Acrylic and PLA: where plastic is the better material

Acrylic glass is brighter than glass

Acrylic glass, chemically polymethyl methacrylate, lets through 92 per cent of light. An ordinary window pane reaches around 80. The difference is visible to the naked eye as soon as the two lie side by side: acrylic looks clearer because it has no green tinge at the edge.

Then there is the weight. The density is about 1.20 grams per cubic centimetre, roughly half that of glass. An object of the same size weighs half as much and is at the same time considerably more impact-resistant. It does not break into shards.

That makes acrylic the right choice where glass reaches its limit: for large, thin panels, for objects that are meant to hang, for anything within reach of children.

The only mistake that really destroys acrylic

Acrylic is softer than mineral glass and scratches more easily. That is the well-known drawback and the more harmless one – fine scratches can often be removed completely with acrylic polish, which is not possible with glass.

The serious mistake is a different one: acrylic does not tolerate alcohols, strong solvents or harsh cleaners. They do not attack the material on the surface but cause stress cracks – fine cracks that grow from within and turn milky. They cannot be polished away, because they are not surface damage.

Window cleaner almost always contains alcohol. Wipe an acrylic surface with it once and you usually see nothing. Do it regularly and after a few months the panel turns cloudy.

The right way: lukewarm water, a drop of mild washing-up liquid, a soft cloth. No paper towels, whose wood fibres scratch. And plenty of water, so that dust is washed away rather than rubbed in.

PLA: made from plants, but not compostable in the garden

PLA stands for polylactic acid. It is a thermoplastic that is made not from petroleum but from corn starch or sugar cane. That is why it is so widespread in 3D printing and why objects made from it can have shapes that no casting and no lathe can produce.

And this is where the misunderstanding begins. PLA is considered biodegradable, and that is true – under conditions that occur in no garden. The standard for this is called EN 13432, and it refers to industrial composting: around 70 °C and about 70 per cent humidity. Such facilities exist, but household organic waste does not necessarily end up there.

In home compost or out in nature, PLA breaks down many times more slowly. So it is not a material that simply disappears after use. What it is: a plastic made from a renewable raw material instead of petroleum. That is a real difference, just a different one from the one claimed.

What PLA cannot cope with: heat

The heat deflection temperature of PLA is around 60 °C. That is low, and it has direct consequences for the living space.

A PLA object does not belong in direct sunlight behind a pane of glass, not on a windowsill above the radiator, not in a car and not in the dishwasher. In all four cases 60 °C is easily reached, and the object warps – not dramatically, but visibly and permanently.

Conversely, in a room at normal temperature nothing happens at all. A PLA object on a sideboard stays as it is.

How to recognise 3D printing

Printed objects are built up layer by layer, and these layers remain visible. Fine horizontal lines run across curved surfaces, and on sloping transitions the spacing becomes larger.

With many objects this is intended and part of the design – the lines catch light and create a texture that no other process produces in the same way. Where a completely smooth surface is wanted, printing is the wrong process.

What printing can do instead: cavities, undercuts and wall thicknesses that can be neither cast nor turned. An object with a shape that would not come out of any mould is almost always printed.

When plastic is the honest choice

There are three cases in which plastic is not the cheaper but the objectively right decision.

First, weight: where something is to hang or stand on a weak shelf, the density of acrylic, half that of glass, is a tangible advantage.

Second, breakage: in a household with children or animals, an object that gets a dent instead of shattering is simply the more sensible one.

Third, form: if a shape can only be produced by printing, there is no alternative that would deliver the same object in a different material.

The case in which plastic is not the right choice is just as clear: as an imitation. Plastic in a wood look, stone look or metal look is a promise the material cannot keep – it ages differently, it feels different, and you notice it the first time you touch it.