Knowledge monthly report: summary of experience and lessons of mold failure in recent months

The following is the company's engineering department based on the recent months of mold problems summed up by some experience, colleagues please study carefully.
Difficult unloading at gate: in the process of injection molding, the gate is stuck in the cover of gate, not easy to get out. Crack damage occurs when the product is opened. In addition, the operator must use the copper rod tip to knock out from the nozzle, so that it can be loose before the mold can be removed, seriously affecting the production efficiency. This fault is mainly caused by the poor finish of the gate taper hole and the knife marks in the circumference of the inner hole. Also, the material is too soft, the small end of the taper hole deforms or damages after a period of use, and the radius of the nozzle sphere is too small, causing the gate material to produce riveting head here. The tapered hole of the gate sleeve is difficult to be processed. Standard parts should be adopted as far as possible. Tapered holes need to be ground below Ra0.4. In addition, the gate pull rod or gate ejection mechanism must be set.

Guide column damage: guide column plays a leading role in the injection mold to ensure that the molding surface of the mold core and the cavity do not touch each other under any circumstances, and the guide column should not be used as the force or positioning part. Injections in the following cases, the dynamic and fixed mode will produce a great lateral migration force:

(1) the plastic parts wall thickness nonuniformity, the material flow through rate is big, thick wall place here produce larger pressure;
(2) the side of the plastic parts is asymmetric, such as the mold on the terraced parting face, and the opposite two sides are not subject to the same reverse pressure.

Dynamic and fixed mold migration: for large mould, due to different charging rates in different directions, and due to the influence of mold self-weight during mold loading, dynamic and fixed mold migration occurs. In these cases, the lateral offset force will be added to the guide column during injection. The surface of the guide column will be brushed when the die is opened, and the guide column will be damaged. In order to solve the above problems, a high strength locating key is added on the parting surface of the mould. The perpendicularity of guide hole and parting surface is very important. In the process is used to move, fixed mold alignment position clamping, in the boring machine last boring, so that the movement, fixed mold hole concentricity, and the vertical degree error is minimized. In addition, the heat treatment hardness of guide column and guide sleeve must meet the design requirements.

Flexible formwork bending: during injection of injection mold, the molten plastic in the mold cavity generates enormous reverse pressure, usually between 600 ~ 1000 kg/cm 2. Mold makers sometimes don't attach importance to this problem, often change the original design size, or replace with low strength steel plate, the dynamic template in the mould of material with a plunger, because on both sides of the large span, cause the injection template under bending. So the dynamic template must choose high quality steel, must have the enough thickness, must not with A3 steel such as low intensity, when necessary, or support block should be set at the bottom of the dynamic template shores, to reduce the thickness of the template, improve the bearing capacity.

Top bar bending, breaking or leakage: the quality of the homemade top bar is better, that is, the processing cost is too high. Now, the standard parts are usually chosen, and the quality is general. If the gap between the top rod and the hole is too large, material leakage will occur, but if the gap is too small, the top rod will become stuck due to the increase of mold temperature and expansion of the top rod during injection. What is more dangerous is that the top bar is sometimes broken when the top bar is pushed out at a normal distance. As a result, the top bar exposed in this section cannot be reset and the concave die is damaged in the next mold closing. In order to solve this problem, the top rod is resharpened, and the 10-15mm mating segment is retained at the front of the top rod, and the middle part is ground down by 0.2mm. After the assembly of all ejector rods, the mating clearance must be strictly checked, generally within 0.05~0.08 mm, to ensure that the entire ejector mechanism can advance and retreat freely.

Cooling bad or channel leakage: injection mold cooling effect directly affect the quality and production efficiency of products, such as poor cooling, product shrinkage, or uneven contraction become warped surface defects such as deformation occurs. On the other hand, the whole or part of the mold overheat, so that the mold can not be formed normally and stop production, serious so that the top bar and other moving parts thermal expansion die and damage. The design of the cooling system, processing to the product shape, don't cry because it is complicated or difficult to machining mould structure and eliminate the system, especially in large and medium-sized mold must give full consideration to the cooling problem.

Distance taut institutions failed: the pendulum hook, fastener spacing taut institutions is generally used in die core-pulling or some secondary ejection mould, because such institutions in the mold of the two sides set in pairs, its movement request must be synchronous, namely the clamping buckle at the same time, mould to a certain position at the same time. Without synchronization, it is bound to cause pulled mold template skewed and damage, the institutions of the parts to have higher rigidity and wear resistance, adjustment is hard, short life institutions, to avoid as far as possible, you can switch to other institutions. Can be used in the case of relatively small pumping heart spring launch mode method, the core pulling force is larger when the dynamic model can be used under the condition of the back slide cores, finish core-pulling action again after the structure of the die, the hydraulic oil cylinder can be used on large mould core pulling. The inclined pin slider core pulling mechanism is damaged. Such mechanisms are often appear problem mostly processing does not reach the designated position and the material is too small, basically has the following two questions: Angle A large pin, advantage is that can be generated within shorter cavity travel relatively large core-pulling distance. However, if the oversize inclination Angle A is adopted, when the extraction force F is A certain value, the bending force P of the inclined pin in the core pulling process is equal to F/COSA, which is also larger, resulting in inclined pin deformation and inclined hole wear. At the same time, the upward thrust N=FTGA produced by the inclined pin on the slider is also larger, which increases the positive pressure of the slider on the guide face in the guide slot, thus increasing the friction resistance of the slider during sliding. Easy to cause sliding friction, guide wear. According to the experience, dip A should not be greater than 25 °.

Guide groove length is too small, some die because of restricted by template area, the length of the guide groove is too small, the slider on the outside of the core-pulling action after the show guide, so that after the core-pulling and mould reset at the beginning of the phase is likely to cause the slider tilt, especially when clamping, slider reset, make the slider damage, even bending failure. As a rule of thumb, the length of the slider left in the chute should not be less than 2/3 of the length of the guide chute after the core pulling is completed. In design, mold manufacturing, according to the requirement of the quality of plastic parts, the size of the batch, make deadline requirements such as the specific circumstances, can not only meet the requirements of products, in the mold structure and the most convenient and reliable, easy processing, low cost, this is the most perfect mold.