Designing the Light Guides for the Digital Dash

Manufacturing Tradeoffs, Geometry Limits, and Why I Ended Up with SLS Nylon

One of the trickiest parts of this digital dash project hasn't been the LEDs, the PCBs, or the firmware—it's been the light guides.

These parts sit between dense SK6805-EC10 LED arrays and the visible gauge indicators. Their job is to:

• Confine light into well-defined channels
• Prevent light bleed between segments
• Shape what you see on the gauge into crisp, readable indicators

On paper, that sounds straightforward. In reality, it forces some brutal constraints on geometry and manufacturing, especially around thin, opaque walls and sharp interior corners.

This writeup walks through the manufacturing methods I evaluated, the constraints I ran into, and why I ultimately ended up with single-part SLS black nylon for the light guides.

System Architecture

1. What the Light Guides Actually Need to Do

The requirements driving the design:

Thin internal walls (~0.5 mm):

To fit within the gauge stackup and keep channel spacing tight.

Opaque at those thicknesses:

Any glow or bleed through a wall means ghosting between adjacent segments.

Sharp, well-defined internal features:

Internal corners define where one segment stops and the next begins.

Geometry that wraps around LEDs and PCB features:

The light guides have to respect LED positions, component clearances, and mechanical mounting.

And all of this has to survive automotive conditions: vibration, temperature swings, and long-term reliability.

2. Why Sharp Vertical Interior Corners Are a Problem for Machining

I initially considered CNC machining the light guides out of materials like PMMA or polycarbonate. Mechanically, these would be great. Optically, they can be polished to very high quality.

But there's a fundamental issue:

Any process using a rotating endmill cannot cut a perfectly sharp internal corner aligned with the tool axis.

Because the cutter is round:

• Every interior corner parallel to the tool axis ends up with a radius.
• Even tiny endmills still leave a noticeable fillet.
• Tool deflection and fragility get worse as the tool size shrinks.

For light separation, those radii become unwanted light paths.

For the light guides, that means:

• Rounded internal partitions → light leaks around corners
• Worse segment separation
• Visible ghosting and halos

You can try to work around this with multi-part assemblies or inserts, but that complicates the design and manufacturing a lot.

This limitation pushed me to look harder at additive methods—especially those that can create sharp internal features without tooling access.

3. Additive Manufacturing Methods I Explored

FDM (Fused Deposition Modeling)

Pros:

• Cheap
• Fast iteration
• Good for rough, initial geometry exploration

Cons:

• Layer lines and surface roughness scatter light
• Poor dimensional precision for thin walls
• Thin features tend to warp
• Structural + optical performance is not good enough

Verdict:
Great for early prototypes of shape, but not even close to acceptable for production or real optical behavior.

SLA (Resin Printing)

On paper, SLA looks perfect:

• Fantastic resolution
• Very sharp internal features
• Smooth surfaces
• Tight tolerances

But there's one non-negotiable requirement it fails:

I need walls around 0.5 mm thick to be truly opaque in the visible spectrum.

The problem:
Despite being marketed as "opaque," SLA resins just aren't fully opaque at those thicknesses. At ~0.5 mm:

• "Black" resins glow gray under LED light
• "Opaque" colored resins become weird diffusers
• Light bleeds right through the walls

It isn't just pigment loading; it's also the underlying polymer network. UV-cured resin simply doesn't block light efficiently enough in thin sections for this use.

Verdict:
SLA is fantastic for form and fit, but fundamentally fails the light-blocking requirement. It's useful for quick visual/fit prototypes, but not as a functional light guide material here.

SLS (Selective Laser Sintering – Black Nylon)

This is the method I ended up using.

Pros:

Single-part geometry:

No need for multi-piece assemblies to work around tool access. Internal channels and baffles can be built in one shot.

Opaque at thin walls:

With the black nylon SLS material, walls around 0.5 mm thick are opaque enough to prevent light bleed between channels. This is crucial: the material actually does what the datasheets and marketing often hand-wave.

Good dimensional consistency:

Plenty accurate for aligning with LED arrays and PCB edges.

Slightly textured surface that works in my favor:

Inside the baffles, a slightly matte surface helps absorb stray light instead of reflecting it.

Cons:

• Not optically clear (which is fine here—I want opaque baffles).
• Surface isn't as smooth as SLA or machined acrylic, but that's more of a non-issue for light-blocking cavities than it would be for a transmissive light pipe.

Verdict:
For this use case—opaque, thin-walled light channels with complex internal geometry—black SLS nylon hits the sweet spot between function and manufacturability. It's the process I chose for the actual light-guide parts.

SLM / DMLS (Metal Printing)

SLM is interesting because it solves one major geometric problem:

• No endmills → no tool-radius limitations
• Extremely complex 3D internal geometry is possible
• Internal corners can be very sharp

From a pure geometry perspective, SLM would actually be excellent for this: crisp internal corners, robust structure, no tool access concerns.

But there are some big deal-breakers:

Weight & Cost

• Metal prints are way heavier than nylon.
• Per-part cost is significantly higher, especially for larger pieces.

Incompatible with the rest of the stackup

• These light guides live right above PCBs and soldered components.
• A heavy, metal light guide is not ideal in that environment.

Vibration and chipping risk

• Automotive environments are high vibration by definition.
• If a metal SLM part ever chips or frets against the PCB, it can damage solder mask.
• Worst-case: small conductive flakes end up on the board → intermittent shorts over time.

That long-term reliability concern is a big one: I don't want the light guide itself to become a failure mode.

Verdict:
Geometrically wonderful, but too heavy, too expensive, and potentially risky in a high-vibration automotive environment due to the chance of chipping solder mask or creating metal debris.

4. Why SLS Nylon Won

After working through all of the above, the decision to use single-part SLS black nylon came down to a few key points:

• It's opaque enough at ~0.5 mm to prevent light bleed between segments.
• It allows single-piece geometry with all the internal channels and baffles I need, without worrying about tool access.
• The internal surface texture actually helps absorb stray light, improving contrast.
• It's lightweight, which is important when it's mounted on top of PCBs inside a car.
• It avoids the debonding/chipping/shorting risks associated with a metal part bouncing around over solder mask in a high-vibration environment.

In other words: SLS isn't the most visually glamorous process, but for opaque, thin-walled, complex internal geometry in a high-vibration automotive application, it does the job better than anything else I tested.

5. Big Picture: What This Says About the Design

This light-guide work ended up being a good example of how:

• "Best resolution" (SLA) doesn't always mean "best part."
• "Best geometry freedom" (SLM) doesn't always mean "best system-level choice."
• Classic machining hits fundamental geometry limits with internal corners.
• The "rougher" process (SLS) can be the right answer when the physics of light, wall thickness, and real-world environment are all accounted for.

The final light guides aren't just shaped by LED placement or aesthetic design—they're the result of a careful balance between optics, mechanics, manufacturing limits, and automotive reliability.