DFM Checklist for Small Enclosures

A small enclosure looks like the easy part of a product: a board, a battery, a couple of buttons and a box around them. It's also where a lot of first builds go wrong. The lid doesn't close over the connector, a screw boss cracks the first time it's tightened, the USB cable won't plug in because its plug body hits the wall, or the molder sends the files back with a list of problems.

This is the checklist we run before an enclosure goes to a printer or a molder. It covers both, because most enclosures start as 3D prints and some go on to injection molding, and designing with the second in mind from the start saves a redesign later. If you want the bigger picture first, start with what design for manufacturing actually means.

The numbers below are general ranges from common design guidance, not specifications for your part. Every material, printer, molder and product is different, so treat them as a first draft, confirm them with test prints and with your manufacturer, and have anything safety-critical or regulated reviewed by an engineer who knows the application.

Before you model anything

Know the process and the quantity. Ten units printed on an FDM printer, two hundred printed in nylon by a service, and two thousand molded are three different designs. Decide which one you're designing for, and which one comes next.
Get the board as a 3D model. Export a STEP file from your ECAD tool with the components on it, not just the board outline. Most enclosure mistakes are a tall capacitor or a connector nobody modeled.
List everything that goes through the wall. Connectors, buttons, switches, LEDs, displays, sensors that need air or light, antennas, mounting holes, cable exits.
Decide how it opens. Screws, snaps or a sliding lid, and whether anyone ever needs to open it again for a battery or a repair.
Know where it lives. On a desk, in a pocket, on a vehicle, outdoors, near water or chemicals. That sets the material, the wall thickness and whether it needs to seal.

Walls, ribs and bosses

Keep walls a uniform thickness. In molding, thick spots cool slower than thin ones and show up as sink marks and warp. Published ranges for molded ABS and polycarbonate run roughly 1 to 3.5 mm, and small enclosures often land somewhere in the middle; your molder will tell you what works for their material and tool.
For FDM prints, size walls in whole extrusion widths. Slicers lay down lines roughly as wide as the nozzle (often about 0.4 to 0.45 mm for a 0.4 mm nozzle; check yours). A wall that's a whole number of line widths prints as clean perimeters instead of leaving thin gaps filled with zig-zag infill.
Stiffen with ribs, not thickness. Guidelines for molded ribs generally put their base at roughly 40 to 60% of the wall they stand on (some materials allow more), and toward the thin end behind a visible surface, so they don't cause sink marks on the outside face.
Don't merge screw bosses into the wall. A boss fused into a wall makes a thick spot. Stand it a little off the wall and tie it in with a rib or two.
Round the inside corners. Sharp inside corners concentrate stress and are where cracks start. A common guideline is an inside radius of about half the wall thickness, with the outside radius equal to the inside radius plus the wall thickness so the wall stays even around the corner.

Draft, parting line and undercuts (for molding)

Add draft to every face that slides out of the mold. Around one to two degrees is a common starting point for smooth surfaces; textured surfaces need more, and your molder will say how much. A part with no draft can drag, scuff or stick in the tool.
Decide where the parting line goes. It leaves a visible seam. Put it on an edge where nobody will look or touch, not across the face with your logo.
Avoid undercuts you don't need. A hook, a side hole or a snap that can't pull straight out of the mold needs a side action or a lifter, which adds tooling cost. Often a small change, such as a slot through the wall under a snap hook, lets it mold without one.

Fasteners and how it closes

Printed parts: use heat-set inserts for any screw that gets tightened more than once. Size the hole from the insert maker's chart, not by guessing: a hole that's too big leaves the insert loose, and one that's too small cracks the boss.
Molded parts: use screws made for plastic (thread-forming screws) in a boss sized to the screw maker's recommendation, or molded-in inserts if the joint gets opened often.
Snap fits are springs. They work when the bending strain stays inside what the material tolerates, and they wear if they're opened often. Prototype them in the real material: a snap that works in PLA can crack in a stiffer plastic or never hold in a softer one.
Use one screw size wherever you can, all driven from the same side. Assembly gets faster and nobody grabs the wrong screw.

Clearances and tolerances

Print a fit test first. Printed parts that slide together typically need somewhere around 0.1 to 0.3 mm of clearance per side, and more for a loose fit or a less precise printer. Published figures vary, so a short test print of a peg in a few holes is how you find the number for your machine and material.
Holes print small. On FDM printers, round holes usually come out slightly undersized. Print holes that need to be round with their axis vertical, and plan to ream or drill critical ones.
Leave room around the board. A gap of around a millimeter between the board edge and the wall is a common starting point; it absorbs board tolerance and print variation. Check component heights against the lid, including the solder joints underneath.
Only tolerance what matters. Tight tolerances cost money on molded and machined parts. Put them on the features that locate the board, seal or mate, and leave the rest open.

Openings, buttons, light and heat

Size connector openings for the cable, not the connector. The plastic body of the plug is usually bigger than the socket on the board. Measure the actual cables your customers will use.
Buttons need an actuator. A flexure or a separate plunger in the case pushes the switch on the board. Check the travel: it should press the switch fully without bottoming out on it hard.
LEDs: a light pipe or a thin window reads better than a hole, and keeps dust out.
Respect the antenna keep-out. Radio modules list an area that must stay clear of metal and copper. Metal screws, plating or a metal lid in that zone can cut range dramatically.
Let heat out. If anything on the board gets warm, give it vents or a path to the case, and remember that a sealed plastic box holds heat.

Sealing, if it has to seal

If the enclosure needs to keep water or dust out, design the gasket groove from the gasket or O-ring maker's sizing chart rather than eyeballing it, and make the lid stiff enough that it clamps the seal evenly between screws. Printed parts can leak through the walls themselves, so seal-critical prints need solid walls and often a coating. And an IP rating comes from testing the finished product, not from the design alone.

Assembly and service

Build it in CAD in the order you'll build it. If the board has to go in at an angle past a boss, or a cable has to be plugged in after the lid is on, you'll find it here instead of on the bench.
Give cables a path and a strain relief. A cable that pulls on its solder joints every time someone tugs it will fail first.
Make it obvious which way parts go together. Asymmetric features keep a lid from going on backward.
Think about the second opening. If someone replaces a battery, the screws and inserts should survive being undone.

Before you order a mold

Print the final design and assemble several units yourself, start to finish.
Plug in every connector with the real cables and press every button.
Drop it, carry it around and use it the way your customers will.
Ask your molder for a DFM review of the files before any steel is cut. A good one will flag thick spots, missing draft and undercuts, and changes are cheap at that stage and expensive after.

Most of these checks take minutes in CAD and save weeks later. If you'd like a second set of eyes on an enclosure, or the whole thing designed and prototyped, tell us what you're building.