Common Injection Mold Faults and Troubleshooting Methods

Common Injection Mold Faults and Troubleshooting Methods

 

The structure and processing of injection molds affect the quality of plastic parts. Sometimes, injection molds malfunction during production. How can we troubleshoot these? Today, DaLiang will share common injection mold faults and troubleshooting methods.

 

1. Guide Pillar Damage.

 

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. In the following situations, the moving and fixed molds will generate significant lateral offset forces during injection:

 

(1) When the wall thickness of the plastic part is uneven, the material flow rate is high through the thicker wall, generating greater pressure at that point;

 

(2) The sides of the plastic part are asymmetrical, such as in molds with stepped parting surfaces, where the counter-pressure on opposite sides is unequal.

 

2. Moving and Fixed Mold Misalignment.

 

Large molds, due to different filling rates in each direction and the influence of the mold's own weight during assembly, experience moving and fixed mold misalignment. In the aforementioned situations, 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, making mold opening impossible. To solve these problems, high-strength locating keys are added to each of the four sides of the mold parting surface; cylindrical keys are a simple and effective method. The perpendicularity of the guide pillar holes to the parting surface is crucial. During machining, 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.

 

3. Bending of the moving mold platen.

 

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 a low-strength steel plate. 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 plate must be made of high-quality steel with sufficient thickness. Low-strength steel plates such as A3 should never be used. If necessary, support columns or blocks should be installed below the moving mold plate to reduce the thickness of the mold plate and improve its load-bearing capacity.

 

4. Bending, breakage, or material leakage of the ejector pin.

 

Self-made ejector pins are of better quality, but the processing cost is too high. Currently, standard parts are generally used, but their quality is poor. If the gap between the ejector pin and the hole is too large, material leakage will occur. However, if the gap is too small, the ejector pin will expand and jam during injection due to the increased mold temperature. More dangerously, sometimes the ejector pin breaks after being ejected a certain distance and cannot be pushed back up. As a result, this exposed section of the ejector pin cannot return to its original position during the next mold closing and will damage the die. 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 strictly checked for mating clearance, generally within 0.05-0.08 mm, to ensure that the entire ejection mechanism can move freely forward and backward.

 

5. Difficulty in gate removal.

 

During injection molding, the gate may stick inside the sprue bushing and be difficult to remove. This results in cracks and damage to the product upon mold opening. 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. The main causes of this problem are poor finish of the sprue's tapered hole and tool marks on the circumference of the inner hole. Secondly, the material may be too soft, causing deformation or damage to the small end of the tapered hole after a period of use, and the nozzle's spherical curvature may be too small, resulting in rivet heads formed at the sprue. The tapered 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 tapered hole must be ground to Ra 0.4 or higher. Additionally, a sprue pull rod or sprue ejection mechanism must be installed.

 

6. Poor cooling or water leakage in the water 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 halt production. In severe cases, it can cause moving parts such as ejector pins to expand and seize, leading to damage. 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. Malfunction of the fixed-distance tensioning mechanism.

 

Fixed-distance tensioning mechanisms such as hooks and latches are generally used in molds with core pulling or 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 being pulled will inevitably become skewed and damaged. The parts of these mechanisms must have high rigidity and wear resistance, and adjustment is difficult. The lifespan of these mechanisms is relatively short, so their use should be avoided as much as possible. Other mechanisms should be used instead. 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 can be used where the core slides as the moving mold retracts, completing the core-pulling action before mold separation. For large molds, hydraulic cylinders can be used for core pulling. Damage to the inclined pin slider type core-pulling mechanism is a concern.

 

Some molds, due to limitations in mold platen area, have guide groove lengths 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 later stages of core pulling and the initial stage of mold closing and reset, especially during mold closing, where the slider's reset is not smooth, leading to slider damage or even bending. Experience suggests that 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. When designing and manufacturing molds, the specific requirements of the plastic part quality, batch size, and manufacturing deadline should be considered. A good mold should meet the product requirements while being the simplest, most reliable, and easiest to manufacture in terms of cost.

 

Previous Page: Causes and Solutions for Vibration Marks in Injection Molding Products