Design of air extraction hole for vacuum forming is the key to mold design. Air extraction hole should be located at place where sheet is finally attached to mold. such as around bottom of concave mold and places where there are depressions when concave mold is formed, and around the bottom of punch when punch is formed, etc. Specific situation depends on shape and size of molded plastic part.
For plastic parts with complex contours, exhaust holes should be concentrated, and for large flat plastic parts, exhaust holes should be evenly distributed. Hole spacing depends on size of plastic parts. For small plastic parts, hole spacing can be selected from 20 to 30mm, and for large plastic parts, distance should be appropriately increased.
Usually, molding plastic has good fluidity and high molding temperature, so air holes are smaller; if thickness of bad material is large, air holes are larger; thickness of blank plate is smaller, and air holes are smaller. In short, requirement for size of air extraction hole is that air between blank and molding surface of mold can be extracted in a short time without leaving traces of air extraction hole on plastic part.
Generally, diameter of air extraction hole is 0.5 to 1 mm, maximum diameter of air extraction hole should not exceed 50% of thickness of sheet. However, for sheets smaller than 0.2 mm, too small air extraction holes cannot be processed.

 

Cavity size of vacuum forming mold should also consider shrinkage rate of plastic, calculation method is same as that of injection mold cavity size. About 50% of shrinkage of vacuum-molded plastic parts is generated after plastic parts are demolded, 25% is generated within 1 hour after demolding and kept at room temperature, remaining 25% is generated within next 8 to 24 hours.
Shrinkage of plastic parts formed by concave molds is 25% to 50% larger than that of plastic parts formed by punch molds. There are many factors that affect dimensional accuracy of plastic parts. In addition to reducing dimensional accuracy of cavity, it is also related to molding temperature, mold temperature and variety of plastic parts. Therefore, it is difficult to accurately determine shrinkage rate in advance.
If production batch is relatively large and dimensional accuracy requirements are high, it is best to use gypsum to make a mold to trial-produce product and measure its shrinkage rate. Above is basis for designing mold cavity.

Generally, there is no ejector device in vacuum forming mold, and mold is released by compressed air after forming. When surface roughness of vacuum forming mold is too low, it is very unfavorable for demoulding after vacuum forming, plastic parts are easy to adhere to molding surface of mold and are not easy to be demolded. Even if there is an ejector device that can be ejected, it is still easy to deform after demoulding. Therefore, surface roughness of vacuum forming mold is relatively high. After surface is processed, it is best to carry out sandblasting.

During vacuum forming, in order to prevent air outside cavity from entering vacuum chamber, a sealing device should be provided on the edge of plastic sheet in contact with mold. For flat parting surface, it is easier to seal plastic sheet and contact surface of mold, but for curved or folded parting surface, it is difficult to seal.

Heating of plastic sheet used in vacuum forming usually adopts resistance wire or infrared rays. Temperature of resistance wire can reach 350℃ to 450℃. Different molding temperatures required for different plastic sheets are generally achieved by adjusting distance between heater and sheet. Distance usually used is 80-120mm.
Mold temperature has an impact on quality and productivity of plastic parts. If mold temperature is too low, cold spots or stress will occur as soon as plastic sheet comes into contact with cavity, resulting in cracks; when mold temperature is too high, plastic sheet may adhere to cavity, plastic will deform when it is released from mold, and production cycle will be prolonged.
Therefore, mold temperature should be controlled within a certain range, generally around 50℃. Control of mold temperature generally relies on natural cooling after contact between plastic and mold, adding an air cooling device to accelerate cooling and water cooling. Opening a cooling water channel in mold is the most effective and commonly used method to control temperature of mold. Cooling water channel should be more than 8mm away from the surface of mold to avoid cold spots.
There are different methods for opening of cooling water channel. Copper pipe or steel pipe can be cast into mold, or mold can be punched or grooved. In method of groove milling, a sealing element and a cover plate must be used.