5 Key Considerations for Mold Design

5 Key Considerations for Mold Design

1. Design of Evacuation Holes

The design of evacuation holes in vacuum forming is crucial to mold design. Evacuation holes should be located where the sheet metal finally contacts the mold, such as around the bottom of the die and in recessed areas during die forming, or around the bottom of the punch during punch forming. The specific location depends on the shape and size of the molded part.

For parts with complex contours, evacuation holes should be concentrated. For large, flat parts, evacuation holes need to be evenly distributed. The hole spacing depends on the size of the part. For small parts, a spacing of 20-30mm is suitable, while for large parts, the spacing should be increased.

Generally, for plastics with good flowability and high molding temperatures, smaller evacuation holes are needed; for thicker sheet metal, larger evacuation holes are needed; and for thinner sheet metal, smaller evacuation holes are needed. In short, the requirement for evacuation hole size is to allow air to be extracted from between the sheet metal and the mold forming surface in a short time without leaving any trace of the evacuation holes on the part.

The diameter of a typical evacuation hole is 0.5–1 mm. It is advisable that the maximum evacuation hole diameter should not exceed 50% of the sheet thickness. However, for sheets smaller than 0.2 mm, excessively small evacuation holes cannot be processed.

2. Cavity Dimensions The cavity dimensions of vacuum forming molds should also consider the shrinkage rate of the plastic. The calculation method is the same as for injection mold cavity dimensions. Approximately 50% of the shrinkage in vacuum-formed plastic parts occurs after demolding, 25% occurs within 1 hour of demolding at room temperature, and the remaining 25% occurs within the subsequent 8–24 hours.

Plastic parts formed using a concave mold shrink by 25%–50% more than those formed using a convex mold. Many factors affect the dimensional accuracy of plastic parts. Besides reducing cavity dimensional accuracy, factors such as molding temperature, mold temperature, and the type of plastic part also play a role. Therefore, accurately determining the shrinkage rate in advance is very difficult.

If the production batch is large and the dimensional accuracy requirements are high, it is best to first create a trial product using a plaster mold to measure its shrinkage rate. This will serve as the basis for designing the mold cavity.

3. Cavity Surface Roughness
Generally, vacuum forming molds do not have ejector devices; demolding is achieved using compressed air after forming. If the surface roughness of the vacuum forming mold is too low, it is very detrimental to demolding after vacuum forming. The plastic part is prone to adhering to the mold surface and is difficult to demold. Even with an ejector device, it is still prone to deformation after demolding. Therefore, the surface roughness of vacuum forming molds should be relatively high. After surface processing, sandblasting is recommended.

4. Edge Sealing Device
During vacuum forming, to prevent air from entering the vacuum chamber, sealing devices must be installed at the edges where the plastic sheet contacts the mold. Sealing the contact surface between the plastic sheet and the mold is relatively easy for straight parting surfaces, but sealing is more difficult for curved or folded parting surfaces.

5. Heating and Cooling Equipment Heating the plastic sheet used in vacuum molding typically employs resistance wire or infrared radiation. Resistance wire temperatures can reach 350℃～450℃. Different molding temperatures are required for different plastic sheets, generally achieved by adjusting the distance between the heater and the sheet. A distance of 80～120mm is commonly used.

Mold temperature affects both the quality of the plastic part and production rate. If the mold temperature is too low, cold spots or stress will occur upon contact between the plastic sheet and the mold cavity, potentially leading to cracks. Conversely, if the mold temperature is too high, the plastic sheet may adhere to the cavity, deforming during demolding and prolonging the production cycle.

Therefore, the mold temperature should be controlled within a certain range, generally around 50℃. Mold temperature control methods include natural cooling after the plastic contacts the mold, adding air cooling devices to accelerate cooling, and water cooling. Incorporating cooling channels within the mold is the most effective and commonly used method for controlling mold temperature. These cooling channels should be at least 8mm away from the mold surface to prevent cold spots.

There are different methods for opening cooling water channels. Copper or steel pipes can be cast into the mold, or holes can be drilled or grooves can be milled in the mold. When using the milling method, sealing elements must be used and a cover plate must be added.