Nine Mold Failures and Solutions

Solutions for the 9 mold failures

1. Difficulty in gate stripping

During the injection molding process, the gate sticks 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 with the tip of the copper rod from the nozzle to loosen it before demoulding. And that seriously affects the production efficiency.

The main reason for this failure is the poor smoothness of the gate taper hole with knife marks in the circumferential direction of the inner hole.

Second, the material is too soft, the small end of the taper hole is deformed or damaged after a period of use.

Besides, the curvature of the nozzle spherical surface is too small, causing the gate material to generate riveting heads.

The taper hole of the sprue bushing is difficult to process, so we should use standard parts as much as possible. If you need to process it yourself, you should use a special reamer. The taper hole needs to be ground to a Ra0.4 or more. In addition, a gate pull rod or gate ejection must be set.

2.  Large-scale mold moving and fixed mold offset

Large-scale molds have different moving and fixed mold deviations due to the different filling rates in each direction and the influence of the mold’s own weight.

In the above cases, the lateral offset force will be added to the guide post during injection. And the surface of the guide post will be roughened and damaged when the mold is opened. In the severe cases, the guide post may be bent or cut off, and the mold cannot even be opened.

To solve the above problems, a high-strength positioning key is added on the four sides of the mold parting surface. The most simple and effective way is to use a cylindrical pole. The verticality of the guide pin hole and the parting surface is very important.

In the processing, the moving and fixed molds are aligned and clamped, and the boring machine is completed the whole for the cylindrical pole at one time. This 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 brush must meet the design requirements.

3. Guide post damage

The guide post 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. The guide post cannot be used as a force-bearing part or a positioning part.

In several cases, the moving and fixed molds will produce huge lateral deflection force during injection. When the wall thickness of the plastic part is not uniform, the material flow through the thick wall has a large speed, generating a large pressure. Besides, the side of the plastic part is asymmetric, such as the back pressure on the two opposite sides of the mold with a stepped parting surface not equal.

4. Movable template bending

When the mold is injected, the molten plastic in the cavity produces a huge back pressure, generally 600~1000 kg/cm. Mold makers sometimes do not pay attention to this problem. They often change the original design size. Or they may replace the movable template with low-strength steel plate. In the mold with ejector rod ejection, the template is bent down during injection due to the large span between the two sides of the seat.

Therefore, we should use the movable formwork made of high-quality steel, with sufficient thickness. While avoiding to use the low-strength steel plates such as A3. When necessary, support columns or support blocks should be set under the movable formwork. It can reduce the thickness of the formwork and increase the load-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 but with a standard quality. If the gap between the ejector pin and the hole is too large, leakage will occur. However, if it’s too small, the ejector pin will become stuck for the increase of mold temperature during injection.

What’s more dangerous is that sometimes the ejector rod will not move and break when ejected for a certain distance. As a result, the exposed ejector rod cannot be reset during the next mold clamping and will damage the die.

To solve that problem, the ejector rod need to be re-ground. Besides, the mating section of 10~15 mm need to be retained at the front end of the ejector rod. While the middle part need to be ground down by 0.2 mm. After all ejector rods are assembled, the fit clearance must be strictly checked, generally within 0.05~0.08 mm. It’s to ensure that the entire ejector mechanism can advance and retreat freely.

6. Poor cooling or water leakage

The cooling effect of the mold directly affects the quality and production efficiency of the product. Problems like poor cooling, product shrinkage, or uneven shrinkage can cause defects such as warping deformation.

On the other hand, the overheated whole or part of the mold will make the mold cannot be formed successfully and stop the production. In severe cases, the movable parts like the ejector pin will be thermally expanded and become damaged.

The design and processing of the cooling system is determined by the shape of the product. Don’t omit this system because of the complex structure of the mold or the processing difficulty. Especially for large and medium-sized molds, the cooling problem must be fully considered.

7. The slider is tilted and the reset is not smooth

Some molds are limited by the template area, making the guide groove length too small. The slider will be exposed outside the guide groove after completing the core pulling action. 

This will easily cause the tilting of slider during the core pulling stage and the initial stage of mold resetting, especially during mold clamping.

When the slide block is not reset smoothly, the slide block will be damaged or even broken under bending.

Knowing from 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.

8. 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 these mechanisms are set in pairs on both sides of the mold. Their actions must be synchronized. That is, close the mold and make it buckled at the same time. Then, open the mold to a certain position and make it unhooked at the same time.

Once out of synchronization, it will inevitably cause the template of the drawn die to be skewed and damaged. The parts of these mechanisms must have higher rigidity and wear resistance, and its adjustment is difficult. And the life of the mechanism is short. Therefore, avoid using it as much as possible. We can use other mechanisms.

When the core pulling force is relatively small, the spring can be used to push out the fixed mold. While the force is relatively large, the core can be used to slide when the movable mold is retracted. Completing the core pulling action before the mold is divided. A hydraulic cylinder can be used to pull the core on the large mold.

In the case of the oblique pin slider core pulling mechanism is damaged, the more common problems of this kind of mechanism are mostly inadequate processing and too small materials. There are mainly the following two problems:

The large oblique pin inclination angle A 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 will be larger. Then, 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 oblique pin on the slider is also greater. This force increases the positive pressure of the slider on the guide surface in the guide 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.

9. Poor air-exhaust in the mold

Air is often generated in injection molds. What are the causes?

  • The air in the ejection system and the mold cavity
  • Some raw materials are dried inadequately, then the moisturewill vaporize into water vapor at high temperatures
  • The overly high injection molding temperature causescertain unstable plastics to decompose and produce gas.
  • Gas generated by volatilization or chemical reaction of certain additives in plastic raw material.

At the same time, the cause of poor air-exhaust also needs to be found out as soon as possible. Poor venting of the injection mold will bring a series of hazards to the quality of the plastic parts and many other aspects.

The main performances:

  • The melt plastic will replace the air in the cavity during the injection molding process. If the airis not discharged in time, it will be difficult to fill the melt into the cavity, resulting in insufficient injection volume and unable to fill the cavity. 
  • Poorly drained gas will form high pressure in the cavity and penetrate into the plastic under a certain degree of compression. And then causes quality defects such as voids, stomata, sparse organization and silver streaks.

Because the gas is highly compressed, the temperature in the cavity rises sharply. And that will further make the surrounding melt decompose and burn, causing local carbonization and scorching of the plastic parts. It mainly appears at the confluence of two melts and the flange of the gate. The poor removal of air makes the speed of the melt plastic entering each cavity different. Therefore, it is easy to form flow marks and fusion marks, and reduce the mechanics of the plastic part performance. Due to the obstruction of the gas in the cavity, the filling speed will be reduced. What’ s more, molding cycle will be affected, and the production efficiency will be reduced.

The distribution of air bubbles in the plastic parts

  • The air bubbles generated by the accumulation of air in the mold cavity, are often distributed on the position opposite to the gate
  • The air bubbles generated by the decomposition or chemical reaction in the plastic raw materials are distributed along the thickness of the plastic parts
  • The air bubbles produced by water vaporization remaining in the plastic raw materials are irregularly distributed throughout the plastic parts.

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