Thermoforming heats a plastic sheet until it is pliable, forms it over or into a tool using vacuum or pressure, cools it and trims the excess. The tooling is a fraction of the cost of an injection mould because it only has to define one side of the part and does not have to contain injection pressure, and that single fact explains most of where the process is used.
Vacuum, pressure and twin sheet
Vacuum forming pulls the softened sheet onto the tool and is the simplest and cheapest. Pressure forming pushes from the other side as well, which gives sharper detail, crisper radii and moulded in texture, and is what makes a thermoformed enclosure look like an injection moulded one. Twin sheet forming joins two formed sheets into a hollow part, which is how large hollow structures get made.
What it is genuinely better at
Large parts, where an injection tool and the press to hold it would be enormous. Low to medium volumes, because tooling is cheap and quick. Thin walled packaging at very high speed on roll fed machines. And parts where only one surface has to be right, which is a great many covers, trays, liners and enclosures.
The comparison with injection moulding
Injection moulding gives detail on both faces, tight tolerances, bosses, ribs and metal inserts, at high tooling cost. Thermoforming gives one good face, looser tolerances, a wall thickness that varies with draw depth, and cheap tooling. The comparison is only meaningful once volume and part size are known, because they usually decide it before anything else does.
Design realities to plan for
Wall thickness thins as the sheet stretches, so deep draws are thinner at the bottom, which has to be allowed for structurally. Generous radii and generous draft are essential. Undercuts need moving tool sections or must be trimmed in afterwards. And trimming is a real operation with its own tooling, often a router or a die, that is easy to leave out of a budget.
Questions people ask about thermoforming services
What is thermoforming?
Heating a plastic sheet until pliable, forming it against a tool with vacuum or pressure, cooling it and trimming the excess material away.
Is thermoforming cheaper than injection moulding?
Much cheaper to tool and more expensive per part. The crossover depends on part size and volume, and for large parts thermoforming can win at surprisingly high volumes.
Does the wall thickness stay even?
No. The sheet thins as it stretches, so deep sections are thinner than the starting sheet. The design has to account for that where strength matters.
Can thermoformed parts look moulded?
Pressure formed parts with moulded in texture and sharp radii come close, and are used for enclosures and covers where an injection tool cannot be justified.