As the extruder can produce a variety of shapes of plastic products, each shape and different specifications of plastic products in the extrusion molding, there is a set of molding specific shapes, sizes of special moulds, so the extruder with molding plastic products mold types are more, such as a variety of specifications of the pipe, bar, profile, plate (sheet) material, film, silk, net and flat belt products with molding moulds, all have different There are different sizes of special moulds.

  The quality of the design and manufacture of moulds has a major impact on the quality of plastic moulded products. If the structure and dimensions are not selected and designed properly, sometimes they cannot even be produced. The requirements for the manufacture and working conditions of moulds are as follows.

1) Requirements for steel used in the manufacture of moulds. ① Good heat resistance, small deformation in the high temperature working environment. ② Strong abrasion resistance. ③ Corrosion resistance, long working hours in the molten environment with corrosive gases, not easy to be corroded, rusted and oxidised. ④ Good machinability. ⑤ Thin oxide layer and low deformation during heat treatment. ⑥ The alloy steel used in the manufacture of mould lips and flow control strips, in addition to the above conditions, should also have a good elasticity. Commonly used steel for mould parts manufacturing are: 45, 40Cr, 65Mn, T8A, T10A, 2Cd3, 3Crl3 and 38CrMoAlA etc.

  2) When moulding plastic products from different raw materials, the mould structure should be chosen to suit them better. For example, if PVC resin is used for the production and moulding of pipes, a straight-through mould structure should be used; if PE or PP resin is used for the extrusion and moulding of films, a mould structure with a spiral grooved mandrel should be used, which is at right angles to the direction of the extruded melt flow.

3) Within the mould, the melt flow cavity should be streamlined and the transitions between different diameter sections should be connected by circular arcs, i.e. there should be no dead ends or stagnant flow in the melt flow section.

  4) The cross section of the melt flow cavity in the mould should be gradually reduced to a certain compression ratio at the mouth of the melt forming mould, so as to eliminate the melt bonding seam caused by the manifold cone holder and to make the product dense. The compression ratio is the ratio between the cross-sectional area of the flow at the outlet of the manifold cone holder and the cross-sectional area of the flow at the outlet of the mould. Depending on the raw material used for the moulded product, the compression ratio is generally taken to be in the range of 3 to 10.

  5) The mould should have as few parts as possible, be compact and symmetrical in shape to facilitate installation, dismantling and heater arrangement. 6) The parts of the mould should have sufficient working strength. The main parts should be tempered and the working surface of the mouth mould should have a high hardness, generally not less than 40~45 HRC.

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