Bending & Thermoforming of Plastics

Thermoforming encompasses all processes where a plastic sheet is heated to its softening point and then shaped using mechanical force, vacuum, or gravity. From simple line bends in acrylic to complex vacuum-formed enclosures, understanding the principles of thermal forming is essential for designing functional plastic products. This guide covers the main techniques, temperature parameters, material behavior, and common pitfalls.

Key terms

Glass transition temperature (Tg): the temperature at which an amorphous plastic transitions from rigid to rubbery. For PMMA: ~105 °C; PET-G: ~80 °C; PC: ~150 °C.
Forming temperature: the optimal temperature range for shaping. Typically 10–40 °C above Tg.
Spring-back: the tendency of a formed part to partially return to its original shape after cooling. More pronounced in cast PMMA than in extruded grades.
Draw ratio: in vacuum forming, the ratio of the depth of the formed part to its width. Higher ratios require thicker starting material.

Line bending (strip heating)

Line bending is the most common thermoforming technique in acrylic fabrication. A narrow heating element (typically a nichrome wire or ceramic strip) heats the sheet along a single line, allowing it to be folded to a precise angle. At PlexiSystem, we use line bending to create display case components, brochure holders, shelf dividers, and product stands.

Process steps

Position the sheet on the bending machine with the bend line aligned over the heating element
Heat both sides of the sheet (dual-sided heating prevents internal stress)
When the material reaches forming temperature (soft and pliable along the bend line), remove from heat
Fold to the desired angle using a jig or protractor guide
Hold in position until the material cools below Tg (~1–3 minutes depending on thickness)

Bending temperatures and minimum radii

Material	Bending temp. [°C]	Heating time (3 mm) [s]	Min. bend radius	Notes
PMMA (cast)	150–170	90–150	1.5 × thickness	Higher spring-back; overbend by 5–10°
PMMA (extruded)	140–160	60–120	1 × thickness	Less spring-back, easier forming
PET-G	110–140	40–80	1 × thickness	Excellent formability, very low spring-back
Polycarbonate	155–175	120–180	2 × thickness	Requires slow cooling to avoid stress
HIPS	100–130	30–60	0.5 × thickness	Very easy to bend, forgiving material
ABS	120–150	50–90	1 × thickness	Good formability, watch for surface marks

Avoiding bubbles in PMMA

Cast PMMA absorbs moisture from the atmosphere. If the sheet has been stored in humid conditions, water trapped in the material can vaporize during heating, causing small bubbles along the bend line. To prevent this, pre-dry the sheet in an oven at 80 °C for 2–4 hours per millimeter of thickness before bending. This is especially important for thicknesses above 5 mm.

Oven forming (free forming)

In oven forming, the entire sheet is heated uniformly in a convection or infrared oven, then quickly transferred to a mold or jig for shaping. This technique is used when the bend radius is large, or when multiple bends must be made simultaneously (e.g., a U-shaped channel or a curved display panel).

Key considerations

Uniform heating: the sheet must reach forming temperature evenly across its entire surface. Temperature variations cause uneven thickness distribution and stress.
Transfer time: once removed from the oven, the sheet cools rapidly. The window for forming is typically 10–30 seconds depending on thickness and material. Speed is critical.
Sag: heated sheets will sag under their own weight. Support on a frame or use a sag-resistant material (polycarbonate sags less than PMMA at forming temperature).
Mold material: wood (MDF), aluminum, or high-temperature epoxy tooling. Mold surface finish transfers directly to the part — polish molds for glossy parts.

Vacuum forming

How vacuum forming works

A heated plastic sheet is draped over (or into) a mold. A vacuum pump removes air between the sheet and the mold, forcing the softened plastic to conform to the mold surface. Atmospheric pressure (~10 N/cm²) provides the forming force. After cooling, the part retains the mold shape.

Design rules for vacuum forming

Draft angle: minimum 3–5° on vertical walls to allow part release from the mold
Draw ratio: keep below 1:1 (depth ≤ width) for uniform wall thickness. Deeper draws require pre-stretching or plug-assist.
Corner radii: minimum 1–2 × material thickness for all internal and external corners
Wall thickness: expect 30–50% thinning at the deepest points. Start with thicker material to ensure minimum wall thickness is met.
Undercuts: not possible without split molds or collapsible cores

Material suitability for vacuum forming

Material	Vacuum forming suitability	Forming temperature [°C]	Max. draw ratio
HIPS	Excellent	130–170	1.5:1
PET-G	Excellent	120–160	1.2:1
ABS	Very good	140–180	1.3:1
PMMA (extruded)	Good	150–180	0.8:1
Polycarbonate	Good	180–210	1.0:1
PMMA (cast)	Limited	160–190	0.5:1

Pressure forming

Pressure forming combines vacuum on the mold side with compressed air (2–6 bar) on the opposite side. This additional force produces sharper detail, better surface definition, and tighter corners than vacuum alone. It is used for high-quality housings and enclosures that approach injection molding aesthetics at lower tooling cost.

Common defects and solutions

Defect	Cause	Solution
Bubbles along bend line	Moisture in material	Pre-dry at 80 °C (2–4 h/mm)
Whitening / stress marks	Bending too cold or too fast	Increase heating time, bend slowly
Uneven bend angle	Non-uniform heating	Use dual-sided heater, check element alignment
Spring-back	Material memory (especially cast PMMA)	Overbend by expected amount, or use jig during cooling
Thin walls in vacuum forming	Draw ratio too high, uneven heating	Use thicker starting material, pre-stretch, plug-assist
Surface marks / texture	Mold surface imperfections	Sand and polish mold, use release agent
Wrinkles / webbing	Excess material in corners	Add relief cuts, adjust clamp frame tension

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