Vacuum Forming with 3D Printed Moulds: The Complete Guide

Short answer: vacuum forming heats a plastic sheet until it is soft, then a vacuum pulls it tight over a mould. A 3D printer is an ideal way to make that mould (also called a buck, master or mold): print it with a draft angle above 3°, 1.5–2 mm vent holes in recesses, rounded edges and no undercuts, smooth the surface, and you can form packaging, masks, casting moulds and chocolate moulds from one print. This guide covers mould design, which filament to print in, sheet choice, the forming process and how to fix the usual defects.

What is vacuum forming?

Vacuum forming is the simplest type of thermoforming. A thermoplastic sheet is clamped in a frame, heated until it is pliable, lowered over a mould and pulled onto it by suction. Once the plastic cools it keeps the shape, and the excess is trimmed off. It shapes one side of a part, so it suits shells, trays, blister packs, covers, masks and moulds rather than solid objects.

A male (positive) mould sits on the forming bed and the sheet wraps over it, which gives accurate inside dimensions. A female (negative) mould is a cavity the sheet is pulled into, which gives accurate outside dimensions but needs vent holes in every recess. Tooling is cheap and quick to change, so it suits prototypes and short runs.

Why 3D printed moulds work so well

Moulds are traditionally machined from wood, MDF, foam or aluminium. For small and medium parts, a 3D printer is faster and more flexible:

• Any shape, overnight. Organic forms, text, logos and textures are as easy to print as a box.
• Design rules built in. Draft angles, fillets and vent holes go straight into the CAD model, so there is no drilling afterwards.
• One print, many parts. MY YARD confirms that 3D printed parts and everyday objects can both be used as moulds on its FORMART machines.

The sheet is thin and cools quickly, so a printed mould is not necessarily damaged by forming. It does wear, though, and how fast depends on the material (see the filament section below).

Designing a mould for vacuum forming

Draft angles

Give every vertical wall a draft angle: MY YARD recommends more than 3°. Without it, the cooling sheet shrinks onto the mould and grips it.

Avoid undercuts

An undercut is any overhang that the sheet can wrap behind. Once it cools, the part locks onto the mould and you have to cut or break one of them to separate them. Fill undercuts in CAD or split complex parts into two simpler moulds. MY YARD notes that the automatic mould release on the FORMART 2 is less effective on moulds with undercut (concave) sides.

Vent holes

Air trapped in a recess stops the sheet from reaching the bottom. Add 1.5–2 mm vent holes in recessed areas, as MY YARD advises, so the vacuum can pull the sheet in. Model them as channels through to the underside of the print.

Fillets, spacing and proportions

Round sharp outside edges and inside corners. Fillets help the sheet flow, reduce thinning on edges and make webbing less likely. Leave space between several moulds on the bed, and keep a moderate height-to-width ratio: tall, narrow objects are harder to form and stretch the sheet thin. Every FORMART accepts objects up to 160 mm high.

Surface finish

The sheet copies everything, including layer lines. Print at a fine layer height with the best surface facing up, then sand, fill or coat it. A smooth master also releases more easily.

Strength and a flat base

The vacuum pushes down on the mould with real force. Use enough walls and infill, and add internal supports to thin-walled or fragile prints, as MY YARD recommends. Give the mould a flat, solid base so it sits firmly on the bed and the sheet cannot be sucked underneath.

Which filament should you print the mould in?

The mould is heated by the hot sheet on every cycle, so heat resistance decides how long it lasts. In MY YARD's tests, PLA prints withstood about 3–5 formings, ABS prints about 30–50 and resin prints hundreds. The figures below are general guidance; results depend on the sheet temperature, the part geometry and your print settings.

Our practical advice: prototype the mould in PLA, which is fast and accurate, and reprint it in PETG, ABS or ASA once the design works and you need more cycles. Higher-temperature sheets such as HIPS (best forming temperature 155–160 °C) are harder on a PLA mould than PET or LDPE (115–125 °C). For longer runs, MY YARD suggests forming a first shell, casting a plaster or cement copy of the mould from it and producing from that. If ABS or ASA warps on your printer, see our ABS and ASA warping guide.

Choosing a sheet material and thickness

FORMART machines take sheets from 0.3 to 3.0 mm thick, from MY YARD or other suppliers. MY YARD's own sheets cover the most common jobs:

As a rule of thumb, thin sheets (0.3–0.5 mm) capture fine detail and suit packaging and chocolate moulds; thicker sheets (0.8–1.5 mm) give stiffer parts and survive deeper draws, because the sheet thins as it stretches over tall features. HIPS is the sheet supplied with every FORMART and the easiest one to learn on. LDPE can break above 60 kPa, so reduce the suction and use the Microporous Forming Bed. For a full comparison of sheets and sizes, read our FORMART and sheet buyer's guide.

