Specifying Injection-Moulded Housings: Wall Thickness, Draft and Colour

Most housing projects that run late do not fail because of the moulding machine. They fail at the specification stage, where a drawing shows a shape but not the process conditions that shape implies. Four parameters carry most of the risk: wall thickness, draft, colour and tool steel. None of them is exotic, and all of them are cheaper to decide before the tool is cut.

Wall thickness and shrinkage

Plastic shrinks as it cools, and thick sections shrink differently from thin ones. A wall that steps from 1.5 mm to 4 mm creates a thick mass that stays molten while the surrounding skin solidifies. The result is a sink mark on the visible face, internal voids, or a warp that only appears after the part has sat overnight. The usual remedy is to keep nominal wall thickness as uniform as the function allows, and to replace thick masses with ribs, bosses or a cored-out section.

For small electrical enclosures in ABS or polycarbonate, 1.8 mm to 2.5 mm is a common starting range. Thin walls below 1.2 mm demand higher injection pressure and a more rigid tool; very thick walls extend cycle time and increase the risk of voids. Where a thick section is unavoidable, a gas-assist or a two-stage moulding may be worth discussing, but the first move is always to redesign the section rather than to compensate downstream.

Draft angles and ejection

A vertical wall will drag against the tool as it opens. Draft is the taper that lets the part release cleanly. As a rule of thumb, one degree per side is a reasonable minimum for smooth surfaces, with more for textured finishes; a heavy texture can require two to three degrees because the texture itself creates undercuts at a microscopic scale. Deep ribs and bosses need their own draft, and any feature that runs parallel to the parting line should be checked for scraping.

Draft also interacts with surface finish. A polished cavity releases more easily than a sandblasted one, but polish adds cost and can show every flow line. The practical approach is to specify texture and draft together on the drawing, then let the toolmaker confirm the release angle before cutting steel.

Colour control and masterbatch

Colour is a specification, not an afterthought. The same pigment can look different on a textured surface, on a thin wall, and under a different light source. For a housing that must match an existing product, supply a physical sample rather than a screen colour, and agree the measurement geometry: a spectrophotometer reading at 45 degrees will not agree with a visual check under office lighting.

Masterbatch dosage, resin lot and drying conditions all shift the final shade. If the housing is moulded in more than one cavity or on more than one machine, define an acceptable colour difference and check first-off parts against it. For outdoor enclosures, also confirm the pigment package is suitable for UV exposure; a colour that matches indoors may chalk within a season.

Tool steel and expected life

Tool steel selection follows volume and material. A P20-type steel is adequate for pilot and medium-volume tools in ABS. For glass-filled engineering plastics, abrasive fillers wear gates and cavities quickly, and a harder steel with a proper heat treatment is cheaper over the tool's life than repeated repair. Stainless inserts help where corrosive gases or aggressive resins are involved.

Ask for the tool life assumption in writing. A 100,000-shot tool and a 500,000-shot tool look identical on a quotation sheet but not in the maintenance budget. Cooling layout belongs in the same conversation: unbalanced cooling is a common cause of warp that no amount of process tweaking will fix.

Our injection moulding service covers tool design, mould making and production for electrical and industrial housings, with first-article reports and colour samples agreed before the first batch.