How to prepare a product for injection moulding

Injection moulding is the process behind most of the plastic parts around you, and the one where design mistakes are most expensive. A machined part can be changed next week. A mould is cut from steel, costs real money, and once the first shot comes out, every change means re-cutting it. So we prepare moulded parts with more care than any other, and we prepare them early, while the design can still move.

These are the rules we apply. None of them are secret. The skill is in applying all of them at once, on a part that also has to look right.

Uniform wall thickness

Plastic shrinks as it cools. Where a wall is thicker, it cools slower and shrinks more, and the surface above it sinks. Where it is thinner, the melt may not reach. The single most important rule is a consistent wall, typically between 1.5 and 3 mm for most housings, with every transition gradual. Thick sections become ribs. Solid bosses become cored bosses. A beautiful chunky corner becomes a shell with a rib behind it.

Draft on every face that leaves the tool

A moulded part has to slide out of the mould. Any surface parallel to the direction the tool opens needs a small angle, usually one to two degrees, more on textured surfaces. Designing draft in from the start is easy. Adding it afterwards to a part that was modelled with vertical walls changes every dimension and every mating surface, which is why it is the first thing we check in CAD that arrives from elsewhere.

Ribs, bosses and radii that behave

Ribs add stiffness without thickness: around half to two thirds of the wall thickness at the base, with draft, and never so tall that they fill badly. Bosses for screws are cored, supported by gussets and connected to the wall by a rib rather than a solid block. Every inside corner gets a radius, because sharp corners concentrate stress, trap air and make the tool harder to fill. These rules sound dull until a part cracks at the one corner that missed them.

Decide where the parting line and gate go

Every moulded part carries two marks: the line where the two halves of the tool meet, and the point where plastic entered. Both are visible, and both can be placed on purpose. We decide them with the design, so the parting line falls on an edge that wants to be there and the gate sits where a small witness mark does not matter. Ejector pin marks are placed the same way. Leave it to the toolmaker and you get them where they are easiest for him, which is rarely where you want them.

Design the fit, not just the part

Housings are usually two or more parts that meet. The joint needs a lip or tongue to locate, a small gap to allow for tolerance and shrinkage, and a decision on how it is held: snap fits, screws, ultrasonic welding or adhesive. Snap fits are elegant and need to be designed with the material's allowable strain in mind, or they break on the tenth assembly. Screws are forgiving and cost assembly time. We choose per product, and we prototype the joint before the tool is ordered.

Choose the material with the supplier in the room

ABS, PC, PC/ABS, PP, PA, POM: each shrinks differently, flows differently and takes colour and texture differently. The choice depends on what the part has to survive, how it has to look, whether it touches food or skin, what it costs, and which grades the moulder actually runs. A material picked from a datasheet and never discussed with the supplier is a recurring cause of surprise at the first shots.

Specify the surface

Gloss, matte, textured, painted, in-mould colour. A high-gloss surface shows every flow line and sink mark; a fine texture hides most of them. We specify surface finish per face, using the standard texture references the toolmaker works with, and we make the first shots in the final finish, because a part that looks fine in grey resin can look very different in glossy black.

Think about the tool itself

Cavity count, tool steel, cooling channels, whether a slide or lifter is needed for an undercut. Each undercut adds a moving part to the tool and real cost. Often a small design change removes it. We review the tool design with the moulder before steel is cut, because the tool design is the last point at which a decision is still cheap.

The three mistakes we see most often

1. Vertical walls with no draft, discovered after the design is approved. The fix touches everything. The cure is to model draft from the first CAD file.
2. Thick sections where the design wanted visual mass. They sink, warp and take minutes instead of seconds to cool, which multiplies the unit cost. The cure is a shell with ribs that gives the same look.
3. Undercuts nobody noticed. A clip, a hole in a side wall, a lip in the wrong direction. Each one needs a slide in the tool. The cure is a draft-angle analysis in CAD before quotation, which takes an hour and saves thousands.

When to involve us

The cheapest moment for a DFM review is before the tool is quoted and after the design is stable enough to be worth reviewing. Send us the CAD. We come back with a marked-up list of what has to change for tooling, what should change for cost, and what can stay.