Which transparent plastic should you choose for your machine cover?
A machine cover has to protect, but often also has to provide visibility. Operators want to be able to see what is happening inside the machine without coming into contact with moving parts, splashes or flying material. That is why protective covers and guards in mechanical engineering are often made of crystal-clear plastic rather than glass or metal.
Among the transparent plastics, polycarbonate and acrylic (PMMA) are the two most widely used materials. At first glance they look very similar: both are clear, light and easy to machine. In practice, they differ in ways that can be decisive for a machine cover.
In this blog we set out the differences and show what to look out for when choosing and processing.
Why transparent plastic for a machine cover?
Transparent plastics are used in many applications as an alternative to glass. They are considerably lighter, less prone to breaking and much easier to machine. A plastic sheet can be milled to size, bent and glued into a cover that fits the machine exactly, including cut-outs, drilled holes and fixing points.
That makes transparent plastic particularly suitable for protective covers, guards, viewing windows and partition walls in machine and equipment manufacturing.
Polycarbonate: tough and virtually unbreakable
Polycarbonate stands out for its extremely high impact strength and mechanical strength. In many cases the material is practically unbreakable and retains its properties over a wide temperature range.
That is why polycarbonate is often the first choice for machine covers where safety is paramount. Think of guards around moving parts, or situations where a cover has to absorb a blow from a tool, a part or an operator.
The surface of polycarbonate is, however, softer than that of acrylic, which makes it more susceptible to scratches. If a cover is used outdoors, UV resistance is also something to watch.
Without protection, UV radiation can degrade the material over time and cause it to yellow. For outdoor applications, versions with a UV protective coating are therefore available.
Acrylic: clear, scratch-resistant and UV-resistant
Acrylic is chosen mainly when optical quality and appearance are important. The material has very high light transmission and is often perceived as clearer than glass.
In addition, acrylic is stiffer and more scratch-resistant than polycarbonate and inherently highly resistant to UV radiation, so it stays clear for a long time.
The downside is that acrylic is harder but also more brittle. The material is less able to withstand impacts and tears more easily, including during processing such as bending. Acrylic is therefore less suitable for covers that are subject to a lot of impact.
An advantage of acrylic is that the edges can be made crystal-clear. A milled edge is naturally somewhat rough and dull.
Through flame polishing or high-quality polishing, the end faces get the same clear look as the surface. This is not possible with polycarbonate.
Polycarbonate or acrylic for your application?
The choice between the two materials comes down, at its core, to the balance between toughness and hardness.
Choose polycarbonate when impact strength and safety come first. That applies to most machine guards, especially when the cover is in a busy production environment or shields moving parts.
Choose acrylic when optical clarity, scratch resistance and appearance matter most, and the cover is not subject to heavy mechanical loads. Think of viewing windows, visible cover hoods or applications where a sleek, glass-clear finish is desired.
The conditions also play a part. Will the cover be used indoors or outdoors? Will it come into contact with chemicals or cleaning agents? And does it need to be watertight? These questions help determine which material and which joining technique are the best fit.
From sheet to cover: milling, bending and gluing
A transparent machine cover is almost always made to measure on the basis of a PDF working drawing and a 3D model, such as a STEP file. The production process usually consists of three steps.
Milling. First the sheet is milled to size. The bending that follows is already taken into account here. Based on the desired internal dimension, we calculate the flat pattern of the product and the so-called bend allowance.
During bending, the length of the material changes slightly, so that deviation is factored into the dimensions in advance.
Bending. Next the cover is bent. In heat bending, the bend line is heated with a heating wire so that the material becomes plastic locally and can be set at the desired angle. This way we bend to the millimetre, up to a length of 3 metres. Up to a thickness of approximately 12 millimetres, bending is quite manageable. With thicker material or very small flanges, a V-groove may be needed to fold the material over neatly.
Bending takes craftsmanship. If the bend seam is heated for too long, the material can burn and air bubbles can form.
Thin material of two to three millimetres can warp over long lengths, and with narrow lips there is no leverage to fold the material over. In such cases a counter mould is used. Humidity also plays a part: it determines how long the material needs to be heated before it can be bent.
Gluing. Many covers are U-shaped, with two sides bent over and the side panels then glued.
UV adhesive is often used for transparent covers. It cures instantly under UV light and gives a neat, glass-clear seam. Polycarbonate and acrylic can also be glued to each other.
If the cover has to be watertight, we opt for two-component adhesive, because UV adhesive is not watertight in the long term. During gluing, static charge is removed from the material with ionised air, so that the adhesive does not run and the seam stays neat and tight.
Quality you can see
With a transparent cover, every imperfection is visible. An air bubble in the glue seam, a burnt bend line or a dull edge stands out immediately.
That is why the machinist checks the dimensions against the working drawing during bending. For complex products we first make a test piece or a trial bend from offcut material. The material is thoroughly cleaned and degreased before gluing.
This produces a cover that is not only dimensionally accurate and functional, but also looks sleek and professional.
Advice on your machine cover
Which transparent plastic suits best depends on the application. Does the cover mainly have to protect, or is appearance the priority? Will it be used indoors or outdoors? And does it need to be watertight?
Lakwijk is happy to think along with you about the right choice of material and the most suitable processing, from sheet to complete, glued cover. Contact us for advice on your machine cover or guard.
