• 2023-10-16
• 619
• 5 Comments

Five key mold design points

1. Design of suction holes

The design of the suction hole of the vacuum molding is the key to the design of the mold, the suction hole should be located in the final place of the sheet mold, such as the bottom of the concave mold and the concave place, the punch molding in the bottom of the punch around the mold, etc., depending on the shape and size of the molded plastic parts.

For plastic parts with complex contours, the extraction holes should be concentrated, and for large flat plastic parts, the extraction 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 the large plastic parts should be appropriately increased.

Usually the molding plastic fluidity is good, the molding temperature is high, then the suction hole is smaller; If the thickness of the bad material plate is large, the suction hole is larger; The thickness of the billet sheet is small, and the porosity is smaller. In short, the requirement for the size of the suction hole is that the air can be extracted between the blank and the mold forming surface in a short time, and no trace of the suction hole is left on the plastic part.

Generally, the diameter of the extraction hole is 0.5 ~ 1mm, and the maximum extraction hole diameter size does not exceed 50% of the sheet thickness, but for sheets less than 0.2mm, too small extraction holes can not be processed.

2. Cavity size

The cavity size of the vacuum forming die should also consider the shrinkage rate of the plastic, and its calculation method is the same as that of the injection model cavity size. About 50% of the shrinkage of the vacuum molded plastic parts is generated after the release of the plastic parts, 25% is generated within 1h after the release at room temperature, and the remaining 25% is generated in the following 8-24h.

The shrinkage of the plastic parts formed by the concave die is 25% to 50% greater than that of the plastic parts formed by the punch die. There are many factors that affect 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 varieties, so it is difficult to accurately determine the shrinkage rate in advance.

If the production batch is relatively large, the dimensional accuracy requirements are high, it is best to first use gypsum to make molds to produce products, measured its shrinkage rate, the above is the basis for the design of mold cavity.

3. Cavity surface roughness

The general vacuum molding mold has no ejector device, and the mold is demoulded by compressed air after forming. When the surface roughness of the vacuum molding mold is too low, it is very unfavorable to the release after the vacuum molding, and the plastic parts are easy to adhere to the molding surface of the mold and are not easy to release, even if the ejecting device can be ejecting, it is still easy to deform after the release. Therefore, the surface roughness of the vacuum forming die is higher. After its surface processing, it is best to carry out sandblasting treatment.

4. Edge sealing device

During vacuum molding, in order to prevent the air outside the cavity from entering the vacuum chamber, a sealing device should be arranged on the edge of the plastic sheet in contact with the mold. For the flat parting surface, it is easier 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 devices

The heating of plastic sheets used in vacuum forming is usually made of resistance wire or infrared ray. The temperature of the resistance wire can reach 350 ° C ~ 450 ° C, and 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 effect on the quality and productivity of plastic parts. If the mold temperature is too low, the contact between the plastic plate and the mold cavity will produce cold spots or stress resulting in cracks; When the mold temperature is too high, the plastic sheet may adhere to the mold cavity, and the plastic will be deformed when the mold is removed, and the production cycle is prolonged.

Therefore, the mold temperature should be controlled within a certain range, generally around 50 ° C. The control of mold temperature generally relies on the natural cooling after the contact of plastic and mold, the addition of air cooling device to accelerate cooling and water cooling methods, opening a cooling water channel in the mold is the most effective and most commonly used method to control the temperature of the mold, and the cooling water channel should be more than 8mm from the surface of the mold to avoid cold spots.

There are different ways to open the cooling channel, you can cast copper pipe or steel pipe into the mold, you can also punch holes in the mold or mill grooves, with the method of milling grooves must use sealing elements and cover plate.