How is the injection mold made up?

  Although the structure of the mold 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 mould is mainly composed of gating system, temperature regulating system, forming parts and structural parts. Among them, the gating system and molding parts are the parts that directly contact with plastics and change with plastics and products, which are the most complex and changeable parts in the plastic mold and require the highest machining finish and precision.
The injection mold is composed of moving mold and 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. In injection molding, the moving mold and the fixed mold are closed to form the gating system and the cavity. When the mold is opened, 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, the injection mold mostly adopts the standard mold base.
gating system
Gating system refers to the part of the runner before the plastic enters the cavity from the nozzle, including the sprue, cold cavity, splitter and gate.
Gating system, also known as runner system, is a group of feeding channels leading the plastic melt from the nozzle of the injection molding machine to the cavity, which is usually composed of main flow channel, shunt channel, gate and cold material cavity. It is directly related to the molding quality and production efficiency of plastic products.
Mainstream
It is a channel connecting the nozzle of injection molding machine to the porthole or cavity in the mold. 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 (0.8 mm) to avoid material overflow and to prevent blockage due to inaccurate connection. The import diameter depends on the size of the product, generally 4-8mm. The sprue diameter should be enlarged inwards at an angle of 3 ° to 5 ° to facilitate the demoulding of the channel debris.
Cold material hole
It is a hole at the end of the main flow channel, which is used to catch the cold material produced between two injections at the end of the nozzle, so as to prevent the blockage of the diversion channel or gate. If the cold material is mixed into the cavity, the internal stress will be easily produced in the products. The diameter of the cold material cavity 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. The top of the demoulding rod should be designed into a zigzag hook shape or a sink groove, so that the debris in the main channel can be pulled out smoothly during demoulding.
Shunt channel
It is the channel connecting the sprue and each cavity in the multi groove die. In order to make the melt fill all the cavities at the same speed, the distribution of the portholes on the mold should be symmetrical and equidistant. The shape and size of the splitter section have an effect on the flow of plastic melt, the difficulty of product demoulding and mold manufacturing. If the flow rate is equal, the resistance of circular section is the least. However, because of the small specific surface area of the cylindrical runner, it is unfavorable to the cooling of the porthole, and the porthole must be set on two half dies, which is labor-consuming and difficult to align. Therefore, the trapezoidal or semicircular cross-section shunt channel is often used, and it is set on the half mold with 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 width of shunt channel section is not more than 8 mm, the extra large can reach 10-12 mm, and the extra small can reach 2-3 mm. On the premise of meeting the needs, the cross-sectional area should be reduced as much as possible, so as to increase the diversion channel vegetation and prolong the cooling time.
Gate
It is the channel connecting the main flow 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 runner system. The shape and size of gate have great influence on the quality of products.
The function of gate is:
A. Control material flow speed:
B. During injection, backflow can be prevented due to premature solidification of the melt in this part
C. In order to increase the temperature of the molten material, the apparent viscosity is reduced and the fluidity is improved
D. It is easy to separate the product from the runner system. The design of gate shape, size and location depends on the nature of the plastic, the size and structure of the product. Generally, the section shape of gate is rectangle or circle, 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, but it is difficult for large gate to become smaller. The gate position should be the place where the product is thickest and does not affect the appearance. Gate size design should take into account the nature of the plastic melt. The cavity is the space for forming plastic products in the mold. The components used to form the cavity are collectively referred to as forming parts. Each molding part often has a special name. The forming parts forming the product shape are called female mold (also called female mold), and those forming the internal shape of the product (such as hole, groove, etc.) are called core or male mold (also called male mold). When designing molding parts, the overall structure of the cavity should be determined according to the performance of the plastic, the geometric shape of the product, the dimensional tolerance and the use requirements. Secondly, according to the determined structure, the location of parting surface, gate and vent and demoulding method are selected. Finally, according to the size of the control product, the design of each part and the combination of each part are determined. The 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 Ra of surfaces in contact with plastic products is more than 0.32um, and corrosion resistance is required. Forming parts are generally through heat treatment to improve the hardness, and choose corrosion-resistant steel manufacturing.
Temperature regulating system
In order to meet the requirements of injection process for mold temperature, a temperature regulating system is needed to adjust the mold temperature. For thermoplastic injection mold, the cooling system is designed to cool the mold. The common method of mold cooling is to set up cooling water channel in the mold, and use circulating cooling water to take away the heat of the mold; besides using hot water or steam in the cooling water channel, electric heating elements can also be installed inside and around the mold.
Forming parts
Forming parts refer to all kinds of parts forming the shape of products, including moving die, fixed die, cavity, core, forming rod and exhaust port, etc. The forming part is composed of core and 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 cavity of the mold. 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 inserts are used in the parts that are easy to damage and difficult to process.
Exhaust port
The utility model is a groove-shaped air outlet opened in the mould, which is used for discharging the original gas and the gas brought in by the molten material. When the molten material is injected into the cavity, the air in the 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 product will have pores, poor connection, insufficient filling, and even the accumulated air will burn the product due to high temperature caused by compression. In general, the vent can be set at either the end of the melt flow in the mold cavity or 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 injection, there will not be a lot of molten material seeping from the exhaust hole, because the molten material will cool and solidify there, blocking the channel. The opening position of the exhaust port should not face the operator, in order to prevent accidental spraying of molten material to hurt people. In addition, the fit clearance between ejector rod and ejector hole, the fit clearance between ejector block and demoulding plate and core can also be used to exhaust.
Structural parts
It refers to the various parts of the mold structure, including: guide, demoulding, core pulling and parting parts. Such as front and rear splint, front and rear buckle template, pressure plate, pressure column, guide column, demoulding rod, return rod, etc.
1. Guide part
In order to ensure that the moving die and the fixed die can be accurately aligned when closing the die, the guide part must be set in the die. In the injection mold, four groups of guide pillars and guide sleeves are usually used to form the guide parts, and sometimes it is necessary to set the inner and outer conical surfaces on the moving mold and the fixed mold respectively to assist the positioning.
2. Launching organization
In the process of mold opening, a push out mechanism is needed to push out or pull out the plastic product and its condensate in the flow channel. Push out the fixed plate and the push plate to hold the push rod. The push rod is generally fixed with a reset rod, which makes the push plate reset when the moving and fixed dies are closed.
3. Side core pulling mechanism
Some plastic products with side concave or side hole must be separated laterally before being pushed out, and then demoulding can be carried out smoothly after pulling out the side core. At this time, it is necessary to set the side core pulling mechanism in the mold.