Plastic Mold Machining Methods: How Cavities and Cores Are Made
A complete injection mold is built from purchased mold-base components plus cavity and core sections that must be machined to precise, product-specific geometry. This guide covers the equipment and methods moldmakers use — milling, grinding, turning, deep-hole drilling and CNC — and how shrinkage is handled.
- Milling, grinding, turning, drilling
- 3- and 5-axis CNC machining
- Gun drilling for cooling lines
- Shrinkage set at machining, not design
Standard moldmaking equipment
Some mold components can be bought finished or semi-finished from mold-base suppliers such as DME or Mold Base Industries. But the cavity and core sections, and any detail specific to a product, must be made on equipment built to produce accurate, precise components.
Milling
Horizontal and vertical milling machines remove bulk material and cut the primary cavity form — today mostly on CNC machining centres.
Grinding
Surface and cylindrical grinders bring mating faces and round details to precise, flat, square finishes.
Turning & drilling
Lathes produce round components; horizontal and vertical drilling equipment cuts holes, ejector and cooling bores.
Work a shop cannot do in-house — deep-hole waterline drilling is the classic example — is subcontracted to specialists rather than tying up equipment used only occasionally.
Actual vs virtual models
In most cases the cavity and core are fabricated from a model — either actual or virtual.
Actual model
A physical master in wood, plaster, epoxy or another easily machined material, made to the exact shape and contours of the product — sometimes with shrinkage applied, sometimes to finished size.
Virtual model
A 3D CAD drawing downloaded to a machining-centre computer, which converts the files into cutting paths. Here the shrinkage factor is usually calculated by the machine.
Handling shrinkage the right way
Deep-hole drilling for waterlines
Cooling-water lines demand long, straight, accurate holes. The process usually needs a horizontal gun drill or boring machine.
Some machine shops specialise in this service for moldmakers who prefer not to invest in special deep-hole equipment that might only be used occasionally. Accurate waterline placement is what lets a mold pull heat out evenly and hold cycle time.
Why the mold designer must know machining
The mold designer — and the novice moldmaker — should be familiar with the machining methods used to produce mold components.
Feeds and speeds are covered in machining reference books, but the principle matters more than the numbers: a cavity has to be designed so it can actually be cut accurately with the equipment available. Understanding reach, tool access, grinding and deep-hole limits is part of designing a manufacturable tool — and one of the golden rules of custom mold design.
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- 3- and 5-axis CNC machining in-house
- Deep-hole drilling for cooling lines
- DFM before steel is cut
Mold machining FAQ
How are injection mold cavities and cores machined?
Cavity and core sections are machined from tool-steel blocks using milling machines, surface and cylindrical grinders, lathes, drilling equipment and, today, CNC machining centres driven from a 3D model. Roughing, semi-finishing, finishing and hand benching take the block to final geometry and surface finish.
Should shrinkage factors be built into the mold model?
It is good practice not to bake shrinkage into the original model. If the final molding material changes, its shrinkage differs, so applying the shrink factor during the machining phase lets the same model serve any plastic. See our note on differential shrinkage and cooling effects.
What is gun drilling used for in mold making?
Deep, straight holes — especially cooling water lines — are produced with a horizontal gun drill or boring machine for accuracy. Many shops subcontract this to specialists rather than tie up dedicated deep-hole equipment.
What is the difference between an actual and a virtual model?
An actual model is a physical master in wood, plaster or epoxy with the product’s shape. A virtual model is a 3D CAD file downloaded to a CNC machining centre, which converts it to cutting paths. Most modern tooling uses virtual models.
Why should a mold designer understand machining methods?
Because the design has to be manufacturable. A designer who knows the feeds, speeds and reach of milling, grinding, EDM and drilling equipment designs cavities that can actually be cut accurately — which is one of the golden rules of mold design.
