Laser-Cut Forms for Thermoforming (Vacuum Forming)




Introduction: Laser-Cut Forms for Thermoforming (Vacuum Forming)

About: Instructor at Maker Works, a 14,000 sq ft membership-based prototyping facility. I enjoy helping people make things.

Thermoforming (Vacuum Forming) is a fast, easy, and cheap way to form sheet plastic into useful shapes.  Widely used to produce costumes, masks, theatrical props, enclosures, and packaging, the heated plastic sheet is drawn down over/into a mold using a vacuum.

This mold can be made of many different materials.  At the high end, metal is the longest lasting.  But wood, high-temperature epoxy, and even plaster and modeling clay (for limited runs) can be used.

If you have access to a 3D milling machine (or 3D printer), plus skill and access to 3D software, you can create a 3D model and then an actual 3D mold directly from your model.  You'll may need to add vent holes, but the mold should be largely useable as is.

However, and the point of this Instructable, If your design consists of basically flat surfaces at different heights, a fast way of accurately making thermoforming molds (especially multiple copies) is to use a laser engraver to cut layers out of acrylic, then glue them together in a stack.  The result can even include vent holes.

In this Instructable, we'll use the example of a Power Supply for a Ferrous Gentleman (from the third motion picture).  First, the design will be created in a vector graphics program.  This will be separated by height to produce separate cut geometries for each layer (including vent holes).  After cutting, the layers are glued.  Finally, draft is added (if required).

Step 1: Create the (vector) Design

Begin by creating a vector drawing of the model.  Tools such as Inkscape, Corel Draw, Adobe Illustrator, and LibreCAD could all be used.  In some cases you may be able to use a bitmap image (e.g., photo) as a guide when creating your 2D model.

Simplify when possible.  For a positive mold--that is, where the plastic is formed down over the mold--you may not get a lot of detail on the outside.  Consider too that you must have vent holes to create any features that are concave.  If the air is not being drawn out, the plastic won't just droop into the feature.

Acrylic is most easily found in 1/8" (actually often a little thinner), and often 1/16", 3/32", and 1/4".  I've found that it is often easier to just settle on one thickness, if possible, so you can cut a bunch of layers out of the same thickness.

If you have not used the Layers feature of a drawing or drafting tool, now is the time to use it.  As you refine your design, you may move elements between layers, but it's useful starting off with an idea of the layers early on.  The Layers feature will let you turn on and off visibility between layers, and this will be helpful in letting you locate vent holes (the tiny holes in the figure above).

What size vent holes?  A common guide is that the diameter of vent holes should be no more than 1/2 the thickness of the plastic you are forming, but larger may be necessary.  That's the diameter at the top of the mold--below the vent hole, the hole should be as wide as you can manage.

In the mold design here, I could probably have added a lot more vent holes in corners.  It's not like it takes a lot of effort (each hole cuts very quickly on the laser), and probably made them larger.

I also experimented with using very large "vent holes" to create features (bolt pattern) on the top ring.  It worked well.

Step 2: Create the Layers

The next step is to take the 2D design and create the individual cuts for each layer.

Since we're building up layer by layer, and need to glue the layers, keep the following in mind:
* do not have overhangs for the plastic to get under
* if you have a vent hole on an upper layer, you need an opening in layers below it (and aligned)
* if there is any issue of aligning layers with respect to each other, feel free to add tiny (engraved?) marks

The image above shows the different layers to be cut from 1/8" acrylic, from top (left) to bottom.  The smaller image shows pieces to be cut from 1/16" acrylic.  The bottom pieces (extreme right) of the mold are just rings for height with some support (probably not necessary for this tiny mold, only 3" or so in diameter), and are hollow because the stage the mold is set on has holes on perhaps 1" or 3/4" centers, which would be unlikely to line up with arbitrary vent holes if the lower pieces were essentially just copies of the middle piece.  In addition, as noted earlier, vent holes should only be narrow at the mold face--below they should been as large as possible.

The PDFs attached are to the files shown, and include an alternative design for the bottom rings (thicker outside, easier to glue).

Step 3: Glue the Layers, Add Draft

Use acrylic glue in a well-ventilated space.  Align vent holes carefully (an un-vented hole will be, of course, useless.)

This technique will work best with molds that are flattish.  The taller/thinner, the more critical it will become that there is some degree of "draft" to the sides.  No draft (0 degrees) means the walls of the mold are straight up and down.  The problem is that with no draft, the mold may not be easy to release from the cooled plastic.  For positive molds (plastic formed over the outside of mold), 5-7 degrees of draft is recommended.  A negative mold (plastic drawn down into the mold) can have 2-3 degrees of draft.

