Structure and processing quality of injection molds directly affect quality and production efficiency of plastic parts. There are many faults in mold production process. Following are solutions to several common faults.

 

During injection molding process, gate is stuck in sprue sleeve and is not easy to come out. When mold is opened, product is cracked and damaged. In addition, operator must knock out tip of copper rod from nozzle to loosen it before demoulding, which seriously affects production efficiency. Main reason for this failure is poor finish of gate taper hole, and there are knife marks in circumferential direction of inner hole.
In addition, material is too soft, small end of taper hole is deformed or damaged after a period of use, curvature of nozzle spherical surface is too small, causing gate material to produce riveting heads here. Taper hole of sprue bushing is more difficult to process, so standard parts should be used as much as possible. Taper hole needs to be ground to below Ra0.4. In addition, a gate pull rod or gate ejection mechanism must be provided.

 

Guide post mainly plays a guiding role in injection mold to ensure that molding surfaces of core and cavity do not collide with each other under any circumstances. Guide post cannot be used as a force-receiving part or a positioning part. In following situations, moving and fixed molds will produce huge lateral deflection force during injection: (1) When wall thickness of plastic part is not uniform, material flow through thick wall has a large speed, and a large pressure is generated here; (2) Side surface of plastic part is asymmetrical, such as a stepped parting surface mold, opposite pressure on two opposite sides is not equal.

 

For large molds, due to different filling rates in all directions and influence of mold's own weight during mold assembly, movable and fixed molds will deviate. In these cases, lateral offset force will be added to guide post during injection, surface of guide post will be roughed and damaged when mold is opened. In severe cases, guide post may be bent or cut off, and mold cannot even be opened. In order to solve above problems, a high-strength positioning key is added on each of four sides of mold parting surface. The most simple and effective way is to use a cylindrical key. Perpendicularity between guide hole and parting surface is very important. In processing, moving and fixed molds are aligned and clamped, and boring is completed at one time on boring machine. This can ensure concentricity of moving and fixed mold holes, minimize verticality error. In addition, heat treatment hardness of guide post and guide sleeve must meet design requirements.

 

When injection mold is injected, molten plastic in cavity produces a huge back pressure, generally 600~1000 kg/cm2. Mold makers sometimes do not pay attention to this problem, and often change original design size, or replace movable mold plate with low-strength steel plate. In mold with ejector pin, because of large span between two sides of seat, mold plate is bent down during injection. Therefore, movable mold plate must be made of high-quality steel, with sufficient thickness, and low-strength steel plates such as A3 should not be used. When necessary, support columns or support blocks should be set under movable mold plate to reduce thickness of mold plate and increase bearing capacity.

 

Self-made ejector rods are of better quality, but processing cost is too high. Nowadays, standard parts are usually used and quality is average. If gap between ejector pin and hole is too large, leakage will occur, but if gap is too small, ejector pin will become stuck due to increase in mold temperature during injection.
What's more dangerous is that sometimes ejector rod will not move and break when it is ejected for a normal distance. As a result, exposed ejector rod cannot be reset during next mold clamping and will damage die. In order to solve this problem, ejector rod was re-ground, and mating section of 10~15 mm was retained at the front end of ejector rod, and middle part was ground 0.2mm smaller. After all ejector rods are assembled, fit clearance must be strictly checked, generally within 0.05~0.08 mm, to ensure that entire ejector mechanism can move forward and backward freely.

 

Cooling effect of injection mold directly affects quality and production efficiency of product, such as poor cooling, large product shrinkage, or uneven shrinkage, resulting in warping deformation and other defects. On the other hand, overall or partial overheating of mold prevents mold from forming normally and stops production. In severe cases, movable parts such as ejector pin are thermally expanded and become damaged. Design and processing of cooling system depends on shape of product. Don't omit this system because of complex structure of mold or difficulty of processing. Especially for large and medium-sized molds, cooling problem must be fully considered.

 

Swing hooks, buckles and other fixed-distance tensioning mechanisms are generally used in fixed mold core pulling or some secondary demolding molds. Because such mechanisms are set in pairs on both sides of mold, their actions must be synchronized, that is, mold is clamped at the same time, mold is opened to a certain position and released at the same time.
Once synchronization is lost, it will inevitably cause mold plate of drawn die to be skewed and damaged. Parts of these mechanisms must have higher rigidity and wear resistance, adjustment is difficult. Life of mechanism is short, so avoid using it as much as possible and you can switch to other mechanisms. When core pulling force is relatively small, spring can be used to push out fixed mold. When core pulling force is relatively large, core sliding when movable mold is retracted can be used, core pulling action is completed first and then mold is split. A hydraulic cylinder can be used to pull core on mold.

 

Common problems of this kind of mechanism are mostly inadequate processing and too small materials. Main two problems are as follows: Tilt angle A is large, and advantage is that it can produce a larger core pulling in a shorter mold opening stroke. However, if inclination angle A is too large, when extraction force F is a certain value, bending force P=F/COSA of inclined pin during core pulling process is larger, inclined pin deformation and inclined hole wear are prone to occur.
At the same time, upward thrust N=FTGA generated by oblique pin on slider is also greater. This force increases positive pressure of slider on guide surface in guide groove, thereby increasing frictional resistance of slider when sliding. It is easy to cause uneven sliding and wear of guide groove. According to experience, inclination angle A should not be greater than 25°.

 

Some molds are limited by area of mold plate and guide groove length is too small. Slider will be exposed outside guide groove after core pulling action is completed. This will easily cause slider to tilt during core pulling stage and initial stage of mold closing and resetting, especially during mold clamping. At this time, slider is not reset smoothly, causing damage to slider, or even bending damage. According to experience, after slider completes core pulling action, length left the chute should not be less than 2/3 of full length of guide groove.
When designing and manufacturing molds, it should be based on requirements of quality of plastic parts, size of batch, requirements of manufacturing period, etc., which can not only meet requirements of product, but also the most simple and reliable mold structure, easy processing, and low cost. This is the most perfect mold.