1. Design of air suction hole

The design of the suction hole for vacuum forming is the key to the mold design. The suction hole should be located in the place where the sheet is stuck, such as around the bottom of the concave mold and the recessed place when the concave mold is formed, around the bottom of the convex mold when the convex mold is formed, etc. The specific situation depends on the shape and size of the molded plastic part.

For plastic parts with complex contours, the suction holes should be concentrated, and for large planar plastic parts, the suction holes need to be evenly distributed. The hole spacing can be determined by the size of the plastic parts. For small plastic parts, the hole spacing can be selected from 20 to 30mm, and large plastic parts should be appropriately increased.

Generally, the molding plastic has good fluidity and high molding temperature, so the suction hole is smaller. The thickness of the bad material plate is large, the suction hole is larger; The thickness of the blank plate is small, and the suction hole is smaller. In short, the requirement for the size of the suction hole is not only to extract the air between the blank and the mold forming surface in a short time, but also not to leave traces of suction holes on the plastic parts.

Generally, the diameter of the suction hole is 0.5~1mm, and the diameter of the larger suction hole should not exceed 50% of the thickness of the sheet, but for plates less than 0.2mm, too small suction 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 calculation of the cavity size of the injection model. About 50% of the shrinkage of vacuum molded plastic parts is produced after demoulding, 25% is produced within 1h at room temperature after demoulding, and the remaining 25% is produced within 8~24h later.

The shrinkage of plastic parts molded by concave mold is 25% ~ 50% larger than that of plastic parts molded by convex mold. There are many factors that affect the dimensional accuracy of plastic parts, which reduce the dimensional accuracy of the cavity, but also related to the molding temperature, mold temperature and plastic parts varieties, so it is very difficult to determine the shrinkage rate in advance.

If the production batch is relatively large and the dimensional accuracy requirements are high, it is better to use gypsum to manufacture the mold and test the product, and measure its shrinkage rate. The above is the basis for designing the mold cavity.

3. Cavity surface roughness

Generally, there is no ejection device for the vacuum forming mold, and the mold is demoulded by compressed air after molding. When the surface roughness of the vacuum forming mold is too low, it is very unfavorable to the demoulding after vacuum forming. The plastic part is easy to adhere to the forming surface of the mold and is not easy to demoulding. Even if the ejection device can be ejected, it is still easy to deform after demoulding. Therefore, the surface roughness of the vacuum forming mold is high. After its surface processing, it is better to carry out sand blasting treatment.

4. Edge sealing device

During vacuum forming, in order to prevent the air outside the cavity from entering the vacuum chamber, a sealing device is provided at the edge where the plastic sheet contacts the mold. For the flat parting surface, it is easier to seal the plastic sheet with the mold contact surface, while for the curved surface or the folding surface parting surface, the sealing is difficult.

5. Heating and cooling device

The heating of the plastic sheet used in vacuum forming usually uses resistance wire or infrared rays. The temperature of the resistance wire can reach 350 ℃ ~ 450 ℃. For different molding temperatures required for different plastic plates, it is generally realized by adjusting the distance between the heater and the plate. The distance usually used is 80 to 120mm.

Mold temperature has an impact on the quality and productivity of plastic parts. If the mold temperature is too low, a contact between the plastic plate and the cavity will produce cold spots or stress resulting in cracks; and when the mold temperature is too high, the plastic sheet may adhere to the cavity, the plastic will deform during demoulding, and the production cycle is prolonged.

Therefore, the mold temperature should be controlled within a certain range, generally around 50 ℃. The control of mold temperature generally depends on natural cooling after contact between plastic and mold, accelerated cooling by adding air cooling device and water cooling. Opening cooling water channel in mold is a more effective and common method to control mold temperature. The cooling water channel should be more than 8mm away from the mold surface to avoid cold spots.

There are different methods for the opening of the cooling water channel. Copper pipes or steel pipes can be cast into the mold, and the mold can also be drilled or milled. The method of milling grooves must use sealing elements and add cover plates.