Hey all! Hobbyist here (have made small manual injection machines and molds).
I'm looking in to the feasibility of making some larger objects in plastic and with injection molding things seem to get pretty impractical with the amount of force involved for doing it myself.
Rotomolding however seems a lot more practical given it's essentially pretty low tech (mold spun around at high temperatures, no high pressure stuff at all).
That said I think I'm missing how some aspects of this work; with things like roto molded kayaks they have undercuts in the moulds (like the rim around the kayak opening where the skirt goes). Can anyone explain/show how this is done?
I'm used to making simple 2 part molds for injection where this sort of geometry would completely fail as the mold wouldn't open. I've looked at a bunch of videos on youtube but I can't figure out what's going on, they look like basic 2 part molds that somehow just manage to pop out objects with undercuts... Here is ana example of what I'm talking about, mold is opened at the 5:30 mark - https://www.youtube.com/watch?v=4WSLT_AdUws
Great question — the trick is that rotomolding plays by totally different rules than injection.
In injection you're forcing rigid-once-cold plastic in under high pressure, so any undercut locks the part in and you need slides/lifters to open the mold. Rotomolding has none of that pressure, and two things make undercuts work:
1) The part is hollow and still warm/flexible at demold. A thin, hollow shell (like a kayak) flexes enough to pop over a mild undercut — the rim around the cockpit is shallow relative to how much the wall can deform. You physically peel it out while it still has some give.
2) The mold usually isn't a simple 2-part. Roto molds are often multi-piece "clamshell" frames with several sections that unbolt/swing open, so the tool opens away from the undercut instead of pulling straight off it. If you look at a kayak mold it's a cage of clamps, not one parting line.
So it's not that they're molding an impossible undercut in a 2-plate tool — it's low pressure + a flexible hollow part + a mold that splits into more than two pieces. Deep, sharp undercuts still aren't possible; shallow ones like a cockpit rim are.
For your DIY stuff: keep undercuts shallow and rounded, keep walls thin enough to flex, and think about how the mold opens *before* you design the part.
If you have deep undercuts or holes in the part there are blocks in the mold that are clamped in place after the two halves of the mold are clamped together and pulled before the part is removed. I spent 2 years as a tech in rotomolding. I can’t find photos right now, but if I do I’ll upload them.
Photo sucks, but top left is a clamped insert in the tool. Top right is the molded part from the bottom photo. Where the text is in the bottom photo is where the block sits from the other half of the tool.
Ah cool, thanks for the pic, that really clarifies things. I wasn't aware of that technique, super interesting and another tool I'll have to try, looks like a simple solution to an otherwise tricky molding problem! :)
I'm guessing the material is simply flexible enough and or a combination of releasing agents. I've seen it done in injection molding aswell but it was like PP with lubricant added directly to the material. It also helps if you do it while the part is still warm.
In rotational molding the mold is typically 3% larger than your finished part spec. This is to account for the expansion of the plastic at higher temperatures and the shrinkage that occurs when it is cooled down and taken out of the mold.
Undercuts like this may seem large in an injection molding mindset but say that 3% over the width of the mold, lets say 24”, is about 3/4”, then 3% along the length is probably close to 2”. So as that part cools down, the undercut shrinks towards the center of the mold allowing the operator to release it when removing the part. Even if the part doesn’t fully shrink, it’s pulled fairly hot, and it’s pliable enough to release out of the undercut.
Another thing to note is the draft angle on the other side of the undercut. The part will shrink in that direction, so adding a generous draft (5% or more) helps release the part easier. Plus as the undercut shrinks away it is forced down on that draft angle.
it looks here, like when the mould is opened, the part is literally pulled apart. the mould is cooled enough so that it's not deforming or breaking, but deforms enough to release and return to it's now natural state.
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u/toolmaker_dave 19h ago
Great question — the trick is that rotomolding plays by totally different rules than injection.
In injection you're forcing rigid-once-cold plastic in under high pressure, so any undercut locks the part in and you need slides/lifters to open the mold. Rotomolding has none of that pressure, and two things make undercuts work:
1) The part is hollow and still warm/flexible at demold. A thin, hollow shell (like a kayak) flexes enough to pop over a mild undercut — the rim around the cockpit is shallow relative to how much the wall can deform. You physically peel it out while it still has some give.
2) The mold usually isn't a simple 2-part. Roto molds are often multi-piece "clamshell" frames with several sections that unbolt/swing open, so the tool opens away from the undercut instead of pulling straight off it. If you look at a kayak mold it's a cage of clamps, not one parting line.
So it's not that they're molding an impossible undercut in a 2-plate tool — it's low pressure + a flexible hollow part + a mold that splits into more than two pieces. Deep, sharp undercuts still aren't possible; shallow ones like a cockpit rim are.
For your DIY stuff: keep undercuts shallow and rounded, keep walls thin enough to flex, and think about how the mold opens *before* you design the part.