What is vacuum forming, and when should you use it?

Vacuum forming heats a plastic sheet until it is soft, then sucks it down over a mould so it cools in the mould’s shape. It makes thin shells that are open on one side, such as trays, covers, packaging inserts and sign faces, and it pays off when you need many of the same shape. We take vacuum forming jobs in Singapore and can 3D print the mould from your file, which is where it meets 3D printing.

Vacuum forming at a glance: thin shells from one mould

See our vacuum forming service

How vacuum forming works: heat, pull down, trim

Vacuum forming is the simplest kind of thermoforming, the family name for shaping a heated plastic sheet over a mould (Formech, a maker of vacuum forming machines). It runs in four steps:

1. Heat. A flat sheet is clamped in a frame and heated until it sags and goes soft.
2. Form. The mould rises into the soft sheet and a vacuum sucks out the air underneath, pulling the plastic tight against the mould.
3. Cool. The plastic cools and keeps the shape, then lifts off the mould.
4. Trim. The flat edge around the part is cut away, leaving the finished shell.

Because the sheet is only pulled by suction, the pressure is low. That is why a mould can be printed, cast or carved instead of cut from steel.

What vacuum forming is good for: many identical thin shells

• Packaging: trays, blister packs and inserts that hold a product in place. Formlabs calls vacuum forming one of the most common ways to make packaging (Formlabs).
• Covers and guards: clear covers over a display or a machine, domes and light covers.
• Sign faces and displays: a shaped face for a lightbox, often in clear PETG or acrylic printed on the back so the print shows through.
• Light shells: casings, props and panels where only the outside has to look right.

The mould costs far less than an injection moulding tool, so prototypes and short to medium runs of large parts become good value (Formech). The limit is the shape: one even sheet, open on one side, never a solid part.

A tray, cover or sign face in mind? Send us the shape and quantity

Sheet plastics: pick clear, low-cost or tough

Each plastic forms differently. This table follows Formech’s guide to popular vacuum forming materials.

ABS, acrylic and polycarbonate soak up moisture from the air, so the sheet is baked dry before forming; PETG and HIPS are not. HIPS is high-impact polystyrene, and acrylic is also sold as PMMA. Cast acrylic only forms gentle, shallow shapes. Tell us whether the part must be clear, live outdoors or take knocks, and we suggest the sheet.

Design rules: taper the walls and keep it shallow

These five rules decide whether a shape forms cleanly. We check your file against them before the mould is made.

• Raised or hollow mould. A male mould is a raised shape the sheet drapes over; a female mould is a hollow the sheet is pulled into. Male moulds usually cost less, and a female mould puts sharp detail on the outside of the part (Formech on male and female moulds).
• Only one face is exact. The face against the mould takes its exact shape. The other face sits a sheet’s thickness away, so its edges and detail come out softer. Decide which face people will see, and that sets the mould.
• Draft. Walls need a slight taper so the part lifts off: about 4 to 5° on a male mould and 1.5 to 3° on a female one, more for deep or textured shapes (Formlabs).
• Depth. The sheet stretches to reach the bottom, so deep shapes come out thinner. Syracuse University’s workshop guide keeps a hollow’s depth under about 75% of its width, or the sheet may tear.
• Webbing and overhangs. Raised shapes set close together can pull the sheet into folds called webbing. Rounded corners, wider spacing or a female mould prevent it (Formech on webbing). Avoid overhangs (undercuts), which lock the part onto the mould.

Not sure your shape will form? Send the file and we check it

3D printing the mould: quick to make and to change

The mould, also called a buck or a tool, used to be machined or cast, which takes time and money. A 3D printed mould saves both, and text or texture on it costs nothing extra (Formlabs).

We print the mould from your 3D file. Five things are worth knowing about it:

• Changes are quick. Change the file and reprint the mould, with no tool to re-cut.
• It must take the heat. A hot sheet lands on it every cycle, and a poorly chosen print material can deform, crack or even melt (Formech), so we print it in one that holds its shape.
• It is smoothed first. The vacuum pulls the sheet into fine detail, tool marks included (Syracuse University), so layer lines are sanded off before forming.
• Vent holes are built in. Small holes let the vacuum reach every pocket; without them the sheet drapes over a pocket instead of following it (Formlabs).
• It suits a run, not years of production. Formlabs reports printed moulds lasting for dozens of parts with care. Tell us your quantity and we say whether a printed mould suits it.

No 3D file? Our design team can model the part from a sketch, a photo or a sample.

See how we 3D print moulds and parts

When to use something else: four better routes

Despite the name, vacuum casting is a different process: liquid resin is poured into a silicone mould under vacuum to make solid parts. For how printing, casting and injection moulding compare by quantity, see 3D printing vs silicone vs injection moulding. For a clear part with a true 3D shape, see how clear a 3D printed part can get.

Not sure which route suits your part? Ask us