The mold core and mold sleeve are extrusion molding molds, which are fixed on the mold core seat. Their function is to fix and support the wire core or cable core, forming a circular shape for the plastic, and entering the mold sleeve in a certain direction. The relative position of the mold core and mold sleeve is adjusted by adjusting the bolts of the mold core seat. The mold sleeve is fixed to the machine head with the help of the mold sleeve cover, and the function of the mold sleeve is to allow plastic to enter the hole channel for molding through the gap formed between its inner cone hole and the outer cone of the mold core.

1、 There are three forms of plastic extrusion molds:
1. Extrusion mold

The core of the extrusion mold does not have a tubular bearing part, and the core shrinks behind the bearing diameter of the mold sleeve. Molten plastic (hereinafter referred to as material flow) is finally shaped by pressure through the mold sleeve. The extruded plastic layer has a tight structure and a flat surface.

1.1 Advantages of extrusion molds:
a.  The extruded plastic layer has a tight structure and a flat surface.  
b. Suitable for extruding plastic with a tensile ratio that is too small.
1.2 Disadvantages of extrusion molds:
a. The amount of glue produced is much lower than that of extrusion tube type
b. It is easy to cause serious eccentricity of the plastic layer,
c. Product quality heavily relies on molds
d. Extrusion molding requires high accuracy in mold matching
e. Poor bending performance of extruded wire cores
2. Extrusion tube mold

The extrusion method in which the mold core has a tubular bearing part during extrusion, and the end face of the mold core mouth extends out of the end face of the mold sleeve mouth or is level with the end face of the mold sleeve mouth is called extrusion tube type. During tube extrusion, due to the presence of the tubular bearing part of the mold core, the plastic does not directly press on the cable core, but moves forward along the tubular bearing part to form a tubular shape, which is then stretched and wrapped on the wire core or cable core.
2.1 Advantages of extrusion tube molds:
a. Fast extrusion speed;                         
b. Simple production operation and easy adjustment of core deviation;
C. Convenient mold matching;  
d. After stretching, the molecules of plastic undergo directional arrangement, improving the mechanical properties of the sheath;
e. The thickness of the sheath is easy to control;
2.2 Disadvantages of extrusion tube molds:
a. Poor density of plastic layer;
b. Poor adhesion between plastic layer and core;
c. The appearance of extrusion is not as round as that of extrusion;
3. Semi extrusion mold

The mold core has a tubular bearing section, but it is relatively short. The extrusion method of shrinking the end face of the mold core bearing diameter into the end face of the mold sleeve is called semi extrusion, which is a transitional form between extrusion tube and extrusion.

2、 Tensile ratio (S) and equilibrium coefficient (K)
The extrusion theory is a practical theory that began in the 1970s when DuPont mass-produced F46. In order to promote this material, after repeated extrusion practices, the theory of tensile ratio S and equilibrium coefficient K was introduced. At the same time, it was found that this theory is applicable to other plastics and became one of the basis for extrusion theory. But it still belongs to typical empirical theory. It is necessary to continuously flexibly apply different materials, equipment characteristics, extrusion front material conditions, environment and other factors in actual production. There is no most reasonable, only more reasonable. Wire and cable extrusion generally adopts tight wrapping or balanced stretching, with K=0.95-1.1.
Stretch ratio: The so-called stretch ratio refers to the ratio of the area of the plastic ring at the mold mouth to the area of the ring covering the wire and cable, which is the ratio of the cross-sectional area of the gap formed by the mold core and sleeve to the nominal thickness cross-sectional area of the product.

Tensile ratio (DDR)=(DD 2-D T 2)/(db2-dc2)
Stretch Balance (DRB)=(DD/db)/(DT/dc)
DD: Inner diameter of the mold
DT: Outer diameter of the mold core
Db: Outer diameter of the wrapped wire
DC: Outer diameter of the core wire

3、 Principles of hot extrusion molding
1. Low melting viscosity: can be extruded or extruded
2. High melting viscosity: extruded tube
3. Poor fluidity after melting: extrusion or semi extrusion
4. Need to extrude dense: extrusion

4、 Selection of extrusion mold size
1. Squeeze type:
Internal mold: The maximum diameter that can pass through the core wire, usually small diameter=d+0.05~0.5mm, large diameter=d+0.5~2mm
External mold: consistent with the extrusion diameter, generally small diameter=D+0-0.5mm, large diameter=D+0.5-1.5mm
2. Squeezing type:
Internal mold: The maximum diameter that can pass through the core wire, usually small diameter=d+0.05~1mm, large diameter=d+0.5~8mm
Outer mold:=outer diameter of inner mold tube+(2.5-4) t

5、 Design requirements for extrusion molds
1. The surface of molds that come into contact with plastic should be smooth and have a high finish. If they are chrome plated and polished, it is more ideal.
2. The mold should have interchangeability and consider the dimensional tolerance requirements of various parts.
3. Plastic has a certain pressure inside the mold, and the angle of the mold sleeve must be greater than the angle of the mold core.
4. Proper selection of mold materials can improve the service life. The mold should have certain hardness, strength, rust resistance, and corrosion resistance.
When designing extrusion type mold cores and sleeves, the relationship between the gap area formed by the mold core and the cross-sectional area of the product pipe should be considered, which is called the stretching ratio.    

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