/Common troubleshooting in plastic injection molds

Common troubleshooting in plastic injection molds

Structural form of plastic injection molds and quality of plastic molding services directly affect quality and production efficiency of plastic parts. The most common mold failures and their main reasons are excluded as follows

Difficult to remove gate

During plastic injection molds process, gate sticks to sprue bushing and is not easily peeled off. When mold is opened, product is cracked. In addition, plastic molding manufacturer to provide plastic molding services 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. The main cause of this failure is poor smoothness of gate cone and presence of tool marks in the circumferential direction of bore. Secondly, material is too soft. After a period of use, small end of tapered hole is deformed or damaged, and spherical curvature of nozzle is too small, so that gate material produces a rivet head here. Taper hole of sprue bushing 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. Tapered hole needs to be ground to Ra0.4 or higher. In addition, a gate pull rod or gate ejection mechanism must be provided.

Damaged guide column

Guide post mainly plays a guiding role in plastic injection molds to ensure that molding surfaces of core and cavity do not touch each other under any circumstances, and guide post cannot be used as force receiving member or positioning member. In the following cases, dynamic and fixed mode will produce a huge lateral offset force during injection:

(1) When wall thickness of plastic parts is not uniform, flow rate of material flow through thick wall is large, and a large pressure is generated here; (2) Side of plastic part is asymmetrical, such as mold of stepped parting surface, opposite pressure on opposite sides is not equal.

Large molds are subject to dynamic and fixed mode offset due to different filling rates and influence of mold weight during mold loading

In above cases, lateral offset force will be applied to guide post during injection. When plastic injection molds is opened, surface of guide post will be pulled, damaged, and guide post will be bent or cut in severe cases, and even mold cannot be opened. In order to solve above problem, a high-strength positioning key is added on each side of mold parting surface, and 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 adopted that after fixed mold is clamped at the position, it is finished once on trampoline, which can ensure concentricity of moving and fixed hole and minimize verticality error. In addition, heat treatment hardness of guide post and guide sleeve must meet design requirements.

Moving template is bent

When mold is injected, molten plastic in the cavity generates a huge back pressure, generally 600 to 1000 kg/cm2. Plastic molding manufacturer to provide plastic molding services sometimes do not pay attention to this problem, often change original design size, or replace moving template 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 template must be made of high-quality steel. If it has 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, and now standard parts are generally used with poor quality. 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 at time of injection. More dangerously, sometimes ejector pin is pulled out at a normal distance and 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 was re-ground, and a 10-15 mm mating section was retained at the front end of ejector pin, and middle portion was ground by 0.2 mm. All ejector rods 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 plastic injection molds directly affects quality and production efficiency of product, such as poor cooling, large shrinkage of product, or uneven shrinkage and deformation of warp surface. On the other hand, plastic molds for sale is overheated in whole or in part, so that plastic molds for sale 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.

Distance tensioning mechanism is out of order

Fixed-length tensioning mechanism such as swing hook and buckle is generally used in mold of fixed mold core or some secondary mold release. Since mechanism is arranged in pairs on both sides of mold, action requirements must be synchronized, that is plastic injection molds is clamped at the same time, plastic molds for sale 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 which should avoid to use have higher rigidity and wear resistance, difficult adjustment, maybe other mechanisms can be used. In the case where pumping force is relatively small, spring can be used to push fixed mold. When core pulling force is relatively large, core can be slid when movable mold is retracted, and structure of splitting after core pulling operation is completed. A hydraulic cylinder can be used for core pulling on mold. Skew pin slider core pulling mechanism is damaged. The most common problems in this kind of mechanism are that processing is not in place and material is too small. There are two main problems: inclination angle A of oblique pin is large, and advantage is that a relatively large core distance can be produced in a shorter opening stroke. However, when excessive inclination angle A is adopted and when extraction force F is a certain value, bending force P=F/COSA received by oblique pin during core pulling process is also larger, and skew pin deformation and oblique hole wear are prone to occur. At the same time, oblique pin produces an upward thrust on the slider. N=FTGA is also larger. This force increases positive pressure of slider on guide surface in guide groove, thereby increasing frictional resistance when slider slides. It is easy to cause slippage and guide groove is worn. According to experience, inclination A should not be greater than 25 degree.

Length of mold changes

Some plastic injection molds are limited by area of stencil, length of guide groove is too small, and slider is exposed outside guide groove after core pulling operation is completed, so that slider is inclined in the post-core pulling stage and initial stage of mold clamping reset, especially in mold clamping. When slider is not reset properly, slider is damaged and even bending is broken. According to experience, after slider is finished, length left in chute should not be less than 2/3 of total length of guide.

Finally, when in process of plastic molding development, it should be based on requirements of quality of plastic parts, size of batch, requirements of manufacturing period, etc., which can meet requirements of products, the simplest and most reliable in the mold structure, easy to process, and low cost, this is a plastic injection molds.

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