Common problems and solutions for injection molds

Molds are an important part of the manufacturing process for many products. However, they are also prone to problems that can affect production. 

This blog post will discuss some of the most common problems with injection molds and how to solve them. We hope this information will help you keep your molds running smoothly and efficiently. Thank you for reading!

Difficult to take off the sprue

During the molding process, the sprue sticks in the sprue sleeve and is not easy to be taken off.

When the injection mold is opened, the product is cracked and damaged. Besides, the operator must knock out the nozzle with the tip of the copper rod to loosen it before demoulding, which seriously affects production efficiency.

The main reasons for this failure are: a poor finish of the conical hole of the sprue, knife sink marks in the circumferential direction of the inner hole;

Secondly, the material is too soft and the small end of the conical hole is deformed or damaged after using it for some time, as well as the curvature 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 a special reamer.

The tapered hole should be ground to Ra0.4 or more; in addition, a sprue puller or sprue ejector must be installed.

Large mold moving and fixed mold offset

Large molds have different charging rates in each direction, as well as the influence of the mold’s self-weight when loading the mold to produce dynamic and fixed mold deflection. 

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 injection mold cannot be opened.

To solve the above problems, a high-strength positioning key is added to the mold parting surface, one on each of the four sides, and the easiest and most effective is to use a cylindrical key. The perpendicularity of the guide pillar hole and the parting surface is crucial. 

In the processing, use the moving and fixed die alignment after clamping, in the boring injection molding machine once boring, to ensure the concentricity of the moving and fixed die hole, and make the verticality error minimum. In addition, the heat treatment hardness of the guide pillar and guide bush must meet the design requirements.

Guide pillar damage

The guide pillar mainly plays a guiding role in the mold to ensure that the molding surface of the core and cavity do not touch each other under any circumstances, and it cannot be used as a force-bearing or positioning part.

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

Dynamic mold plate bending

When the mold is injected, the molten plastic in the mold cavity generates huge back pressure, generally in the range of 600-1000 kg/cm. 

Mold makers sometimes do not pay attention to this problem, and 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 injection when the template down bending. 

Therefore, the moving template must use high-quality steel, to have enough thickness, do not use A3, and another low-strength steel plate, when necessary, should be set up below the moving template support column or support block, to reduce the thickness of the template, improve the bearing capacity.

Top bar bending, fracture, or leakage

The quality of the homemade top bars is better, that is, the processing cost is too high, now generally use standard parts, and the quality is worse.

If the gap between the top bar and the hole is too big, there will be material leakage; but if the gap is too small, the top bar will be stuck due to mold temperature rise during injection and expansion. 

What’s more dangerous is that sometimes the top bar is jacked out for a distance and then broken, as a result, when the mold is closed next time, this exposed top bar can’t be reset and hit the concave mold.

To solve this problem, the top bar should be resharpened by keeping a 10-15mm mating section at the front of the top bar and grinding the middle part 0.2mm smaller.

All the ejector rods in the assembly must be strictly checked for their fit clearance, generally within 0.05-0.08 mm, to ensure that the entire ejector mechanism can enter and exit freely.

Poor cooling or water leakage

The cooling effect of the mold directly affects the quality and productivity of the products. If the cooling is poor, the product shrinkage is large, or shrinkage is not uniform and warped surface deformation and other injection molding 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 ejector rod and other moving parts of thermal expansion are stuck and damaged.

Cooling system injection mold design, and processing, to the shape of the product, do not omit this system because of the complex structure of the mold or processing difficulties, especially for large and medium-sized molds, must be fully considered cooling problems.

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, and the slider in the core action is completed to reveal the outside of the guide slot, so that after the core stage and the initial stage of the injection mold reset are easy to cause the slider tilt, especially in the mold, the slider reset is not smooth, so 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.

Fixed distance tensioning mechanism lost

Pendulum hook, buckle, and other fixed distance tensioning mechanisms are generally used in the fixed die core or some secondary demolding mold because of these institutions in the mold on both sides of the face of the pair set.

Its action must be synchronized, that is, close the mold at the same time buckle, open the injection mold to a certain position at the same time off the hook, once the loss of synchronization, will inevitably cause the mold to be pulled by the template skew and damage.

The parts of these institutions should have high rigidity and wear resistance, adjustment is also very difficult, the life of the institution is short, try to avoid the use, can be replaced with other institutions.

In the case of relatively small extraction force, the spring can be used to push out the fixed die; in the case of relatively large extraction force, the core slides when the moving die is backed up, first completing the core extraction action and then the mold structure; in large molds, the hydraulic cylinder can be used to extract the core.

Inclined pin slider type core extraction mechanism damage. The more common fault of this mechanism, most of the processing is not in place, as well as the material used is too small, there are two main problems: inclined pin inclination angle A large, the advantage is that it can produce a larger core extraction distance in a shorter mold opening stroke.

However, take too large inclination angle A, when the extraction force F is a certain value, in the process of core extraction, the bending force P = F/COSA, also the larger the tilt pin, easy to deformation of the tilt pin and tilt hole wear;

At the same time, the tilt pin on the slider to produce upward thrust N = FTGA is also larger, this force makes the slider on the guide groove guide surface pressure increase, thus increasing the friction resistance of the slider sliding, easy to cause sliding Not smooth, guide groove wear. According to experience, the inclination angle A should not be greater than 25 degrees.

Poor exhaust in the injection mold

There is often gas in the injection mold, what is the cause of this?

(1) Air in the casting system and mold cavity.

(2) Some raw materials contain water that has not been dried out, they will vaporize into water vapor at high temperatures.

(3) Due to the high melt temperature in the plastic injection molding process, some unstable plastic will decompose and produce gas.

(4) Certain additives in plastic raw materials volatilization, or mutual chemical reaction generated by the gas.

At the same time, the cause of poor exhaust gas, also need to find out as soon as possible. The poor exhaust of injection molds will bring a series of hazards to the quality of plastic parts and many other aspects, mainly in the following ways.

(1) In the injection process, the melt will replace the gas in the cavity, if the gas is not discharged promptly, it will cause difficulties in filling the melt, resulting in insufficient injection volume and can not fill the cavity.

(2) Poorly excluded gas will form high pressure in the cavity and penetrate the interior of the plastic under a certain degree of compression, causing quality defects such as cavities, porosity, tissue looseness, and silver weld lines.

(3) As the gas is highly compressed, the material temperature inside the cavity rises sharply, which causes the surrounding melt to decompose and burn, resulting in local carbonization and scorching of the plastic part. It mainly appears in the merging of two melts, the gate flange.

(4) Poor gas removal makes the melting speed in each cavity different, therefore, it is easy to form flow marks and fusion marks, and make the mechanical properties of the plastic parts lower.

(5) Due to the obstruction of gas in the cavity, it will reduce the mold filling injection speed, affect the injection molding cycle and reduce the molding efficiency.

In plastic parts, the main distribution of air bubbles is：

(1) Air bubbles generated by the accumulation of trapped air in the injection mold cavity, often distributed on the part opposite the gate.

(2) Bubbles generated by decomposition or chemical reaction in the plastic raw material, then distributed along the thickness of the plastic part.

(3) Bubbles are generated by the vaporization of residual water in the plastic material, which is irregularly distributed on the whole plastic part.