Nine most common failures of injection molds

Time:2020-09-28 10:44:10 / Popularity: / Source:

injection molds 

1 Difficulty in gate discharge

During injection molding process, gate is stuck in gate sleeve and is not easy to come off. When mold is opened, crack damage occurs. In addition, operator must knock out nozzle with tip of a copper rod to loosen it before demoulding, which seriously affects production efficiency.

Main cause of failure

Taper holes of gate has a poor finish, and there are knife marks in circumferential direction of inner hole. Secondly, material is too soft, small end of tapered hole is deformed or damaged after a period of use, and spherical curvature of nozzle is too small, which causes gate material to produce rivets here. Taper holes of gate sleeve is difficult to process. Standard parts should be used as much as possible. If you need to process it yourself, you should also make or buy a special reamer. Taper holes need to be ground to Ra0.4 or more. In addition, a gate pull rod or gate ejection must be provided.

2 Large molds dynamic fixed mold offset

Large molds are subject to different feed rates in different directions, and are affected by weight of mold during mold loading, resulting in movement and fixed mold displacement. In above cases, lateral offset force will be added to guide post during injection, surface of guide post will be fuzzed and damaged when mold is opened. In severe cases, guide post will be bent or cut off, and even mold cannot be opened.


In order to solve above problems, a high-strength positioning key is added on each side of mold parting surface. The most convenient and effective method is to use cylindrical keys. Verticality of guide post hole and parting surface is very important. During processing, moving and fixed molds are aligned and clamped, boring is completed on boring machine at one time. This can ensure concentricity of moving and fixed mold holes, and minimize verticality errors. In addition, heat treatment hardness of guide post and guide sleeve must meet design requirements.

3 Guide post damage

Guide post mainly plays a guiding role in mold to ensure that molding surfaces of core and cavity do not touch each other under any circumstances, and guide post cannot be used as a force member or a positioning member.

Main cause of failure

In several cases, moving and fixing mold during injection will generate a huge lateral offset force. When wall thickness of plastic parts is required to be uneven, velocity of material passing through thick wall is large, and a large pressure is generated here; sides of plastic parts are asymmetric, such as counter pressure on opposite sides of mold with a stepped parting surface are not equal.

4 Bending of moving mold plate

When mold is injected, molten plastic in mold cavity generates a huge back pressure, which is generally 600 ~ 1000 kg / cm. Mold maker sometimes does not pay attention to this problem, often changes original design size, or replaces movable template with a low-strength steel plate. In a mold that uses ejector pins, template has a large bending span during injection, which causes template to bend down during injection.


Therefore, moving mold plate must be made of high-quality steel, and it must have sufficient thickness. Low-strength steel plates such as A3 must not be used. When necessary, support columns or blocks should be set under moving mold plate to reduce thickness of mold plate and improve carrying capacity.

5 Jack is bent, broken or leaking

Quality of self-made ejector is better, that is, processing cost is too high. Nowadays, standard parts are generally used, and quality is worse. If gap between ejector pin and hole is too large, material leakage will occur, but if clearance is too small, ejector pin will swell due to rise of mold temperature during injection.
What is more dangerous is that sometimes ejector pin is ejected at a normal distance and breaks. As a result, exposed ejector pin cannot be reset when next mold is closed, and concave mold is damaged.


In order to solve this problem, ejector rod was reground, a fitting section of 10-15 mm was retained at the front end of ejector rod, and middle part was milled 0.2 mm smaller. After assembly of all ejectors, fit clearance must be strictly checked, generally within 0.05 ~ 0.08 mm, to ensure that entire ejection mechanism can move forward and backward freely.

6 Poor cooling or water leaks

Cooling effect of mold directly affects quality and production efficiency of product. Defects such as poor cooling, large shrinkage of product, or uneven shrinkage may cause warpage deformation. On the other hand, mold is overheated in whole or in part, making mold unable to be molded normally and stopping production. In severe cases, movable parts such as ejector pin can be stuck and damaged due to thermal expansion.

