Although the structure of the mould may vary greatly due to the variety and performance of plastics, the shape and structure of plastics products and the type of injection machine, the basic structure is the same. The die is mainly composed of pouring system, temperature regulating system, forming parts and structural parts. Among them, the casting system and the forming parts are the parts directly contacted with the plastics, and change with the plastics and products. They are the most complex and most changeable parts in the plastic mould, requiring the highest processing smoothness and accuracy.

The injection mould consists of two parts: the moving mould and the fixed mould. The moving mould is installed on the moving template of the injection moulding machine and the fixed mould is installed on the fixed template of the injection moulding machine. In injection moulding, the gating system and the cavity are formed by the closure of the movable and fixed moulds, and the movable and fixed moulds are separated when the moulds are opened to remove the plastic products. In order to reduce the heavy workload of mold design and manufacturing, most injection moulds adopt standard mold base.

gating system

The pouring system refers to the runner part of the plastic before it enters the cavity from the nozzle, including the main runner, the cold material hole, the shunt runner and the gate, etc.

Gating system, also known as runner system, is a group of feeding channels that lead plastic melt from injector nozzle to cavity, usually composed of main runner, shunt runner, gate and cold hole. It is directly related to the forming quality and production efficiency of plastic products.

Main Street

It is a passage connecting the injector nozzle of injection moulding machine to the shunt channel or cavity in the mould. The top of the main channel is concave to connect with the nozzle. The inlet diameter of the main channel should be slightly larger than that of the nozzle (0.8mm) to avoid spills and to prevent blockage due to improper connection between the two. Import diameter depends on the size of the product, generally 4-8 mm. The diameter of the main channel should be enlarged inward at an angle of 3 to 5 degrees in order to demould the runner excrescence.

Cold Cave

It is a hole located at the end of the main channel to trap the cold material produced between the two injections at the end of the nozzle, thus preventing the blockage of the diversion channel or gate. If the cold material is mixed into the cavity, the internal stress will easily occur in the product. The diameter of the cold material hole is about 8-10 mm and the depth is 6 mm. In order to facilitate demoulding, the bottom of the demoulding rod is usually assumed by the demoulding rod. The top of the demoulding rod should be designed as a zigzag hook or a sinking groove so as to pull out the bulge of the main channel smoothly when demoulding.

Shunt

It is the channel connecting the main channel and each cavity in the multi-groove die. In order to make the melt fill all the cavities at the same speed, the arrangement of the shunts on the plastic mould should be symmetrical and equidistant. The shape and size of the shunt section have an effect on the flow of plastic melt, the demoulding of products and the difficulty of die manufacturing. If the flow rate is equal, the resistance of the circular section is the smallest. However, because of the small specific surface of the cylindrical channel, it is not conducive to the cooling of the diversion channel, and the diversion channel must be set on the two-half die, which is labor-intensive and difficult to align. Therefore, trapezoidal or semi-circular cross-section dividers are often used, and they are arranged on half of the die with a demoulding rod. The runner surface must be polished to reduce flow resistance and provide faster filling speed. The size of the runner depends on the type of plastic, the size and thickness of the product. For most thermoplastics, the cross-section width of the shunt is not more than 8 mm, the super-large can reach 10-12 mm, and the super-small can reach 2-3 mm. On the premise of meeting the requirements, the section area should be reduced as far as possible to increase the shunt redundancy and prolong the cooling time.

Gate

It is the channel connecting the main channel (or shunt channel) and the cavity. The cross-sectional area of the channel can be equal to that of the main channel (or shunt channel), but it is usually reduced. So it is the smallest section in the whole channel system. The shape and size of gate have a great influence on the quality of products.

The function of gate is:

A. Controlling material flow speed:

B. Preventing reflux during injection due to premature solidification of the molten material in this part:

C. Increase the temperature by shearing the passed melt, thereby reducing the apparent viscosity and improving the fluidity:

D. Easy to separate the product from the runner system. The design of gate shape, size and location depends on the properties of plastics, the size and structure of products. Generally, the section shape of gate is rectangular or circular, the section area should be small and the length should be short. This is not only based on the above effect, but also because it is easier for small gate to become larger, while it is difficult for large gate to shrink. The gate position should be chosen where the product is thickest without affecting the appearance. The design of gate size should take into account the properties of plastic melt. The cavity is the space for forming plastic products in the mould. The components used to form the cavity are collectively called forming parts. Each forming part often has a special name. The forming part that forms the shape of the product is called the concave die (also known as the negative die), and the inner shape of the product (such as holes, grooves, etc.) is called the core or the punch (also known as the positive die). When designing forming parts, the overall structure of the cavity should be determined according to the performance of plastics, the geometric shape of the product, the dimensional tolerance and the requirements of use. Secondly, according to the determined structure, the location of parting surface, gate and exhaust hole and demoulding mode are selected. Finally, according to the size of the control product, the design of each part and the combination mode of each part are determined. Plastic melt has a high pressure when it enters the cavity, so the forming parts should be reasonably selected and checked for strength and stiffness. In order to ensure the smooth and beautiful surface of plastic products and easy demoulding, the roughness of surfaces contacted with plastics is Ra > 0.32 um, and it should be corrosion resistant. Formed parts are generally made of corrosion-resistant steel by heat treatment to improve hardness.