Nine kinds of mold failure solutions!

Time:2020-07-28 08:19:20 / Popularity: / Source:

1. Difficulty in gate stripping

During injection molding process, gate is stuck in sprue sleeve and is not easy to come out. When mold is opened, product is cracked and damaged. In addition, operator must knock out tip of copper rod from nozzle to loosen it before demolding, which seriously affects production efficiency.
Main reason for this failure is poor finish of gate taper hole, and there are knife marks in circumferential direction of inner hole. Second is that material is too soft, small end of taper hole is deformed or damaged after a period of use, and curvature of nozzle spherical surface is too small, resulting in riveting head of gate material. Taper hole of sprue sleeve is more difficult to process, so standard parts should be used as much as possible. If you need to process it yourself, you should also make your own or buy a special reamer. Taper hole needs to be ground to Ra0.4 or more. In addition, a gate pull rod or gate ejection must be provided.
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2. Large mold moving and fixed mold offset

Large-scale molds have different dynamic and fixed mold deviations due to different filling rates in each direction and influence of mold's own weight during mold assembly. In above cases, lateral offset force will be added to guide post during injection, surface of guide post will be roughened and damaged when mold is opened. In severe cases, guide post may be bent or cut off, mold cannot even be opened.
In order to solve above problems, a high-strength positioning key is added on each of four sides of mold parting surface. The most simple and effective way is to use a cylindrical key. Verticality of guide pin hole and mold parting surface is very important. During processing, moving and fixed molds are aligned and clamped, then boring on boring machine at one time. This can ensure concentricity of moving and fixed mold holes, minimize verticality error. In addition, heat treatment hardness of guide post and guide sleeve must meet design requirements.

3. Guide post injury

Guide post mainly plays a guiding role in mold to ensure that molding surfaces of core and cavity do not collide with each other under any circumstances. Guide post cannot be used as a force-receiving part or a positioning part.
In several cases, moving and fixed molds will produce huge lateral deflection force during injection. When wall thickness of plastic part is not uniform, material flow through thick wall has a large speed, and a large pressure is generated here; side of plastic part is asymmetric, such as back pressure on opposite sides of mold with a stepped mold parting surface not equal.

4. Movable mold plate bending

When mold is being injected, molten plastic in cavity produces a huge back pressure, generally 600-1000 kg/cm. Mold makers sometimes do not pay attention to this problem, and often change original design size, or replace movable mold plate with low-strength steel plate. In mold with ejector pin, mold plate bends down during injection due to large span between two sides of seat. Therefore, movable mold plate must be made of high-quality steel, with sufficient thickness, and low-strength steel plates such as A3 should not be used. When necessary, support columns or support blocks should be set under movable mold plate to reduce thickness of formwork and increase bearing capacity.

5. Ejector rod is bent, broken or leaking

Self-made ejector rods are of better quality, but processing cost is too high. Nowadays, standard parts are generally used, and quality is worse. If gap between ejector pin and hole is too large, material leakage will occur, but if gap is too small, ejector pin will become stuck due to increase in mold temperature during injection.
What's more dangerous is that sometimes ejector rod will not move and break when it is ejected half of distance. As a result, exposed ejector rod cannot be reset during next mold clamping and will damage cavity mold. In order to solve this problem, ejector rod was re-ground, leaving a 10-15 mm mating section at the front end of ejector rod, and middle part was ground down by 0.2 mm. After all ejector rods are assembled, fit clearance must be strictly checked, generally within 0.05 to 0.08 mm, to ensure that entire ejector mechanism can advance and retreat freely.

6. Poor cooling or water leakage

Cooling effect of mold directly affects quality and production efficiency of product, such as poor cooling, large product shrinkage, or uneven shrinkage, resulting in warping deformation and other defects. On the other hand, the whole or part of mold is overheated, so that mold cannot be formed normally and production is stopped. In severe cases, movable parts such as ejector rod will be thermally expanded and become damaged. Design and processing of cooling system depends on shape of product. Don't omit this system because of complex structure of mold or processing difficulties. Especially for large and medium-sized molds, cooling problem must be fully considered.

