How is an injection mold made?

Although the mold structure may vary due to the variety and performance of plastics, the shape and structure of plastic products and the type of injection machine, the basic structure is the same. The mold is mainly composed of pouring system, temperature-regulating system, forming parts and structural parts. Pouring system and molding parts are directly in contact with plastic parts, and change with plastic and products, is the most complex plastic mold, the biggest change, processing requirements of the highest degree of smoothness and precision.

The injection mold is composed of moving mold and fixed mold. The moving mold is installed on the moving mold of the injection molding machine, and the fixed mold is installed on the fixed mold of the injection molding machine. During injection molding, the moving mold and the fixed mold are closed to form the pouring system and cavity, and the moving mold and the fixed mold are separated to take out the plastic products. In order to reduce the heavy die design and manufacturing workload, injection mold mostly adopts the standard die frame.

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

The gating system is also known as the runner system, which is a group of feeding channels that lead the plastic melt from the injection machine nozzle to the mold cavity, usually composed of the main channel, the shunt channel, the gate and the cold feed hole. It is directly related to the molding quality and production efficiency of plastic products.

The mainstream way

It is a passage in the mold connecting the injection molding machine nozzle to the shunt or cavity. The top of the main channel is concave for connection with the nozzle. The inlet diameter of the main channel should be slightly larger than the nozzle diameter (0.8mm) to avoid material overflow and to prevent blocking due to incorrect connection between the two. Import diameter according to the size of the product, generally 4-8mm. The diameter of the main channel should be expanded inward at an Angle of 3° to 5° to facilitate the release of the flow passage details.

Cold material point

It is a hole at the end of the main stream to trap the cold material generated between two injections at the nozzle end, thus preventing the shunt or gate from blocking. If the cold material once mixed with the cavity, the manufactured products will easily produce internal stress. The diameter of the cold feed hole is about 8-10mm and the depth is 6mm. In order to facilitate demoulding, the bottom is usually carried by the demoulding rod. The top of the demoulding rod should be designed into a zigzag hook or a sunken groove, so that the demoulding can be successfully pulled out of the main channel.

Shunt way

It is the channel connecting the main channel and each cavity in the multi-groove mold. In order to make the molten material fill each cavity at the same speed, the distribution of the shunt duct on the mold should be symmetrical and equidistant. The shape and size of the shunt section affect the flow of the plastic melt, the product demould and the difficulty of mold making. If the flow of the same amount of material, then the circular section of the channel resistance is the least. However, because the cylindrical flow passage is smaller than the surface, it is not good for the cooling of the flow passage redundancy, and the flow passage must be set up on the two halves of the die, which is both laborious and difficult to align. Therefore, a trapezoidal or semicircular diverter is often used, and is set on half of the mold with a stripper. The runner surface must be polished to reduce flow resistance and provide faster mold 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 duct is no more than 8mm, the extra-large one 10-12mm, and the extra-small one 2-3mm. On the premise of satisfying the needs, the cross-sectional area should be reduced as far as possible to increase the redundancy of the shunt duct and extend the cooling time.

gate

It is a channel connecting the main (or shunt) channel to the cavity. The cross-sectional area of a passage may be equal to the main (or shunt) passage, but is usually reduced. So it's the least cross-sectional area of the whole runner system. The shape and size of the gate have great influence on the quality of the product.

The function of the gate is:

A. Control the material flow velocity:

B. In the injection, backflow can be prevented due to early coagulation of the melt stored in this part:

C. Make the passing melt subjected to strong shear and raise the temperature, thus reducing the apparent viscosity and improving the fluidity:

