Two Clear Plastics With Completely Different Personalities

Every prototype eventually needs a clear part: a window, a lens, a light pipe, a guard, a sight glass, a display cover. There are two default answers in every shop, cast acrylic and polycarbonate sheet, and they behave so differently in fabrication that choosing the wrong one shows up as a cracked, hazy, or crazed part on the day of the review.

The short version: acrylic is the optical material, clearer and glossier and easier to polish and bond, while polycarbonate is the structural material, essentially unbreakable but soft-surfaced and chemically fussy. The datasheet comparison is in acrylic vs polycarbonate. What follows is what happens when you actually make parts out of them.

The Parameters That Decide It

  • Optical clarity. Cast acrylic transmits roughly 92 percent of visible light with almost no color cast. Polycarbonate transmits about 88 percent and carries a slight yellow-green tint that stacks up visibly in thick sections. For a lens, a light pipe, or anything a customer will look through, acrylic wins.
  • Impact strength. Polycarbonate is roughly 20 to 30 times more impact resistant. Acrylic shatters into sharp pieces. If a person could be hurt, or the part is a machine guard, polycarbonate is not optional.
  • Surface hardness. Acrylic is much harder and holds a polish. Polycarbonate scratches easily, which is why glazing grades ship with a hard coat. On a display model handled by hundreds of people, an uncoated polycarbonate window looks tired within a week.
  • Chemical resistance. Both are vulnerable, differently. Acetone, alcohols, and many solvents attack acrylic. Alkalis, ammonia-based glass cleaner, and certain oils attack polycarbonate, which crazes badly when solvent hits it under stress.
  • Temperature and UV. Polycarbonate holds up to roughly 240 degrees F continuous against about 180 degrees F for acrylic, so proximity to an LED array or a power supply matters. Outdoors the order reverses: acrylic stays clear for decades, polycarbonate yellows without a UV-stabilized coating.
  • Cost. Acrylic sheet runs roughly 40 to 60 percent of the price of polycarbonate in comparable thickness.

The Quick Rule

Use acrylic when the part is being looked at. Windows, lenses, display covers, light guides, sight glasses, showpiece parts, anything that must look like production glass on camera.

Use polycarbonate when the part is being loaded or hit. Guards, structural panels, snap-in covers that flex during assembly, anything within reach of a moving mechanism, anything that gets dropped.

When both matter, the usual answer is polycarbonate with a hard coat, accepting the optical penalty, or a two-part design with an acrylic optical element protected behind a polycarbonate structure.

Fabrication: Where the Real Differences Bite

Laser cutting. Acrylic laser cuts beautifully and comes off the bed with a flame-polished, ready-to-use edge. Polycarbonate does not: it chars, discolors along the cut, and releases unpleasant fumes. If your part is a flat panel and you want it cut today, that difference alone often decides the material.

Machining. Both cut on a router or mill, but they want different tooling. Acrylic wants sharp single-flute or O-flute cutters, high spindle speed, and steady feed; it chips if you dwell and melts if you rub. Polycarbonate is gummy and wants sharp tools, aggressive chip evacuation, and often coolant or air blast. Neither tolerates a dull cutter. Job-shop pricing for either is driven by the same factors as any machined part, laid out in CNC machining cost, plus extra time for finishing.

Bonding and polishing. Acrylic solvent welds with acrylic cement into a joint that is nearly invisible and as strong as the parent material, and it sands and buffs to optical clarity. Polycarbonate does not solvent weld well, needs a structural adhesive whose joint line always shows, and polishes slowly because the soft surface overheats. Seamless bonded corners are a strong argument for acrylic.

Thermoforming. Both form, but polycarbonate must be dried before heating or it bubbles, and acrylic forms at a lower temperature with less spring-back.

What About Parts That Are Not Flat?

A curved or freeform clear part cannot come off a laser bed, and there are four practical routes.

Machined from solid block, then polished. Highest optical quality, right for lenses and small light pipes, slow and expensive on anything large because polishing hours dominate.

Thermoformed from sheet. Cheap tooling and fast, good for domes and shallow curves. Wall thickness varies with draw depth and optical quality is only fair.

Cast in clear urethane from a silicone mold. The workhorse for complex clear parts in small quantities. Clear urethanes reach good transparency and take tints well, but they yellow over time and are softer than acrylic, so they suit models rather than long-service parts. Costs and quantities are in urethane casting.

Printed in clear resin, then finished. Straight off the printer a clear SLA part is translucent at best. It becomes genuinely transparent only after wet sanding through several thousand grit and buffing or clear coating, which is hours of hand work per part. Worth it for one lens, not for twenty. The process tradeoff is in FDM vs SLA and the technique overlaps with painting and finishing prototypes.

Three Mistakes That Ruin Clear Parts

Crazing from stress plus solvent. A clear part clamped by an overtightened screw and then wiped with the wrong cleaner develops a spiderweb of fine cracks within days. This is the number one cause of ruined clear prototypes. Prevent it with clearance holes and shoulder washers instead of screws bearing on the plastic, generous radii instead of sharp internal corners, annealing after machining, and a written instruction to clean only with mild soap and water.

Removing the masking too early. Both sheet materials ship with protective film. Leave it on through every operation and peel it at the last possible moment.

Ignoring internal stress. Sheet stock carries residual stress from manufacture and machining releases it, so parts warp after cutting and craze around holes. Annealing, a controlled soak below the softening point followed by slow cooling, relieves it. Budget that cycle on any clear part with tight flatness or a bonded joint.

Thinking One Step Ahead to Production

A prototype clear part made from cast acrylic sheet does not automatically translate to production. Injection molded clear parts are usually PMMA, polycarbonate, or a clear copolyester, and they bring their own problems: flow lines and weld lines that are invisible in an opaque part and glaring in a clear one, sink marks that distort optics, and gate vestige placement that has to move out of the viewing area. Those defects and their causes are covered in injection molding defects.

So decide early. If the production part will be molded polycarbonate, prototype in polycarbonate even though acrylic would be easier, because you want to see the tint and the scratch behavior now rather than at first article. Matching the prototype material to the intended production material is cheap insurance.

Get Your Clear Parts Made Right

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