Step by step: vacuum forming over a 3D print

1. Prepare the mould. Make sure the print is clean and dry: water or debris drawn into the vacuum line can damage the pump.
2. Place it on the bed with space around it; add the webbing remover parts for tall or sharp-cornered moulds.
3. Clamp the sheet in the quick-lock frame with a turn of the handle.
4. Choose the material settings. Pick the sheet from the material database or a user preset; the FORMART 2 reads the QR code on MY YARD sheets itself.
5. Dry and heat. Use the dehumidify or preheat programme if the sheet may have absorbed moisture, then heat it to its forming temperature.
6. Form. Lower the frame over the mould and let the vacuum pull the sheet tight. With PET, do this within three seconds of reaching temperature.
7. Cool and release. Let the part cool on the mould, then lift it off. The FORMART 2 blows air to help release it.
8. Trim and finish. Cut away the flange, sand the edges and paint HIPS if needed.

Troubleshooting common defects

Webbing

Webbing is when the soft sheet folds onto itself instead of following the mould, leaving ridges between tall features or at corners. MY YARD notes that mould corners and tall moulds are the usual causes. Space moulds further apart, add fillets, and use the webbing remover kit included with every FORMART: the rectangular parts for corners and the ring-shaped parts for tall moulds. On the FORMART SE and FORMART 2, the Volume Control Kit adds an adjustable separator for difficult layouts.

Thinning

The sheet stretches as it is drawn over a tall mould, so walls and top edges come out thin. Lower the height-to-width ratio, round the edges, use a thicker sheet, heat evenly, and consider switching from a female to a male mould for deep parts, as MY YARD suggests.

Poor detail

Soft edges and missing texture usually mean the sheet was not hot enough, a recess had no vent hole or the sheet was too thick. Check the temperature, add vent holes, try a thinner sheet and smooth any layer lines you do not want copied.

Bubbles and whitening

Bubbles come from moisture in the sheet: dry it first with the dehumidifying function. PET turns white when it is overheated or heated for too long; lower the temperature slightly and form as soon as it reaches the target.

Which FORMART should you choose?

All three MY YARD FORMART machines have a built-in heater and -90 kPa vacuum pump, take sheets from 0.3 to 3.0 mm and form objects up to 160 mm high. The difference is mainly the format and the level of automation:

• FORMART S (€1,197.90): A4-size sheets (300 × 205 mm), 230 × 130 mm work area. For jewellery, chocolates, soap and classrooms.
• FORMART SE (€2,413.95): 500 × 300 mm sheets, 430 × 230 mm work area, and a size-switching kit down to A4. For masks, lampshades and larger packaging.
• FORMART 2 (€3,018.95): the same format plus QR-code material detection, automatic mould release and stepless suction. For regular forming.

Each machine is available on its own or in Starter and Advanced bundles with sheets. For a detailed comparison, see FORMART S vs SE vs FORMART 2, and if you want to work with food, read how to make chocolate moulds with a 3D printer and a vacuum former.

Frequently asked questions

Can you vacuum form over a 3D printed PLA mould?

Yes. The sheet is thin and cools quickly, so a PLA mould is not necessarily damaged. In MY YARD's tests a PLA mould lasted about 3–5 formings, ABS about 30–50 and resin hundreds, so PLA is best for tests and one-offs.

Which filament is best for vacuum forming moulds?

Print prototypes in PLA because it is fast and accurate. For more cycles, use a more heat-resistant filament such as PETG, ABS or ASA, or a resin print. For long runs, cast a plaster or cement copy of the mould from the first formed shell.

What draft angle does a vacuum forming mould need?

MY YARD recommends a draft angle of more than 3° on vertical walls. More draft makes it easier to release the formed sheet, especially on deep parts and female moulds.

How big should vent holes be in a vacuum forming mould?

MY YARD recommends vent holes of 1.5–2 mm in diameter in recessed areas, so the vacuum can pull the sheet into them. Model them as channels through to the underside of the print.

What sheet thickness should I use for vacuum forming?

FORMART machines take sheets from 0.3 to 3.0 mm. Thin sheets of 0.3–0.5 mm capture fine detail for packaging and chocolate moulds; thicker sheets of 0.8–1.5 mm give stiffer parts and cope better with deep draws.

How do I stop webbing when vacuum forming?

Space moulds further apart, round sharp corners, keep tall moulds in proportion and use the webbing remover kit supplied with every FORMART. On the FORMART SE and FORMART 2, the Volume Control Kit adds an adjustable separator.