Injection molds are parts that impart shape and size to plastic during molding. Although structure of mold may vary due to variety and performance of plastics, shape and structure of plastic products, and type of injection machine, basic structure is same.

 

Mold is mainly composed of three parts: gating system, forming parts and structural parts. Among them, gating system and molding parts are parts that are in direct contact with plastic, change with plastic and product. They are the most complex and changeable parts in mold, and require the highest processing finish and precision.
Gating system refers to part of runner before plastic enters cavity from nozzle, including main runner, cold material cavity, runner and gate. Molded parts refer to various parts that make up shape of product, including movable molds, fixed molds and cavities, cores, molding rods, and exhaust ports. A typical mold structure is shown in the figure.

It is a passage in mold that connects nozzle of injection machine to runner or cavity. Top of sprue is concave for engagement with nozzle. Inlet diameter of main channel should be slightly larger than diameter of nozzle (0.8mm) to avoid overflow and prevent the two from being blocked due to inaccurate connection. Diameter of inlet depends on size of product, generally 4-8mm. Diameter of sprue should be expanded inward at an angle of 3° to 5° to facilitate release of runner debris.

It is a cavity at the end of main channel to capture cold material generated between two injections at the end of nozzle, thereby preventing blockage of branch channel or gate. If cold material is mixed into cavity, internal stress is easily generated in manufactured product. Diameter of cold material hole is about 8-10mm, and depth is 6mm. In order to facilitate demoulding, bottom is often borne by demoulding rod. Top of the demoulding rod should be designed as a zigzag hook or a sunken groove, so that sprue can be pulled out smoothly when demoulding.

It is channel connecting main channel and each cavity in multi-slot mold. In order to make melt fill each cavity at same speed, arrangement of runners on mold should be symmetrical and equidistant. Shape and size of runner section have an impact on flow of plastic melt, demolding of product and ease of mold manufacturing.
If flow of same amount of material is considered, flow channel resistance with a circular cross-section is the smallest. However, because specific surface of cylindrical runner is small, it is not good for cooling of runner excess, and runner must be opened on the two halves of mold, which is labor-intensive and easy to align.
Therefore, trapezoidal or semi-circular cross-section runners are often used, and are opened on half of mold with ejector bars. Runner surface must be polished to reduce flow resistance and provide faster filling speed. Size of runner depends on type of plastic, size and thickness of product. For most thermoplastics, cross-sectional width of runner is not more than 8m, extra large can reach 10-12m, and extra small is 2-3m. Under premise of meeting needs, cross-sectional area should be reduced as much as possible, so as to avoid increasing shunting duct debris and prolonging cooling time.

 

It is channel that connects main channel (or shunt channel) and cavity. Cross-sectional area of channel can be equal to main channel (or branch channel), but it is usually reduced. So it is part with the smallest cross-sectional area in the entire runner system. Shape and size of gate have a great influence on quality of product.
Function of gate is: A, control flow rate; B, during injection, melt stored in this part can be prematurely solidified to prevent backflow; C, passing melt is subjected to strong shear to increase temperature, thereby reducing apparent viscosity to improve fluidity, D. it is convenient to separate product from runner system. Design of gate shape, size and location depends on nature of plastic, size and structure of product.
Generally, cross-sectional shape of gate is rectangular or circular, cross-sectional area should be small and length should be short. This is not only based on above effects, but also because it is easier for a small gate to become larger, while it is difficult to shrink a large gate. Gate position should generally be selected at place where product is thickest without affecting appearance. Design of gate size should take into account properties of plastic melt.

It is space in mold for molding plastic products. Components used to form cavity are collectively referred to as molded parts. Each formed part often has a special name. Molding parts that constitute shape of product are called concave molds (also known as female molds), and those that constitute internal shapes of products (such as holes, grooves, etc.) are called cores or punches (also known as male molds).
When designing a molded part, the overall structure of cavity should be determined first according to performance of plastic, geometry of product, dimensional tolerances and usage requirements. Second is to select parting surface, position of gate, exhaust hole and demoulding method according the determined structure. Finally, according to size of control product, design of each part and combination of parts are determined. Plastic melt has a high pressure when it enters cavity, so molding parts should be reasonably selected, checked for strength and stiffness.
In order to ensure surface of plastic products is smooth, beautiful and easy to demould, all surfaces in contact with plastics should have a roughness Ra>0.32um and be resistant to corrosion. Formed parts are generally heat treated to increase hardness and made of corrosion-resistant steel.

It is a groove-shaped air outlet opened in mold to discharge original gas and gas brought in by melt. When melt is injected into cavity, air originally stored in cavity and gas brought in by melt must be discharged out of mold through exhaust port at the end of material flow, otherwise product will have pores, poor welding, underfilling, and even accumulated air will burn product due to high temperature caused by compression.
Under normal circumstances, vent hole can be set either at the end of flow of melt in cavity, or at parting surface of mold. The latter is a shallow groove with a depth of 0.03-0.2mm and a width of 1.5-6mm on one side of die. During injection, there will not be a lot of molten material oozing out of vent hole, because molten material will cool and solidify there, block channel. Opening position of exhaust port should not face operator to prevent accidental ejection of molten material and injury.
In addition, matching gap between ejector rod and ejector hole, matching gap between ejector block and stripper plate, core can also be used to exhaust air.

It refers to various parts that make up mold structure, including: guiding, demoulding, core pulling and parting various parts. Such as front and rear splints, front and rear buckle templates, bearing plates, bearing columns, guide columns, stripping plates, stripping rods and return rods, etc.

This is a device for solidifying molten material in mold. For thermoplastics, it is generally channel of cooling medium in male and female molds, and cooling purpose is achieved by circulating flow of cooling medium. Cooling medium introduced varies with type of plastic and structure of product, including cold water, hot water, hot oil and steam. Key is high-efficiency uniform cooling, uneven cooling will directly affect quality and size of product. Arrangement of cooling channels and selection of cooling medium should be considered according to thermal properties of melt (including crystallization), shape of product and structure of mold.

Plastic products are usually produced in batches or in large quantities, so molds are required to be used with high efficiency and high quality, and less or no processing after molding. Therefore, mold design must consider:
1. Determine parting surface and gate position according to performance and molding performance of plastic part.
2. Considering craftsmanship in mold manufacturing project, determine design plan according to equipment condition and technical strength, to ensure that mold is easy to process from the whole to parts, and it is easy to ensure dimensional accuracy.
3. Consider injection productivity, increase number of injections per unit time, and shorten molding cycle.
4. Dimensions and structures of holes, columns, convexes, and concavities that require precision are displayed in mold, that is, plastic parts are not processed or less processed after they are formed.

 

5. Mold structure strives to be simple and applicable, stable and reliable, with short cycle and low cost, which is convenient for assembly, maintenance and replacement of wearing parts.
6. Selection and processing of mold materials.
7. Standardized production of molds: try to use standard mold bases, commonly used ejector pins, guide parts, sprue sleeves, positioning rings and other standard parts.

1. Gating system: Channel through which molten material flows from nozzle of injection machine into cavity, including main channel, runner, gate, cold well, and pull rod.
2. Molding parts: parts for molding plastic parts, such as cores, cavities and other auxiliary parts.
3. Temperature control system: used to adjust temperature of mold.
4. Plastic parts ejection system: including side parting mechanism, secondary ejection mechanism, first reset mechanism and several sequential fixed-distance parting mechanisms, etc.
5. Installation part: part that reliably installs mold body on injection machine.
6. Connection system: a connection system that integrates each structural component into a whole.
7. Guidance system: to ensure movement accuracy of each structural component, such as guide posts, guide chutes, etc.