Mold problems can be avoided during manufacturing
Mold problems can be avoided during manufacturing
The guide post mainly plays a guiding role in the mold to ensure that the forming surfaces of the core and cavity do not collide with each other under any circumstances. The guide post cannot be used as a stress part or positioning part.In the following two cases, the moving and fixed mold will produce huge lateral offset force during injection:
When the wall thickness of plastic parts is required to be uneven, the rate of material flow passing through the thick wall is large, resulting in large pressure here;
The side of the plastic part is asymmetric, such as the mold with stepped parting surface, and the back pressure on the opposite two sides is not equal.
2. Difficult gate stripping
In the process of injection molding, the gate is stuck in the gate sleeve and is not easy to come out. When opening the mold, the product is cracked and damaged. In addition, the operator must knock out the tip of the copper rod from the nozzle to loosen it before demoulding, which seriously affects the production efficiency.
The main reason for this failure is the poor finish of the gate taper hole and the knife mark in the circumferential direction of the inner hole. Secondly, the material is too soft, the small end of the conical hole is deformed or damaged after use for a period of time, and the spherical radian of the nozzle is too small, resulting in the rivet head of the gate material here. The taper hole of gate sleeve is difficult to process, so standard parts shall be used as far as possible. If it needs to be processed by ourselves, special reamers shall also be made or purchased. The taper hole shall be ground to Ra0 Less than 4.
In addition, a gate pull rod or gate ejection mechanism must be set.
3. Dynamic and fixed mold offset
For large molds, due to different filling rates in all directions and the influence of die self weight during die installation, there are dynamic and fixed die offsets.
In these cases, the lateral offset force will be added to the guide column during injection. During mold opening, the surface of the guide column will be roughened and damaged. In serious cases, the guide column will be bent or cut off, or even unable to open the mold.
In order to solve the above problems, a high-strength positioning key is added on the parting surface of the die, one on each side, and the most simple and effective is to use the cylindrical key. The perpendicularity between the guide post hole and the parting surface is very important.
During processing, the moving and fixed die are aligned and clamped, and then the boring is completed on the boring machine at one time, so as to ensure the concentricity of the moving and fixed die holes and minimize the perpendicularity error. In addition, the heat treatment hardness of guide post and guide sleeve must meet the design requirements.
4. Moving formwork bending
When the mold is injected, the molten plastic in the mold cavity produces a huge back pressure, generally 600 ~ 1000 kg / cm2. Sometimes mold manufacturers do not pay attention to this problem, often change the original design size, or replace the moving template with low-strength steel plate. In the mold with ejector rod, the template is bent down during injection due to the large span of seats on both sides.
Therefore, the movable formwork must be made of high-quality steel with sufficient thickness. A3 and other low-strength steel plates shall not be used. If necessary, support columns or blocks shall be set under the movable formwork to reduce the thickness of the formwork and improve the bearing capacity.
5. The ejector rod is bent, broken or leaked
The self-made ejector rod is of good quality, but the processing cost is too high. Now standard parts are usually selected, and the quality is average. If the gap between the ejector rod and the hole is too large, there will be material leakage, but if the gap is too small, the ejector rod will expand and jam due to the increase of mold temperature during injection. What's more dangerous is that sometimes the ejector rod can't move and break when it is pushed out for a general distance. As a result, the exposed ejector rod can't be reset at the next mold closing, and the die is damaged.
In order to solve this problem, the ejector rod shall be reground, the matching section of 10 ~ 15mm shall be reserved at the front end of the ejector rod, and the middle part shall be ground smaller by 0.2mm. After all ejector rods are assembled, the fit clearance must be strictly checked, generally within 0.05 ~ 0.08mm, to ensure that the whole ejector mechanism can move forward and backward freely.
6. Poor cooling or water leakage
The cooling effect of the mold directly affects the quality and production efficiency of the products, such as poor cooling, large shrinkage of the products, or warping deformation caused by uneven shrinkage. On the other hand, the overall or local overheating of the die makes the die unable to form normally and stop production. In serious cases, the movable parts such as the ejector rod are blocked by thermal expansion and damaged.
The design and processing of the cooling system depend on the product shape. Do not omit this system because of the complex die structure or difficult processing. In particular, the cooling problem must be fully considered for large and medium-sized dies.
7. The length of the guide groove is too small
Some molds are limited by the area of the template and the length of the guide groove is too small. After the core pulling action is completed, the slider is exposed outside the guide groove. In this way, the slider is easy to tilt in the post core pulling stage and the initial stage of mold closing and resetting. Especially during mold closing, the slider is not reset smoothly, resulting in damage to the slider and even bending damage.
According to experience, after the slider completes the core pulling action, the length left in the chute shall not be less than 2 / 3 of the total length of the guide groove.
8. Failure of fixed distance tensioning mechanism
Fixed distance tensioning mechanisms such as swing hook and buckle are generally used in fixed mold core pulling or some secondary demoulding molds. Because these mechanisms are set in pairs on both sides of the mold, their actions must be synchronized, that is, the mold is buckled at the same time, and the mold is decoupled at the same time when it is opened to a certain position.
Once the synchronization is lost, it is bound to cause the formwork of the drawn die to be skewed and damaged. The parts of these mechanisms should have high stiffness and wear resistance. It is also difficult to adjust. The service life of the mechanism is short. Try to avoid using it and use other mechanisms instead. When the core pulling force is relatively small, the method of spring pushing out the fixed die can be adopted. When the core pulling force is relatively large, the structure of core sliding when the moving die retreats, first completing the core pulling action and then dividing the die can be adopted. The core pulling of hydraulic cylinder can be adopted on large dies.
9. The inclined pin sliding block core pulling mechanism is damaged.
Most of the common problems of this mechanism are that the processing is not in place and the material is too small. There are mainly the following two problems:
The inclination angle of oblique pin a is large;
The advantage is that it can produce a large core pulling distance in a short mold opening stroke.
However, if the inclination angle a is too large, when the pulling force F is a certain value, the bending force P = f / cosa of the inclined pin in the core pulling process is also larger, which is prone to deformation of the inclined pin and wear of the inclined hole.
At the same time, the upward thrust n = ftga generated by the inclined pin on the slider is also greater, which increases the positive pressure of the slider on the guide surface in the guide groove, thus increasing the friction resistance when the slider slides. It is easy to cause unsmooth sliding and wear of guide groove. According to experience, the inclination A shall not be greater than 25 °.
