Let's talk about principle of injection molds, take a look at structure disassembly

Time:2020-05-23 09:04:18 / Popularity: / Source:

injection molds 
Injection molds are parts that give plastics shape and size 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 consistent. Mold is mainly composed of three parts: gating system, molding parts and structural parts. Among them, gating system and molded parts are parts that are in direct contact with plastic, change with plastic and product. They are the most complicated and most varied parts of mold, require the highest processing finish and precision.
Gating system refers to part of flow path before plastic enters cavity from nozzle, including main runner, cold cavity, sub runner and gate. Molded parts refer to various parts that form shape of product, including moving mold, fixed mold, cavity, core, molding rod and exhaust port.

Main runner

It is a passage in mold connecting nozzle of injection machine to shunt or cavity. Top of main runner is concave so as to connect with nozzle. Inlet diameter of main runner should be slightly larger than nozzle diameter (0.8 mm) to avoid flashing, prevent the two from blocking due to inaccurate connection. Inlet diameter depends on the size of product, which is generally 4-8mm. Diameter of main runner should be enlarged inwards at an angle of 3 ° to 5 ° to facilitate removal of debris from runner.

Cold cavity

It is a cavity located at the end of main runner to capture cold material generated between two injections at the end of nozzle, thereby preventing clogging of runner or gate. If cold material is mixed into cavity, internal stress is likely to occur in manufactured product. Diameter of cold cavity is about 8-10 mm and depth is 6 mm. In order to facilitate demoulding, bottom is often borne by demolding rod. Top of demolding rod should be designed into a zigzag hook shape or set with a sag groove, so that main roadway can be pulled out smoothly when demoulding.

Sub runner

It is a channel connecting main runner and each cavity in multi-slot mold. In order to make molten material fill each cavity at an equal speed, layout of sub runners on mold should be symmetrical and equidistant. Shape and size of sub runner cross-section have an impact on flow of plastic melt, product demolding, and mold manufacturing difficulty. If flow of same amount of material is used, resistance of runner with a circular cross section is the smallest. But because cylindrical runner is smaller than surface, it is not good for cooling debris of sub runner, and such a sub runner must be opened on two mold halves, which is laborious and easy to align. Therefore, it is often used a trapezoidal or semicircular cross-section sub runner, and opened on half mold with a release rod. Surface of runner must be polished to reduce flow resistance and provide a faster filling speed. Size of runner depends on type of plastic, size and thickness of product. For most thermoplastics, cross-sectional width of runner does not exceed 8m, extra large can reach 10-12m, and extra small 2-3m. Under premise of satisfying needs, cross-sectional area should be reduced as much as possible, so as not to increase shunts and increase cooling time.


It is a channel connecting main runner (or sub runner) and cavity. Cross-sectional area of channel can be equal to that of main runner (or sub runner), but it is usually reduced. Therefore, it is the smallest cross-sectional area in entire flow channel system. Shape and size of gate greatly affect quality of product. Function of gate is: A. Control flow rate of material: B. In injection, backflow can be prevented due to premature coagulation of molten material present in this part: C. Molten material passing through is subjected to strong shear to increase temperature , So as to reduce apparent viscosity to improve fluidity: D, easy to separate product and 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, which is not only based on above effect, but also because it is easier to make small gate larger, and it is difficult to reduce large gate. Gate location should generally be selected where product is the thickest without affecting appearance. Design of gate size should take into account nature of plastic melt.
mold manufacturing 


It is space where plastic products are molded in mold. Components used to form cavity are collectively called molded parts. Each molded part often has a special name. Molded parts that make up shape of product are called concave molds (also called female molds), those that make up internal shape of product (such as holes, grooves, etc.) are called cores or punches (also called male molds). When designing molded parts, overall structure of cavity must be determined according to performance of plastic, geometric shape of product, dimensional tolerances, and requirements for use. Secondly, according to determined structure, select parting surface, gate, vent position and demoulding method. Finally, according to size of control product, design and combination of each part are determined. Plastic melt has high pressure when entering cavity, so molded parts should be properly selected and checked for strength and rigidity. In order to ensure that surface of plastic products is clean, beautiful and easy to demold, all surfaces in contact with plastics have a roughness Ra> 0.32um, and must be resistant to corrosion. Molded parts are generally heat-treated to increase hardness and are made of corrosion-resistant steel.

