Common faults of injection mold

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Common Faults Of Injection Mold
Injection mold is a tool for producing plastic products; it is also a tool for giving plastic products a
complete structure and precise dimensions. The structure of the injection mold and the quality of
mold processing directly affects the quality and production efficiency of plastic parts. The most
common and most common causes of injection mold failures and troubleshooting methods in
the production of injection molds and plastic products are described in detail as follows:
1. Difficulties in gate stripping
During the injection molding process, the gate stacks to the sprue sleeve and is not easy to come
out. When the mold is opened, the product is cracked and damaged. In addition, the operator
must knock out the tip of the copper rod from the nozzle to loosen it before demoulding, which
seriously affects production efficiency.
The main reason for this failure is the poor finish of the gate taper hole, and there are knife marks
in the circumferential direction of the inner hole. The second is that the material is too soft, the
small end of the taper hole is deformed or damaged after a period of use, and the curvature of
the nozzle spherical surface is too small, resulting in the riveting head of the gate material.
The taper hole of the sprue bushing is more difficult to process. Standard parts should be used as
much as possible. If you need to process it yourself, you should also make your own or buy a
special reamer. The taper hole needs to be ground to Ra0.4 or more. In addition, a gate pull rod
or gate ejection mechanism must be provided.
2. Guide pin damage

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The guide pin mainly plays a guiding role in the mold to ensure that the molding surfaces of the
core and the cavity do not collide with each other under any circumstances, and the guide post
cannot be used as a force-bearing part or a positioning part.
In the following situations, when the injection is moving, the fixed mold will generate a huge
lateral offset force:
(1) When the wall thickness of the plastic part is not uniform, the material flow through the thick
wall has a large speed and the large pressure;
(2). The side of the plastic part is asymmetrical, such as the back pressure on the opposite sides
of the mold with a stepped parting surface.
3. Large mold
Due to the different filling rates in each direction and the influence of the die weight during mold
loading, the moving and fixed mold deviations occur. In the above cases, the lateral offset force
will be added to the guide pin during injection, and the surface of the guide pin will be roughed
and damaged when the mold is opened. In severe cases, the guide pin may be bent or cut off,
and the mold cannot even be opened.
In order to solve the above problems, a high-strength positioning key is added on each of the four
sides of the mold parting surface. The most simple and effective way is to use a cylindrical key.
The verticality of the guide pin hole and the parting surface is very important.
In the processing, the moving is used. After the fixed mold is aligned and clamped, the boring
machine is finished at one time on the boring machine, which can ensure the concentricity of the
moving and fixed mold holes, and minimize the verticality error. In addition, the heat treatment
hardness of the guide post and guide sleeve must meet the design requirements.
4. Movable template bending
When the mold is injected, the molten plastic in the cavity produces huge backpressure,
generally 600~1000 kg/cm. Mold makers sometimes do not pay attention to this problem, often
change the original design size, or replace the movable template with a low-strength steel plate.
In the mold with ejector rods, the mold plate is bent during injection due to the large span
between the two sides.
Therefore, the movable formwork must be made of high-quality steel, with sufficient thickness,
and low-strength steel plates such as A3 should not be used. When necessary, support columns
or support blocks should be set under the movable formwork to reduce the thickness of the
formwork and improve the bearing capacity.
5. The ejector rod is bent, broken, or leaking
The self-made ejector rods are of better quality, but the processing cost is too high. Nowadays,
standard parts are generally used and the quality is poor. If the gap between the ejector pin and
the hole is too large, material leakage will occur, but if the gap is too small, the ejector pin will
become stuck due to the increase in mold temperature during the injection.
What's more dangerous is that sometimes the ejector rod will not move and break when it is

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ejected for a normal distance. As a result, the exposed ejector rod cannot be reset during the
next mold clamping and will damage the die.
In order to solve this problem, the ejector rod was re-ground, and the mating section of 10~15
mm was retained at the front end of the ejector rod, and the middle part was ground 0.2 mm
smaller. After all ejector rods are assembled, the fit clearance must be strictly checked, generally
within 0.05~0.08 mm, to ensure that the entire ejector mechanism can move forward and
backward freely.
6. Poor cooling or water leakage
The cooling effect of the mold directly affects the quality and production efficiency of the product,
such as poor cooling, large product shrinkage, or uneven shrinkage, resulting in warping
deformation and other defects. On the other hand, the overall or partial overheating of the mold
prevents the mold from forming normally and stops production. In severe cases, the movable
parts such as the ejector pin are thermally expanded and become damaged.
The design and processing of the cooling system depend on the shape of the product. Don't omit
this system because of the complex structure of the mold or the difficulty of processing.
Especially for large and medium-sized molds, the cooling problem must be fully considered.
7. The fixed distance tensioning mechanism fails
Fixed-pitch tensioning mechanisms such as swing hooks and buckles are generally used in fixed
mold core pulling or some secondary demolding molds. Because such mechanisms are set in
pairs on both sides of the mold, their actions must be synchronized, that is, The mold is buckled
at the same time when the mold is closed, and the mold is released to a certain position at the
same time.
Once the synchronization is lost, it will inevitably cause the template of the drawn die to be
skewed and damaged. The parts of these mechanisms must have higher stiffness and wear
resistance, and adjustment is difficult. The life of the mechanism is short. Avoid using it as much
as possible. You can switch to other mechanisms.
In the case of relatively small core-pulling force, the spring can be used to push out the fixed
mold. In the case of relatively large core-pulling force, the core can be used to slide when the
movable mold is retracted. The core-pulling action is completed first and then the mold is divided.
A hydraulic cylinder can be used to pull the core on the mold. The oblique pin slider core pulling
mechanism is damaged. The more common problems of this kind of organization are mostly
inadequate processing and too small materials. The main two problems are as follows.
The oblique pin inclination angle A is large, and the advantage is that it can produce a larger core
pulling distance in a shorter mold opening stroke. However, if the inclination angle A is too large
when the extraction force F is a certain value, the bending force P=F/COSA of the inclined pin
during the core pulling process is larger, and the inclined pin deformation and the inclined hole
wear are prone to occur.
At the same time, the upward thrust N=FTGA generated by the inclined pin on the slider is also
greater, which increases the positive pressure of the slider on the guide surface in the guide

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groove, thereby increasing the frictional resistance of the slider when sliding. It is easy to cause
uneven sliding and wear of the guide groove. According to experience, the inclination angle A
should not be greater than 25.
8. Some molds are limited by the template area
The length of the guide groove is too small, and the slider will be exposed outside the guide
groove after the core pulling action is completed. This will easily cause the slider to tilt in the
post-core pulling stage and the initial stage of the mold closing reset, especially when the mold is
closed, the slider reset is not smooth.
Damage the slider or even damage it under bending. According to experience, after the slider
completes the core pulling action, the length left in the chute should not be less than 2/3 of the
full length of the guide groove.
9. Finally in the design
When manufacturing a mold, it should be based on the requirements of the quality of the plastic
parts, the size of the batch, the requirements of the manufacturing period, etc., which can not
only meet the requirements of the product, but also the most simple and reliable mold structure,
easy to process, and low cost. The mold is excellent.

Common faults of injection mold

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Common faults of injection mold