Short answer: choose abrasion-resistant acrylic when the window lives on wipe-downs and coolant splash, and polycarbonate when it can take a hit. Then mount the sheet so it floats: clamp strips or a frame over rubber liners, fastener holes about 1/8 in [3 mm] oversize, and roughly 0.03 in [0.75 mm] of clearance per foot of window across a 35 C swing. Clear sheet rarely fails in the middle; it cracks at tight bolt holes that gave it no room to move. The table below puts typical datasheet numbers side by side, the worked example sizes the clearance, and the last table makes the call by environment. The same trade repeats on full door glazing, which the acrylic vs polycarbonate window article covers for door windows.
Acrylic and polycarbonate side by side
Impact is the headline number. At 1/4 in [6 mm] thickness, polycarbonate takes about 30 times the notched-Izod hit of acrylic, which is why machine-guard specifications default to it. Acrylic answers with a harder, clearer surface, about 92% light transmission against 88%, that survives years of rag work. Everything else sits close enough that two questions carry the choice: can the sheet be struck, and how often do you wipe it?
| Property | Acrylic (PMMA) | Polycarbonate (PC) |
|---|---|---|
| Notched Izod impact at 73 F, 1/4 in sheet | about 0.4 ft lb/in | about 13 ft lb/in |
| Light transmission, clear sheet | about 92% | about 88% |
| Continuous service temperature | about 160 F [71 C] | about 240 F [116 C] |
| UL 94 flammability grade | HB | V-0 |
| Toughness in cold | brittle approaching freezing | ductile to about -40 F [-40 C] |
| Surface against wiping | hard, wipes clean for years | scores readily unless hard-coated |
| Coolants and chemicals | solvent crazing at stress points; tolerates coolant splash | some water-based coolants cloud it; hot alkali attacks it |
| Linear expansion coefficient | about 70 × 10⁻⁶ per °C | about 65 × 10⁻⁶ per °C |
Impact, temperature, transmission and flammability figures are typical datasheet values for clear sheet; the datasheet of the specific grade wins where it differs. The expansion coefficients are handbook values for the base resin.
Where acrylic wins
Acrylic is the visibility material. Its harder surface shrugs off the dry-rag and mild-cleaner wipe-downs that score polycarbonate, so the window you can still see through after a year of daily cleaning is usually acrylic. In coolant-mist areas it holds that clarity: the splash that clouds polycarbonate leaves acrylic alone, provided solvents stay off it near stress points. It is also the stiffer sheet, around 3.2 GPa flexural modulus against 2.4 GPa at typical datasheet values, so a large window bows less across its frame. For observation windows up to about 24 x 18 in, 3/16 in [5 mm] acrylic is plenty; step to 1/4 in [6 mm] on bigger openings or wherever a knock is possible.
Where polycarbonate wins
Polycarbonate is the impact material, and the margin is not close: about 13 ft lb/in of notched Izod against 0.4 ft lb/in for acrylic at 73 F. It also runs about 80 F hotter in continuous service, 240 F [116 C] against 160 F [71 C], carries a V-0 flammability grade under UL 94 where acrylic sits at HB, and stays ductile to about -40 F [-40 C] while acrylic turns brittle near freezing. Those numbers make it the default beside drives, in full sun, and anywhere a part or tool can reach the window. Treat a guard window as a safety part: a cracked polycarbonate sheet comes off the machine that day, and a cracked acrylic guard never goes on at all.
How to mount the sheet so it floats
Both materials fail at holes and clamped edges long before they fail mid-span. Mount the sheet so it can move: clamp strips top and bottom, a pair of clips at each end, or a full frame capturing the sheet over rubber. Line every metal-to-sheet face with EPDM or neoprene strip so the clamp spreads its load, drill fastener holes 1/8 in [3 mm] oversize (a 0.25 in [6 mm] hole for a #10 screw), and snug the screws until the rubber compresses. Do not pull the sheet hard against dry metal through its holes; that combination of point load and zero clearance is where crazing rings start.
Leave free play on every edge: about 0.03 in [0.75 mm] per foot of window across a 35 C swing, worked through in the next section. A 16 in acrylic sheet grows 0.04 in between a 10 C morning and a 45 C afternoon, and a sheet bolted tight has nowhere to put that growth except into its holes.
A window is also the door's seal. A stainless door comes rated NEMA 4X / IP66 by the manufacturer; not UL listed. That rating holds while the frame compresses the window's perimeter gasket evenly, about 25-30% squeeze, over the full cutout; one dry corner sets the whole door at that corner's performance. The IP rating chart guide explains what the second digit tests. The cutouts, glands and HMI windows guide carries the cutout fundamentals, mounting glass in a door cutout with a hidden frame floats glass the same way, and what a field hole does to the rating covers openings cut after the box arrives.
Clearance for a 16 x 12 in window across a 35 C swing
The inputs, assumed so you can swap in your own: a 16 x 12 in acrylic sheet, 1/4 in [6 mm] thick, in a stainless door that sits at 10 C at morning start-up and reaches 45 C after an afternoon of sun and spindle heat, a 35 C swing. The handbook linear-expansion coefficient for acrylic is about 70 × 10⁻⁶ per °C; polycarbonate runs about 65 × 10⁻⁶ per °C.
The rule in words: growth equals coefficient times length times temperature change, ΔL = α × L × ΔT, on every dimension of the sheet.
