An injection mold is a tool comprised of a series of parts that allows molten plastic to be formed and cooled in such a way as to create a discrete part shape. Although the structure of the plastic mold may vary due to the variety and performance of the plastic, the shape and structure of the plastic product, and the type of the injection machine, the basic structure is the same. If you didn’t understand how an injection mold worked before and how is the injection mold made up, you will after reading this.
Components Of An Plastic Injection Mold
Casting system
It refers to the part of the flow path before the plastic enters the cavity from the nozzle, including the main flow path, the cold material hole, the runner, and the gate. The casting system is directly related to the molding quality and production efficiency of plastic products.
Main Flow Path
It is a section of runner connecting the injection nozzle of injection molding machine to the shunt runner or cavity in the mold. The top of the main flow path is concave to connect with the nozzle. The diameter of the inlet of the main flow path should be slightly larger than the diameter of the nozzle (0.8mm) to avoid overflow, and to prevent blocking due to improper connection between the two. The diameter of the inlet depends on the size of the product, generally 4-8mm.
Cold Material Tole
It’s a hole at the end of the main flow path, which is used to catch the cold material produced between the two injections at the end of the injection nozzle, so as to prevent the blockage of the shunt runner or the gate. Once the cold material is mixed into the cavity, the internal stress will be easily produced in the products. The diameter of the cold charging 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 borne by the demoulding rod.
Sub-Runner
It is the channel connecting the main flow path and each cavity in the multi groove die. In order to make the melt fill the cavities at the same speed, the distribution of the sub-runner on the mold should be symmetrical and equidistant. The shape and size of the cross section of the sub-runner affect the flow of the plastic melt, the difficulty of product demoulding and mold manufacturing. If the flow rate is equal, the flow resistance of circular section is smaller. However, because the specific surface of the cylindrical channel is small, it is not good for the cooling of the debris in the sub-runner. Therefore, a trapezoid or semicircle cross-section shunt channel is often used, and is arranged on a half mold with a demoulding rod. The runner surface must be polished to reduce flow resistance and provide a 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 section width of the shunt channel is not more than 8mm, the extra large one can reach 10-12mm, and the extra small one can reach 2-3mm.
Gate & Sprue
It is the channel connecting the main flow path(or sub-runner) and the cavity. The cross-sectional area of the channel can be equal to the main flow path of sub-runner, but it is usually reduced. So it is a small part of the whole flow passage system. The shape and size of gate have a great influence on the quality of products.
The purpose of the sprue is to:
– Control the material flow speed.
– Backflow can be prevented during injection due to the early setting of the molten material in this part.
– The temperature of the molten material passing through is increased due to strong shearing, so as to reduce the apparent viscosity and improve the fluidity.
Temperature Regulation System
In order to meet the requirements of injection process for mold temperature, a temperature regulating system is needed to regulate the mold temperature. For the injection mold of thermoplastic, the cooling system is mainly designed to cool the mold. The common method of mold cooling is to set up cooling water channel in the mold, using circulating cooling water to take away the heat of the mold. The heating of the mold can not only use hot water or steam in the cooling water channel, but also install electric heating elements in and around the mold.
Exhaust Port
It is a groove type air outlet opened in the mold to discharge the original gas and the gas brought in by the melt. When the molten material is injected into the mold cavity, the air originally stored in the mold cavity and the gas brought in by the melt must be discharged to the outside of the mold through the exhaust port at the end of the material flow, otherwise, the products will have air holes, poor connection, insufficient filling of the mold, or even the accumulated air will burn the products due to high temperature caused by compression. In general, the vent hole can be located at the end of the melt flow in the mold cavity or on the parting surface of the mold. The latter is to open a shallow groove with a depth of 0.03-0.2mm and a width of 1.5-6mm on one side of the die. During the injection, there will be no leakage of a lot of molten material from the vent hole, because the molten material will cool and solidify there to block the channel.
Molding Parts
Molding parts refer to all kinds of parts forming the shape of products, including moving die, fixed die and cavity, core, forming rod and exhaust port. The forming part consists of core and die. The core forms the inner surface of the product, and the concave die forms the outer surface shape of the product. After the mold is closed, the core and cavity form the mold cavity. According to the process and manufacturing requirements, sometimes the core and die are composed of several pieces, sometimes they are made as a whole, and only insert parts that are easy to be damaged and difficult to process are used.
The Plastic Mold According To The Structure
It refers to various parts that constitute the plastic injection mold structure, including: various parts such as guide, demolding, core pulling and parting. Such as front and rear splint, front and back buckle template, pressure plate, pressure column, guide column, stripping template, demolding rod and return rod, etc.
