Although the structure of the mold may vary due to the variety and performance of the plastic, the shape and structure of the plastic products and the type of the injection machine, the basic structure is the same. The mold is mainly composed of gating system, temperature regulating system, forming parts and structural parts. Among them, the pouring system and molding parts are the parts that directly contact the plastic and change with the plastic and products. They are the most complex parts in the plastic mold, which change the most, and require the highest machining finish and precision.


The injection mold consists of a moving mold and a fixed mold. The moving mold is installed on the moving template of the injection molding machine, and the fixed mold is installed on the fixed template of the injection molding machine. During injection molding, the moving mold and the fixed mold are closed to form a gating system and a mold cavity. During mold opening, the moving mold and the fixed mold are separated to take out the plastic products. In order to reduce the heavy workload of mold design and manufacturing, most injection molds use standard mold bases.


1、 Gating system

The gating system refers to the part of the flow channel before the plastic enters the mold cavity from the nozzle, including the main flow channel, cold material cavity, shunt channel and gate.

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


1. Sprue

It is a section of channel in the mold connecting the injection nozzle of the injection molding machine to the shunt channel or cavity. The top of the main runner is concave to connect with the nozzle. The inlet diameter of the main flow channel shall be slightly larger than the nozzle diameter (0.8mm) to avoid material overflow and prevent blockage due to incorrect connection between the two. The inlet diameter is determined according to the size of the product, generally 4-8mm. The diameter of the main flow channel shall be expanded inwards at an angle of 3 ° to 5 ° to facilitate the demoulding of the flow channel vegetation.


2. Cold material cave

It is a cavity at the end of the main runner, which is used to capture the cold material produced between two injections at the end of the nozzle, so as to prevent the blockage of the shunt or gate. Once the cold material is mixed into the mold cavity, internal stress is easy to occur in the product. The diameter of the cold material cavity is about 8-, and the depth is 6mm. In order to facilitate demoulding, its bottom is usually borne by the demoulding rod. The top of the demoulding rod should be designed as a zigzag hook or set with a sunken groove, so that the sprue can be pulled out smoothly during demoulding.


3. Shunt channel

It is the channel connecting the main flow channel and each cavity in the multi slot die. In order to make the molten material fill the cavities at the same speed, the arrangement of shunts on the plastic mold should be symmetrical and equidistant. The shape and size of the cross-section of the splitter have an influence on the flow of the plastic melt, the demoulding of the product and the difficulty of the mold manufacturing. If the flow rate is equal, the resistance of the flow passage with circular section is the smallest. However, because the specific surface of the cylindrical runner is small, it is unfavorable to the cooling of the runner's superfluous matter, and this runner must be set on the two halves of the mold, which is both labor-intensive and difficult to align. Therefore, a trapezoidal or semicircular section shunt channel is often used, and it is set on half of the mold with a demoulding rod. 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 section width of the shunt channel is no more than 8mm, the extra large can reach 10-, and the extra small can reach 2-3mm. On the premise of meeting the needs, the cross-sectional area shall be reduced as far as possible, so as to increase the debris in the diversion channel and prolong the cooling time.


4. Gate

It is the channel connecting the main flow channel (or shunt channel) and the mold 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 part of the whole runner system. The shape and size of the gate have a great influence on the product quality.


The function of gate is:

A. Control material flow speed:

B. During the injection, the reverse flow can be prevented due to the early solidification of the molten material in this part:

C. Make the passed melt subject to strong shear and increase the temperature, so as to reduce the apparent viscosity and improve the fluidity:

D. It is convenient to separate the product from the runner system. The design of gate shape, size and position depends on the nature of the plastic, the size and structure of the product. Generally, the cross-section shape of the gate is rectangular or circular, and the cross-sectional area should be small and the length should be short. This is not only based on the above functions, but also because it is easy for the small gate to become larger, while it is difficult for the large gate to shrink. Generally, the gate position shall be the thickest part of the product without affecting the appearance. The gate size shall be designed taking into account the properties of the plastic melt. The cavity is the space where plastic products are molded in the mold. The components used to form the cavity are collectively referred to as formed parts. Each formed part often has a special name.

