Nine kinds of mold failure

1. Difficult to take off the material from the sprue

During the injection process, the sprue sticks in the sprue sleeve and is not easy to be taken off. When the mold is opened, the product is cracked and damaged. Besides, the operator must use the tip of copper rod to knock out from the nozzle to make it loose before demoulding, which seriously affects the production efficiency.

The main reason for this failure is the poor finish of the conical hole of the gate and the knife marks in the circumferential direction of the inner hole. Secondly, the material is too soft, and the small end of the cone hole is deformed or damaged after using for a period of time, and the arc of the nozzle sphere is too small, resulting in the riveting head of the sprue material here. The tapered hole of the sprue is difficult to process, so standard parts should be used as much as possible, if you need to process it by yourself, you should also make your own or buy special reamer. The tapered hole should be ground to Ra0.4 or more. In addition, the sprue puller or sprue ejector must be set.
 

2. Large moulds moving and fixed mould offset

Large molds have different charging rates in each direction and the influence of the mold's self-weight during mold loading, resulting in moving and fixed mold deflections. In the above cases, the lateral offset force will be added to the guide column during injection, and the surface of the guide column will be stretched and damaged when the mold is opened, and in serious cases, the guide column will be bent or cut off, and even the mold cannot be opened.

In order to solve the above problems, a high strength positioning key is added on each side of the mold parting surface, and the easiest and most effective is to use a cylindrical key. The perpendicularity of the guide pillar hole and the parting surface is very important, in the processing is to use the dynamic and fixed die alignment position after clamping, in the boring machine once boring, so as to ensure the concentricity of the dynamic and fixed die hole, and make the verticality error minimum. In addition, the guide pillar and guide sleeve heat treatment hardness must meet the design requirements.
 

3. Guide pillar damage

Guide pillar in the mold mainly plays a guiding role to ensure that the core and cavity molding surface in any case do not touch each other, can not be used as a guide pillar force or positioning parts.

In several cases, the dynamic and fixed mold will produce huge lateral deflection force during injection. When the wall thickness of the plastic part is not uniform, the rate of material flow through the thick wall is large, and a large pressure is generated here; the side of the plastic part is not symmetrical, such as the step-shaped parting surface of the mold on the opposite sides of the counter pressure is not equal.
 

4. Dynamic template bending

When the mold is injected, the molten plastic in the mold cavity generates a huge back pressure, generally in the range of 600~1000kg/cm. Mold makers sometimes do not pay attention to this problem, often change the original design size, or the dynamic template with low-strength steel plate instead, in the mold with the top bar top material, due to the large span between the two sides of the seat, resulting in the injection of the template down bending. Therefore, the moving template must use high quality steel, to have enough thickness, do not use A3 and other low strength steel plate, when necessary, should be set up below the moving template support column or support block, in order to reduce the thickness of the template, improve the bearing capacity.
 

5. Bending, breaking or leakage of the top bar

The quality of homemade top bar is better, is too high processing costs, now generally use standard parts, the quality is worse. If the gap between the top bar and the hole is too large, there will be material leakage, but if the gap is too small, the top bar will be stuck due to mold temperature increase during injection, and the top bar will expand.

What is more dangerous is that sometimes the top bar is jacked out of the general distance and broken, as a result, this section of the exposed top bar can not be reset and hit the concave mold during the next mold closing. In order to solve this problem, the top bar is resharpened by keeping 10~15mm mating section at the front end of the top bar and grinding the middle part smaller by 0.2mm. All the top bar in the assembly, must be strictly checked with the clearance, generally in 0.05 ~ 0.08 mm, to ensure that the entire ejector mechanism can enter and exit freely.

