9 Common Injection Molding Defects

This article mainly describes 9 common injection molding defects and the causes and solutions for these injection molding defects.

Difficulty in gate stripping in injection molding defects

During the injection molding process, the gate stacks to the sprue bushing, making it difficult to pull out. Opening the injection mold will crack and damage the product. In addition, the tip of the copper rod must be knocked out from the nozzle to loosen it, which greatly affects production efficiency.

Main causes of injection mold defect

The taper hole of the gate has a poor finish, and there is a knife mark in the circumferential direction of the inner hole. The second is that the gate material creates a riveted head here because the material is too soft, the smaller end of the tapered hole is deformed or damaged after use, and the spherical curvature of the nozzle is too small.

Since it is difficult to machine the tapered hole of the sprue bush, you need to use standard parts as much as possible, and if you need to machine it yourself, you need to make it yourself or purchase a special reamer. The taper hole needs to be polished to Ra0.4 or higher. In addition, you need to have a gate pull rod or gate eject.

Large mold movement and fixed mold offset in injection molding defects

Large-scale molds have different filling factors in each direction, and the dynamic and fixed mold deviations differ due to the influence of the mold’s own weight when the mold is attached. In the above case, a lateral offset force is applied to the guidepost during injection, and when the mold is opened, the surface of the guidepost becomes rough and damaged. In severe cases, the guidepost may bend or break. , And you can’t even open the mold.


To solve the above problems, high-strength positioning keys have been added to each of the four sides of the mold parting surface. The easiest and most effective way is to use a cylindrical key. The verticality of the guide pin holes and the parting surface is very important. In processing, moving dies and fixed dies are aligned and clamped to complete the boring machine at once. This ensures the concentricity between the moving mold and the fixed mold. Minimize holes and vertical errors. In addition, the heat treatment hardness of the guide pins and guide sleeves must meet the design requirements.

Damage to the guide pin in injection molding defects

The guide pin acts primarily as a guide in the injection mold, preventing the core and cavity molding surfaces from colliding under any circumstances. The guide pin cannot be used as a force-supporting or positioning component.

Main causes of injection mold defect

In some cases, mobile and fixed types generate a large lateral deflection force during the mold injection. If the wall thickness of the plastic part is not uniform, the material flow will pass through the thick wall at high speed, where a large pressure will be generated. The sides of the plastic part are asymmetric, like two back pressures. On the opposite side of the injection mold where the stepped parting surfaces are not equal.

Movable template bending in injection molding defects

When the injection mold is ejected, the molten plastic in the cavity typically creates a large back pressure of 600-1000 kg/cm. Mold makers may not pay attention to this issue and may change the original mold design size or replace the movable template with a low-strength steel plate. The mold that ejects the ejector rod is large, so the template bends down during injection. Straddle both sides of the seat.


Therefore, the movable formwork must be made of high-quality steel of sufficient thickness and do not use low-strength steel sheets such as A3. If necessary, install stanchions or support blocks under the movable formwork to increase the formwork thickness and load-bearing capacity.

The ejector rod is bent, broken, or leaking in injection molding defects

The quality of the self-made ejector rod is good, but the processing cost is too high, and standard parts are commonly used these days, and the quality is getting worse. If the gap between the ejector pin and the hole is too large, leakage will occur, but if the gap is too small, the ejector pin will be clogged due to the rise in mold temperature during the injection.

Even more dangerous is that the ejector rods may move and not break when ejected a certain distance, resulting in the exposed ejector rods not being able to be reset during the next mold clamping and damaging the die.


To solve this problem, the ejector rod was re-polished, leaving a 10-15 mm mating area at the front end of the ejector rod and 0.2 mm at the center. After assembling all ejector rods, the fit clearance should be rigorously checked (usually within 0.05-0.08 mm) to ensure that the entire ejector mechanism can move back and forth freely.

Poor cooling or water leakage in injection molding defects

The cooling effect of the injection mold directly affects the quality and production efficiency of the product, such as poor cooling, large shrinkage of the product, and uneven shrinkage, and causes defects such as warpage deformation. On the other hand, because all or part of the mold is overheated, the injection mold cannot be molded normally and production stops. In severe cases, moving parts such as ejector pins will thermally expand and be damaged.

