The short version: acrylic (PMMA) is the better-looking, more scratch-resistant, cheaper, more weather-stable clear plastic, and polycarbonate is the far tougher one. If a clear part is a lens, a light pipe, a display window that will not be struck, or an outdoor sign, acrylic is usually right. If it can be dropped, impacted, flexed, or has to survive a safety test, polycarbonate is usually right. Transparent parts are the least forgiving features in a product, so the choice deserves more thought than it usually gets.
Why Clear Parts Punish Sloppy Design
On an opaque housing, a slight sink mark, a flow line, or a knit line is invisible under texture and paint. On a transparent part, every one of those defects is a visible feature that a customer reads as low quality. Worse, they are visible through the part, so internal geometry, gate location, and even the polish of the mold steel all end up on display. A clear part is where molding discipline becomes a cosmetic requirement.
Acrylic vs Polycarbonate: What Actually Separates Them
- Optical clarity. Acrylic transmits slightly more light and has lower haze and less yellow tint, especially in thick sections. For a lens or a thick light guide, this is a real, visible advantage.
- Impact strength. Polycarbonate is dramatically tougher — this is not a close comparison. Acrylic is brittle and cracks; polycarbonate deforms and absorbs. Machine guards, face shields, and drop-tested handhelds are polycarbonate.
- Scratch resistance. Acrylic is noticeably harder at the surface. Bare polycarbonate scuffs easily and generally needs a hard coat if it is a touched surface.
- UV and weathering. Acrylic is inherently stable outdoors and stays clear for years. Polycarbonate yellows and embrittles in sunlight unless you specify a UV-stabilized grade or a coated sheet.
- Heat resistance. Polycarbonate handles higher service temperatures. If the clear part sits over an LED array, a power supply, or a heat source, that gap matters.
- Chemical resistance. Both have vulnerabilities. Acrylic is attacked by many solvents; polycarbonate is prone to environmental stress cracking with certain oils, cleaners, and alcohols — a common field failure in medical and kitchen products cleaned with the wrong wipe. Test with the actual cleaning agent your customer will use.
- Cost. Acrylic is generally cheaper per pound and easier to process.
- Machinability and bonding. Acrylic cuts, polishes, and solvent-bonds beautifully — a gift for prototypes. Polycarbonate machines fine but polishes less easily.
Optical Quality Starts With the Mold
A clear molded part inherits the surface of the tool. That means high-polish steel, careful venting, and a gate placed where the flow front will not leave a visible mark across the viewing area. Weld lines around a boss or a hole in the middle of a clear window are permanent. Both materials must be dried thoroughly before molding — moisture produces splay and streaks that no amount of process tweaking removes, and polycarbonate is especially unforgiving here.
Molded-in stress is the other invisible enemy. Both materials show internal stress under polarized light, and stressed polycarbonate is the part that cracks six months later at exactly the point where the flow was constricted. Slow, even fill, generous radii, and balanced cooling matter more than usual. Where the parting line falls decides where a visible witness mark lands on a clear face, so plan that deliberately — see draft angles and parting lines.
Design Rules for a Transparent Part
- Keep the viewing area uninterrupted. Move bosses, ribs, and snaps to a peripheral flange outside the clear window.
- Hold wall thickness uniform across the optical area. Thickness variation distorts the image and shows as banding.
- Avoid sharp internal corners. They concentrate stress in a material that will show you exactly where the stress is.
- Design a light seal. If the part covers an illuminated display, plan an opaque bezel or a masked border so internal light does not leak around the edge.
- Decide the finish deliberately. A polished surface looks premium and shows every fingerprint. A lightly textured or matte surface hides handling marks and diffuses glare. A first-surface anti-glare or anti-fingerprint treatment is often worth its cost on a touched window.
- Plan for tolerance to the mating housing. A clear window that pinches against the housing will craze along the contact line.
Where a clear window sits over a screen, the interaction between the lens and the panel underneath matters as much as the plastic itself — refer to LCD vs OLED vs E-Ink for how each display type behaves behind a cover lens, and consider optical bonding if reflections between the lens and the panel are unacceptable.
Scratches, Coatings, and Field Life
If a clear part is going to be wiped, pocketed, or touched daily, plan a hard coat. Coated polycarbonate becomes competitive with acrylic on scratch resistance while keeping its toughness, which is why so many premium products use exactly that combination. For an outdoor product, acrylic's inherent UV stability often removes a whole coating requirement — one of the few cases where the cheaper material is also the more durable one. And if you need both toughness and clarity in a thin section, look at co-extruded or laminated stacks and at PC/PMMA capped sheet before assuming you have to compromise.
Cost, Packaging, and Inspection
Clear parts have higher scrap rates than opaque ones because cosmetic tolerance is tighter, and that shows up in your piece price. Build it into your cost model rather than discovering it at first production. Protect them properly: apply a peel film at the molder, avoid loose bulk packing, and require individual protection in the packaging spec. A perfect lens scratched in a shipping box is a reject you paid full price for.
Write an explicit cosmetic inspection standard — what size and quantity of specks, scratches, and flow marks are acceptable, in which zones, viewed from what distance under what light. Without it, every shipment becomes an argument. Choosing between clear materials is a subset of the broader question of how to choose plastic for your product, and where impact matters more than clarity you may be comparing ABS vs polycarbonate instead. The surface treatment of the surrounding opaque parts should be selected in the same pass — see mold texture selection — so the clear window and its bezel read as one designed object.
Projects House designs and specifies transparent parts — lens geometry, material grade, mold finish, cosmetic standards, and the supplier conversations that keep them consistent. If your product has a clear window, lens, or light pipe and you want it to look right in production rather than only in CAD, contact us through our form.