Air extraction hole design for vacuum forming is the key to mold design. Air extraction hole should be located at place wher sheet is finally attached to mold, such as around bottom of concave mold and recessed place when concave mold is formed, around bottom of convex mold when convex mold is formed. 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 need to be evenly distributed. Hole spacing can be determined by size of plastic part. For small plastic parts, hole spacing can be selected from 20 to 30mm. For large plastic parts, distance should be increased appropriately.
Generally, molded plastic has good fluidity and molding temperature is high, so vent hole is smaller; bad material plate thickness is larger, so vent hole is larger; blank plate thickness is small, so vent hole is smaller. In short, requirement for size of air extraction hole is to extract air between blank and molding surface in a short time without leaving traces of air extraction hole on plastic part. Generally, diameter of suction hole is 0.5 to 1mm, maximum diameter of  suction hole should not exceed 50% of thickness of sheet. However, for plates less than 0.2mm, too small a suction hole cannot be processed.

 

Cavity size of vacuum forming mold should also consider shrinkage rate of plastic, and calculation method is same as that of injection mold cavity size. About 50% of shrinkage of vacuum molded plastic part is produced after plastic part is demolded, 25% is produced within 1 hour after demolding and kept at room temperature, and remaining 25% is produced within 8 to 24 hours.
Plastic parts formed by concave molds shrink by 25% to 50% than those formed by convex molds. There are many factors that affect dimensional accuracy of plastic parts. In addition to dimensional accuracy of cavity, it is also related to molding temperature, mold temperature and type 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 plaster to make a mold to try out product and measure its shrinkage. Above is basis for designing mold cavity.

Generally, there is no ejection device for vacuum forming mold, and it is demoulded by compressed air after forming. When surface roughness of vacuum forming mold is too low, it is very unfavorable for demolding after vacuum forming. Plastic part is easy to adhere to mold forming surface and is not easy to demold. Even with an ejector device that can be ejected, it is still easy to deform after demolding. Therefore, surface roughness of vacuum forming mold is relatively high. After surface is processed, sandblasting is best.

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 contact surface of plastic sheet with mold, while 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℃～450℃, different molding temperature required for different plastic plates is generally achieved by adjusting distance between heater andplate. 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 when plastic plate contacts cavity, causing cracks; When mold temperature is too high, plastic sheet may adhere to cavity, plastic will be deformed during demolding, and production cycle is prolonged.

 

Therefore, mold temperature should be controlled within a certain range, generally around 50°Mold temperature control generally includes natural cooling after plastic is in contact with mold, additional air-cooled devices to accelerate cooling and water cooling, etc. Cooling water channel in mold is the most effective and commonly used method to control mold temperature. Cooling channel should be at least 8mm away from mould surface to avoid cold spots. There are different ways to open cooling water channel. You can cast copper pipes or steel pipes into mold, or punch holes and mill grooves in mold. Method of milling grooves must use sealing elements and cover plates.