Main causes of injection molding defects

The design and processing of the cooling system depend on the shape of the product. Do not omit this system due to the complicated structure of the injection mold or the difficulty of processing. Especially for large and medium injection molds, cooling issues need to be taken into consideration.

Slider is tilted and reset is not smooth in injection molding defects

Some injection molds are limited by the template area and the guide groove length is too short. When the core pulling motion is complete, the slider is exposed to the outside of the guide groove. This makes it easy for the slider to tilt between the core pulling stage and the initial stage. Mold reset stage, especially during mold clamping. At this time, the sliding block will not be reset smoothly, and the sliding block will be damaged or even if it is bent, it will be damaged.


Experience has shown that after the slider has completed the pulling motion of the core, the length remaining on the chute should be at least 2/3 of the total length of the guide groove.

Fixed distance tension mechanism is out of order in injection molding defects

Fixed pitch tension mechanisms such as swing hooks and buckles are typically used in fixed mold core pulls or some secondary mold release molds. These mechanisms are paired on both sides of the mold, so their behavior must be synchronized. At the same time, it buckles.

If you open the injection mold in a specific position and disconnect it at the same time, the loss of synchronization will inevitably distort and damage the template of the pulled mold. The components of these mechanisms need to be more rigid and wear-resistant. It is difficult to adjust. Avoid using it as it has a short life. You can switch to another institution.

Main causes of injection molding defect

Damage to the core pull mechanism of tilted pin sliders, a more common obstacle, is most often improper handling and too small material. The main issues are:

There is an advantage that the pulling distance of the core can be increased by shortening the mold opening stroke because the diagonal inclination angle A of the pin is large. However, if the tilt angle A is too large, if the pull-out force F is a constant value, the bending force P = F / COSA of the tilt pin during the core pulling process will increase, and the deformation of the tilt pin and the tilting hole will increase. Wear is likely to occur.

At the same time, the upward thrust N = FTGA generated by the diagonal pin of the slider also increases. This force increases the positive pressure of the slider on the guide surface of the guide groove and increases the frictional resistance of the slider sliding. The guide groove is prone to slip unevenness and wear. Experience has shown that the tilt angle A should not exceed 25.


If the tensile force of the core is relatively small, a spring can be used to extrude the fixed mold. If the tensile force of the core is relatively high, the core can be slid when the movable mold is retracted. The core pulling operation is complete. Before splitting the injection mold, you can use the hydraulic cylinder to pull the core of the injection mold.

The poor exhaust of injection mold in injection molding defects

Gas is often generated in injection molds. What is the cause?

Main causes of injection molding defect

The air in the pouring system and injection mold cavity, some raw materials contain moisture that was not removed by drying, and the temperature during injection molding is too high, so it evaporates into steam at high temperatures and some instability. Plastic decomposes gas. Certain additives in the plastic raw material volatilize or chemically react to generate gas.

Poor ventilation in injection molds poses a series of hazards in many aspects, including the quality of plastic parts. The main symptoms are:

During the injection molding process, the molten resin replaces the gas in the cavity. If the gas is not exhausted in time, it will be difficult to fill the molten resin and the injection volume to fill the cavity will be inadequate.

Gas that is not removed smoothly creates high pressure in the cavity that penetrates the plastic under some compression, causing quality defects such as cavities, pores, sparse texture, and silver streaks.

Due to the high degree of compression of the gas, the temperature inside the cavity rises sharply, resulting in the decomposition and combustion of the surrounding melt, resulting in local carbonization and charring of the plastic parts.

This mainly appears at the confluence of the two melts and the flange of the gate. Due to insufficient gas removal, the rate of melt entering each cavity is different, which makes it easier to form flow marks and fusion marks, which reduces the performance of plastic parts.

Blockage of gas in the cavity slows down the filling rate, impacts the molding cycle, and reduces tax efficiency.

The distribution of air bubbles in plastic parts, that is, the air bubbles created by the air trapped in the cavity, is often distributed on the opposite side of the gate. The bubbles generated by the decomposition or chemical reaction of the plastic raw material are irregularly distributed throughout the plastic part, and the bubbles generated by the vaporization of the remaining plastic raw material water are irregularly distributed throughout the plastic part.

Besides these injection molding defects, there are other injection molding defects, contact us if you want to know more about the injection molding defects.