How on Earth: Getting down and dirty with acrylics

08/02/2018

Acrylic, Perspex, Plexiglas, they’re all the same, well sort of. Acrylic is the simple name for Polymethylmethacrylate (PMMA). Perspex and Plexiglas on the other hand are brand names.

 

 

 

Discovered in 1865, but not sold commercially until 1933, the process for making acrylic has changed little. Lighter than glass, but with a higher optical quality it was a favoured for use in aircraft and submarines in WW2. This incredible material has since been used for point of sale displays, visors, fibre optic cables, bulletproof panels, and even corneal replacement!

Acrylic is fascinating. It’s an amorphous polymer (long strands mixed together); you can usually tell amorphous material by optical clarity, other examples include PET and Polycarbonate (PC). There are two main processes to make acrylic sheet (Cast and Extrusion), but how it’s made changes the mechanical properties of the material. Here’s why:

Imagine you have two bowls of spaghetti. One is all mixed up nice in a bowl with the Bolognese sauce, its imperfect. You aren’t going to trace one piece end-to-end without getting in there with your fork. That’s cast acrylic.

Now imagine another bowl, it arrives on your table served on a six foot plate, stretched across like some sort of Heston Blumenthal gastro pub creation. The strands have been pulled in one direction. You can see where one piece starts and ends. This is extruded acrylic.

To make things easier we’ve created a table to help inform design decisions.

  

 

Cast

Extruded

Chemical resistance

More resistant to solvents

Less resistant to solvents

Laser cutting

Almost no burr

Burr on one side, can look different depending on which direction the acrylic was extruded

Laser engraving colour

Matt white

Matt grey

Heat bending direction

No difference

Behaves different depending on extrusion direction

Colour change when heated

Can change colour

Less likely to change colour

Ease of bending/shaping

Harder to bend/shape

Easier to bend/shape

Thickness tolerances

High. At 3mm (+/- 15%)

Lower. At 3mm (+/- 5%)

Scratch resistance

More scratch resistant

Less scratch resistant

Colour

Many options available (even colour matching)

Limited choices (due to high volume production run)

Cost

More expensive

Low

 

Take a look at our range of cast acrylic material.

Now available in 1000x500mm and 1000x600mm sheet sizes, perfect for maximising your bed space on your laser cutter.

 

OK so you know the history, and what material works best for your project. The big question, how do they make it?

It starts with petroleum (to produce 1kg of acrylic, you need 2kg of petrolium). The by-product of the refinement process creates what we need to produce acrylic.

Here’s the sciency bit (hang in there):

Acetone (used in nail polish remover) is reacted with sodium cyanide (used to extract gold from ore). This produces acetone cyanhydrin, which is reacted with methyl alcohol (the same stuff the ancient Egyptians used while embalming!). This creates methyl methacrylate, a monomer. When more than one monomer chain is connected together (using a catalyst) this creates a polymer. And so we have Polymethylmethacrylate or Acrylic.

 

Cast acrylic sheet

It’s pretty simple really; a mould is created with two sheets of glass separated by a window spacer. This forms a cavity. During the polymerisation process shrinkage occurs, so the glass plates must be free to move.

A pre-measured volume of methyl methacrylate and the catalyst is added.

The moulds are moved to an oven or autoclave for polymerisation, this runs on a pre-programmed temperature cycle to prevent bubbles forming. Once out of the mould, sheets are annealed to remove residual stress (like warping), they do this by running an additional heat cycle at around 140°C.

 

 

Find out how much you can save with our cast acrylic sheet offers.

 

Extruded acrylic sheet

Extrusion is an industrial process where material is forced through a die (which has a pre-defined shape), and at the other end comes out as sheet. Here’s how it works:

Material is loaded into a hopper, and below it a feeder allows a quantity of material to drop onto a barrel and screw below. The screw uses sheer force (like the force exerted when cutting paper with scissors) to mix the methyl methacrylate and the catalyst. The screw pushes the material to the die forme.

As the material is shaped it is pulled through a three-roll stack which handles the acrylic as it cools, it also measures thickness to ensure tolerances are met. The sheet is covered with a protective layer and then cut to size. The conditions are set to prevent residual stress, and so exhibit low shrinkage.

 

 

Find out how much you can save with our extruded acrylic sheets.

 

We hope you’ve enjoyed learning about acrylic sheet and our guide supports design thinking in the future.

Did you know?

We have over three hundred different types of acrylic materials from sheet which is perfect for laser cutting, vacuum forming, bending and much more. As well as acrylic rod, tube and balls. Everything you need for your D&T workshop online.

See the range of acrylic products for D&T.

 

 

Did you know – you can now get 3D printer filament made from PMMA, a beautiful and durable material thanks to its transparency and stiffness and is as easy to print with as PLA.

 

Check out the range of PMMA materials.