Guide pillars in molds primarily serve a guiding function to ensure that the molding surfaces of the core and cavity do not collide under any circumstances. They cannot be used as load-bearing or positioning components.
During injection, the moving and stationary molds will generate significant lateral offset forces in the following two situations:
When the wall thickness of the plastic part is uneven, the material flow rate is high through the thicker walls, generating greater pressure at these points;
When the sides of the plastic part are asymmetrical, such as in a mold with a stepped parting surface, the counter-pressure on opposite sides is unequal.
2. Difficulty in gate removal
During injection molding, the gate may stick to the gate bushing and be difficult to remove. Upon mold opening, cracks and damage may occur in the product. Furthermore, the operator must use a pointed copper rod to tap it out from the nozzle to loosen it before demolding, severely impacting production efficiency.
This failure is mainly caused by poor surface finish of the gate taper hole and tool marks along the circumference of the inner hole. Secondly, the material is too soft, causing deformation or damage to the small end of the conical hole after a period of use. Additionally, the nozzle's spherical curvature is too small, leading to rivet heads in the sprue material. The conical hole of the sprue bushing is difficult to machine; standard parts should be used whenever possible. If machining is necessary, a custom-made reamer should be used or purchased. The conical hole needs to be ground to Ra 0.4 or below.
Furthermore, a sprue pull rod or sprue ejection mechanism must be installed.
3. Moving and Fixed Mold Misalignment
Large molds experience moving and fixed mold misalignment due to varying filling rates in different directions and the influence of the mold's own weight during mold assembly.
In these cases, lateral offset forces during injection will be applied to the guide pillars, causing surface roughening and damage to the guide pillars during mold opening. In severe cases, the guide pillars may bend or cut off, or even prevent mold opening altogether.
To solve these problems, high-strength locating keys should be added to all four sides of the mold parting surface. Cylindrical keys are the simplest and most effective method. The perpendicularity of the guide pillar holes to the parting surface is crucial.
During processing, the moving and fixed molds are aligned and clamped, then bored in one pass on a boring machine. This ensures the concentricity of the moving and fixed mold holes and minimizes perpendicularity errors. Furthermore, the heat treatment hardness of the guide pillars and guide sleeves must meet design requirements.
4. Moving Mold Platen Bending
During injection, the molten plastic in the mold cavity generates enormous back pressure, typically 600-1000 kg/cm². Mold manufacturers sometimes neglect this issue, often altering the original design dimensions or replacing the moving mold platen with low-strength steel. In molds using ejector pins, the large span between the two side seats causes the mold platen to bend downwards during injection.
Therefore, the moving mold platen must be made of high-quality steel with sufficient thickness. Low-strength steel plates such as A3 should never be used. If necessary, support pillars or blocks should be installed below the moving mold platen to reduce its thickness and increase its load-bearing capacity.
5. Ejector Pin Bending, Breakage, or Material Leakage
Self-made ejector pins are of better quality, but the processing cost is too high. Currently, standard parts are usually used, although their quality is generally lower. If the clearance between the ejector pin and the hole is too large, material leakage will occur. However, if the clearance is too small, the ejector pin will expand and seize during injection due to the increased mold temperature. More dangerously, sometimes the ejector pin will break after being ejected a certain distance and cannot be pushed back, resulting in the exposed section of the ejector pin failing to return to its original position during the next mold closing and damaging the mold cavity.
To solve this problem, the ejector pin is reground, retaining a 10-15 mm mating section at the front end and grinding the middle section down by 0.2 mm. After assembly, all ejector pins must be rigorously checked for clearance, generally within 0.05-0.08 mm, to ensure the entire ejection mechanism can move freely.
6. Poor Cooling or Water Leakage in the Cooling Channels
The cooling effect of the mold directly affects the quality of the product and production efficiency. Poor cooling leads to large shrinkage or uneven shrinkage of the product, resulting in defects such as warping and deformation. On the other hand, overheating of the mold, either as a whole or in parts, can prevent normal molding and cause production stoppages. In severe cases, thermal expansion of moving parts such as ejector pins can cause them to seize and be damaged.
The design and processing of the cooling system should be determined by the product shape. This system should not be omitted due to the complexity of the mold structure or the difficulty of processing, especially for large and medium-sized molds where cooling must be fully considered.
7. Insufficient Guide Groove Length
Some molds, due to limitations in the mold plate area, have guide grooves that are too short. After the core-pulling action is completed, the slider protrudes outside the guide groove. This easily causes slider tilting during the post-core-pulling stage and the initial stage of mold closing and resetting. Especially during mold closing, the slider may not reset smoothly, leading to damage or even bending.
Based on experience, the length of the slider remaining in the guide groove after the core-pulling action should not be less than 2/3 of the total guide groove length.
8. Malfunction of the Fixed-Distance Tensioning Mechanism
Fixed-distance tensioning mechanisms such as hooks and latches are generally used in fixed-mold core-pulling or some molds with secondary demolding. Because these mechanisms are set in pairs on both sides of the mold, their operation must be synchronized; that is, they must latch simultaneously when the mold closes and disengage simultaneously when the mold opens to a certain position.
Once synchronization is lost, the mold platen of the pulled die will inevitably become skewed and damaged. These mechanisms require parts with high rigidity and wear resistance, are difficult to adjust, and have a short lifespan. Their use should be avoided as much as possible; alternative mechanisms can be used. When the core-pulling force is relatively small, a spring-driven method can be used to push out the fixed mold. When the core-pulling force is relatively large, a structure where the core slides as the moving mold retracts, completing the core-pulling action before mold separation, can be used. For large molds, hydraulic cylinders can be used for core pulling.
9. Damage to the Angled Pin Slider Type Core-Pulling Mechanism.
The most common problems with this type of mechanism are inadequate machining and insufficient material. The main issues are as follows:
A large angle A on the angled pin;
The advantage is that it can generate a large core-pulling distance within a short mold opening stroke.
However, with an excessively large angle A, when the pulling force F is constant, the bending force P=F/COSA on the angled pin during the core-pulling process is also larger, easily leading to angled pin deformation and wear of the angled hole.
Simultaneously, the greater the upward thrust N=FTGA generated by the inclined pin on the slider, the greater the force. This force increases the normal pressure of the slider on the guide surface inside the guide groove, thereby increasing the frictional resistance during slider sliding. This can easily lead to uneven sliding and wear of the guide groove. Based on experience, the inclination angle A should not exceed 25°.
