Tool design is where a part drawing becomes a manufacturing cost. Five decisions dominate: where the tool splits, where the melt enters, how heat leaves, how the part comes off the core, and what has to move to release undercuts. Every one of them is influenced by the part geometry, which is why the cheapest time to change a tool is before it exists.
The parting line decides the part
Where the tool splits determines which faces are formed by which half, where the witness line falls, and which features become undercuts. A parting line chosen for the toolmaker's convenience can put a visible line across a cosmetic face; one chosen for appearance can force an action. Agree it in the design review rather than discovering it on the first shot.
Gating: where the melt enters and what it leaves behind
Gate location sets how the cavity fills, where weld lines form, where the part is packed hardest and what mark is left. A gate on a cosmetic face means a visible vestige. A gate far from a thick section means that section is packed through a thin one and will sink. Hot runners remove the runner scrap and add cost and complexity; cold runners are simpler and throw material away.
Cooling is cycle time, and cycle time is price
Heat has to leave through the steel, so the layout of cooling channels sets how fast the tool can run and how evenly the part shrinks. Uneven cooling is the main cause of warp. Conformal cooling channels, which follow the part surface rather than being drilled straight, are one of the strongest practical arguments for printed tool inserts.
Ejection and actions
Pins, sleeves and stripper plates push the part off the core, and where they land leaves a mark, so ejection is a cosmetic decision as much as a mechanical one. Anything the tool cannot simply pull off needs a side action, a lifter, a collapsing core or an unscrewing mechanism, each of which adds design, parts, maintenance and risk. Removing one undercut can pay for a lot of design review.
Questions people ask about injection molding tool design
Who designs the mould, me or the toolmaker?
The toolmaker designs the tool. Your job is the part and the requirements: which faces are cosmetic, which dimensions are critical, and what the expected life is. Give them that and the tool design follows.
Can I change the part after the tool is cut?
Steel can be removed easily and added with difficulty, which is why toolmakers deliberately leave cavities slightly full. Changes that remove plastic are usually cheap; changes that add plastic often mean welding or a new insert.
What is the most common design mistake?
Uneven wall thickness, followed closely by missing draft. Both produce defects that no amount of process tuning removes.
Hot runner or cold runner?
Hot runners save material and cycle time and suit high volume and multi cavity tools. Cold runners are cheaper, simpler and easier to maintain, and suit lower volumes and colour changes.