6. Poor cooling or water leakage

The cooling effect of the mold directly affects the quality of the products and production efficiency, such as poor cooling, the products shrink large, or shrinkage is not uniform and appear warped surface deformation and other defects. On the other hand, the mold as a whole or local overheating, so that the mold can not be formed normally and stop production, and in serious cases, the top bar and other moving parts of thermal expansion jammed and damaged. The design of the cooling system, processing to the shape of the product, do not because of the complex structure of the mold or processing difficulties and omit this system, especially large and medium-sized mold must s fully consider the cooling problem.
 

7. Slider tilt, reset is not smooth

Some molds are limited by the area of the template, the length of the guide slot is too small, the slider in the core action is completed to reveal the outside of the guide slot, so in the core after the stage and the initial stage of the mold reset are easy to cause the slider tilt, especially in the mold, the slider reset is not smooth, so that the slider damage, and even bending damage. According to the experience, the length of the slide should not be less than 2/3 of the full length of the guide groove after the core extraction is completed.
 

8. Fixed distance tensioning mechanism failure

The fixed distance tensioning mechanism such as hook and buckle is generally used in the fixed mold core extraction or some secondary demoulding molds, because these mechanisms are set up in pairs on both sides of the mold, their actions must be synchronized, that is, the mold is closed and buckled at the same time, and the mold is opened to a certain position and decoupled at the same time. Once the synchronization is lost, it will inevitably cause damage to the skewed template of the pulled mold, and the parts of these institutions should have high rigidity and wear resistance, and it is difficult to adjust them.

In the case of relatively small extraction force can be used spring push out the fixed mold method, in the case of relatively large extraction force can be used to move the mold back when the core sliding, first complete the core extraction action before the mold structure, in large molds can be used hydraulic cylinder core extraction. Inclined pin slider type core extraction mechanism damage. Most of the more common problems of this mechanism is not in place on the processing and the material is too small, mainly the following two problems.

The advantage of a large inclination angle A of the slant pin is that it can produce a large core extraction distance within a short 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 in the process of core extraction will be bigger, and the pin will be easily deformed and the hole will be worn. At the same time, the upward thrust force N=FTGA generated by the tilting pin on the slider is also greater, which increases the positive pressure of the slider on the guiding surface in the guide groove, thus increasing the frictional resistance of the slider when sliding. It is easy to cause the sliding is not smooth, guide groove wear. According to experience, the inclination angle A should not be greater than 25.
 

9. Poor exhaust in the injection mold

There is often gas in the injection mold, what is the cause of this? Air in the casting system and mold cavity; some raw materials contain moisture that is not dried out, they will vaporize into water vapor at high temperature; because the temperature is too high during injection molding, some unstable plastic will decompose and produce gas; some additives in plastic raw materials volatilization or mutual chemical reaction to generate gas.

At the same time, the cause of poor exhaust gas, also need to find out as soon as possible. In the injection molding process, the melt will replace the gas in the cavity, if the gas is not discharged in time, it will cause difficulties in filling the melt, resulting in insufficient injection volume and can not fill the cavity; the gas excluded will form a high pressure in the cavity, and in a certain degree of compression penetrate into the interior of the plastic, resulting in cavities, The gas is highly compressed, making it difficult to fill the cavity;

As the gas is highly compressed, the temperature inside the cavity rises sharply, which causes the surrounding melt to decompose and burn, resulting in localized carbonization and scorching of the plastic part. It mainly occurs at the merging of two melt streams and at the flange of the gate; poor gas removal makes the melt speed into each cavity different, so it is easy to form flow marks and fusion marks, and make the mechanical properties of the plastic parts lower; due to the obstruction of gas in the cavity, it will reduce the mold filling speed, affect the molding cycle and reduce production efficiency.

The distribution of air bubbles in the plastic parts, the air bubbles generated by the accumulation of air in the mold cavity, often distributed in the parts opposite to the gate; plastic materials in the decomposition or chemical reaction of the bubbles along the thickness of the plastic parts distribution; plastic materials in the residual water vaporization bubbles, is irregularly distributed in the entire plastic parts.