The formed parts that form the shape of the product are called female dies (also called female dies), and the parts that form the internal shape of the product (such as holes, grooves, etc.) are called cores or male dies (also called male dies). When designing molded parts, the overall structure of the cavity should be determined according to the performance of the plastic, the geometry of the product, the dimensional tolerance and the use requirements. Secondly, according to the determined structure, the position of the parting surface, gate and exhaust hole and the demoulding method are selected. Finally, the design of each part is carried out according to the size of the control product and the combination mode of each part is determined. The plastic melt has high pressure when entering the cavity, so the molded parts should be reasonably selected and the strength and stiffness should be checked. In order to ensure the smooth and beautiful surface of plastic products and easy demoulding, the roughness ra> of the surface in contact with plastic; 0. and corrosion resistance. The formed parts are generally made of corrosion-resistant steel by heat treatment to improve the hardness.


2、 Temperature regulating 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 for thermoplastic, the cooling system is mainly designed to cool the mold. The common method of mold cooling is to set up a cooling water channel in the mold, and use the circulating cooling water to take away the heat of the mold; In addition to using hot water or steam from the cooling water channel, electric heating elements can also be installed inside and around the mold.


3、 Formed parts

Molded parts refer to various parts that form the shape of products, including moving mold, fixed mold, cavity, core, molded rod and exhaust port. The forming part is composed of a core and a female die. The core forms the inner surface of the product, and the die forms the outer surface shape of the product. After clamping, the core and cavity form the mold cavity. According to the process and manufacturing requirements, sometimes the core and female die are combined by several blocks, sometimes they are made into a whole, and only the parts that are easy to be damaged and difficult to process are used with inserts.


1. Exhaust port

The utility model is a groove shaped air outlet opened in the mold to discharge the original gas and the gas brought in by the molten material. When the melt 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 outside the mold through the exhaust port at the end of the material flow, otherwise the product will have pores, poor connection, insufficient mold filling, and even the accumulated air will burn the product due to high temperature caused by compression. Generally, the exhaust hole can be set at the end of molten material flow in the mold cavity or on the parting surface of the plastic mold. The latter is to set a shallow groove 0.03-0.2mm deep and 1.5-6mm wide on one side of the die. During injection, there will not be a lot of molten material exuding from the exhaust hole, because the molten material will cool and solidify there and block the channel. The opening position of the exhaust port shall not face the operator to prevent accidental ejection of molten material from injuring people. In addition, the matching clearance between the ejector rod and the ejector hole, and the matching clearance between the top block, the stripping template and the core can also be used for exhaust.


4、 Structural parts

It refers to various parts constituting the mold structure, including various parts for guiding, demoulding, core pulling and parting. Such as front and rear plywood, front and rear buckle formwork, pressure bearing plate, pressure bearing column, guide column, formwork removal, demoulding rod and return rod, etc.

1. Guide part

In order to ensure that the moving die and the fixed die can be aligned accurately when closing the die, guide parts must be set in the die. In the injection mold, four groups of guide posts and guide sleeves are usually used to form the guide parts, and sometimes the inner and outer conical surfaces that coincide with each other are set on the moving mold and the fixed mold to assist in positioning.

2. Push out mechanism

In the process of mold opening, a pushing mechanism is required to push out or pull out the plastic products and their condensates in the flow channel. Push out the retainer plate and push plate to hold the push rod. In general, the push rod is also fixed with a reset rod, which resets the push plate when the moving and fixed mold are closed.

3. Side core pulling mechanism

Some plastic products with side recesses or side holes must be laterally parting before being pushed out, and the lateral core can be smoothly demoulded after being pulled out. At this time, it is necessary to set a side core pulling mechanism in the mold.

The injection device is a device for injecting the resin material into the mold after being heated and melted. The resin is squeezed into the barrel from the head, and the melt is conveyed to the front end of the barrel through the rotation of the screw. In that process, the resin material in the barrel is heated under the action of the heater, and the resin becomes molten under the action of the shear stress of the screw. The molten resin equivalent to the molded product, the main flow channel and the shunt channel is retained in the front end of the barrel (called metering), and the screw continuously shoots the material into the mold cavity. When the molten resin flows in the mold, it is necessary to control the moving speed (injection speed) of the screw and control it with pressure (pressure maintaining) after the resin fills the mold cavity. When the screw position and injection pressure reach a certain value, we can switch the speed control to pressure control.