5 key points for mold design
1. Design of air extraction hole
The design of air extraction hole for vacuum forming is the key of die design. The air extraction hole should be located at the last place where the sheet is pasted to the die, such as around the bottom of the die and where there is a depression during die forming, around the bottom of the punch during punch forming, etc. the specific situation depends on the shape and size of the formed plastic part.For plastic parts with complex contour, the air extraction holes shall be concentrated, and for large plane plastic parts, the air extraction holes shall be evenly distributed. The hole spacing can be determined according to the size of plastic parts. For small plastic parts, the hole spacing can be selected from 20 ~ 30mm, and for large plastic parts, the distance should be increased appropriately.
Generally, if the molding plastic has good fluidity and high molding temperature, the suction hole is smaller; If the thickness of bad material plate is large, the suction hole is larger; The thickness of the blank plate is small, and the air extraction hole is smaller. In short, the requirement for the size of the air extraction 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 the trace of the air extraction hole on the plastic part.
Generally, the diameter of the air extraction hole is 0.5 ~ 1mm, and the maximum diameter of the air extraction hole should not exceed 50% of the sheet thickness. However, for plates less than 0.2mm, too small air extraction holes cannot be processed.
2. Cavity size
The shrinkage of plastic should also be considered in the cavity size of vacuum forming mold, and its calculation method is the same as that of injection mold. About 50% of the shrinkage of vacuum molded plastic parts is generated after demoulding, 25% is generated within 1h at room temperature after demoulding, and the remaining 25% is generated within 8 ~ 24h.
The shrinkage of plastic parts formed by female die is 25% ~ 50% greater than that formed by male die. There are many factors affecting the dimensional accuracy of plastic parts. In addition to reducing the dimensional accuracy of the cavity, it is also related to the molding temperature, mold temperature and plastic parts variety. Therefore, it is very 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 manufacture the mold with gypsum first, trial produce the product, and measure its shrinkage. The above is the basis for designing the mold cavity.
3. Cavity surface roughness
Generally, there is no ejector for vacuum forming molds, which are demoulded by compressed air after forming. When the surface roughness of the vacuum forming die is too low, it is very unfavorable to the demoulding after vacuum forming. The plastic parts are easy to adhere to the forming surface of the die and are not easy to demould. Even if there is an ejector device to eject, it is still easy to deform after demoulding. Therefore, the surface roughness of vacuum forming die is high. After its surface processing, it is best to carry out sand blasting.
4. Edge sealing device
During vacuum forming, in order to prevent the air outside the cavity from entering the vacuum chamber, a sealing device shall be set at the edge of the plastic sheet in contact with the mold. For the straight parting surface, it is easy to seal the contact surface between the plastic sheet and the mold, but for the curved or folded parting surface, it is difficult to seal.
5. Heating and cooling device
The heating of plastic sheet used in vacuum forming usually adopts resistance wire or infrared ray. The resistance wire temperature can reach 350 ℃ ~ 450 ℃. For different molding temperatures required by different plastic plates, it is generally realized by adjusting the distance between the heater and the plate. The distance usually used is 80 ~ 120mm.
Mold temperature has an effect 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 contacts the mold cavity, resulting in cracks; 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 about 50 ℃. The mold temperature control generally depends on the natural cooling after the contact between the plastic and the mold, the addition of air cooling device to accelerate cooling and water cooling. Opening a cooling water channel in the mold is the most effective and common method to control the mold temperature. The cooling water channel shall be more than 8mm away from the mold surface to avoid cold spots.
There are different ways to open the cooling water channel. The copper pipe or steel pipe can be cast into the mold, or the mold can be drilled or milled. The groove milling method must use the sealing element and add the cover plate.
