone, and then flows into the shaping zone of the die. Meanwhile, the metal material is first formed by bending and fed into the die via a metal guide plate. It travels through an independent channel formed by multiple die plates and converges with the molten plastic at the merging zone of the die before being co-extruded. Finally, after cooling and sizing, the composite profile is produced.
② The metal liner channel inside the mold can be designed either within the core or tail cone, or integrated directly into the mold structure for plastic-metal co-extrusion.
③ A positioning device for the metal liner should be installed inside the mold to ensure high dimensional accuracy, shape precision, and consistency of the co-extruded product. This positioning device is typically placed at the convergence point prior to plastic-metal bonding. Specifically, the inlet section of the metal liner channel is enlarged, and the cross-sectional area is reduced gradually until 1–40 mm before the outlet, maintaining a gap between the metal liner and the surrounding wall of 0.01–1 mm. This ensures precise alignment of the metal material and prevents plastic from backflowing into the metal channel and clogging it.
④ The bonding length between the melt and the metal is generally optimal when it ranges from 1 to 60 mm. In practice, if the bonding length is too long, excessive pressure may build up in the die head, causing the plastic melt to easily backflow into the metal liner channel. Additionally, due to uneven pressure distribution across different surfaces, the metal liner may experience slight vibration, leading to unstable production. Conversely, if the bonding length is too short, the bonding strength between the plastic and metal will decrease.
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