7. Slider is tilted and reset is not smooth

injection molding process 
Some molds are limited by area of mold plate and guide groove length is too small. Slider will be exposed outside guide groove after core pulling action is completed. This will easily cause slider block to tilt during core pulling stage and initial stage of mold closing and resetting. Especially during mold clamping, sliding block is not reset smoothly, which damages sliding block and even breaks under bending. According to experience, after slider completes core pulling action, length left in chute should not be less than 2/3 of full length of guide groove.

8. Failure of fixed distance tensioning mechanism

Fixed-pitch tensioning mechanisms such as swing hooks and buckles are generally used in fixed mold core pulling or some secondary demolding molds. Because such mechanisms are set in pairs on both sides of mold, their actions must be synchronized, that is, mold is closed at the same time, mold is opened to a certain position and released at the same time. Once synchronization is lost, it will inevitably cause mold plate of drawn die to be skewed and damaged. Parts of these mechanisms must have higher rigidity and wear resistance, adjustments are also difficult, mechanism life is short, try to avoid using them, and other mechanisms can be used instead.
When core pulling force is relatively small, spring can be used to push out fixed mold. When core pulling force is relatively large, core sliding when movable mold is retracted can be used,. Core pulling action is completed first and then mold is divided. Hydraulic cylinder can be used for core-pulling on large molds. Oblique pin slider core pulling mechanism is damaged. The most common problems of this kind of organization are that processing is not in place and materials used are too small, mainly due to following two problems.
Oblique pin inclination angle A is large, and advantage is that it can produce a larger core pulling distance in a shorter mold opening stroke. However, if inclination angle A is too large, when extraction force F is a certain value, bending force P=F/COSA of inclined pin during core pulling process is also larger, inclined pin deformation and inclined hole wear are prone to occur. At the same time, upward thrust N=FTGA generated by inclined pin on the slider is also greater. This force increases positive pressure of slider on guide surface in guide groove, thereby increasing frictional resistance of slider when sliding. Easy to cause uneven sliding and wear of guide groove. According to experience, inclination angle A should not be greater than 25.

9. Poor exhaust in injection mold

Gas is often generated in injection molds. What is cause? Air in gating system and mold cavity; some raw materials contain moisture that has not been removed by drying, they will vaporize into water vapor at high temperatures; due to high temperature during injection molding, certain unstable plastics will decompose and produce gas; certain additives in plastic raw materials volatilize or generate gas from chemical reactions with each other.
At the same time, cause of poor exhaust also needs to be found out as soon as possible. Poor venting of injection mold will bring a series of hazards to quality of plastic parts and many other aspects. Main performance is: melt will replace gas in cavity during injection molding process. If gas is not discharged in time, it will be difficult to fill melt, resulting in insufficient injection volume and unable to fill cavity; Unsmooth gas will form high pressure in cavity and penetrate into plastic under a certain degree of compression, causing quality defects such as cavities, pores, sparse tissue and silver streaks, etc.
Because gas is highly compressed, temperature in cavity rises sharply, which in turn causes surrounding melt to decompose and burn, causing local carbonization and scorching of plastic parts. It mainly appears at confluence of two melts and flange of gate; Unsmooth removal gas makes speed of melt entering each cavity different, so it is easy to form flow marks and fusion marks, reduce mechanical properties of plastic parts; due to obstruction of gas in cavity, filling speed will be reduced, molding cycle will be affected, and production efficiency will be reduced.
Distribution of bubbles in plastic parts, bubbles generated by air trapped in mold cavity, are often distributed on position opposite to gate; bubbles generated by decomposition or chemical reactions in plastic raw materials are distributed along thickness of plastic parts; air bubbles produced by gasification of water remaining in plastic raw materials are irregularly distributed throughout plastic parts.
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