General purpose PC or ABS
- Tough, easy to mold, and widely available in many colors
- Sensitive to many disinfectants where the part is under stress
- Works when cleaning is mild or stress is kept very low
Molded part guide: medical device housings
Injection Molded Medical Device Housings
A housing on a bedside monitor, pump, or handheld device is wiped with disinfectant many times a day for years. Most plastics tolerate the chemical on its own. What cracks them is the chemical meeting stress that was molded in or designed in, which makes stress the first thing to design out.
The failure
Cracks that appear months after launch
Monitors, infusion pumps, diagnostic readers, handheld instruments, and carts.
The usual story is a housing that passed every test, then started cracking at screw bosses and snap hooks after some months in service. The cause is environmental stress cracking. Quaternary ammonium wipes, alcohol blends, and hydrogen peroxide cleaners soak into amorphous plastics like polycarbonate and ABS and let polymer chains slip apart wherever the part is under tension. Unstressed areas survive the same wipe.
Stress comes from two places. Some is designed in: a self-tapping screw spreading a boss, a snap hook held deflected, a press-fit insert, or a sharp inside corner. Some is molded in: plastic frozen while still stretched by high packing pressure, a cold tool, or a gate that forces flow through a thin section. Both kinds add together, and the crack starts where the total is highest.
So the design work is to keep sustained stress low where wipes reach. Fasten with threaded inserts rather than self-tapping screws in housings that are cleaned hard, avoid snap hooks that stay flexed after assembly, and give inside corners a generous round. Gate into thick, cosmetic-safe areas so the part packs without stretching thin walls.
Resin choice is the other lever. Some polycarbonate blends, copolyesters, and semicrystalline resins such as PBT resist common disinfectants far better than general purpose PC or ABS. Resin suppliers publish chemical resistance data for named cleaners, and testing molded parts under strain with your actual wipes is the reliable check.
Where cracks start
Stress points on a cleaned housing, and the fix for each
Every row is a place where sustained tension meets a disinfectant.
| Feature | Why it is stressed | Design response |
|---|---|---|
| Self-tapping screw posts | The screw spreads the post outward for its whole life | Threaded inserts, or posts with lower hoop stress |
| Snap hooks | Hooks that stay flexed after assembly hold tension | Hooks that return to rest once engaged |
| Sharp inside corners | Load concentrates at the corner | Generous rounds at every inside corner |
| Areas near the gate | Highest molded-in stress from packing | Gate into a thick area away from wiped faces |
| Weld lines | Weaker bond where flows meet | Place them away from screws, hooks, and wiped edges |
Wipeable design
Seams and crevices hold fluid, so design them out
A housing that is easy to wipe is also one that keeps disinfectant off stressed features.
- Tight, consistent parting gaps stop fluid wicking into the joint where screws and hooks sit
- Smooth or fine textures clean better than coarse grain, which traps residue
- Recessed screws covered by a snap-in cap keep wipes away from the fastener
- Buttons and displays sealed with a gasket or membrane keep liquid out of the electronics
Resin
General purpose PC or ABS, or a chemically resistant grade
Resin choice is judged against the cleaning regime, not the resin price alone.
Chemically resistant blends and copolyesters
- Tolerate common hospital wipes far better under strain
- Some grades shrink differently, so the tool is cut for the chosen resin
- Higher resin cost, usually small next to a field failure
Drawing
What to send with a device housing
The cleaning regime belongs on the drawing as much as the dimensions do.
Housing CAD with fasteners, snap hooks, and inserts shown
The disinfectants and cleaning frequency the device will see
Resin, or the requirements if the resin is still open
Cosmetic faces, color, and texture
Drop, impact, and ingress requirements
Labels, legends, and markings, and how they are applied
Keep reading
Related guides
Other housings, and the rules that keep stress low.
Enclosures
Splitting, fastening, and sealing a shell.
EnclosuresElectronic enclosures
Boards, shielding, and heat.
Electronic enclosuresSurgical instrument handles
Grips that are cleaned and reused.
Surgical instrument handlesSelf-tapping screw bosses
Posts that hold screws without cracking.
Self-tapping screw bossesWeld line strength
Keeping seams away from loaded features.
Weld line strengthPC/ABS blend
A common housing resin and its limits.
PC/ABS blendPBT
A semicrystalline option for chemical exposure.
PBTFamily mold for device housings
A case study on molding housing halves together.
Family mold case studyMolded parts hub
Every part-type guide, grouped by family.
Molded parts hubFAQ
Questions about molded device housings
Our housings crack only at the screw posts. Is the resin wrong?
Maybe, but the posts are the real signal. A self-tapping screw holds the post in tension for life, and a disinfectant finds that tension first. Threaded inserts or a lower-stress post design often fix it without a resin change.
Can molding settings reduce cracking?
They help. A warmer tool, controlled packing, and a gate in a thick area all lower molded-in stress. They cannot cancel stress that the design adds, so both are reviewed together.
Why do white housings turn yellow over time?
Light, heat, and some cleaners discolor certain resins, polycarbonate and ABS in particular. Stabilized grades and color matched to aged samples reduce the visible change.
Can both halves be molded in one tool?
Often, in a family mold, when the halves are similar in size and resin. It keeps color and texture matched between halves.
Next step
Quote the housing with its cleaning regime
Upload the housing and list the disinfectants it will see. DFM feedback covers fasteners, snap hooks, gating, weld lines, and resin.