D. Facilitate the separation of products from the runner system. The design of gate shape, size and position depends on the nature of the plastic, the size of the product and the structure. Generally, the cross section of sprue is rectangular or round, and the cross-sectional area should be small while the length should be short. This is not only based on the above functions, but also because it is easy for small sprue to become larger while it is difficult for large sprue to become smaller. The gate location should generally be selected in the product in the thickest and without affecting the appearance of the place. Gate dimensions should be designed to take into account the nature of the plastic melt. The cavity is the space in which plastic products are formed in a mold. The components used to form the cavity are collectively called molded parts. Each molded part often has a special name. The forming parts that constitute the shape of the product are called die (also called die), and those that constitute the internal shape of the product (such as holes, grooves, etc.) are called core or punch (also called die). The overall structure of the cavity should be determined according to the performance of plastic, the geometrical shape of the product, the tolerance of dimension and the requirement of use. Secondly, according to the determined structure, the parting surface, the location of gate and vent as well as the stripping mode are selected. Finally, the design of each part and the combination of each part are determined according to the size of the control. Plastic melt into the cavity has a high pressure, so the molding parts to be reasonable selection of materials and strength and stiffness of the check. In order to ensure the smooth and beautiful surface of plastic products and easy to dismold, the surface with plastic contact shall be Ra>0.32um with corrosion resistance. Molded parts are generally heat-treated to improve hardness, and the selection of corrosion resistant steel manufacturing.

The thermal control system

In order to meet the requirements of injection process on mold temperature, it is necessary to have a temperature regulating system to adjust the mold temperature. For thermoplastic injection moulds, the cooling system is designed to cool the moulds. The common method of mold cooling is to set cooling water channel in the mold and take away the heat of the mold with the circulating cooling water. In addition to using hot water or steam through cooling water channels, electrical heating elements can be installed in and around the mold.

Molding parts

Forming parts refer to all kinds of parts constituting the shape of products, including moving mold, fixed mold and cavity, core, forming rod and exhaust port, etc. The forming parts consist of core and die. The core forms the inner surface of the product, and the die forms the outer surface of the product. After closing the mold core and cavity constitute the mold cavity. According to the process and manufacturing requirements, sometimes the core and die by a number of pieces combined, sometimes made as a whole, only in the vulnerable parts, difficult to process the use of inserts.

vent

It is a groove opening in the mold for discharging the original and molten gas. When the molten material is injected into the cavity, the air originally stored in the cavity and the gas brought in by the melt must be discharged to the mold at the end of the material flow through the exhaust port, otherwise it will make the product with air holes, poor connection, mold filling dissatisfaction, and even the air is compressed to produce high temperature and burn the product. Under normal circumstances, the vent can be located in the end of the molten material flow in the cavity, but also in the parting surface of the mold. The latter is a shallow slot with a depth of 0.03-0.2mm and width of 1.5-6mm on one side of the die. During injection, the vent does not leak much molten material because the molten material cools and solidifies there, blocking the passage. The opening position of the exhaust port should not be facing the operator in case the molten material spews out accidentally and injures people. In addition, you can also use the ejector rod and ejector hole clearance, ejector block and ejector template and core clearance to exhaust.

Structural parts

It refers to the structure of the various parts of the mold, including: guide, demoulding, core-pulling and parting of various parts. Such as front and rear splint, front and rear buckle formwork, bearing plate, bearing column, guide column, release formwork, release rod and return rod, etc.

1. Guide parts

In order to ensure that the moving mold and the fixed mold can be correctly aligned when closing the mold, guiding parts must be set in the mold. In injection mould, four sets of guide posts and guide sleeve are usually used to form guide parts, and sometimes inner and outer cones matching each other are set on moving mould and fixed mould respectively to assist positioning.

2. Launch organization

In the process of mold opening, a push out mechanism is required to push out or pull out the plastic products and their condensate in the flow passage. Push out the fixed plate and push plate to clamp the push rod. In the push rod in general also fixed a reset rod, reset rod in moving, fixed mode closing the push plate reset.

3. Side core pulling mechanism

Some of the plastic products with side concave or side hole, before being pushed out must first carry out the side parting, draw out the side core to be able to unmold smoothly, at this time need to set up side core pulling mechanism in the mold.

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How is an injection mold made?

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