Lighting samples from another process
- Different surface and section, so the scatter and brightness are different
- No weld lines or gate blush, so lit defects stay hidden until molding
- The lighting sign-off has to be repeated on molded parts
Molded part guide: diffusers
Injection Molded Diffusers
A diffuser is judged with the lights on. That changes what counts as a defect: a section change, a snap hook on the back, or a weld line nobody noticed on the bench can all show up as a dark or bright patch once the LEDs are behind it.
The glow
Turning a row of bright dots into one even surface
Status bars, lit logos, appliance displays, and lamp covers all use molded diffusers.
LEDs are small and intense. Put a clear cover over a row of them and the user sees a row of dots. A diffuser scatters the light so it leaves the surface in every direction, and the dots blur into a band. How well that works depends on two things the part design controls: how far the diffuser sits from the LEDs, and how strongly it scatters.
Distance does a lot of the work. Light from neighboring LEDs spreads and overlaps before it reaches the diffuser, and the farther the diffuser sits relative to the LED spacing, the more even the light already is when it arrives. When the product is thin and the diffuser must sit close, the plastic has to scatter harder, and stronger scattering also absorbs more of the light.
The part itself becomes visible when lit. In a diffusing resin, a thicker section scatters and absorbs more than a thinner one, so a change in section shows up as a change in brightness. Anything on the back, such as a snap hook or a screw post, casts a shadow through the front face.
Lit defects
Problems that only appear with the lights on
Inspect diffusers lit, in the product, or these will be found by the customer instead.
| What you see lit | Cause | Design response |
|---|---|---|
| Visible dots over each LED | Diffuser too close to the LEDs for their spacing | More depth between board and diffuser, closer LED spacing, or stronger diffusion |
| Dark band or darker patch | Thicker section, or sink over a feature on the back | Keep the lit area at an even section; move features outside it |
| Shadow shaped like a hook or post | A snap hook, post, or stiffener behind the lit face | Retain the diffuser at its edges, outside the lit zone |
| Thin line or streak across the glow | Weld line or flow line invisible when unlit | Gate to fill the lit area in one front; place the seam outside it |
| Cloudy halo near one edge | Gate blush at the gate location | Gate on a hidden flange rather than the lit face |
How the scatter is made
Diffusing resin, textured steel, or both
Scattering can come from inside the resin or from the surface the tool puts on it.
- Diffusion grades carry scattering particles through the whole section, so they spread light evenly but are sensitive to section changes
- A textured cavity face scatters at the surface, and the texture can be varied across the part
- Texturing the face toward the LEDs keeps the outer face smooth and easy to clean
- Hold resin supplier and grade constant, since lot changes can shift the color of the glow
Approval
Why the glow is approved on parts from the production tool
Brightness depends on the section and surface the steel produces.
First diffusers from the production steel
- What you see lit is what every production part will look like
- Texture and gating adjustments are made on the tool you keep
- The hardened steel tool is completed in 10 business days on qualified programs
Drawing
What to send so the diffuser is reviewed lit
The diffuser only makes sense with the light source behind it.
Diffuser CAD, housing, and the board with LED positions and part numbers
The lit area marked, and where hooks, posts, and gates are acceptable
A reference for the look you want: photos, a sample, or a target glow
Color of the diffuser unlit, if it is also a cosmetic surface
Heat from the LEDs and any outdoor or UV exposure
Whether users touch or clean the outer face
Keep reading
Related guides
Other optical parts and the defects that show when lit.
Light pipes
Carrying light to a small indicator.
Light pipesLenses
Clear windows instead of frosted ones.
LensesBezels
The frame around a lit panel.
BezelsInstrument panels
Lit icons on a control face.
Instrument panelsGate blush
Cloudy marks near the gate.
Gate blushWeld lines
Seams that appear when lit.
Weld linesFlow lines
Streaks across a translucent face.
Flow linesPolycarbonate vs acrylic
Base resins for diffusers.
Polycarbonate vs acrylicMolded parts hub
Every part-type guide, grouped by family.
Molded parts hubFAQ
Questions about molded diffusers
We can still see each LED through the diffuser. What should change first?
Usually the distance between the LEDs and the diffuser relative to LED spacing. If the product cannot get deeper, a stronger diffusion grade or a textured inner face helps, at some cost in brightness.
There is a shadow of a snap hook in the glow. Can the mold hide it?
Not really, because the hook is in the light path. Move retention to the edges of the part, outside the lit area.
Why do some diffusers look fine unlit and streaky when lit?
Weld lines, flow lines, and gate blush scatter light differently from the rest of the part. They are faint in room light and obvious with a light source behind them.
Can you match a diffuser we already use?
Send the existing part and its CAD. Matching the glow depends on resin grade, section, and surface texture, which are reviewed together.
Next step
Quote the diffuser with its LED layout
Upload the diffuser, housing, and board. DFM feedback covers the lit zone, section changes, retention placement, and gating.