Acrylic vs polycarbonate for an enclosure window

Use polycarbonate anywhere a thrown part, a cart corner or a hose can reach the pane: for a 1200 x 900 mm machine-guard window, 6 mm [0.25 in] polycarbonate is the working floor, and 6 mm acrylic in that same spot cracks on the first tool let go. Acrylic earns its keep on dry sight glasses where nothing can strike it, because its harder surface stays clear under years of wiping. Typical datasheet numbers put the impact gap at 30 to 40 times, while the two plastics weigh within 2% of each other at equal thickness. The sections below compare the two sheet materials, give the thickness and glazing that hold up, and end with the pane to order for each environment.

Which material survives an impact?

Polycarbonate survives impacts that shatter acrylic: typical datasheet notched Izod values run about 12 to 16 ft-lb/in for polycarbonate against about 0.4 ft-lb/in for acrylic, a 30-to-40-times gap, so every window in a machine's firing line glazes with polycarbonate.

Standard notched Izod tests at 23 C [73 F] are how sheet suppliers publish impact, and the published rows sit near 0.4 ft-lb/in for acrylic and 12 to 16 ft-lb/in for polycarbonate. The gap widens in the cold: acrylic loses impact strength sharply as a pane approaches freezing, while polycarbonate keeps most of its room-temperature toughness well below -20 C [-4 F]. Fasteners tell the same story on a real door. An acrylic pane cracks when a screw bites close to an edge or an oversized hole grabs the sheet; a polycarbonate pane flexes, grips and stays in one piece.

Machine practice mirrors the numbers. ANSI B11.19 (2019) is the US standard that sets performance criteria for machine safeguards, guards included, and its 2019 edition was the first major rewrite in ten years. On a bench mill the thrown part is a real load case: an edge finder slung off a 4000 rpm spindle, a parting tool, a drill bit. Polycarbonate takes that hit as a dent; acrylic takes it as a spider-crack across the pane. The same physics picks the housing for a camera watching the cut, worked through in camera enclosure for CNC coolant, and the same comparison run across machine-tool doors is on polycarbonate vs acrylic for CNC mill door windows.

How do acrylic and polycarbonate compare side by side?

At equal thickness the two plastics differ mainly in impact and scratch behaviour: roughly 30 to 40 times the notched Izod impact for polycarbonate, a harder acrylic surface that resists wiping scratches, and continuous service near 120 C for polycarbonate against about 70 C for acrylic.

The rows are typical clear-sheet datasheet values at 23 C [73 F]; the datasheet for the named sheet you buy wins over any row here. Read the table as a decision, not a scorecard. The polycarbonate column decides any pane a machine or a person can reach. The acrylic column decides where the only enemy is dust and a rag: clarity, stiffness, clean solvent-cemented joints, and a lower price at the same size.

Where does polycarbonate win?

Polycarbonate wins every case with an impact path: machine-guard doors, panes beside a spindle, outdoor boxes near traffic, and any cover a cart corner can reach — a quarter-inch (0.25 in) PC pane is the floor for a pane up to 1200 x 900 mm in the firing line.

Containment is the whole job where chips fly. A liberated cutter or a part squeezed out of the vice arrives with real energy, and the pane has to absorb it without becoming fragments. Polycarbonate yields locally, dents, sometimes whitens, and stays in the frame; acrylic absorbs the same hit by cracking edge to edge, and a cracked guard window comes out the same day with the machine down. Any HMI cover a cart corner can reach follows the same rule.

Two references set the tone for heavy cases. OSHA's hazard information bulletin on a machine-tool observation window describes a pane built as a 1/4 in [6.4 mm] laminated glass plate with a 1/2 in [12.7 mm] polycarbonate layer behind it, separated by a gap: layered protection where thrown stock is a design case rather than a surprise. Cold sites stay on polycarbonate too, because an unheated shop at -10 C [14 F] is where acrylic's impact margin is already gone while polycarbonate still bends rather than breaks.

Where does acrylic win?

Acrylic wins where scratches are the only enemy and nothing can strike the pane: dry indoor sight glasses, meter covers behind guarding, display windows out of the traffic path, because its harder surface stays clear under years of wiping while polycarbonate clouds with hairline scratches.

A scratched acrylic pane polishes back; a scratched polycarbonate pane stays scratched. The second win is stiffness: at about 3.2 GPa against 2.4 GPa for polycarbonate (typical datasheets), an acrylic pane bows about 25% less under the same push, which helps on a tall bare sight pane. Third, joining: acrylic solvent-cements into clear, strong joints and machines to a polished edge, while polycarbonate wants PC-specific cement or polyurethane. Over electronics inside a dry cabinet, acrylic also works as a clear cover, with the checks in acrylic mounting and ESD: is a clear PCB mount safe. Acrylic sheet usually prices below hard-coated polycarbonate at the same size, which is why non-impact sight glasses stay acrylic.

How thick should the window be?