The arithmetic:
- Width: 0.000070 per °C × 16 in × 35 °C = 0.0392 in
- Height: 0.000070 per °C × 12 in × 35 °C = 0.0294 in
The result: the sheet grows about 0.04 in along its width and 0.03 in along its height. Polycarbonate at 65 × 10⁻⁶ per °C gives 0.036 in and 0.027 in, the same call to one decimal. Both round to the shop rule of 0.03 in [0.75 mm] per foot of window per 35 C swing, since 70 × 10⁻⁶ per °C × 12 in × 35 °C = 0.029 in per foot.
The decision: give each dimension at least its growth figure as total clearance, split evenly either side of the sheet, and let the rubber liners close the gap while the sheet is cold. Recalculate outside the rule's range. A 50 C swing or a sheet wider than 24 in pushes growth past 0.08 in on the width (0.000070 per °C × 24 in × 50 °C = 0.084 in), and a clamped-tight sheet converts every bit of that into stress at the holes.
The decision by environment
| Environment | Window | Basis |
|---|---|---|
| CNC enclosure, observation side, coolant mist | abrasion-resistant acrylic, 3/16 to 1/4 in [5-6 mm] | 92% transmission survives wipe-downs; splash leaves it clear |
| Splash or chip zone, parts can reach the window | polycarbonate, 1/4 in [6 mm], hard-coated if wiped often | about 13 ft lb/in Izod at 73 F; guard duty |
| Outdoor, full sun, public access | UV-stabilized polycarbonate | 240 F [116 C] continuous; ductile at -40 F [-40 C] |
| Food or pharma washdown on 316L | acrylic where chlorine sanitizers dominate, polycarbonate where hot caustic does | chlorine crazes acrylic; hot alkali attacks polycarbonate |
| Indoor panel, dust only, large window | acrylic, 1/8 to 3/16 in [3-5 mm] | clarity and stiffness, no impact duty |
Two rows deserve a sentence. On a bench-top mill door the same trade plays out at small scale, and polycarbonate vs acrylic for CNC mill door windows walks through it at guard sizes. On washdown, match the sheet to the sanitizer list rather than to habit: a plant running both chlorine and caustic usually puts polycarbonate in the windows nearest the caustic. Stainless is our default body for those rooms; where the walls themselves are plastic, the polycarbonate vs fiberglass guide covers the body choice.
What to put on the order
- Sheet: acrylic (PMMA) or polycarbonate (PC), thickness — 3/16 in [5 mm] for small observation windows, 1/4 in [6 mm] for guard duty
- Grade: abrasion-resistant acrylic, hard-coated polycarbonate, UV-stabilized grade outdoors
- Opening: door cutout sized to the sheet plus its clearance figure — 0.03 in [0.75 mm] per foot across a 35 C swing
- Mount: clamp frame or strips, EPDM or neoprene liners on both faces, fastener holes 1/8 in [3 mm] oversize, perimeter gasket compressed about 25-30%
Our custom enclosures take the door cutout from your drawing, position, size and corner radius, and build the box around it from one piece, delivered in 10 business days from your approval of the drawing. Send the sheet material and the opening size with the drawing and the door arrives ready for the glazing you chose. For a standard box, the stainless enclosure shop filters every wall-mount and HMI size by grade.
Catalog dimensions to compare
Compare these catalog dimensions with your component layout. Open each enclosure for its rating, material and usable space, then check those specifications against your application.
| Enclosure | Material | Outside H x W x D / display | Rating |
|---|---|---|---|
| 16 × 12 × 8 in | 304 stainless | 16 x 12 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 24 × 20 × 8 in | 304 stainless | 24 x 20 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 30 × 24 × 10 in | 304 stainless | 30 x 24 x 10 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 12 × 10 × 6 in | 304 stainless | 12 x 10 x 6 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
How much bigger should the mounting holes be than the screws?
Drill fastener holes about 1/8 in [3 mm] oversize, so a #10 screw sits in a 0.25 in [6 mm] hole. Keep each hole at least 0.5 in [12 mm] from the sheet edge and clamp through EPDM or neoprene strip. That leaves room for the 0.04 in a 16 in acrylic sheet grows across a 35 C swing, so the strip holds the sheet without starting a stress ring at the hole.
Which window material survives coolant mist better?
Abrasion-resistant acrylic. It wipes clean for years in splash and mist, while polycarbonate scratches unless hard-coated and some water-based coolants cloud it. Both craze around tight drilled holes, so float the sheet with about 0.03 in [0.75 mm] of clearance per foot. Clean with mild soap and water, which neither material objects to, and keep acetone and similar solvents well away from acrylic.
Does adding a window affect a NEMA 4X rating?
Only through the gasket. Our stainless doors come rated NEMA 4X / IP66 by the manufacturer; not UL listed, and the rating holds while the window frame compresses its perimeter gasket evenly, around 25-30% squeeze, over the whole cutout. One dry corner lets water track past and sets the whole door at that corner's performance, so clamp evenly and re-check the screws after the first thermal cycle.
Acrylic or polycarbonate for an outdoor enclosure in the sun?
UV-stabilized polycarbonate. It stays ductile through a -40 F [-40 C] night and runs to about 240 F [116 C] continuous service against 160 F [71 C] for acrylic, so a sun-heated door is no problem. Acrylic also yellows and crazes faster under UV. Specify the UV-stabilized grade, not plain sheet, and give the sheet the same 0.03 in per foot of floating clearance.
