Causes of dents in plastic molds

During the plastic injection molding process, there will be some undesirable situations, such as whitening, shrinkage, dents... Then how to solve this situation? It is necessary to analyze the causes of the defects and solve them in a targeted manner. Bad phenomenon.

1. Dent

The reasons for the dents during the injection molding process are:

1. Insufficient cooling of the mold and insufficient cooling time will cause serious deformation.

2. This situation can also occur if the internal pressure of the mold is insufficient.

3. The thickness of each part of the product is different. In this case, the overall treatment method: reduce the temperature of the barrel and the mold, force cooling at the place where the dent is generated, fill the flow edge at the place of the dent, and control the difference in the thickness of the designed product.

4. A place that is too thick will dent due to large shrinkage. Treatment method: by increasing the pressure and increasing the holding time, the depression can be alleviated or even eliminated. The key depends on the degree of the depression and whether the manufacturer is willing (too long pressure will reduce productivity).

To sum up, the main solutions for the dents produced during the plastic injection molding process are as follows:

1. Improve injection energy: pressure, speed, time and material volume, and increase back pressure to make filling full.

2. Increase material temperature and flow smoothly. Lower the material temperature to reduce shrinkage, and appropriately increase the mold temperature, especially the local mold temperature at the part where the vacuum bubble is formed.

3. Set the gate in the thick part of the part to improve the flow conditions of the nozzle, runner and gate, and reduce the consumption of pressure.

4. Improve mold exhaust.

2. Related knowledge

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1. In the process of producing products, dents are the most frequent occurrence of undesirable phenomena. The plastic injected into the mold shrinks in volume when it is cooled. The early cooling part means that the surface hardens first, and bubbles will be generated inside. The so-called dent is cooling. The slow part creates a conspicuous concave surface in the direction of bubble shrinkage.

2. Materials with large shrinkage are also prone to dents. When changing the molding conditions to eliminate dents, the setting conditions should be set in the direction of small shrinkage. That is, the mold temperature and barrel temperature decrease, and the injection pressure increases, but it should be noted that this may cause residual internal stress.

3. Because the dents are better to be inconspicuous, if they do not affect the appearance, they are deliberately processed into a corroded pattern on the mold, such as grains, grains, etc. Also, if the molding material is HIPS, it is also effective to lower the mold temperature to reduce the finish. However, once dents occur in these methods, it is difficult to repair polished products.

Three, the solution

1. Instant: Increase the injection pressure, prolong the injection holding time, lower the barrel temperature and mold temperature, and force cooling where the dents are generated.

2. Short-term: fill up the flow edge where the dent is generated. When the material in the dent area has a narrow place, make this part thick.

3. Long-term: The thickness difference of the designed product should be completely avoided. For ribs that are prone to dents, the long and narrow shape should be as short as possible.

Fourth, the difference in materials

Materials with large molding shrinkage have large dents. For example, PE, PP, even if only a slight reinforcement ribs, will produce dents.

The material molding shrinkage rate is as follows:

PS0.002~0.006

PP0.01~0.02

PE0.02~0.05

5. References:

1. When the temperature is reduced to no dents, if the material in the cavity is still under pressure, it should be considered that dents will not be generated. The pressure in the mold of the material surrounding the mold is the static pressure, which is not always constant everywhere. The pressure at the part close to the gate is high. If the material has a wide through edge, because of the transmission of pressure to all corners, the pressure difference between the part near the gate and the place far away from the gate is very small compared to the overall pressure, and no depression will occur. It is also possible to obtain products without residual internal stress. When part of the material flows into a difficult place, this place has high pressure, and the pressure in other places decreases and dents are generated. This part of the residual high pressure is the internal stress of the product. In an ideal state, the material temperature rises with the mold temperature, the material fluidity is better, and the injection becomes bottom in the static pressure state.

2. When changing the molding conditions, the combination of temperature, pressure and time should be prepared in advance, and the results can be known early if they are performed in order. First of all, after the time has become long, it is easy to know every small change in pressure. It should be noted that the result when the temperature changes is obtained, and the result should be the result when the temperature drops after the injection of the material.

Plastic molds will inevitably have some bad conditions during the injection process. It is necessary to make specific response methods according to the different situations of dents, and make corresponding adjustments to the parameters of the injection molding machine, the use of molds or raw materials to ensure the product Can be produced normally and smoothly.