Which kinds of mold problems can be avoided during manufacturing?

Time:2019-10-30 09:34:22 / Popularity: / Source:

plastic mold 

Pilot column damage

Guide post mainly plays a guiding role in the mold to ensure that molding surfaces of core and cavity do not touch each other under any circumstances, guide post cannot be used as force receiving member or positioning member.
In following two cases, movement and setting of mold will produce a huge lateral displacement force:
When wall thickness of plastic part is not uniform, velocity of material flowing through thick wall is large, and a large pressure is generated therein;
Side of plastic part is asymmetrical. For mold with a stepped parting surface, opposing pressures on the opposite sides are not equal.

Gate is difficult to remove

During injection molding process, gate is stuck in sprue bushing and is not easily peeled off. When mold is opened, product is cracked. In addition, operator must use tip of copper rod to knock out from nozzle, so that it can be released after being loosened, which seriously affects production efficiency.
Main cause of this failure is poor smoothness of gate cone and presence of tool marks in circumferential direction of bore. Secondly, material is too soft. After a period of use, small end of tapered hole is deformed or damaged, spherical curvature of nozzle is too small, so that gate material produces a rivet head here. Taper hole of sprue sleeve is difficult to process. Standard parts should be used as much as possible. If you need to process it yourself, you should also make a special reamer. Taper hole needs to be ground to below Ra0.4.
In addition, a gate pull rod or gate ejection mechanism must be provided.

Dynamic and fixed mold offset

Large molds are subject to dynamic and fixed mold offset due to different filling rates and influence of mold weight during mold loading.
In these cases, lateral offset force will be applied to guide post during injection. When mold is opened, surface of guide post will be pulled and damaged. In severe cases, guide post will be bent or cut, and even mold cannot be opened.
In order to solve above problem, a high-strength positioning key is added on each side of the mold parting surface, the most convenient and effective one is to use a cylindrical key. Perpendicularity of guide post hole to parting surface is critical.
In processing, it is clamped in the position of dynamic and fixed molds, and it is finished once on trampoline. This can ensure concentricity of moving, fixed hole and minimize vertical error. In addition, heat treatment hardness of guide post and guide sleeve must meet design requirements.

Moving mold plate bending

When mold is injected, molten plastic in the cavity generates a huge back pressure, generally 600 to 1000 kg/cm2. Moldmakers sometimes do not pay attention to this problem, often change original design size, or replace moving mold plate with a low-strength steel plate. In the mold with top bar topping, template is bent down during injection due to large span of two sides.
Moving mold plate must be made of high-quality steel. It must have sufficient thickness. It is not allowed to use low-strength steel plates such as A3. If necessary, support columns or support blocks should be placed under moving plate to reduce thickness of template and improve carrying capacity.

Top rod is bent, broken or leaking

Quality of self-made ejector is better, but processing cost is too high. Nowadays standard parts are usually used, quality is generally good. If gap between ejector pin and hole is too large, leakage occurs; but if gap is too small, ejector pin expands and is stuck due to an increase in mold temperature during injection. More dangerously, sometimes ejector pin is pushed out at a normal distance and then it is broken. As a result, exposed ejector pin cannot be reset and die is broken during next clamping.
In order to solve this problem, ejector rod is re-grinded, a 10-15 mm mating section is retained at the front end of ejector pin, and middle portion is ground by 0.2 mm. All ejector rod must be strictly checked for fit clearance after assembly, generally within 0.05~0.08 mm to ensure that entire ejector mechanism can move forward and backward freely.

Poor cooling or water leakage

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

Guide slot length is too small

Some molds are limited by area of stencil, length of guide groove is too small, slider exposes outside of guide groove after core pulling operation is completed, so that slider tilting is easy to occur in post-core pulling stage and initial stage of mold clamping reset. Especially when mold is closed, slider is not reset smoothly, which causes slider to be damaged or even crushed.
According to experience, after slider is finished, length left in the chute should not be less than 2/3 of total length of guide.

Distance tensioning mechanism fails

Fixed distance tensioning mechanism such as swing hook and buckle is generally used in fixed mold core or mold of some secondary demoulding. Since mechanism is arranged in pairs on both sides of mold, action requirements must be synchronized, that is mold is clamped at the same time, mold is opened to a certain position and unhooked at the same time.
Once synchronization is lost, it will inevitably cause template of pulled mold to be skewed and damaged. Parts of these mechanisms have higher rigidity and wear resistance, adjustment is also difficult, life of mechanism is short, and it is avoided as much as possible. Other institutions can be used instead. In the case where core pulling force is relatively small, spring can be used to push fixed mold. When core pulling force is relatively large, structure in which core slides when movable mold is retracted, core splitting operation is performed after split mold is used. Hydraulic cylinders can be used for core punching on large molds.

Skew pin type core pulling mechanism is damaged.

Most of problems that occur in such institutions are that processing is not in place and materials used are too small. There are two main problems:
Angle A of oblique pin is large;
Advantage is that a relatively large core distance can be produced in a shorter mold opening stroke.
However, when excessive inclination angle A is adopted, extraction force F is a certain value, bending force P=F/COSA received by oblique pin during core pulling process is also larger, skew pin deformation and oblique hole wear are prone to occur.
At the same time, oblique pin produces an upward thrust on slider N=FTGA, which increases positive pressure of slider against guide surface in the guide groove, thereby increasing frictional resistance when slider slides. It is easy to cause slipping and guide groove is worn out. According to experience, inclination A should not be greater than 25°.
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