9 Problems That Often Occur in Injection Molds


The following are the causes of 9 problems that often occur in injection molds and the solutions.

  1. Difficulties 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 finished product shows crack damage. In addition, it is necessary for the operator to knock out the top of the copper rod from the nozzle to loosen it before demoulding, which will severely affect the output power.

The main factor of this shortcoming is the poor brightness of the gate taper hole and the knife mark 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 time, and the curvature of the nozzle spherical surface is too small, causing the sprue to be riveted here.

The taper hole of the sprue bushing is more difficult to process, so standard mold components should be used as much as possible. If you need to process it yourself, you should also suppress yourself or buy a special reamer. The taper hole needs to be ground to Ra0.4 or more. In addition, it is necessary to set the gate pull rod or gate ejection.

  1. Large-scale mold moving and fixed mold offset

Due to the different filling rates of large molds, and the influence of the weight of the mold during mold installation, the moving and fixed mold deviations occur. In the above-mentioned situations, the lateral deflection force will be added to the guide pin during injection, the guide pin is roughened and damaged when the mold is opened, and the guide pin is bent or blocked in severe cases, and the mold cannot be opened.

In order to deal with the above questions, a high-strength positioning key is added on each of the four sides of the mold parting surface. The most concise and useful one is to use a cylindrical key. The straightness of the guide pin hole and the parting surface is the most important thing.

In the processing, the moving and fixed molds are aligned and clamped, and then boring on the boring machine at one time. This can ensure the concentricity of the moving and fixed mold holes. Minimize straightness errors. In addition, the heat treatment hardness of the guide pin and the guide sleeve must meet the requirements of the plan.

  1. Guide pin damage

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

In several cases, the moving and fixed molds will have infinite lateral deflection force during the injection. When the wall thickness of the plastic part is not uniform, the material flow passes through the thick wall at a high rate, and greater pressure occurs here; the side surface of the plastic part is asymmetrical, such as the stepped parting surface of the mold on the two opposite sides. The backpressure is not equal.

  1. Dynamic template twists and turns

When the mold is being injected, the molten plastic in the mold cavity generates infinite backpressure, usually 600~1000 kg/cm. Mold makers sometimes don’t pay attention to this question, and usually change the original plan standard, perhaps replace the movable template with a low-strength steel plate. In the mold with ejector rod ejection, due to the large span between the two side seats, the template will bend when the injection is made.

Therefore, it is necessary to use excellent steel for the movable formwork, and the thickness must be met, and low-strength steel plates such as A3 cannot be used. When necessary, support columns or support blocks should be set under the movable formwork to reduce the thickness of the formwork and adjust the forward load.

  1. The ejector rod is twisted, cracked, or leaked

The quality of the self-restraining ejector rod is better, but the processing cost is too high. Nowadays, standard parts are usually used, and the quality is worse. If the gap between the ejector pin and the hole is too large, the material will leak, 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 riskier is that sometimes the ejector pin is pushed out and usually does not move at intervals and breaks. The exposed ejector pin cannot be reset when the mold is clamped once, and the concave mold is damaged.

In order to deal with this question, the ejector rod was reground from the beginning, and a cooperative section of 10 to 15 mm was stored at the front end of the ejector rod, and the base was slightly grounded by 0.2 mm. After all the ejector pins are installed, it is necessary to strictly check the cooperation gap, usually within 0.05~0.08 mm, to ensure that all ejector arrangements can move forward and backward freely.

  1. Poor cooling or water leakage

The cooling effect of the mold directly affects the quality and output of the finished product, such as poor cooling, large shortening of the finished product, or uneven shortening, resulting in warping deformation.

On the other hand, the whole mold may be overheated, so that the mold cannot be formed normally and production is stopped. In severe cases, the movable parts such as ejector rods may be damaged due to thermal expansion.

The cooling system plan depends on the shape of the product. Don’t omit this special system because of the messy structure of the mold or the difficult processing. Especially large and medium-sized molds must be adequately considered for cooling.