We don't have control over the angle of the laser (you'll always get 90-degree cuts), but a quick pass on a disk sander could add draft to the outside.  In some cases you may want to use a file to create draft on interior segments.

Step 4: Use It!

Follow the directions of your thermoformer.

In general terms, you'll first locate the mold on the perforated platform.  If the platform raises and lowers, drop the platform down.  If the frame raises and lowers, raise the frame.  Insert and clamp the plastic in the frame.

Turn on the heater (move the heater into position if needed).  A variety of plastics can be used in thermoforming, but for your first tries hi-impact polystyrene ("HIPS") is a great choice--it is cheap, easily formed, requires no drying, and is widely available.  With HIPS, it is often sufficient to use the droop of the plastic as a guide to when the plastic is ready to form.  On a small thermoformer, a droop of perhaps 1/2" may be a good indication for thin HIPS, while a larger former may be associated with a larger droop.  Each former will have a different sequence of steps for moving the plastic down over the mold, moving the heater out of the way, and turning on the vacuum or fan to draw the plastic down.

Just as we need vent holes on any concave features of the mold itself, there must be a way for air to be sucked out around the bottom of the mold, or plastic will not be drawn down around the mold.  In this example, putting a few thin strips of acrylic under the mold raises it enough to allow the plastic to be pulled down well.  If we didn't do this, the plastic would no be pulled down well, since early on the plastic would likely cover the vacuum holes on the platform leaving no where for the air to go.

Once pulled, remove the plastic.  The mold may be retained in the plastic--gently push it out.

Since the molds can be made of scrap material, it's easy to make multiple units.  If you need lots of thermoformed objects, forming over multiple molds at the same time makes a lot of sense.

That's it.  For simple, "2-1/2D" objects, using the laser cutter to cut multiple layers is a fast and easy way to create thermoforming molds.

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    3 years ago

    Wow - this instructable is impressive ! Would this method of mold making work for an object/mold that was 10" dia x 2.5" high ? I've read that if you are forming a clear plastic (like petg), the surface finish of the mold must be almost perfect, or any imperfections will transfer out onto the surface being formed, and create distortions. I am trying to figure out how to get a mold for a clear piece like this, without it costing an arm and a leg. Would it be possible to put me in touch with someone that could make me a mold like this (~10" dia at base, 2-1/2" tall, 6 degree draft) ?


    Reply 3 years ago

    The general idea--cutting layers quickly and easily on a 2D machine like a laser and stacking--could apply. But if your model doesn't have essentially flat, sharp features, then other methods would be preferable, since the layering technique here has joints and discontinuities between every layer and does not handle curved sides. Any changes in height are discrete steps.

    Tooling board, wood, or similar soft material milled out, covered with Bondo, painted, and coated with release agent might get you better results. Much depends on the actual shape you need--organic shapes usually take longer to mill to a given smoothness than shapes that have flat heights. You could also use the stacked laser-cut model as the basis for something to be smoothed with Bondo--a tool like Autodesk's Slicer for Fusion 360 might be useful there.

    For either laser or 3D machining, check out your local makerspace. If it looks like 3D machining might be the answer, check out Good luck with your project!


    4 years ago

    Cool instructable! I'm trying to make one of my own, based on this design.

    Any chance you have the STL images lying around? I was hoping to turn them into a 3D printable version.

    Thanks for the awesome work!


    Reply 4 years ago

    This was never a 3D model--just the 2D layers I posted above. You can read the PDFs into a vector graphic program like Inkscape, generate DXFs, and then use something like Fusion 360 to make a 3D model by extruding the DXFs. Good luck with your project!


    Reply 4 years ago

    Thanks for the quick reply! I didn't know that it was possible to extra SVG files from PDF's until I read your reply. I ended up creating a model from scratch that resembles your own design. Since it was a 3D printable model, I was able to add about 10 degrees of draft around the edges of the arc reactor. I've attached a picture of my vacuum formed results, spruced up a little bit with some LED's.


    Reply 4 years ago

    Very nice! 3D printing is a great way to make molds for thermoforming. For other folks reading this, I'd make the following comment--the ridges we get with 3D printing (especially when the ridges are parallel to the thermoforming platform) could make it extra difficult to extract the mold from the finished part. So add additional draft (sounds like 10 degrees worked well?), and/or use whatever technique you like to smooth out the ridges so the sides don't grip as much. A mold release agent probably wouldn't hurt either.


    Reply 4 years ago

    Thanks! And, yes, I did have a little trouble removing the 3D printed part from the plastic. The part I designed had lots of vent holes, just as yours did, but still gave me a bit of a fight. In the end all I had to do was let the plastic cool, then flex it some to have it release. The next time I'll try 15-20 degrees to see how that goes.