A kiss-off in rotational molding is a solid, reinforcing structure that transmits a load from one nominal wall to another. These bridge-like structures increase the stiffness of rotomolded parts, which are hollow, and provide dimensional stability.

How are kiss-offs made?

Rotational molds have a convex half (male) and a concave half (female). Kiss-offs, or kiss-off ribs, are produced when plastic powder bridges the gap between the male and female parts of the rotational mold.

Which rotomolded parts use kiss-offs?

Kiss-offs are used in rotomolded parts ranging from plastic bins to baby changing stations. They’re also used with doors. When the surface of a rotomolded door is pushed and flexed, the kiss-off ribbing transfers the load from one side to the other.

Are kiss-offs ever used in plastic tanks?

Yes. They can be used for baffles. Without these part features, the pickup point for a plastic fuel tank might not stay submerged, such as when fuel sloshes backward during a steep uphill climb.

Do kiss-off ribs provide advantages besides strength?

The added strength that kiss-offs provide is important, but these part features also prevent warpage across large, flat surfaces.

How do you determine the thickness of the kiss-off area?

According to the Association of Rotational Molders (ARM), 175% of the nominal wall thickness is a good starting point. These proportions provide a good weld line and also leave enough room for the plastic to move over all of the mold’s surfaces.

Does the distance between the nominal walls matter?

Yes. If the distance is great, heating the mold surfaces in the kiss-off area can be challenging. Additional heating can also increase cycle times and costs. Designing a hole through the mold and the part in the kiss-off area can help to direct hot oven air towards the kiss-off surface.

Which rotational molder offers kiss-off design help?

Gregstrom Corporation is a rotational molder that provides design assistance, including help with kiss-offs. At our Woburn, Massachusetts (USA) manufacturing facility, we also provide value-added services such as parts finishing, assembly, and quality assurance.

For your next plastics project, contact us.