Injection Molding Prototypes

Prototype Moulding Is a Decision About Tooling, Not About Plastic

Every injection moulded prototype raises the same question: how much tooling is justified before the design is settled. Answering it badly is expensive in both directions — a production tool cut against a moving design gets scrapped, while a substitute process can fail to prove the thing that mattered.

There are three honest answers, and the right one depends on what the prototype has to prove.

The Three Routes

Route	Tool	Realistic quantity	What it proves	What it does not prove
Vacuum casting	Silicone mould from a master pattern	A handful to a few dozen	Form, fit, colour, and assembly with parts in a production-like resin	Not the flow behaviour, cycle or shrinkage of the final tool
Bridge tooling	Aluminium tool, single cavity	Tens to a few thousand	Real moulded parts in the production material, real shrinkage, real gate marks	Not the cycle time or lifetime of a hardened production tool
Production tooling	Hardened steel, often multi-cavity	Thousands upward	Everything, at production economics	Nothing — but the commitment is made before the design is settled

Vacuum casting runs on a Mingda MD-650 with a 650 × 600 × 500 mm envelope and ±0.2 mm accuracy — the loosest figure in the shop, and the right one for a process that is about form rather than tolerance.

Why Bridge Tooling Is Usually the Right Answer

Bridge tooling in aluminium exists to resolve a specific tension: the design is nearly settled, a few thousand parts are needed for validation or early sales, and a hardened production tool would be a premature commitment.

It uses the production material. Shrinkage, warpage and flow marks appear as they will in production, which is the whole point.
It exposes tool problems early. Parting lines, ejector marks, gate position and draft angle issues show up in the bridge tool rather than in the production tool, where changes cost far more.
It buys real market evidence. Parts in hand, in the real material, before the tooling budget is committed.

Its limits are equally real: an aluminium tool has a shorter life, fewer cavities and a longer cycle than hardened steel. For a few thousand parts that is acceptable. For a hundred thousand it is not.

Design Rules That Decide Whether the Tool Works

Draft angle. A vertical wall does not release. One degree per side is the usual minimum, more on textured surfaces, because texture holds the part.
Uniform wall thickness. Thick sections sink and warp while thin sections short-shot. Where thickness must change, taper it rather than stepping it.
Gate position. The gate decides where weld lines fall and where the part is strongest. It is a design decision, not a manufacturing afterthought.
Ribs and bosses. A rib as thick as the wall creates a sink mark on the opposite face. Rib thickness is typically 50-60% of the nominal wall.
Undercuts. An undercut needs a lifter, a slider or a side action. Each one adds tool cost and a potential failure point.
Shrinkage allowance. Every material shrinks differently. Aluminium bridge tools are cut with a shrinkage allowance chosen per material, and getting that number wrong scales the whole part.

What Changes Between Vacuum Casting and Bridge Tooling

Factor	Vacuum casting	Bridge tooling
Material	Production-like resin, a facsimile	The actual production grade
Tool life	Silicone degrades after tens of parts	Aluminium holds for thousands
Cost driver	Master pattern plus per-part labour	One-time tool cost, then low part cost
Lead time	Days	Weeks for the tool, then fast parts
Right when	Design still moving, or only a few parts needed	Design near-final and volume is real

Verification

Moulded parts are checked on a Zeiss Contura G2 CMM at ±0.001 mm against the drawing, and surface on a Mitutoyo Surftest SJ-210 across Ra 0.005 to 40 μm. Cosmetic acceptance has to be written down before production — gloss level, colour reference and acceptable defect size — otherwise parts get rejected for reasons nobody agreed on. ISO 9001:2015, qualification ratio above 99%.