Key takeaways
- Keep the nominal wall uniform and transition gradually. Sink and warp usually start at a thick section.
- Draft is not optional. Every vertical face that ejects needs it, including bosses and ribs.
- Electrical housings need material data for flammability, tracking and temperature, not just mechanical strength.
- Agree the tool, the material and the critical dimensions before the mould is cut.
Start with a uniform wall
A moulded housing is a wall with features. The nominal wall sets the fill, the cooling and the shrink. If one section is twice as thick as the rest, it cools more slowly and pulls material from the thinner section. The result is sink, warp or a void.
A common starting range for electrical housings is 2 to 3.5 mm, depending on the material, the flame rating and the mechanical duty. Thin walls need higher injection pressure and a stiffer tool. Thick walls add cycle time and weight. The right number comes from the material and the structural requirement, not a rule of thumb alone.
Ribs and bosses without sink
Ribs add stiffness without adding a thick wall. Keep the rib base at about half to two thirds of the nominal wall, and add draft. A rib that is too thick behaves like a thick section and sinks on the opposite face. A rib that is too thin is hard to fill and may short-shot.
Bosses are the other common feature. A boss should be supported by a rib or a gusset, not stand alone. The wall of the boss should be thin enough to cool but strong enough to take the screw or insert. For a self-tapping screw, the boss diameter and the pilot hole depend on the material and the screw type.
A boss is a heat problem
A solid boss cools slowly and sinks the face behind it. Core it, support it and keep the wall consistent.
Draft, shut-offs and ejection
Every surface that moves against the tool needs draft. A typical value is 1° to 2° per side, more for a textured surface. Without draft, the part drags, scuffs or sticks. The tool may need a side action, which adds cost and maintenance.
Shut-offs are where two tool surfaces meet to close a hole or form an opening. They need enough land and a clean match. A shut-off that is too narrow wears and flashes. If the housing has a large opening, check that the shut-off is accessible for maintenance.
Material choice for electrical housings
An electrical housing is not just a box. It may need flame retardance, tracking resistance, temperature resistance, impact strength and dimensional stability. The material data sheet should be reviewed for the thickness and the condition that apply, not just the headline grade.
PA66 and PBT are common for electrical parts. PA66 is tough and has good electrical properties, but it absorbs moisture and can change dimensions. PBT has good dimensional stability and electrical properties, with lower moisture absorption. PC and ABS are used for enclosures where impact and appearance matter, but the flame rating and the tracking index need to be checked.
If the housing must meet a flammability class, state the standard and the thickness at which the rating applies. A UL 94 rating is not a property of the material alone. It depends on the thickness and the colour.
| Question | Why it matters |
|---|---|
| What is the flame rating and at what thickness? | The rating may not apply at the wall you designed |
| What is the CTI or tracking index? | Creepage and clearance |
| What is the continuous use temperature? | Thermal ageing and derating |
| How much moisture does it absorb? | Dimensional change and electrical properties |
| Does it need UV or chemical resistance? | Outdoor or industrial exposure |
| What colour and finish? | Appearance and laser marking |
Tolerances and the features that move
Moulded parts shrink. The shrink rate depends on the material, the wall, the gate and the cooling. The tool is cut with a shrink allowance, but the final dimensions still vary across the part. Features far from the gate and features in different flow directions can move differently.
Put tolerances on the features that locate the housing, the terminals, the cover and the mounting. Use a general tolerance for the rest. If the housing has a sealing surface, a connector cut-out or a breaker pocket, those are the features to control.
| Feature | What to control |
|---|---|
| Terminal pockets | Position, depth and wall thickness |
| Cover interface | Flatness and step |
| Mounting holes | Position and diameter |
| Breaker or module slot | Width, depth and squareness |
| Sealing surface | Flatness and surface finish |
| Snap fit | Undercut and deflection |
Tooling decisions that set the cost
The cavity count, the steel, the gating and the side actions decide the tool cost. A single-cavity tool is cheaper but has a higher cycle cost per part. A multi-cavity tool spreads the cycle but needs a larger press and more Maintenance.
Ask for a mould flow analysis before the tool is cut. It shows the fill pattern, the weld lines, the air traps and the likely warp. It is cheaper to change a gate or a wall in the analysis than in the steel.
What to send with a moulding RFQ
Send the 3D model, the 2D drawing with the critical dimensions, the material grade, the flame rating, the colour, the annual volume, the tool ownership expectation and the inspection level. Add the assembly drawing if the housing mates with other parts.
If the design is new, ask for a DFM review before the quote. The review should cover wall thickness, draft, ribs, bosses, shut-offs, gate location and the features that will need a side action. That review is the cheapest way to avoid a tool change later.
Frequently asked questions
What wall thickness should an injection moulded housing have?
A common range is 2 to 3.5 mm for electrical housings, depending on the material and the flame rating. The important rule is a uniform nominal wall with gradual transitions. Confirm the thickness with the material data and a mould flow analysis.
How much draft does a moulded part need?
A typical starting point is 1° to 2° per side, more for textured surfaces. Deep ribs, bosses and shut-offs need enough draft to eject without scuffing. The tool designer should confirm the value for the specific geometry.
What is the difference between PA66 and PBT for electrical housings?
PA66 is tough and has good electrical properties, but it absorbs moisture and can change dimensions. PBT has good dimensional stability and lower moisture absorption. The choice depends on the temperature, the flame rating, the tracking index and the dimensional requirement.
Can a moulded housing be changed after the tool is cut?
Small changes can be made by removing steel, but adding material usually means welding or a new insert. Changes to a shut-off, a gate or a side action are more expensive. Complete the DFM review before the tool is cut.
Sources and further reading
- ISO 2768-1:1989, General tolerances
- UL 94, Standard for safety of flammability of plastic materials
- Qumek moulding DFM reviews and first article inspection records
Qumek Engineering Desk
Engineering team
The engineering team behind Qumek drawings, DFM notes, tolerances and inspection standards. Every guide is reviewed against the drawings and processes used on live programmes.
Send a housing model for a moulding DFM review
Include the material, flame rating, volume and assembly drawing. We will review the wall, draft, ribs and shut-offs before tooling.
Send a drawing