Guards
The material choice is decided by what hits the guard. The three problems that follow — fogging, static and thread stripping — are decided by how the guard is designed around it.
Most guard specifications begin with a material and work backwards. That is how acrylic ends up on a machine that throws swarf, and how polycarbonate ends up on a display cabinet that nobody hits.
The question to answer first is: what is the guard protecting against?
| Hazard | Material | Why |
|---|---|---|
| Nothing thrown; visibility and containment | Acrylic | Best clarity, best scratch resistance, best UV life, lower cost |
| Swarf, chips, coolant spray | Polycarbonate | Deforms instead of cracking; acrylic will eventually crack |
| Tool breakage, projectile risk | Polycarbonate, thicker | Impact energy is the design case, not visibility |
| Chemical exposure with impact | Coated polycarbonate | Acrylic crazes under solvent plus stress |
| Heat above 90 °C | Neither — use glass or a high-temp plastic | Both soften well below this |
A middle option worth knowing: impact-modified acrylic. Better impact resistance than standard acrylic, better clarity and UV life than polycarbonate. For guards on moderately fast machinery it is often the right compromise, and it is underspecified because buyers tend to think in two options rather than three.
Thickness follows position and exposure, not a single company standard.
| Position | Typical thickness | Notes |
|---|---|---|
| Fixed side panel, no impact | 3–5 mm acrylic | Visibility only; stiffness from the frame |
| Hinged access door | 5–8 mm acrylic | Handled daily; needs stiffness at the hinge line |
| Window facing the cutting zone | 6–10 mm polycarbonate | Impact exposure; replace on scratching, not breakage |
| Full enclosure on a fast machine | 10–20 mm polycarbonate | Structural as well as protective; needs a metal frame |
| Ceiling panel over the work zone | 8–12 mm polycarbonate | Gravity plus impact; often overlooked |
The ceiling panel is the one people forget. It sees falling debris, hot chips and, on some machines, coolant dripping down. It is also the panel nobody inspects, so a cracked ceiling panel stays cracked.
Coolant mist condenses on the inside of guards and visibility degrades until nobody can see the work. Four fixes, in order of effectiveness:
1. Air purge. A small airflow across the inside face keeps mist from settling. Cheap, effective, and the only solution that actually prevents rather than mitigates.
2. Hydrophilic coating. A coating that spreads droplets into a film rather than letting them bead. Works well until it wears, and it does wear with cleaning.
3. Gap and drain design. A slight slope and a drain at the low point stops accumulation. This is a design decision made at drawing stage and costs nothing.
4. Wiper or removable panel. The fallback. A hinged section that can be wiped, or a panel that lifts out.
What does not work: cleaning more often. It is a symptom treatment that consumes labour and scratches the surface over time.
On electronics assembly, painting and cleanroom equipment, a guard that holds a charge attracts dust and can discharge into sensitive components.
Anti-static acrylic is available and it works, but the additive that provides conductivity also slightly reduces clarity. It is specified only where needed, not as a default.
A three-layer approach is often more robust than the material alone: anti-static sheet for the panel, a conductive frame or gasket to bleed the charge, and a ground path from the frame. The sheet without a bleed path just holds the charge somewhere else.
Note that anti-static performance degrades with cleaning. Abrasive or solvent cleaning removes the surface layer that does the work, which is why the cleaning specification matters as much as the material specification.
You cannot reliably tap a thread into acrylic for anything load-bearing, and guards are usually load-bearing because they are hinged, latched and handled.
Acrylic threads strip under repeated tightening and craze at the thread root. A guard that is opened daily will have stripped fixings within a year.
| Method | How it works | Best for |
|---|---|---|
| Through-bolt + washer + compliant pad | Bolt passes through, load spread by a flexible washer | Most fixed panels |
| Threaded insert | Metal insert pressed or bonded into the hole | Repeated assembly, serviceable joints |
| Bonded bracket | Metal or acrylic bracket solvent-bonded to the panel | Hinges on clear panels |
| Isolated bushing | Compliant bushing between bolt and panel | Vibration environments |
Whichever is used, keep holes at least 1.5× diameter from the panel edge and avoid clamping the panel hard against a metal frame without a compliant layer. Vibration plus direct contact is how guards crack at the fixings.
Acrylic and polycarbonate are not structural materials, and a guard is a structure. The frame carries the load; the panel closes the opening.
Aluminium extrusion frame with panels captured in a groove or clamped by a profile. Standard, modular, and easy to replace a single panel.
Welded steel frame where the guard is part of the machine structure. Heavier, and the panels need compliant mounting to avoid cracking at the fixings.
Self-supporting panel only for small guards where the panel is thick enough to carry itself. Above roughly 500 mm, an unsupported panel needs a frame regardless of thickness.
Guards get cleaned with whatever is available, and this is where most premature failures start.
No acetone or MEK. Both attack acrylic and polycarbonate rapidly. A single wipe can leave a permanent cloudy patch.
No ammonia glass cleaner. Common in workshops, and it crazes acrylic over repeated use.
No abrasive pads. They scratch, and scratches are where cracks start.
Approved: mild soap and water with a soft cloth, or an alcohol-free plastic cleaner. Put the list where the cleaning happens, not in a document nobody reads.
Not on the cutting zone. Acrylic will crack under swarf impact and coolant exposure, usually within months. Polycarbonate is the correct choice there. Acrylic is fine for side panels and covers that do not face the work zone.
Usually chemical attack from cleaning agents, or fine scratching from wiping. Polycarbonate has poor scratch resistance without a hard coat. If the guard is cleaned frequently, specify a coated grade and use the approved cleaning list.
Polycarbonate can sometimes be stop-drilled at the crack tip to slow propagation, but that is a temporary measure. Acrylic cracks are not repairable. In both cases the correct action is replacement, and the reason for the crack — fixing design, vibration, chemical exposure — needs addressing or the new panel will fail the same way.
5–8 mm acrylic or 6 mm polycarbonate for a typical 400 × 600 mm door. The hinge line is where stiffness matters, so a folded return or a bonded stiffener at the hinge edge is often more effective than extra thickness across the whole door.
What hits the guard and what cleans it decide both material and fixing method. Those two facts are worth more than a drawing for getting the specification right first time.
