How does an injection mold go from an idea to parts? Rather than telling, we show: the shots below are from our own mold shop — a 12-cavity injection mold, photographed through a single build day. Plates on the CNC in the morning, press trial at noon, final hand-fitting in the afternoon.

For the economics of our aluminum tooling approach: Aluminum Mold or Steel Mold? · Service details: Plastic Injection Mold Making

1) Cavity and Runner Plates on the CNC

The cavity plates are the heart of the mold. In a multi-cavity mold, the cavities are laid out symmetrically, and the runner channels that distribute molten plastic evenly to every cavity are machined into the same plate.

Multi-cavity injection mold plate pair with H-shaped runner and cavity layout
Plate pair — H-shaped runner and 12-cavity layout
Close-up of tapered cavity pockets machined into an injection mold plate
Tapered cavities up close — milling marks still fresh

The regular milling texture on the surface is intentional: cavity interiors define the part surface, while the shut-off faces define sealing. The two are machined to different tolerances.

2) Assembly, First Mounting and the T0 Trial on the Press

Once the plates are machined, the mold is assembled: guide pillars, springs and the ejection system go in.

Assembled injection mold halves on a pallet
Assembly done — mold halves and die spring
Injection mold halves spaced apart — guide pillars and core group
Halves spaced apart — guide pillars and core group

The mold is then mounted on the injection press. T0 (dry cycling): the mold is opened and closed between the platens while the ejection system and guide pillars are checked.

Injection mold mounted on the press, both halves open
Mold on the press — halves open between the tie bars
Commissioning the mold on the desktop injection press — moving half and core unit
Commissioning — moving half and central core unit

Depending on part and mold size, trials run either on the desktop press or on a 150-ton clamping force injection machine — here the same mold is clamped into the 150-ton press with blue mold clamps:

Mold clamped into a 150-ton injection machine with blue mold clamps
On the 150-ton press — mold clamped between the tie bars
Mold halves open on the 150-ton injection press, cavity face visible
Halves open — cavity face and runner visible

3) Hand-Fitting the Ejector Pins

The final step is handwork. The tips of the ejector pins that push the part out are ground perfectly flush with the cavity surface — a proud pin leaves a mark on the part, a recessed one leaves flash.

Hand-grinding injection mold ejector pins flush, sparks flying
Pin tips being fitted flush to the face
Clean cavity plate after fitting
Result: cavities and shut-off face ready for production

Why Multi-Cavity?

A 12-cavity mold produces 12 parts per cycle. At a ~30-second cycle that is ~1,400 parts per hour — for small plastic parts, cavity count is what really drives unit cost down. More cavities also mean a more expensive mold; the right cavity count sits at the intersection of your target quantity and tooling budget. We work that trade-off out with you at the quoting stage.

Have a similar part? Send your STEP/STL file and we will come back with a cavity-count recommendation, a firm price and lead time → Let’s Talk About Your Mold