Why Do Rotomolded Products Need To Have Vents?

1. Why are there vents on the surface of rotomolded products?

（1）Rigid process requirements - exhaust is the "lifeline" of rotational molding
Gas must escape: During the rotational molding process, the volatiles generated by the air and plastic melt in the mold must be discharged, otherwise it will cause:

Bubbles, holes (internal defects)

Surface bulging (gas expansion lifts up uncured plastic)

Uneven wall thickness (gas hinders the flow of plastic)

The exhaust hole is the only channel: the rotational molding mold is a closed structure, and the exhaust hole is the only outlet for gas escape.

（2）The location of the exhaust hole cannot be completely hidden
It needs to be set at the highest point: the gas naturally gathers upward, and the exhaust hole is usually located at the top of the mold (corresponding to the surface of the product).

Complex structure multi-porous requirements: Deep grooves, protrusions and other areas prone to gas accumulation require additional exhaust holes, which may appear on the visible surface.

（3） Air pressure balance during the cooling stage
During cooling, the plastic shrinks, and external air needs to be replenished through the exhaust hole to prevent the product from collapsing. If the exhaust hole is completely closed, it may cause:

Negative pressure deformation (such as concave stool surface)

Stress concentration (cracking risk)

2.How to reduce the impact of exhaust holes on the surface?
（1）Design optimization
Hidden position: set the exhaust holes on non-appearance surfaces (such as the bottom of the stool, the inner side of the legs).

Integrated exhaust: use the gap between the reinforcing ribs or the buckle structure as an invisible exhaust channel.

Porous dispersion: increase the number of small-diameter exhaust holes (such as Ø0.8mm×10 instead of Ø2mm×1), and reduce the visibility of a single hole.

（2） Process improvement
Stage exhaust: open large holes for exhaust at the beginning of heating, and reduce the aperture to reduce overflow at the end.

Sintered metal plug: use porous metal filter to exhaust, avoid plastic blockage and make the traces more regular.

（3）Post-processing
Filling and polishing: For high-end products, the same material plastic can be used to fill the holes and then polish (higher cost).

Surface coating: spray or film covering (adhesion needs to be considered).