Design Principle

Because different forming molds have been applied in many fields, and the manufacturing technology of professional molds has also undergone certain changes and development in recent years, so in this part, the general design rules of vacuum plastic forming molds are summarized.
The design of vacuum blister forming mold includes batch size, forming equipment, precision conditions, geometric shape design, dimensional stability and surface quality.

1.It is used for batch size experiment. If the output of mold is small, it can be made of wood or resin.

However, if the experimental mold is to obtain data on shrinkage, dimensional stability and cycle time of the product, a single-cavity mold should be used for the experiment, and it can be used under production conditions. Molds are generally made of plaster, copper, aluminum, or aluminum-steel alloy, and aluminum-resin is rarely used.

2.Geometric shape design

When designing, it is often necessary to comprehensively consider dimensional stability and surface quality. For example, product design and dimensional stability require the use of female molds (concave molds), but products with higher surface gloss require the use of male molds (punch molds). In this way, the plastic parts ordering party will take these two into account. Point, so that the product can be produced under the best conditions. Experience has proved that designs that do not meet the actual processing conditions often fail.

3.Dimensional stability

During the mold forming process, the surface of the plastic part in contact with the mold has better dimensional stability than the part leaving the mold. If the material thickness needs to be changed in the future due to the need of material rigidity, it may cause the male mold to be converted to the female mold. The dimensional tolerance of plastic parts cannot be less than 10% of the shrinkage rate.

4.Surface of plastic parts

In terms of the range that the mold forming material can cover, the surface structure of the visible surface of the plastic part should be molded at the contact point with the mold. If possible, the smooth surface of the plastic part should not be in contact with the mold surface. It’s like using female molds to make bathtubs and washtubs.

5.Modification

If a mechanical horizontal saw is used to saw off the clamping edge of the plastic part, there must be at least a 6-8mm margin in the height direction. Other dressing work, such as grinding, laser cutting or jet, must also be left margin. The gap between the cutting lines of the knife edge die is the smallest, and the distribution width of the punching die is also small when trimming. These should be noted.

6.Shrinkage and deformation

Plastics are easy to shrink (such as PE), and some plastic parts are easy to deform. No matter how to prevent them, plastic parts will deform during the cooling stage. Under this condition, it is necessary to change the shape of the forming mold to adapt to the geometric deviation of the plastic part. For example: Although the wall of the plastic part remains straight, its reference center has deviated by 10mm; the mold base can be raised to adjust the shrinkage of this deformation.

7.Shrinkage

The following shrinkage factors must be taken into consideration when manufacturing blister molds.

• ① The molded product shrinks. If the shrinkage rate of the plastic is not clearly known, it must be obtained by sampling or experimenting with a mold of similar shape. Note: Through this method, only the shrinkage rate can be obtained, but the deformed size cannot be obtained.
• ② Shrinkage caused by the adverse effects of intermediate media, such as ceramics, silicone rubber, etc.
• ③ The shrinkage of the material used in the mold, such as the shrinkage when casting aluminum.