1. Design of air extraction holes The design of the air extraction holes for vacuum injection moulding is the key to injection moulding design. The air extraction holes should be located at the last place where the sheet is attached to the mold. For example, when the concave mold is formed, it is around the bottom of the concave mold and where there is a depression. When the punch is formed, it is at the bottom of the punch. The specific situation depends on the shape and size of the plastic
injection mould parts.
For plastic parts with complex contours, the air extraction holes should be concentrated. For large flat plastic parts, the air extraction holes need to be evenly distributed. The hole spacing depends on the size of the plastic part. For small plastic parts, the hole spacing can be selected between 20 and 30mm. For large plastic parts, the distance should be increased appropriately.
Generally, the molded plastic has good fluidity and the molding temperature is high, so the air extraction holes will be smaller; if the thickness of the bad material sheet is large, the air extraction holes will be larger; if the thickness of the blank sheet is small, the air extraction holes will be smaller. In short, the requirement for the size of the air extraction holes is that the air between the blank and the mold forming surface can be extracted in a short time without leaving traces of the air extraction holes on the plastic parts.
Generally, the diameter of the air extraction holes is 0.5~1mm, and the maximum air extraction hole diameter should not exceed 50% of the sheet thickness. However, for plates smaller than 0.2mm, too small air extraction holes cannot be processed.
2. Cavity size The cavity size of the vacuum forming mold should also consider the shrinkage rate of the plastic, and its calculation method is the same as the injection mold cavity size calculation. About 50% of the shrinkage of vacuum molded plastic parts occurs after the plastic part is demoulded, 25% occurs when the plastic part is kept at room temperature for 1 hour after demoulding, and the remaining 25% occurs within the next 8 to 24 hours. of.
Plastic parts formed with concave molds shrink 25% to 50% more than plastic parts formed with male molds. There are many factors that affect the dimensional accuracy of plastic parts. In addition to reducing the dimensional accuracy of the cavity, they are also related to the molding temperature, mold temperature and plastic part type. Therefore, it is difficult to accurately determine the shrinkage rate in advance.
If the production batch is relatively large and the dimensional accuracy requirements are high, it is best to use gypsum to make a mold to trial-produce the product and measure its shrinkage. The above is the basis for designing the mold cavity.
3. Cavity surface roughness Generally, vacuum molding molds do not have an ejection device, and they are demoulded by compressed air after molding. When the surface roughness of the vacuum forming mold is too low, it is very detrimental to demoulding after vacuum molding. Plastic parts tend to adhere to the molding surface and are difficult to demould. Even if there is an ejection device to eject, it is still easy to demould after demoulding. Deformation. Therefore, the surface roughness of the vacuum forming mold is relatively high. After its surface is processed, it is best to carry out sandblasting.
4. Edge sealing device During vacuum molding, in order to prevent the air outside the mold cavity from entering the vacuum chamber, a sealing device must be installed at the edge where the plastic sheet contacts the mold. For straight parting surfaces, it is easier to seal the contact surface between the plastic sheet and the mold, but for curved or folded parting surfaces, sealing is difficult.
5. Heating and cooling devices The heating of plastic sheets used in vacuum molding usually uses resistance wire or infrared rays. The temperature of the resistance wire can reach 350°C ~ 450°C. The different molding temperatures required for different plastic sheets are generally achieved by adjusting the distance between the heater and the sheet. The distance usually used is 80~120mm.
Mold temperature has an impact on the quality and productivity of plastic parts. If the mold temperature is too low, cold spots or stress will occur as soon as the plastic plate comes into contact with the mold cavity, resulting in cracks; if the mold temperature is too high, the plastic sheet may adhere to the mold cavity, and the plastic will deform when it is demoulded, and The production cycle is extended.
Therefore, the mold temperature should be controlled within a certain range, generally around 50°C. Mold temperature control generally relies on natural cooling after contact between the plastic and the mold, adding air cooling devices to accelerate cooling and water cooling. Opening cooling water channels in the mold is the most effective and common method to control the mold temperature. The cooling water channels should be far away from each other. The surface should be above 8mm to avoid cold spots.
There are different methods for opening cooling water channels. You can cast copper pipes or steel pipes into the mold, or you can drill holes or mill slots in the mold. The method of milling slots must use sealing components and add a cover plate.
