Several common fault solutions for structural form and processing of injection moulding

Time:2019-07-31 10:02:52 / Popularity: / Source:

Structural form and processing quality of injection moulding directly affect quality and production efficiency of plastic parts. There are many various injection moulding defects in injection molding production process and following are several common fault solutions.
injection molding production 

Difficult to remove gate

During injection molding production process, gate is stuck in sprue bushing and is not easily peeled off. When mold is opened, product is cracked. In addition, operator must use tip of copper rod to knock out from nozzle, so that it can be released after being loosened, which seriously affects production efficiency. Main cause of this injection moulding defects is poor smoothness of gate cone and presence of tool marks in circumferential direction of bore.
Also material is too soft, small end of tapered hole is deformed or damaged after a period of use, spherical curvature of nozzle is too small, so that gate material produces a rivet head here. Taper hole of sprue sleeve is difficult to process and standard parts should be used as much as possible. If you need to process it yourself, you should also make a special reamer. Taper hole needs to be ground to below Ra0.4. In addition, a gate pull rod or gate ejection mechanism must be provided.

Guide column damage

Guide post mainly plays a guiding role in injection moulding to ensure that molding surfaces of core and cavity do not touch each other under any circumstances, guide post cannot be used as force receiving member or positioning member. In following cases, movement and fixed mode will produce a huge lateral displacement force during injection:
(1) When wall thickness of plastic part is not uniform, velocity of material flowing through large  thick wall, and a large pressure is generated here;
(2) Side of plastic part is asymmetrical. For example, for a mold with a stepped parting surface, opposing pressures on opposite sides are not equal.

Dynamic and fixed mold offset

Large molds are subject to dynamic and fixed mold offset due to different filling rates and influence of mold weight during mold loading. In these cases, lateral offset force will be applied to guide post during injection. When mold is opened, surface of guide post will be pulled and damaged. In severe cases, guide post will be bent or cut, and even mold cannot be opened.
In order to solve above problem, a high-strength positioning key is added on each side of mold parting surface, the most convenient and effective one is to use a cylindrical key. Perpendicularity of guide post hole to parting surface is critical. In processing, it is clamped in the position of moving and fixed molds, and it is finished once on trampoline. This ensures concentricity of moving and fixed holes, minimizes vertical error. In addition, heat treatment hardness of guide post and guide sleeve must meet design requirements.

Moving template bending

When injection moulding is injected, molten plastic in cavity generates a huge back pressure, generally 600~1000 kg/cm2. Moldmakers sometimes do not pay attention to this problem, often change original design size, or replace moving template with a low-strength steel plate. In mold with top bar topping, template is bent down during injection due to large span of two sides. Moving template must be made of high-quality steel. It must have sufficient thickness and it is not allowed to use low-strength steel plates such as A3. If necessary, support columns or blocks should be placed under moving plate to reduce thickness of template and improve carrying capacity.

Ram is bent, broken or leaking

Quality of self-made ejector is better, but processing cost is too high. Now standard parts are usually used, and quality is generally good. If gap between ejector pin and hole is too large, leakage occurs, but if gap is too small, ejector pin expands and is stuck due to an increase in mold temperature at the time of injection.
More dangerously, sometimes ejector pin is pulled out at a normal distance and is broken. As a result, exposed ejector pin cannot be reset and die is broken during next clamping. In order to solve this problem, ejector rod is re-grinded, a 10-15 mm mating section is retained at the front end of ejector pin, and middle portion is ground by 0.2 mm. All ejector rods must be strictly checked for fit clearance after assembly, generally within 0.05~0.08 mm, to ensure that entire ejector mechanism can move forward and backward freely.

Poor cooling or water leaks

Cooling effect of injection moulding directly affects quality and production efficiency of product. If cooling is poor, product shrinks greatly, or shrinkage is uneven, and defects such as warping deformation occur. On the other hand, mold is overheated in whole or in part, so that mold cannot be formed normally and production is stopped. In severe cases, movable parts such as ejector are thermally expanded and damaged. Design of cooling system depends on shape of product. Do not omit this system because of complicated structure of mold or difficulty of processing especially for large and medium-sized molds, cooling problem must be fully considered.

Distance tensioning mechanism fails

Fixed distance tensioning mechanism such as swing hook and buckle is generally used in the mold of fixed mold core or some secondary demoulding. Since such mechanisms are arranged in pairs on both sides of mold, action requirements must be synchronized, that is mold is simultaneously buckled, mold is opened to a certain position and unhooked at the same time.
Once synchronization is lost, it will inevitably cause template of pulled mold to be skewed and damaged. Parts of these mechanisms have higher rigidity and wear resistance, adjustment is also difficult, life of mechanism is short, it is avoided as much as possible. Other mechanisms can be used instead. In the case where pumping force is relatively small, spring can be used to push fixed mold. When core pulling force is relatively large, core sliding can be used when moving mold is retracted, and structure of splitting after core pulling operation is completed first, and hydraulic cylinder can be used for core pulling on large mold.

Oblique pin slider core pulling mechanism is damaged

Most of problems that occur in this kind of mechanism are that processing is not in place and material is too small. There are two main problems: inclination angle A is large, and advantage is that a relatively large core distance can be generated in a shorter opening stroke. However, when excessive inclination angle A is adopted and extraction force F is a certain value, bending force P=F/COSA received by oblique pin during core pulling process is also larger, skew pin deformation and oblique hole wear are prone to occur.
At the same time, oblique pin produces an upward thrust on slider N=FTGA, which increases positive pressure of slider on guide surface in guide groove, thereby increasing frictional resistance when slider slides. It is easy to cause slipping and guide groove is worn out. According to experience, inclination A should not be greater than 25°.

Guide slot length is too small

Some molds are limited by area of stencil, length of guide groove is too small, slider exposes outside of guide groove after core pulling operation is completed, so that slider is inclined in post-core phase and initial stage of mold clamping reset, especially in mold clamping. When slider is not reset properly, slider is damaged and even bending is broken. According to experience, after slider is finished, length left in chute should not be less than 2/3 of total length of guide.
When designing and manufacturing molds, it should be able to meet requirements of products according to requirements of quality of plastic parts, size of batch, requirements of manufacturing period, etc. Meanwhile mold structure is the easiest and most reliable, easy to process, an cost is low, this is the most perfect mold.
injection molding production 

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