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Injection mold making principle

update:15-07-2019
abstract:

An injection mold is a part that imparts shape and size […]

An injection mold is a part that imparts shape and size to a plastic during molding. Although the structure of the mold may vary depending on the type and performance of the plastic, the shape and structure of the plastic product, and the type of the injection machine, the basic structure is uniform. The mold is mainly composed of a casting system, a molded part and a structural part. The gating system and the molded part are in direct contact with the plastic and vary with the plastic and the product. It is the most complicated and most varied part of the mold, and requires the highest degree of smoothness and precision.
The pouring system refers to the part of the flow path before the plastic enters the cavity from the nozzle, including the main channel, the cold material hole, the split channel and the gate. Molded parts refer to various parts that make up the shape of the product, including moving molds, fixed molds and cavities, cores, forming rods, and exhaust ports.
1, the mainstream road
It is a passage in the mold that connects the nozzle of the injection machine to the splitter or cavity. The top of the main flow path is concave to engage the nozzle. The inlet diameter of the main flow channel should be slightly larger than the nozzle diameter (O.8mm) to avoid flashing and prevent the two from intercepting due to inaccurate connection. The diameter of the inlet depends on the size of the product, generally 4-8mm. The diameter of the main flow path should be enlarged inward by an angle of 3° to 5° in order to demold the flow path.
2, the runner Z
It is a channel connecting the main channel and each cavity in a multi-slot die. In order for the melt to fill the cavities at a constant velocity, the arrangement of the runners on the mold should be symmetric and equidistant. The shape and size of the cross-section of the runner have an effect on the flow of the plastic melt, the release of the product, and the ease of mold manufacture. If the flow is of equal amount, the flow path resistance with a circular cross section is the smallest. However, because the specific surface of the cylindrical flow passage is small, it is unfavorable for the cooling of the shunt passage, and the shunting passage must be opened on the two mold halves, which is laborious and easy to align. Therefore, a trapezoidal or semi-circular cross-section runner is often used and is placed on one half of the mold with the stripper. The runner surface must be polished to reduce flow resistance to provide faster filling speed. The size of the runner is determined by the variety of plastics, the size and thickness of the product. For most thermoplastics, the cross-sectional section width is no more than 8mm, and the extra large can reach 10-12mm, and the extra small is 2-3mm. The cross-sectional area should be minimized to meet the need, so as to avoid increasing the shunting and extending the cooling time.
3, gate
It is the channel that connects the main channel (or shunt) to the cavity. The cross-sectional area of ​​the channel can be equal to the main channel (or shunt), but it is usually reduced. So it is the smallest part of the cross-sectional area of ​​the entire runner system. The shape and size of the gate have a great influence on the quality of the product. The function of the gate is: A. Control the flow velocity: B. In the injection, the molten material stored in this part can be condensed early to prevent backflow: C, the molten material is subjected to strong shearing and the temperature is raised. , thereby reducing the apparent viscosity to improve fluidity: D, to facilitate separation of the product from the runner system. The design of the gate shape, size and location depends on the nature of the plastic, the size and structure of the article. Generally, the cross-sectional shape of the gate is rectangular or circular, and the cross-sectional area should be small and the length should be short, which is not only based on the above effects, but also because the small gate becomes larger, and the large gate is difficult to be reduced. The location of the gate should generally be chosen where the product is thickest and does not affect the appearance. The gate size should be designed to take into account the nature of the plastic melt.
4, cavity
It is the space for molding plastic products in the mold. The components used to form the cavity are collectively referred to as molded parts. Each molded part often has a unique name. The molded part constituting the outer shape of the product is called a concave mold (also called a female mold), and the inner shape (such as a hole, a groove, etc.) constituting the inner shape of the product is called a core or a punch (also called a male mold). When designing a molded part, the overall structure of the cavity is first determined according to the properties of the plastic, the geometry of the product, the dimensional tolerances, and the requirements for use. Secondly, the position of the parting surface, the gate and the vent hole, and the demolding mode are selected according to the determined structure. Finally, the design of each part is carried out according to the size of the control item and the combination between the parts is determined. The plastic melt has a high pressure when it enters the cavity, so the molded parts should be properly selected and checked for strength and stiffness. In order to ensure the smooth and beautiful surface of the plastic products and easy demoulding, the surface in contact with the plastic has a roughness Ra>0.32um and is resistant to corrosion. Molded parts are generally heat treated to increase hardness and are made of corrosion-resistant steel.
5, the exhaust port
It is a trough-shaped air outlet opened in the mold to discharge the original and the gas brought in by the melt. 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 outside of the die through the exhaust port at the end of the flow, otherwise the product will have pores, poor fusion, The filling is not full, and even the accumulated air burns the product due to the high temperature generated by compression. In general, the vent hole can be disposed at the end of the flow of the melt in the cavity or on the parting surface of the mold. The latter is a shallow groove having a depth of 0.03-0.2 mm and a width of 1.5-6 mm on one side of the die. During the injection, the vent hole does not have a lot of melt oozing out, because the melt will cool and solidify there to block the passage. Do not point the opening of the exhaust port to the operator to prevent the melt from accidentally squirting and injuring people. In addition, the matching gap between the ejector rod and the ejector hole, the matching gap between the top block and the stripper and the core may be utilized to exhaust.
6, structural parts D#i
It refers to the various parts that make up the mold structure, including: parts for guiding, demoulding, core pulling and parting. Such as front and rear splint, front and rear buckle template, bearing plate, pressure column, guide column, stripping plate, stripping rod and return rod.
7. Heating or cooling device
This is a device for solidifying and shaping the melt in the mold. For thermoplastics, generally the passage of the cooling medium in the male and female molds, the circulating flow of the cooling medium is used for cooling purposes. The cooling medium to be introduced varies depending on the type of plastic and the structure of the product, such as cold water, hot water, hot oil and steam. The key is high-efficiency uniform cooling, which will directly affect the quality and size of the product. The arrangement of the cooling channels and the choice of cooling medium should be considered based on the thermal properties of the melt (including crystallization), the shape of the article, and the mold structure.

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