The Requirements For Fillets in Vacuum Forming Molds

To avoid stress concentration in the workpiece and enhance impact strength, sharp corners and edges on the mold surface should not be used; instead, they should be replaced with rounded corners. The radius of the rounded corners is preferably equal to or greater than the thickness of the sheet material, but should not be less than 1.5mm. The radius of the rounded corners in the female mold is usually selected in such a way that during the molding process, the plastic is completely stationary relative to the mold .

Where the plastic does not come into contact with the mold, the heat dissipation effect is very poor, which leads to an extended cooling time. Additionally, due to the different contraction rates in the fast and slow cooling zones, the workpiece may deform. In Figure (a), the material does not fully contact the radius of the rounded corner of the female mold, which will result in the inability to achieve a repeatable molding radius, as the radius formed here is random. The reference value for a repeatable molding radius is: R = 1.5S (initial thickness of the material). 

For the final product, the sharp edge on the upper surface of the male mold is easier to obtain than that in the bottom area of the female mold. For almost all thermoplastics, the minimum radius R of the female mold area is approximately between 0.2 and 0.5 mm, and it is independent of the material thickness S. The prerequisite is a relatively low stretch rate during molding (molding ratio H:W approximately reaching 0.4:1), a high material temperature, and an appropriate mold temperature. For pressure molding, a higher molding pressure is beneficial, but it is not required in all cases.