Main cause of failure

Design and processing of cooling system are determined by shape of product. Do not omit this system because of complex mold structure or difficult processing, especially large and medium molds must fully consider cooling problem.

7 Slider is tilted, reset is not smooth

Some molds are limited by area of mold plate and length of guide groove is too small. Slider is exposed to outside of guide groove after core pulling operation is completed. In this way, slider is likely to be inclined during core pulling stage and initial stage of mold resetting, especially when mold is closed, slider is not reset smoothly, causing damage to slider and even bending damage.
According to experience, after slider completes core pulling action, length remaining in slide groove should not be less than 2/3 of total length of guide groove.
Large molds 

8 Failure of fixed tensioning mechanism

Fixed distance tensioning mechanisms such as pendulum hooks and buckles are generally used in fixed mold core pulling or some secondary demoulding molds. Because these mechanisms are set in pairs on both sides of mold, their actions must be synchronized, that is, mold is closed at the same time, mold is opened to a certain position and decoupled at the same time. Once synchronization is lost, mold plate of drawn mold will inevitably be skewed and damaged. Parts of these mechanisms must have high rigidity and abrasion resistance. It is also difficult to adjust and life of mechanism is short. Avoid using it as much as possible and use other mechanisms.

Main cause of failure

More common faults of oblique pin slider core pulling mechanism are that processing is not in place and material is too small. Main problems are as follows:
Oblique pin inclination angle A is large, advantage is that a larger core pulling distance can be generated in a short mold opening stroke. However, if inclination angle A is too large, when pulling force F is a certain value, bending force P = F / COSA that oblique pin receives during core drawing process is larger, oblique pin deformation and oblique hole wear are likely to occur. At the same time, oblique pin produces an upward thrust N = FTGA on slider. This force increases positive pressure of slider on guide surface in guide groove, thereby increasing frictional resistance when slider slides. Easy to cause sliding irregularities, guide groove wear. According to experience, inclination angle A should not be greater than 25.


In the case of relatively small core-pulling force, a spring can be used to push fixed mold. In the case of relatively large core-pulling force, core can slide when movable mold is retracted, core pulling action is completed after core pulling action is completed. On large molds, hydraulic cylinder core pulling can be used.

9 Poor ventilation in injection molds

Gas is often generated in injection molds. What causes it?

Main cause of failure

Air stored in casting system and mold cavity, some raw materials contain moisture that has not been dried out, they will vaporize into water vapor at high temperatures. Due to high temperature during injection, some unstable plastics will decompose and produce gas; some additives in plastic raw materials are volatilized or gas generated by chemical reaction with each other.
At the same time, cause of poor venting needs to be found out as soon as possible. Poor exhaust of injection mold will bring a series of hazards to quality of plastic part and other aspects. Main performance is that melt will replace gas in cavity during injection process. If gas is not discharged in time, it will cause difficulty in filling melt, resulting in insufficient injection volume and unable to fill cavity. Exhausted gas will form high pressure in cavity and penetrate into plastic with a certain degree of compression, causing quality such as voids, pores, sparse tissue, and silver streaks.
Because gas is highly compressed, temperature in cavity rises sharply, which causes surrounding melt to decompose and burn, local carbonization and scorching of plastic parts. It mainly appears at the confluence of two melts, at gate flange; poor removal of gas makes speed of melt entering each cavity different, so it is easy to form flow marks and weld marks, reduce mechanical properties of plastic parts. Due to obstruction of gas in cavity, filling speed will be reduced, molding cycle will be affected, and tax collection efficiency will be reduced.
Distribution of air bubbles in plastic parts. Air bubbles generated by accumulation of air in mold cavity are often distributed on part opposite to gate; bubbles generated by decomposition or chemical reaction in plastic raw material are distributed along thickness of plastic part; Bubbles generated by water vaporization are irregularly distributed throughout plastic part.

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