The structure and processing quality of injection molds directly affect the quality of plastic parts and production efficiency.
This article provides a brief analysis of some of the most common faults in mold production.
Difficulty in removing material from the gate.
During injection molding, the gate becomes stuck inside the gate bushing and is difficult to remove. When the mold is opened, the product develops cracks and damage.
Furthermore, the operator must use the tip of a copper rod to tap it out from the nozzle to loosen it before demolding, severely impacting production efficiency.The main cause of this failure is poor surface finish of the gate conical hole, with 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 conical hole after a period of use.
Additionally, the nozzle’s spherical curvature may be too small, resulting in rivet heads forming at this point in the gate material. The conical hole of the gate 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 must be ground to a Ra value below 0.4. Furthermore, a gate pull rod or gate ejection mechanism must be installed.
Guide post 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
- When the wall thickness of the plastic part is not uniform, the material flow rate is high when passing through the thick wall, which generates greater pressure at this point;
- The sides of the plastic part are asymmetrical, such as the mold with a stepped parting surface, where the opposing sides are not subjected to equal counter pressure.
Moving and fixed mold offset
Large molds, due to varying filling rates in different directions and the influence of the mold’s own weight during assembly, can experience displacement between the moving and stationary molds.
In these cases, lateral offset forces will be applied to the guide pillars during injection, causing surface roughening and damage to the guide pillars during mold opening. In severe cases, the guide pillars may bend or be cut off, or even prevent mold opening altogether.
To address the above issues, high-strength locating keys are added to each of the 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 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 bushings must meet the design requirements.
Bending of the moving template
During injection molding, the molten plastic inside the mold cavity generates enormous back pressure, typically between 600 and 1000 kg/cm². Mold manufacturers sometimes neglect this issue, frequently altering the original design dimensions or replacing the moving mold plate with low-strength steel.
In molds using ejector pins, the large span between the two side supports causes the mold plate 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.
Where necessary, support columns or blocks should be installed below the moving mold plate to reduce its thickness and increase its load-bearing capacity.
The push rod is bent, broken, or leaking material
Self-made ejector pins are of better quality, but the processing cost is too high. Currently, standard parts are usually used, but their quality is generally lower. 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.
Even more dangerously, sometimes the ejector pin breaks after being ejected a certain distance, and as a result, this exposed section of the ejector pin cannot return to its original position during the next mold closing, damaging the mold cavity.
To solve this problem, the ejector rods were reground, retaining a 10-15 mm mating section at the front end and reducing the middle section by 0.2 mm.
After assembly, all ejector rods must have their mating clearance strictly checked, generally within 0.05-0.08 mm, to ensure the entire ejection mechanism can move forward and backward freely.
Poor cooling or water leakage in the water system
The cooling effect of a mold directly affects the quality of the finished product and production efficiency. Poor cooling can lead to excessive or uneven shrinkage, resulting in defects such as warping and deformation.
Furthermore, overheating of the mold, either as a whole or in specific areas, can prevent proper molding and halt production. In severe cases, it can cause moving parts like ejector pins to seize due to thermal expansion and damage.
The design and manufacturing of the cooling system should be determined by the shape of the product.
This system should not be omitted simply because of the complexity of the mold structure or the difficulty of manufacturing, especially for large and medium-sized molds where cooling must be fully considered.
Malfunction of the fixed-distance tensioning mechanism
Hooks, latches, and similar spacer-type tensioning mechanisms are generally used in molds with fixed-mold core pulling or some secondary demolding processes. 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 plate being pulled will inevitably become skewed and damaged. The parts of these mechanisms must have high rigidity and wear resistance, and adjustment is also difficult. The lifespan of these mechanisms is relatively short, so their use should be avoided as much as possible, and 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, a hydraulic cylinder can be used for core pulling. The inclined pin-slider type core-pulling mechanism is damaged.
The most common problems with this type of mechanism are inadequate machining and insufficient material usage. The main issues are as follows:
A large tilt angle A on the slant pin has the advantage of generating a larger core-pulling distance within a shorter mold opening stroke.
However, with an excessively large tilt angle A, when the pulling force F is constant, the bending force P=F/COSA on the slant pin during the core-pulling process is also greater, making it prone to slant pin deformation and wear of the slant 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°.
The guide groove length is too small
Some molds, due to limitations in template area, have guide groove lengths that are too short. After the core-pulling action is complete, the slider protrudes outside the guide groove.
This can easily cause slider tilting during the later stages of core pulling and the initial stages 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.
When designing and manufacturing molds, the most perfect mold should be designed based on the specific requirements of the plastic parts, the batch size, and the manufacturing time limit.
This design should not only meet the product requirements but also be the simplest, most reliable, and easiest to process in terms of mold structure, while keeping the cost low.
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