3

I need to create few dozen opaque domes/hemispheres of various sizes from 80cm to 120cm. The shape is pretty basic, example: http://www.cmspinning.com/images/hemisphere_landing_large.jpg The domes have to be foldable into smaller pieces for easy transportation and storage. Think heavy duty rubber gloves but just dome shape.

Would like to know if there are elastic sheet materials available to create these domes I could vacuum form like we can vacuum form rigid materials such as ABS plastic.

I understand there might be rubber domes available for sale so I wont waste time making my own but I'm going to make slight changes to the perfect dome shape so I need to make my own.

Possible candidates: TPE (thermoplastic elastomer), TPR (thermoplastic rubber), TPO (thermoplastic olefin), low density polypropylene, low density polyethylene, vinyl aka PVC, Polyurethane, EVA (Ethylene Vinyl Acetate). Some seller websites claim these can be thermoformed but there is next to none other source, video, etc. available online.

0
1

I'm not aware of any flexible rubbers which can be formed by vacuforming. In my experience, when you start with a flat sheet of flexible rubber and press it against a mold, it pretty much returns to being a flat sheet of rubber when the pressure is released.

You can however cast flexible rubber (in liquid form) into a mold with your desired hollow hemisphere shape and then let it cure in the mold. This will result in your hemisphere being the natural shape of the rubber to which it will return after each deformation.

You will want to use a two part mold (one part to provide the outer shape of the bowl and the other to occupy the hollow interior while the foam is curing). Two nesting bowls can be used together as the mold with the difference in the bowl sizes defining the wall thickness of your final rubber part.

3
  • Thank you. But could you please tell which rubbers you have tried? Thermoplastic Olefin (TPO) Thermoplastic Elastomer (TPE) Thermoplastic Rubber (TPR)? I could only find info on whether these can be vacuum formed in one place, a Stratasys document, where it says it is possible. But cant find actual examples anywhere. usglobalimages.stratasys.com/Main/Secure/Applications/… (check the "VACUUM FORMING MATERIALS" table)
    – Leo Ervin
    Oct 6 '17 at 13:20
  • Quote from above document: "Virtually any thermoplastic that is available as extruded sheet stock may be used (see figure 1). And unlike injection or blow molding processes, wall thicknesses can range from foils to thick-gauge stock—thicknesses ranging from 0.0005 to 0.50 inch (0.0127 to 12.7 mm)" . But hey its Stratasys, they might be using some very powerful vacuums maybe, or pressure, or long cooling times. Dunno.
    – Leo Ervin
    Oct 6 '17 at 13:22
  • I haven't used any of those products, ...yet. Thanks for sharing the link. Those definitely look interesting. My vacuum forming experience is limited to clear PE. In my experience, the quality of the heat source matters more than that of the vacuum, but that may not be true for everyone. Wonder if Stratasys has a sampler that lets potential customers try small pieces of all of the sheet materials? I would definitely be interested in getting (even paying a small price for) something like that. Oct 6 '17 at 13:47
1

There's a technique I've used for a similar requirement that doesn't require thermoforming or vacuum forming. You end up with a flexible, rubbery dome (or other shape per the mold).

The first step is to stretch a stretchy cloth material over the mold (needs to be a convex mold for this, where you can stretch the cloth over the outside). Stretch it where needed so that it is completely flat against the mold with no bulges. Cloth like old tee shirt material works well, and cotton absorbs well for the next step. Shapes with smooth, regular surfaces like domes work well. Something like a cube would probably require alterations to the cloth for it to lay flat on the form.

The second step is to impregnate the cloth, which holds it in that shape and gives it the plastic or rubbery feel. The form composition will affect what you can use for this, and whether you will need a release agent (never hurts). Here are some of the materials I've used:

  • Hot melt glue (generally use a metal form for this). Apply enough glue to cover the surface with a thin layer. Use the glue gun tip to spread the glue around. Then use a heat gun at a low setting to heat the glue to the point that it becomes watery (but not so hot that it scorches). The glue will soak into the cloth and the surface will self-level. When it cools, you have your shape. I sometimes turn it inside out, put it back on the form, and repeat the process on the other side. If you want a high-quality surface, turn it inside out when you're done, as the inside surface molds itself to the form and retains the surface detail and finish.

    The result is flexible and rubbery but stiff enough to keep its shape. This is also a handy way to make small, formed things like phone covers. Large items the size described in the question would be flexible enough to fold within reason for storage or transportation. As long as you don't press a sharp crease into it, it won't retain the fold line when you reopen it.

  • Silicone caulk thinned with mineral spirits to paint consistency. Apply as many coats as needed to saturate the cloth and leave a surface layer. Then turn it inside out, put it back on the form and coat the other side to catch any spots without a good coating. The first side needs to cure to the point that it's stable (mostly cured) before flipping it to the second side, especially if you might stretch or distort it getting it off the form. Once it's finished, let it dry and cure for several days. Then remove it from the form and let it dry another day or two.

    The result is very soft and rubbery. It will retain it's shape, but on something large, may not be stiff enough to not sag or collapse from its own weight without filler. It is soft and flexible enough to fold as needed for storage and transportation.

  • PVA glue. Thin it with water so it absorbs into the cloth. Once the cloth is saturated, you can add a coating of undiluted glue. This dries harder and less flexible than hot melt glue, but still flexible (i.e., not rigid) and strong. This might not be sufficiently flexible to fold for storage and transportation. But you could do something like cut it into sections joined by a hinge of the same (untreated) cloth.

I've also used this technique with other materials to produce a hard plastic or air-dry clay or cold porcelain shell, but those wouldn't be applicable to the requirements here. A single layer of cloth has always been adequate for my own needs, but you can repeat the process to add a second layer of cloth if you need something even stronger or more rigid.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.