Acrylic sheet

Acrylic is also known by the trade names Perspex and Plexiglas. Sold mainly as a clear glazing sheet, it must be worked correctly to avoid cracking and breaking.

Main properties

Acrylic looks like glass, but has 10-20 times the impact resistance for the same size, as it's able to flex when hit, absorbing the impact before breaking. It's much more able to survive stray footballs, bricks etc. Thinner is not as tough of course.

Acrylic insulates better than glass, but less than twinwall polycarbonate.

It's not recommended for use as the sole layer of domestic glazing, as it melts & burns in a fire. This creates a security issue. It also can be cut more quietly than glass, though not silently.

Acrylic scratches & clouds if rubbed repeatedly. This includes cleaning with unsuitable materials, such as scourers.

Acrylic provides much less noise reduction than the same thickness of glass, due to lower weight & much less rigidity.

Acrylic loves to crack & break. Beware of buying sheets with small cracks at the edges, these are liable to spread, sometimes splitting the whole sheet in 2.

Main uses

Secondary glazing
Shed glazing
Child safe interior glazing
Tanks for fish & vivariums
Display cases, racks, stands etc
Gardening bells
Decorative lighting with LEDs
Plastic optics

Thickness & type

4mm is rigid, and suitable for secondary glazing, shed glazing, tanks, stands etc.

2mm isn't fully rigid. Its usable for shed windows, but less secure. It can be used in a frame as secondary glazing for little windows, but doesn't give the noise reduction of heavier stiffer glazing. 2mm can be used where it won't lie entirely flat, such as non-frame type shed windows, and mildly curved applications. Mild curving can be used to increase rigidity.

Various other sizes and shapes are available. Thinner sheet isn't always cheaper.

Cast acrylic costs more than extruded, and has rather poorer tolerances. It has less internal stresses and glues better. It has superior flatness, better surface finish and more thermal stability.

Extruded is cheaper, more ductile, and preferred for bending and forming.

Acrylic is also available in colours. It has been spotted at eye watering prices of around £1000 a piece at one diy suppliers!

Other options

Polycarbonate is softer with worse scratch resistance, tending to go cloudy over time. But it has 10-20 times the impact strength of acrylic.

Twinwall & multiwall polycarbonate provide a lot more insulation than clear sheet, but aren't clear like glass, and cost a lot more.

Down the food chain in clear sheet plastics is polystyrene, available in clear sheets but usually not super optically clear, tends to have visible swirling in it, brittle and decays in UV, but considerably cheaper than acrylic.

Cutting

Breaking and cracking often happen due to wrong cutting technique. The usual problem is basically applying too much force, or using a saw with too coarse teeth.

There are several effective ways to cut it, including:

Score and snap. Fast, but a little risk of it going wrong.
Electric saw - always take it gently, use a fine tooth metal cutting blade, clamp the acrylic between 2 strips of wood, support the sheet well near the end of the cut, and remove the blade from the work as soon as it stops.
Hand sawing - as above
Die grinder - use a small flat abrasive wheel to melt your way along.
Hot wire cutter
Router - but keep the cutting bit cool if you're going to glue it. Go gently
Have a machine shop cut it
Laser cutting services available online - smooth polished edges, but makes gluing harder

Score & snap: Clamp the metal guide edge in place, it will move otherwise. Make a deep score with several passes of the knife. Repeat on the other side. Place the sheet over a square edge to break, otherwise the crack will wander off course. It's worth clamping 18mm ply onto both sides to avoid the crack going off course. Engage maximum optimism and snap it. Go and buy another sheet.

Sawblades should have at least 3 teeth in the work at any one time.

Drilling

A regular twist bit will likely crack it. Rregrind a [[drill bit] to zero rake angle. Be very gentle. Be gentle when the bit breaks out, be prepared to pull upwards to take the weight of the drill during this phase. When deep drilling pieces, use coolant & clear swarf reguarly.

Its also possible to make holes with a small pointed grindstone in a die grinder. These just melt through with far less crack risk. Withdraw the stone as soon as it goes through, or it won't then withdraw as plastic hardens on it.

Machining

The same basic principles apply to all types of machining. Heat is the main problem. Keep pressure or feed rate low and speed moderate. Water, air or water mist coolants allow more feed rate.

Laser etching and cutting work well.

With hand tools, use nonaggressive cutting angles to avoid cracking.

Edge finishing

Edges can be filed, sanded, or routed gently. Once suitably shaped, the simplest way to get a good finish is to wipe once with solvent. This restores shine. Don't wipe it a 2nd time, the plastic goes temporarily gooey.

Edges can also be flame polished, but this isn't suitable for edges due to be glued.

Gluing

Solvent can often be used alone for gluing. The surfaces should be close mating, and the joint pressed closed during setting to ensure a gap free joint.

Methylene chloride is the solvent of choice for acrylic. Acetone, cellulose thinners, and some other solvents also work. Commercial mixes for gluing acrylic are available too. These solvents all have sizeable safety issues and good ventilation is necessary.

Dissolve some shavings in the solvent for a quick drying gap filling glue.

Superglue can also be used, but it mars the appearance.

Laser cut edges and flame polished edges need annealing before gluing, or it stress cracks. More

Bending

Acrylic softens at 80C. The following work:

An IR heating element close to the acrylic is good for bending along a straight line. Use a jig to control the bend. A more precise bending line can be achieved by covering an element except for a 3/8" slot.
Hot air gun with nozzle
Boiling water for bending large areas or slump moulding

Cleaning

A lot of cleaning products damage acrylic. Use cool water with a little detergent. Avoid all types of scourer or scraper, and minimise cloth use, as loose grit can scratch.

Refinishing

If the surface is scratched or cloudy, either grind & polish, or wipe once with solvent. Commercial scratch removing polishes are very fine abrasives. Polishing blemishes out can take a lot of work.

Safety

Acrylic has no specific safety issues, other than melting and flammability in a fire, which limits its use as glazing.

The solvents used with it have more serious issues, one should be properly informed before using them.