Injection mould design today is a digital workflow from the first part model to the final trial. Here is a general walk through how CAD/CAM shapes each stage.
1. 3D part modelling and design-for-moulding checks
Everything starts with a 3D model of the plastic part. Tools like SolidWorks, AutoCAD, Mastercam and Siemens NX are commonly used across the industry for modelling, drafting and machining preparation. Once the model exists, it is checked for moulding problems before any steel is cut:
- Draft angles: vertical faces need a small taper so the part releases from the mould without drag marks or damage.
- Wall thickness: walls that are fairly even fill and cool more predictably. Sudden thick sections can cause sink marks and warpage, and very thin ones may not fill.
- Undercuts: features that block straight ejection need side actions, lifters or a redesign of the part. Spotting them early saves cost later.
These checks are easiest and cheapest to fix on screen, while the part is still only a model.
2. Parting line and core/cavity split
The parting line is where the two mould halves meet. Choosing it well affects appearance, flash risk, ejection and how complex the mould becomes. In CAD the part is scaled for material shrinkage, the parting surfaces are built, and the model is split into core and cavity. The designer then adds inserts, slides, ejector pins and sealing (shut-off) faces so the split is practical to machine and to maintain.
3. Mould flow thinking, gating and cooling layout
Even without running a full simulation, a designer thinks about how molten plastic will travel through the tool:
- Gating: the gate position and type decide how the cavity fills, where weld lines appear and how the part is packed.
- Runners: balanced and reasonably short runners help each cavity fill in a similar way.
- Cooling: channels placed close to the cavity surface and spread evenly help the part cool uniformly, which supports consistent cycle times and less warpage.
- Venting: small vents let trapped air escape so the cavity fills cleanly.
Flow simulation software can support these choices, but experience with the material and the part geometry still matters.
4. CAM toolpaths for CNC machining, electrodes and EDM
Once the 3D mould design is finalised, CAM software such as Mastercam or the CAM modules in Siemens NX turns it into toolpaths for CNC machines.
- Roughing removes bulk material quickly from the core and cavity blocks.
- Semi-finishing and finishing bring surfaces close to final shape and finish, using suitable cutters and step-overs.
- Electrodes: sharp internal corners, deep ribs and fine details that a cutter cannot reach are often made with EDM. The electrodes are designed in CAD and machined, commonly from graphite or copper.
- EDM then burns those features into the hardened or hard-to-cut steel.
Good CAM planning includes tool selection, workholding and checking toolpaths in simulation to avoid collisions before the real machine runs.
5. Trial and revision loop
When the tool is assembled, it goes for a trial on an injection moulding machine. The first shots show how the design behaves in practice: fill pattern, short shots, flash, sink, warpage, ejection and cycle time. The team compares the sample part with the drawing, then adjusts process settings first. If the tool itself needs changes, such as steel-safe corrections, gate changes, venting or cooling updates, these are made and the tool is trialled again. Because the design lives in CAD, revisions are easy to track and carry across to the machining programs.
Closing thoughts
CAD/CAM does not replace moulding experience; it connects design decisions to what the machines actually cut. Checking draft, walls and undercuts early, splitting the mould sensibly, planning gates and cooling, and machining with well-prepared toolpaths all reduce surprises at trial. A short, disciplined trial and revision loop then takes the mould from good to production-ready.
Madhav Plasto Mould in Ahmedabad designs and builds moulds in house; see our process at https://madhavplastomould.com/process.html
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