Exhaust vent

It is a kind of groove-shaped gas outlet opened in mold to discharge original gas brought by melt. When molten material is injected into cavity, air originally in cavity and gas brought by melt must be discharged out of mold at the end of flow through exhaust port, otherwise product will have pores, poor welding, underfilling, even accumulated air will burn product due to high temperature caused by compression. In general, vent hole can be located either at the end of melt flow in cavity or on 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. During injection, there will not be a lot of molten material leaking from vent hole, because molten material will cool and solidify at this place and block channel. Opening of exhaust port must not face operator to prevent accidental injection of molten material and injury. In addition, matching gap between ejector rod and ejector hole, matching gap between top block, stripper plate and core can also be used for exhaust.

Structural parts

It refers to various parts that constitute mold structure, including: various parts such as guide, demolding, core pulling and parting. Such as front and rear splint, front and back buckle mold plate, pressure plate, pressure column, guide column, stripping template, demolding rod and return rod, etc.

Heating or cooling device

This is a device for solidifying and shaping melt in mold. For thermoplastics, cooling medium channels in male and female molds are generally used to achieve cooling purposes by circulating cooling medium. Cooling medium introduced varies with type of plastic and product structure, 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. Layout of cooling channels and choice of cooling medium should be considered according to thermal properties of melt (including crystallization), shape of product and mold structure.
mold manufacturing 

Introduction and composition of injection molds

Plastic products are usually produced in batches or large quantities. Therefore, high efficiency and high quality are required when mold is used, little or no processing after molding, so mold design must consider:
1. Determine parting surface and gate position according to use performance and molding performance of plastic parts.
2. Consider manufacturability in mold manufacturing project, determine design plan according to equipment status 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. Show size and hole, column, convex, concave and other structures with accuracy requirements in mold, that is, plastic parts are not processed or processed less.
5. Mold structure strives to be simple and applicable, stable and reliable, short cycle and low cost, which is convenient for assembly maintenance and replacement of wearing parts.
6. Selection and treatment of mold materials.
7. Standardized production of molds: try to use standard mold bases, common jacks, guide parts, gate sleeves, positioning rings and other standard parts.

Basic composition of injection mold

1. Molded parts

Parts that give shape, structure, and size to molding materials are usually made up of cores (plugs), concave mold cavities, threaded cores, and inserts.

2. Gating system

It is a channel that leads molten plastic from nozzle of injection machine to closed mold cavity, usually composed of main runner, sub runner, gate and cold material well.

3. Guide parts

In order to ensure that movable mold and fixed mold can be accurately aligned when they are closed, guide parts are provided for guiding and positioning. It is composed of guide posts and guide sleeves, some molds are also provided with guide parts on ejection plate to ensure smooth and reliable movement of demolding mechanism.

4. Demoulding mechanism

Device for realizing demoulding of plastic parts and gating system has many structural forms. The most commonly used demolding mechanisms are ejector rod, ejector tube, ejector plate and pneumatic ejector, which are generally composed of ejector rod, reset lever, slingshot, ejector rod fixing plate , and a top plate guide post / guide sleeve.
injection molds 

5. Core pulling mechanism

For plastic parts with side holes or undercuts, before they are ejected and demolded, they must be pulled sideways or separated from slider (side parting) to be demolded smoothly.

6. Mold temperature adjustment system

In order to meet mold temperature requirements of injection molding process, a mold temperature adjustment system (such as cooling water, hot water, hot oil, and electric heating system, etc.) is needed to adjust mold temperature.

7. Exhaust system

In order to expel gas in mold cavity smoothly, an exhaust groove is often set at mold parting surface, and gap between push rods or other moving parts (such as sliders) of many molds can also play role of exhaust.

8. Other structural parts

Parts that meet requirements of mold structure (such as: fixed plate, moving / fixing template, brace, support plate and connecting screws, etc.).

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