1. Design of air extraction hole
The design of the air extraction hole for vacuum forming is the key to the mold design.
The air extraction hole should be located at the last place where the sheet is pasted to the mold, such as around the bottom of the concave mold and where there is a depression when forming the concave mold, around the bottom of the convex mold when forming the convex mold, 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. For large plastic parts, the distance should be increased appropriately.
Generally, the fluidity of molding plastic is good, and the suction hole is smaller when the molding temperature is high;
If the thickness of the 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 that the air between the blank and the mold forming surface can be extracted in a short time without leaving 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 thickness of the sheet.
However, for sheet 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 produced after demolding, 25% is produced within 1h at room temperature after demolding, and the remaining 25% is produced within 8 ~ 24h.
The shrinkage of plastic parts formed by the female die is 25% ~ 50% greater than that formed by male 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 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 demolded by compressed air after forming.
When the surface roughness of the vacuum forming mold is too low, it is very unfavorable to the demolding after vacuum forming.
The plastic parts are easy to adhere to the forming surface of the mold and are not easy to demold.
Even if there is an ejector device to eject, it is still easy to deform after demolding.
Therefore, the surface roughness of the vacuum forming die is high.
After its surface processing, it is best to carry out sandblasting.
4. Edge sealing device
During vacuum forming, in order to prevent the air outside the cavity from entering the vacuum chamber, a sealing device should 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 sheets used in vacuum forming usually adopts resistance wire or infrared ray.
The temperature of the resistance wire 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 impact 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, and the plastic will deform during demolding, which prolongs the production cycle.
Therefore, the mold temperature should be controlled within a certain range, generally around 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 the cooling and water cooling.
Opening the cooling water channel in the mold is the most effective and commonly used method to control the mold temperature.
The cooling water channel should be more than 8mm away from the mold surface to avoid cold spots.
There are different ways to open the cooling water channel.
You can cast the copper pipe or steel pipe into the mold, or punch holes or mill grooves on the mold.
The method of milling grooves must use sealing elements and add cover plates.