The injection mold is composed 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. In injection molding, the moving mold and the fixed mold are closed to form the gating system and cavity. When the mold is opened, the movable mold and the fixed mold are separated to take out the plastic products.
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 consistent. The mold is mainly composed of gating system, temperature regulating system, forming parts and structural parts. The gating system and molding parts are the parts that directly contact with the plastics and change with the plastics and products. They are the most complicated and most changeable parts in the plastic mold, which require the highest machining finish and precision.
Gating system refers to the part of the runner before the plastic enters the cavity from the nozzle, including the sprue, cold pocket, splitter and gate. Forming parts refer to various parts that form the shape of products, including moving die, fixed die and cavity, core, forming rod and exhaust port, etc.
1, Gating system
Gating system, also known as runner system, is a group of feeding channels leading the plastic melt from the injection nozzle to the cavity, which is usually composed of main channel, shunt channel, gate and cold material hole. It is directly related to the molding quality and production efficiency of plastic products.
1. Main channel
It is a section of channel connecting the injection nozzle of the injection machine to the shunt channel or cavity in the mold. The top of the sprue is concave to connect with the nozzle. The diameter of the inlet of the main channel should be slightly larger than that of the nozzle (o.8mm) to avoid overflow and to prevent the two from blocking due to improper connection. The inlet diameter depends on the size of the product, generally 4-8mm. The diameter of the sprue should be enlarged inward at an angle of 3 ° to 5 ° to facilitate the demoulding of the runner vegetation.
2. Cold hole
It is a hole at the end of the sprue to catch the cold material produced between two injections at the end of the nozzle, so as to prevent the blockage of the splitter or gate. If the cold material once mixed into the cavity, the product is easy to produce internal stress. The diameter of the cold hole is about 8-10 mm and the depth is 6 mm.
In order to facilitate demoulding, the bottom of the mould is usually borne by a demoulding rod. The top of the demoulding rod should be designed as a zigzag hook or a depression groove, so that the main channel vegetation can be pulled out smoothly during demoulding.
3. Diversion channel
It is the channel connecting the main channel and each cavity in the multi slot die. In order to make the melt fill each cavity at the same speed, the distributary channel should be arranged symmetrically and equidistantly on the mold.
The shape and size of splitter section have influence on the flow of plastic melt, demoulding of products and difficulty of 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 channel, it is not conducive to the cooling of the shunt channel vegetations. Moreover, this kind of shunt channel must be set on two half dies, which is labor-intensive and easy 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 a faster filling speed. The size of the runner depends on the type of plastic, the size and thickness of the product. For most thermoplastic plastics, the section width of diversion channel is no more than 8 m, the extra large one can reach 10-12 m, and the very small one is 2-3 M. On the premise of meeting the needs, the cross-sectional area should be reduced as far as possible, so as to avoid the increase of the shunt duct bulks and the extension of cooling time.
4. Gate
It is the channel connecting the main 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), but it is usually reduced. So it is the smallest section in the whole flow channel system. The shape and size of gate have great influence on product quality.
The purpose of the gate is to:
Control material flow speed
In the injection process, backflow can be prevented due to the early setting of the molten material in this part
The temperature of the molten material is increased due to strong shear, thus reducing the apparent viscosity and improving the fluidity
It is easy to separate the products 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 cross-section shape of 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 effect, but also because it is easier for small gate to become larger, while it is difficult for large gate to shrink. Gate position should be selected in the thickest part of the product without affecting the appearance. Gate size design should take into account the nature of the plastic melt. The cavity is the space in the mold for forming plastic products.
The components used to form the cavity are collectively referred to as molded parts. Each molding part often has a special name. The forming parts forming the shape of the product are called female die (also known as female die), and those forming the internal shape of the product (such as holes, grooves, etc.) are called core or punch (also known as male mold). When designing the molding parts, the overall structure of the cavity should be determined according to the properties of the plastics, the geometrical shape of the products, the dimensional tolerance and the use requirements.
Secondly, the location of parting surface, gate and vent hole and demoulding method are selected according to the determined structure. Finally, according to the size of the control product, the design of each part and the combination mode of each part are determined.
When the plastic melt enters the cavity, it has a high pressure, so the molding 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 surface in contact with plastic should have a roughness Ra > 0.32um and be re
