PMMA Thermoforming: Forming Acrylic Without Losing the Clarity You Paid For

Why form acrylic rather than mould it

Injection moulding gives a more precise acrylic part with a better surface and no trimming. Thermoforming gives a cheaper part at low volume, in sizes that no mould could economically produce, from sheet that is already optically perfect.

The economics decide it. A 1,200 mm display window cannot be injection moulded at any reasonable cost. A run of 200 covers for a machine needs a tool in two weeks, not twelve. And a formed part starts from sheet that has already been polished or textured by the extruder, so the former inherits an optical quality it does not have to create.

The trade is that the former has to preserve that quality through a heating cycle, and acrylic is unforgiving about it. The failure modes are specific: stress whitening at radii, bubbles from moisture, and crazing that appears weeks later.

Drying: the defect you cannot see at first

PMMA absorbs about 0.3 percent moisture. At forming temperature that moisture becomes steam, and steam in a softening sheet produces bubbles that are visible through the thickness — a defect that is fatal in a transparent part.

Cast acrylic absorbs less than extruded and dries faster. Extruded sheet can hold enough moisture to bubble after a week in a humid warehouse, and it gives no warning. The target is 0.05 percent, reached at 80 °C in four to six hours for a 3 mm sheet.

Cast sheet should be dried slowly and evenly. A rapid heat-up on a thick cast sheet can produce internal stress that shows as a faint haze after forming, and it cannot be removed.

The forming window and the colour of the failure

PMMA forms between 155 and 190 °C at the surface. Below 155 the sheet is stiff and the material whitens at every bend. Above 190 it sags badly, the surface takes on a fine orange-peel texture, and bubbles appear more readily.

The failure at the cold end is distinctive and worth learning to read. Stress whitening appears as a white, slightly opaque band on the outside of a radius, and it is permanent. It is not a surface mark that can be polished out; the polymer has yielded and craze lines have formed through the thickness.

The failure at the hot end is sag. A 3 mm acrylic sheet at 190 °C will sag 30 to 50 mm across a 600 mm span, which means the sheet is no longer flat when the mould arrives. The part that results has uneven wall and often a visible flow pattern where the sagging sheet touched the mould first.

Table 1 — PMMA forming parameters. The window is 25 to 30 °C wide, and the cold end is the one that produces permanent damage.

Draw ratio and the corners that fail first

Acrylic loses wall faster than most thermoforming materials because it strain-hardens less at forming temperature. At a draw ratio of 0.6 the base retains about 55 percent; at 1.0 it retains 38; at 1.2 it is 33 and the part has lost the rigidity that justified the material.

The corner is worse. A square or rectangular cavity has four corners where the sheet stretches in two directions at once, and the corner wall retains about 70 percent of what the base does. That is where acrylic tears during demoulding, and it is where a stress crack starts in service.

The practical limit for acrylic is a draw ratio of 0.9 for a part that must stay clear, and 1.1 for a part where some thinning is acceptable. Beyond that, plug assist is required, and a plug on a transparent part leaves a mark that may be visible through the finished component.

Table 2 — Wall retained in PMMA against draw ratio, vacuum only. Percentages are of original sheet thickness.

Moulds: polished aluminium and a hot surface

Acrylic moulds are aluminium, machined and polished. The cavity surface finish transfers directly to the part, and on a transparent component any tool mark is magnified. A mould for a clear part should be polished to at least 800 grit and preferably to a mirror.

Mould temperature is 70 to 90 °C, which is hot by thermoforming standards and requires cartridge heaters or a hot oil circuit. A cold mould chills the sheet on contact and produces stress whitening at the point of first contact, which on a clear part is a visible white spot in the middle of the product.

Draft must be generous — 3 degrees minimum on a polished surface, more on a textured one. Acrylic grips a polished tool, and a part that is dragged out will be scratched on a surface that cannot be repaired.

Stress, crazing and annealing

A formed acrylic part carries two kinds of stress: the frozen orientation from stretching, and the thermal gradient from cooling. Both raise the likelihood of crazing, and crazing in acrylic is the beginning of a crack.

Crazing appears as a fine network of silvery lines, usually at an edge, a hole or a radius, and it appears after the part has been in service for days or weeks. It is accelerated by solvents, by assembly stress and by ultraviolet light.

Annealing removes most of the frozen stress. Two hours at 80 °C for a 3 mm part, four hours for 6 mm, then cooling at 10 °C per hour. The part comes out slightly smaller — about 0.3 percent — and markedly more resistant to crazing.

For a display window or a machine guard, annealing is optional and cheap insurance. For a part that will be drilled, bonded with solvent or mounted under stress, it is mandatory.

Trimming and bonding

Acrylic trims cleanly with a router or a saw. A steel rule die works on thin sheet and leaves a slightly ragged edge on thick sheet. The cut edge should be sanded and, for a stressed part, polished, because a rough edge is a crack initiator.

Solvent bonding with methylene chloride or a proprietary acrylic cement gives a joint that is optically clear and as strong as the sheet, if the edges are machined flat and the parts are held without movement for the full cure time. A joint that is moved while the solvent is still working will craze.

Adhesive bonding with a two-part acrylic or a UV-curing adhesive works for dissimilar materials. The surface must be cleaned with isopropanol, never with a ketone, which attacks the acrylic.