Industry information

Solutions to typical problems of injection molds

2021-08-07
Solutions to typical problems of injection molds
1. Difficulties in gate stripping
During the injection molding process, the gate is stuck in 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 so that it can be demolded after it is loosened, which severely affects the production power.

The main element 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 use, and the curvature of the nozzle spherical surface is too small, causing the riveting of the gate material to occur here. The taper hole of the sprue bushing is more difficult to process, so standard parts should be used as much as possible. The taper hole needs to be ground to a Ra0.4 or more. In addition, it is necessary to set the gate pull rod or gate ejection.

2. 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 movable and fixed molds will deviate. In the above-mentioned situations, the lateral deflection force will be added to the guide post during injection, the guide post will be roughened and damaged when the mold is opened, and the guide post will be bent or blocked when the mold is severe, 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 the boring machine is finished 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 post and the guide sleeve must meet the requirements of the plan.

3. Damage to the guide post
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, and the guide post 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 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 back pressure is not equal.

4. Dynamic template twists and turns
When the mold is being injected, the molten plastic in the mold cavity generates infinite back pressure, 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 installed under the movable formwork to reduce the thickness of the formwork and adjust the forward load.

5. The ejector rod is tortuous, cracking or leaking
The quality of the self-restrained 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 injection.

What's more risky is that sometimes the ejector pin is pushed out and breaks normally at intervals, and 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 re-ground, and a 10~15 mm cooperative section was stored at the front end of the ejector rod, and the base was slightly reduced 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.

6. 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 and other shortcomings. 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, processing depends on the shape of the product, do not omit this special system due to the messy structure of the mold or the difficult processing, except for the large and medium-sized molds, you must fully consider the cooling problem.

7. The slider is tilted and the reset is not smooth
Some molds are bound by the area of the template, and the length of the guide groove 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 inclined, which is particularly close. During the die, the sliding block is not reset smoothly, causing damage to the sliding block and even bending damage. According to experience, after the slider finishes the core-pulling action, the length left in the chute should not be less than 2/3 of the total length of the guide groove.

8. 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 buckled together when the mold is closed, and the mold is unhooked when the mold is opened to a certain position.

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 switch to other parts. arrange.

In the case of a small core pulling force comparison, the method of using a tension spring to push out the fixed mold can be selected. 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 as follows:

The oblique 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 oblique pin during the core pulling process is larger, and the oblique pin deformation and oblique hole wear are prone to appear.

At the same time, the upward thrust N=FTGA of 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, and then increases the resistance when the slider slides. It is easy to form 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.

9. Poor exhaust in the injection mold
Gas often occurs 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 Differentiation produces gas; the gas produced 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 cause 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 process. If the gas is not discharged in time, it will form a melt. The filling is difficult, resulting in the shortage of injection volume and the inability to fill the cavity; the poorly swept air will cause high pressure in the cavity, and enter the inside of the plastic under a certain degree of contraction, forming voids, pores, sparseness and silver streaks, etc. Quality shortcomings;

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 appear to be carbonized and burnt. It is mainly present at the junction 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 movable marks and fusion marks, and make the plastic parts The mechanical performance is reduced; due to the obstruction of the gas in the cavity, the filling speed will be reduced, the molding cycle will be affected, and the tax power will be reduced.

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