Thickness follows impact and span: 6 mm [0.25 in] polycarbonate is the floor for a 1200 x 900 mm guard door, 3 mm [0.12 in] suits a small sight window away from the spindle, and heavy machines step up to 12.7 mm [0.5 in] PC or a layered pane.

Span is the quiet variable alongside thickness.

Worked example, the weight of the big pane: a 1200 x 900 mm [about 47 x 35 in] door window at 0.25 in (0.006 m) thick has a volume of 1.2 m x 0.9 m x 0.006 m = 0.00648 m³. Acrylic at 1.18 g/cm³ (1180 kg/m³) weighs 0.00648 x 1180 = 7.6 kg [17 lb]. Polycarbonate at 1.20 g/cm³ weighs 0.00648 x 1200 = 7.8 kg [17 lb]. Glass at 2.5 g/cm³ weighs 0.00648 x 2500 = 16.2 kg [36 lb]. The two plastics land within 2% of each other, so weight never picks between them; it says plastic beats glass on a vertical sliding door, halving the pane load the gas struts carry.

Span then sets the bow. Treating a 100 mm wide strip of quarter-inch polycarbonate as simply supported over a 900 mm span, the centre deflection under a 50 N hand push is F L³ / 48 E I = 50 x 0.729 / (48 x 2.4 x 10⁹ x 1.8 x 10⁻⁹) = 36.5 / 207 = 0.18 m. That is an upper bound, because four-edge support in a frame cuts it several-fold, but the pane still bows visibly under a firm hand. Glaze all four edges, break a long span with a mid rib so no unsupported run exceeds about 600 mm, and accept the flex: containment comes from strength, not stiffness. The clamp spacing and mid-support rules for a long pane sit in long enclosure window seal: spacing and support.

How do you glaze and seal a plastic window?

Glaze plastic in a gasketed frame that clamps all four edges — a 1/8 in [3 mm] vinyl cord takes up an extrusion slot, and a bead of neutral-cure silicone or polyurethane beds the pane — because an unsealed pane leaks coolant, chips and noise at every gap.

In an extrusion slot, the seal is a cord: press the 1/8 in [3 mm] vinyl cord into the slot alongside the pane to take up the tolerance and stop the rattle, on a bed of neutral-cure silicone or polyurethane. On a machined or sheet-metal door, run a soft rubber gasket on both faces and clamp through bolts or screws; a soft gasket works best at about 25% compression, the usual design band, so build the frame to close on a known gap instead of squeezing the sheet hard.

Let the sheet move. Acrylic and polycarbonate expand at roughly 70 x 10⁻⁶ per K (typical datasheets), so the 1.2 m pane grows 1.2 x 70 x 10⁻⁶ x 30 = 2.5 mm over a 30 C swing. A pane captured rigidly in an aluminium frame bows or cracks at the fixings; sealant beds and slotted screw holes absorb that growth. Water then has nowhere to go but past the gasket, and the checks that find the leak — gasket set, fastener torque, corner gaps — sit in enclosure display window leaks: gasket and fastener checks, with the adhesive pick for a glued pane in bonding a polycarbonate lens to anodized aluminium. When the pane runs cold, fog follows, and the fixes are in prevent condensation near a cold window.

Which material for which environment?

Pick by the worst thing the pane will meet: impact calls for polycarbonate at quarter-inch or more, scratch-only duty earns acrylic, and outdoor or washdown duty needs UV-stabilized hard-coated polycarbonate sealed with neutral-cure silicone.

Acrylic appears on exactly one row, and it is the row where nothing can hit the pane. Every wet, cold, outdoor or machine-side row is polycarbonate, hard-coated where wiping or sun is part of the duty.

What goes on the order for a window?

For a machine you build, order clear UV-stabilized polycarbonate at quarter-inch thickness with a gasket frame; for an enclosure from us, put the window on the drawing so the cutout, gasket and rating travel together.

On a machine you build, the order line is a sheet: clear, UV-stabilized polycarbonate at 0.25 in [6.4 mm], cut oversize by the frame, with gasket, cord and sealant on the same list.

On an enclosure, the window is a cutout, and a cutout belongs on the drawing. We build 304 and 316L stainless boxes to your width, height, depth and cutouts from one piece; a 16 x 12 x 6 in wall-mount enclosure is a common host for a gasketed viewing window. Delivery is 10 business days, anywhere in the world, counted from your order and, for custom work, from your approval of the drawing. A cutout is also where a rating is kept or lost: with the pane in its clamped, gasketed frame the box holds its manufacturer-rated NEMA 4X / IP66 (not UL listed), and what NEMA rating a panel keeps after you cut it and mount devices walks that check. The cutout fundamentals sit in the cutouts, glands and HMI windows guide.

When the window is a display, skip the loose pane entirely. An HMI housing takes the panel PC as the front wall: a 7 in panel PC HMI housing or a 10.4 in panel PC HMI housing in 304 stainless, gasketed rear cover, IP65-type front seal with the display installed, manufacturer rated. You set size and cutouts in the custom enclosures shop, pick stainless grades in 304 stainless enclosures, and the display range in HMI housings.

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