How would it be manufactured?

What would be the best approach when manufacturing a double walled bucket? It would be made out of vinyl. In the jpeg I constructed it in one piece but I’m assuming you would have to break it down into pieces for practicality and economical purposes.

What’s the best approach? The most economical?

blow mold eot.

Rotational molding.

My vote is for rotomolding… perfect for vinyl parts.

I’d say blowmolding but both will do.

But seriously get this book:

And stop asking silly questions :wink:

Two primary issues that will drive your decision between rotomolding and blowmolding: production volume and ruggedness.

Blowmolding is better for higher volumes, but the tooling is generally more expensive and the nominal wall is typically thinner (think thermorming and how the material is stretched). Rotomolding can create much more rugged parts with much thicker nominal walls and less expensive tooling (depending on on whether you go with cast or fabricated tooling).

The other issue is: how big is this part? That’s also critical since small, fine details are more challenging in rotomolding, but really large parts require a big extruder to create a large parison.


Looks like i’m a little late to the party, but I cast my vote for rotomolding.

But if you are looking at high volumes and still want strength you could injection mold two parts and assemble it top to bottom.

If you do go with rotomolding, remember to have huge radii everywhere (as huge as you can get). It lets the material flow (slosh?) easier.

Not necessarily. I’ve designed quite a few rotomolded products and while the molder would love you to design your product with big radii everywhere, your product might end up looking like a child’s toy (think Step2). It’s one of the reasons why the rotomolded industry labors under the “tanks and toys” perception of their process where, in reality, you can do some pretty elegant things.

I’ve designed parts where I have some radii as sharp as .090". The trick is to make sure the material can roll into those features. (It doesn’t really “slosh” because it starts as a powder and begins to stick to the side walls of the mold. The rest of material actually sinters, or fuses together as the powder rolls around in the mold.) The idea is not to end up with a phenomon called “bridging” where the material creates pockets and feature doesn’t fill properly.

It really depends on what you do, but there’s always tricks to use the deal with it as long as you have a good molder who has a good toolmaker.

Hope this helps,



… as long as you have a good molder …

Which begs the question; after machining operations, what is the weirdest thing you’ve ever found inside of a roto-molded part?

I’ll start; the operator’s wedding band.

That’s what I call finished by hand… awesome…


Yeah, I figure it must have been a tough week for some factory rat; really pi**ed him off this time she did I’d say… .

Your tanks ‘n’ toys comment got me Warren. It was back in 1981 that we were roto-molding motorcycle saddlebags and top boxes. Our vendor, down in LA, mostly made dock floats and buoys for the Coast Guard. Their dock floats were a their own priority product and didn’t have any machining steps (they were lashed onto the dock structure) so their shop help didn’t think much of using their product as a trash can.

Frito bags, wax paper sandwich wrappers, cigarette butts, and aluminum beverage cans (and not always soft drinks) also showed up. Of course the last thing our customers would want is an embedded Taco Chip in their new motorcycle luggage. We finally broke them of their sloppy shop practices but we probably rejected a hundred sets of bags the first three month of production.

  • unjack -

Dimensional tolerance was a big problem for us; imprecise material thickness control, poor material accounting on the part of the molder (what’s another cup of resin … more or less … it’s a dock float), accurate process temperature control, etc. Our mounting hardware for the motorcycle was well controlled, but since the PP was dimensionally all over the place we had a helluva time; sometimes the fit would be loose, some times, tight, some times completely unacceptable. It was a QC nightmare, to say the least. A strapped on dock float always fits…

Since process temperature directly affects mechanical properties, we would occasionally find that the parts had a dull “dry” looking surface finish, not unlike charcoal… which essentially what it was; over heated. When this situation occurred it also resulted in a brittle part and QC eventually learned to automatically rejected parts with this characteristic out of hand.

+1 for rotomold

Trying to think of a synonym for slosh that implies powder…

I know, it’s difficult to explain accurately. The powder actually does “flow” kinda like a liquid and the heated mold is rotated so that the “pool” of powder coats all the surfaces. But it’s the sintering that unifies all the particles into a solid wall.

I think the important thing for designers to understand is the way the material rolls into the features without any pressure other than gravity. That tells you a lot about the kinds of details and features you can create with it.

Check out this flash animation created by the SPE Rotational Molding Division:


gas-assist injection molding.

Depends on how big it is and your quantities, but sure, gas-assist can make hollow parts. This process is best for coring out thick sections and adding structure while removing sinks. One exception (a truly hollow part) is inexpensive faucets which are molded in plastic as a hollow part and then chromed to appear like metal.


Gas-assist is cool, but Water-assist is where it’s at.

Check out the link, it’s actually got a pretty good animation of the water-assist process (pretty much same as gas-assist as far as macro process goes).

I know one of the guys who helped develop that process. I remember when it was first introduced at an NPE several years ago. It is pretty cool.

But with all of those processes, gas or water, you have to find a molder that’s familiar with and has a lot of experience dialing it in. And obviously, you have to have the volumes to justify it.


Are you insinuating that I can’t just shoot some CAD over to China and have them Gas assist? I don’t believe it.

Be my guest. Overseas manufacturing has its own advantages and disadvantages. Just make sure your Chinese molder has experience using it (get samples); don’t just take their word for it. And, you need to make sure you design you gas (or water) channel properly so you don’t get “fingering” (which means the bubble starts to scoot into the nominal wall).

That’s all I’m saying…