Key Points of The Structural Design for Rotomolding Mould

The structure of the mould directly determines the molding effect of the product. The key design elements include:

1. Cavity design

l It must be completely matched with the 3D structure of the product. Consider the fluidity of the plastic melt (round corners R should be made at the corners to avoid stress concentration).

l For products with inserts (such as metal interfaces), a positioning structure for the inserts needs to be designed (to prevent displacement during rotation)

2. Generative surface design

l The rotomolding molds are mostly split type (upper/lower / left/right parting), and the parting surfaces need to be smooth (to reduce flash) and are equipped with positioning pins / guides (to ensure the accuracy of mold closure)

l The position of the parting surface should avoid the product's appearance surface (to prevent the molding line from affecting the aesthetics). For complex products, multiple parting surfaces can be adopted (such as three-way pipe fittings).

3. Exhaust structure

l During the rotomolding process, gases (such as air and plastic volatiles) will be generated inside the mold. Therefore, it is necessary to design vent slots (with a width of 0.5-1mm and a depth of 0.1-0.2mm) or vent holes (with a diameter of 0.5-1mm) to prevent the products from having air bubbles or missing materials.

l The exhaust position is usually at the highest point of the cavity or where the gas tends to accumulate (such as the top of a sealed cavity).

4. Heating / Cooling Adaptation Structure

l The wall thickness of the mold should be uniform (to avoid local overheating / uneven cooling). Generally, the wall thickness of aluminum alloy molds is 8-15mm, while that of steel molds is 10-20mm.

l The large molds can be designed with reinforcing ribs (to ensure strength while reducing weight), and connection holes for the clamping device of the rotomolding machine are reserved (for easy fixation).

5. Demolding assistance design

l The mold cavity needs to be designed with a demolding slope (usually 1° - 3°, and the slope of the outer surface can be smaller), to prevent the product from being scratched during demolding.

l For deep cavity products (such as barrel-shaped ones), a ejection mechanism can be designed at the bottom of the cavity (or an elastic mold can be used, and it will naturally contract and be ejected after cooling)