Crystalline engineering plastics such as POM, Pa, PBT, and PET tend to be much more warped than non-crystalline ones, and improvements should be considered when designing molds and forming. If this is not done, it will be almost impossible to correct in the subsequent injection molding stage.
1. What is the main reason for warping?
The shrinkage of crystalline engineering plastics is relatively high and is affected by many factors. Such as demoulding too fast, mold temperature is too high, mold temperature is not uniform and the runner system is asymmetrical.
For crystalline engineering plastic materials, warpage is greatly affected by wall thickness and mold temperature. The wall thickness of the workpiece is not uniform or the corner is not smooth, and the workpiece cannot be warped and deformed by the average cooling shrinkage during molding. In order to make the surface beautiful, some runners are placed on the corners of the product. When molding, the melt can only be injected into the cavity from one end at a high speed. The solidified plastics are straightened and aligned in the same direction. When the mold is released, the raw materials are pulled. Back to the original state to form a warp. Inappropriate mold temperatures can also cause warpage.
Due to the directionality of the glass fiber, the glass fiber reinforced engineering plastic exhibits completely different shrinkage characteristics during injection molding, and the shrinkage effect caused by the difference in wall thickness is relatively slight. Therefore, the main cause of warpage is the difference in shrinkage of the reinforcing fibers flowing in the horizontal direction and in the vertical direction.
From the mold gate analysis, the design of the mold should ensure that the plastic can smoothly enter the cavity, the split runner should avoid the use of right-angle turning, and the turning point should adopt the curved transition zone. The short and thick split runner is ideal for reducing fluid resistance and fluid orientation. In order to avoid the tightness of the melt filler, it is difficult to demould and cause deformation. The cross-sectional shape and size of the runner should be based on the amount of glue. The shape of the product changes. After the product is more difficult to form part of the runner, the main channel is correspondingly enlarged, so that the cross-sectional area of the main channel is equal to the sum of the cross-section of the runner.
In summary, the factors affecting the warpage deformation are the wall thickness distribution, the gate position, the flow inhibition, the runner, and the rigidity of the molded article. These different causes lead to warpage of the injection molded article. Depending on whether the material contains reinforcing fibers, the same part often produces opposite warpage.
2. How to prevent warpage?
When injection molding of crystalline engineering plastics, the wall thickness of the parts is required to be uniform, and the accumulation of melted glue is avoided as much as possible. Multi-point gates are used to meet high pressure injection molding to minimize shrinkage differences, and mold temperature control systems are designed to minimize heat dissipation.
For glass fiber reinforced plastics, the symmetry of the structure of the shaped article is as important as the uniform wall thickness. Asymmetrical parts impede the flow of the melt and create a deflection that eventually causes warpage. Asymmetrical parts must be balanced by combining hidden cores during the mold planning and design stages. The location of the gate is also important, and each split and weld line is a potential cause of warpage.
In order to make the molten plastic fill the cavity smoothly, the design should avoid the following points:
(1) The thickness difference between the same parts is too large;
(2) Excessive acute angles;
(3) The buffer zone is too short, which makes the difference between thickness and thickness very different.
3. Process measures to prevent warpage during injection molding
Assuming that the molded article, gate, and mold have been properly designed, the warpage of the article can be controlled by adjusting the holding pressure and the mold temperature during injection molding. The mold can also be used with most thermal loop systems to complete heat dispersion.
For reinforced engineering plastics, changing the injection rate or lowering the mold temperature slightly improves the warpage. If possible subsequent warpage is not foreseen during the mold making and molding design phase, subsequent corrections cannot be made by changing the molding process conditions.
4. What can I do when warping has occurred?
For glass fiber reinforced plastics, mold filling research, that is, partial filling of the mold in stages, by studying the front end contour of the melt flow, understanding the fiber direction reconstruction, referring to the shrinkage curve of the reinforcing material, corresponding measures can be taken to reduce the warpage deformation. , for example, with flow aids or obstructions. These can change the flow front profile of the melt, thereby affecting warpage.
This approach requires a lot of practical experience, and increasing the amount of knowledge can provide preventive measures in the future. For crystalline engineering plastics, it is not possible to obtain a flat part like a non-crystalline raw material.
The use of low warpage, semi-crystalline polymers is advocated to improve the warpage of the article due to chemical adjustment or combination of different reinforcing materials. The most expensive method of warping improvement is to modify the mold. If the molding experience is rich, the modification of the mold insert can improve the warpage deformation of the precision parts, and the shaping fixture can also be used for warpage correction when producing the injection molded parts.