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

Some molds are bound by the area of ​​the template and the guide groove length is too small, and the slider is exposed outside the guide groove after the core pulling action is completed.

So in the period after core pulling and the initial period of mold closing and resetting, the slider is simply tilted, which is particularly close. During the die, the slide block is not reset smoothly, causing damage to the slide block and even bending damage.

According to experience, after the slider finishes the core pulling action, the length of the slide groove should not be less than 2/3 of the total length of the guide groove.

  1. The fixed distance tightening arrangement fails

Fixed-distance tensioning arrangements such as swing hooks and buckles are usually used in fixed mold core pulling or some secondary demolding molds. Because this type of arrangement is set in pairs on both sides of the mold, its actions must be synchronized. That is, the mold is closed and buckled together, and the mold is opened to a certain position and released together.

Once the synchronization is lost, the template that constitutes the pulling die must be tilted and damaged. The parts of these arrangements must have higher rigidity and wear resistance, and adjustments are also difficult. The arrangement life is short. Try to prevent use. You can use other parts.

In the case of low core-pulling force comparison, a stretch spring can be used to push out the fixed mold. In the case of high core-pulling force comparison, the core sliding when the movable mold is retracted can be selected. The core-pulling action is terminated first and then the mold is split. Hydraulic cylinders can be used to pull cores on large molds. The oblique pin slider core pulling arrangement is damaged.

The disadvantages of this arrangement are mostly inadequate processing and too small materials. The first two questions are the following:

The inclined pin inclination angle A is large, and the advantage is that a larger core pulling distance can occur in a shorter mold opening stroke. However, an excessively large inclination angle A is adopted. When the extraction force F is a certain value, the bending force P=F/COSA encountered by the inclined pin during the core pulling process is larger, and the inclined pin deforms and the inclined hole wears easily.

At the same time, the upward thrust of the inclined pin on the slider N=FTGA is also greater, this force increases the positive pressure of the slider on the guide surface in the guide groove, and then increases the resistance when the slider slides. It is easy to make up that the sliding is not smooth and the guide groove is worn. According to experience, the inclination angle A should not be greater than 25.

  1. Poor exhaust in the injection mold

Gases often occur in injection molds. Why does this happen?

The air in the pouring system and the mold cavity; some materials are rich in moisture that has not been swept away by drying, and they will vaporize into water vapor at high temperatures; due to the high temperature during injection molding, certain unstable plastics will occur Differentiate and generate gas; the gas generated by the transpiration of certain enhancers in the plastic material may chemically react with each other.

At the same time, the cause of poor exhaust is also required to find out quickly. Poor venting of the injection mold will bring a series of damages to the quality of the plastic parts and many other aspects. The first embodiment is that the melt will replace the gas in the cavity during the injection molding process.

If the gas is not discharged in time, it will form a melt. The filling is difficult, resulting in the short amount of injections and the inability to fill the cavity; the poorly swept air will cause high pressure in the cavity, and enter the interior of the plastic under a certain degree of contraction, forming voids, pores, sparse and silver patterns quality defects;

Because the gas is highly compressed, the temperature in the cavity rises sharply, which in turn causes the surrounding melt to differentiate and burn, which makes the plastic parts show some carbonization and charring.

It is mainly present at the confluence of the two melts and the flange of the gate; the gas is not cleaned smoothly, which makes the speed of the melt entering each cavity different, so it is easy to form moving marks and fusion marks and make the plastic parts mechanical function decreases; due to the obstruction of the gas in the cavity, the filling speed will be reduced, the molding cycle will be affected, and the taxing power will be reduced.

The dispersion of air bubbles in the plastic parts, the air bubbles generated by the accumulation of air in the mold cavity, are often scattered on the part opposite to the gate; the bubbles in the plastic material that differentiate or chemical reactions are scattered along with the thickness of the plastic part; in the plastic material, the air bubbles generated by the gasification of the remaining water are scattered irregularly on all the plastic parts.

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