Polycarbonate (PC)
- Very tough; the usual choice where the window can be hit or dropped
- Scratches more easily and yellows in sunlight without a UV-stabilized grade
- Sensitive to some cleaners and solvents when molded-in stress is high
Molded part guide: lenses
Injection Molded Lenses
An opaque part can hide a lot. A clear part shows everything: flow marks, trapped moisture, stress frozen in during filling, and every ejector pin that touched the wrong face.
Clear parts
In a clear part, the defects are part of the view
Lenses here means cover windows, display covers, indicator lenses, and sensor windows.
Most molded lenses are not imaging optics. They are windows: a cover over a display, a lens over an indicator, a window in front of a camera or sensor. The job is to let light through without adding anything the user can see. That puts the whole molding process on display, because defects that disappear in a black part sit in the middle of the view in a clear one.
Stress is the least obvious. As the melt fills and packs, the polymer chains are stretched and then frozen in place. The part looks clear, but through a polarized screen, such as most LCDs or a pair of sunglasses, that frozen stress shows up as rainbow bands. Stressed areas are also the first to crack when the lens meets a cleaner or a solvent.
Moisture is the most common. Polycarbonate and acrylic both absorb water from the air, and pellets that are not dried before molding turn that water into steam in the barrel. It comes out as silver streaks, called splay, across the window. It is a process discipline problem, and it has to be controlled on every run, not fixed once.
Defects
What goes wrong in a clear window, and what prevents it
Each of these is decided partly in the part design and partly in how it is run.
| What you see | Cause | Prevention |
|---|---|---|
| Rainbow bands under polarized light | Molded-in stress from filling and packing | Even sections, gentle flow path, gate placed to fill the window in one smooth front |
| Silver streaks across the window | Moisture in the resin turning to steam | Drying the resin before every run |
| Distortion when looking through the part | Sink or uneven shrink where a thick mounting flange meets the window | Blend thick to thin gradually, or move heavy features away from the viewing zone |
| Haze or a dull patch | Unpolished or worn steel, or a cold flow front | Polished optical faces in the tool, and a fill that keeps the front moving |
| Round marks on the window face | Ejector pins pushing on the optical surface | Eject on the flange or on tabs outside the window |
Design
Draw a line around the optical zone
The most useful thing on a lens drawing is a boundary between faces that must be clear and faces that can carry marks.
- Keep the gate, the weld line, and every ejector contact outside the marked zone
- Put snap hooks and screw holes on a flange, not on the window, since stress collects around them
- Specify how parts are packed; clear windows scratch against each other in a bulk bag
- List the cleaners the lens will meet, because stressed PC can crack in contact with some of them
Resin
Polycarbonate or acrylic for a molded window
The choice is mostly impact against scratch, and the environment decides which one matters.
Acrylic (PMMA)
- Higher clarity and better scratch and weather resistance
- Brittle; snap features and screw points crack more readily
- A good fit for fixed windows that are protected from impact
Drawing
What to send with a lens or window
Optical requirements rarely survive the trip into a STEP file.
Lens CAD with the optical zone marked on both faces
How the window is inspected: by eye, against a light box, or through a polarizer
The bezel or housing that holds the lens, and how it is retained
Outdoor exposure, impact risk, and cleaners the window will see
Whether a coating is planned after molding, and by whom
Packing requirements to protect the clear faces
Keep reading
Related guides
Other optical parts and the defects that show up in clear resin.
Light pipes
Carrying LED light inside the product.
Light pipesDiffusers
Frosted panels instead of clear ones.
DiffusersBezels
The frame that holds the lens.
BezelsSensor housings
Windows in front of a sensing element.
Sensor housingsPolycarbonate vs acrylic
Toughness against clarity.
Polycarbonate vs acrylicPolycarbonate
Impact-resistant clear resin.
PolycarbonateAcrylic (PMMA)
High clarity, scratch resistant.
Acrylic (PMMA)Surface haze
Dull patches on clear faces.
Surface hazeMolded parts hub
Every part-type guide, grouped by family.
Molded parts hubFAQ
Questions about molded lenses
Our display window shows rainbow patterns. Is the resin bad?
Usually not. Rainbows under a polarized display come from stress frozen in during molding. Gate position, section changes, and process settings reduce it more than a resin swap does.
Some lenses have silver streaks and others do not. Why the difference?
Splay is almost always moisture. It varies with how well the resin was dried before that run, which is why drying has to be controlled every time the tool runs.
Can molded lenses replace glass?
For many cover windows and indicator lenses, yes, with lower weight and better impact resistance. Precision imaging optics are a different discipline; send your acceptance criteria and DFM review will say whether molding fits.
Do you apply hard coats?
Coatings are not part of the molding quote. If a coating is planned, say so in the upload, because it can affect resin and surface choices.
Next step
Quote the lens with its optical zone marked
Upload the lens and its housing with the clear zone and inspection method. DFM feedback covers gate and weld line location, ejection, and section changes.