8 Injection Molding Defects & Troubleshooting Tips – How To Fix Common Injection Molding Issues

Injection mold is a tool for producing plastic products, and also a tool for giving plastic products complete structure and accurate size. The structure and processing quality of injection mold directly affect the quality of injection molding products and production efficiency. The following is a brief analysis of some common injection molding defects and their countermeasures.

Common Defects And Troubleshooting Tips For Plastic Injection Molding

1.Difficulty In Gate Stripping

In the process of injection molding, the gate is stuck in the sprue sleeve, which is not easy to come out. When the mold is opened, the products are damaged by cracks. In addition, the operator must use the tip of the copper bar to knock out the nozzle to loosen it before demoulding, which seriously affects the production efficiency.

Causes and troubleshooting technique of gate stripping in injection molding:
The main reason for this kind of failure is the poor finish of the taper hole of the gate and the knife mark in the peripheral direction of the inner hole. Secondly, the material is too soft, the small end of the taper hole is deformed or damaged after a period of use, and the spherical arc of the nozzle is too small, resulting in the rivet head of the sprue here. The taper hole of the sprue sleeve is difficult to process, so standard parts shall be used as far as possible. If it is necessary to process by itself, special reamer shall be made or purchased by ourselves. The taper hole shall be ground to below Ra0.4. In addition, gate pulling rod or gate ejection mechanism must be set.

2.Guide Pillar Damaged

The guide pillar mainly plays a guiding role in the mold, so as to ensure that the forming surfaces of the core and the cavity do not collide with each other under any circumstances, and the guide pillar cannot be used as a stressed or positioned part. In the following cases, the dynamic and fixed mold will produce huge lateral displacement force during injection:

– When the wall thickness of plastic parts is not uniform, the rate of material flow through the thick wall is high, and the pressure is high;
– The side of the plastic part is asymmetric, such as the mould with stepped parting surface, and the opposite side faces are not subjected to the same counter pressure.

3.Moving Mold Migration

Due to the different filling rate in all directions and the influence of the dead weight of the die, the moving and fixed die offsets of the large-scale die. In these cases, the lateral offset force will be applied to the guide pillar during injection, and the surface of the guide pillar will be roughened and damaged when the mold is opened. In serious cases, the guide pillar will be bent or cut off, and even the mold cannot be opened.

How to troubleshoot this injection molding defect?
In order to solve the above problems, a high-strength positioning key is added on the parting surface of the die, and a cylinder key is more convenient and effective. The perpendicularity of the lead post hole and the parting surface is very important. In the process of machining, the moving and fixed mould are used to align and clamp the position, and then the boring is completed on the boring machine at one time, so as to ensure the concentricity of the moving and fixed mould holes and make the perpendicularity error smaller. In addition, the heat treatment hardness of lead post and guide sleeve must meet the design requirements.

4.Moldboard Is Bent

When the mold is injected, the melting plastic in the mold cavity produces a huge back pressure, generally 600 ~ 1000 kg/cm2. Sometimes the mould maker doesn’t pay attention to this problem, and often changes the original design size, or replaces the moving moldboard with low-strength steel plate. In the mould with ejector pin, the moldboard bends down due to the large span of the two side seats. Therefore, the moving moldboard must be made of high-quality steel with sufficient thickness. A3 and other low-strength steel plates shall not be used. If necessary, support columns or blocks shall be set under the moving moldboard to reduce the thickness of the moldboard and improve the bearing capacity.

5.Bent, Broken Or Missing Ejector Pin

The quality of self-made ejector pin is better, that is, the processing cost is too high. Now, standard parts are usually used, with average quality. If the gap between the ejector pin and the hole is too large, there will be material leakage, but if the gap is too small, the ejector pin will be stuck due to the increase of mold temperature during injection. What’s more dangerous is that sometimes the ejector pin will not move and break when it is pushed out for a general distance. As a result, the exposed ejector pin cannot be reset and the die will be damaged in the next die closing.

How to fix this issue?
In order to solve this problem, the top bar shall be grinded again, and a 10-15 mm fitting section shall be reserved at the front end of the top bar, and the grinding of the middle part shall be 0.2 mm smaller. After assembly, all ejector pins must be strictly checked for the starting and matching clearance, generally within 0.05-0.08mm, to ensure that the whole ejector mechanism can move forward and backward freely.

6.Poor Cooling Or Water Leakage

The cooling effect of injection mold product directly affects the quality and production efficiency of products, such as poor cooling, large shrinkage of products, or uneven shrinkage and warping. On the other hand, the whole or part of the injection mould is overheated, so that the mold can not be formed normally and the production is stopped. In serious cases, the moving parts such as the ejector pin are stuck and damaged due to thermal expansion. The design and processing of the cooling system depends on the shape of the product. Do not omit this system because of the complexity of the mold structure or the difficulty of processing. Especially for large and medium-sized molds, the cooling problem must be fully considered.

7.Burn Marks

Burn marks are discolorations, usually rust colored, that appear on the surface of the injection molded prototypes.

What causes this injection molding issue?
Burn marks are caused either by the degradation of the plastic material due to excessive heating or by injection speeds that are too fast. Burn marks can also be caused by the overheating of trapped air, which etches the surface of the molded part.

Remedies:

Reduce injection speeds.
Optimize gas venting and degassing.
Reduce mold and melt temperatures.

8.Short Shot

As the term implies, short shots can be described as a situation where a molding shot falls short. This means that the molten plastic for some reason does not fully occupy the mold cavity or cavities, resulting in a portion where there is no plastic. The finished product becomes deficient because it is incomplete.

Causes and troubleshooting tips of short shots in molded parts:
Short shots can be caused by a number of things. Incorrect calibration of the shot or plasticizing capacities can result in the plastic material being inadequate to fill the cavities. If the plastic is too viscous, it may solidify before fully occupying all the cavities and result in a short shot. Inadequate degassing or gas venting techniques can also result in short shots because air is trapped and has no way to escape; plastic material cannot occupy the space that air or gas is already occupying.

Remedies:

Select a less viscous plastic with higher flowability. This plastic will fill the hardest-to-reach cavities.
Increase mold or melt temperature so as to increase flowability.
Account for gas generation by designing the mold so that gas is not trapped within the mold and is properly vented.
Increase the material feed in the molding machine or switch to a machine that has a higher material feed in the event that the maximum material feed has been reached.