The parting line is where the two halves of the tool meet, and choosing it is one of the first decisions in tool design. It determines which faces are formed by which half, where a visible witness line falls, which features become undercuts, and where flash will appear if the tool ever wears. It is a design decision with cosmetic and cost consequences.
What the choice decides
Every face on the cavity side is formed by one steel and every face on the core side by another, so a dimension that crosses the line inherits the tool's closing accuracy as well as the machining accuracy. Features below the line in the direction of draw are undercuts and need a mechanism. And the line itself leaves a small witness mark on the finished part.
Cosmetic consequences
On a visible part, the witness line should sit on an edge, in a groove, or along a natural break in the form rather than across a smooth face. That is normally achievable if it is raised early. It is much harder to change once the tool is designed, which is why cosmetic requirements belong in the first design review rather than in the first article inspection.
Stepped and non planar parting lines
The line does not have to be flat. A stepped or contoured parting line follows the shape of the part and can eliminate undercuts that a flat line would create, at the cost of more difficult tool machining and more critical shutoff. It is a normal technique and it is often cheaper than the side action it replaces.
Flash and shutoff
Wherever the two halves meet they must seal against injection pressure, and the seal is the shutoff. A shutoff at a shallow angle wears quickly; a shutoff on a large flat face needs the whole face to be flat. Where flash appears first is usually where the shutoff was hardest to make, which is worth knowing when a tool starts to flash after months of good production.
Questions people ask about injection molding parting line
What is a parting line?
The line where the two halves of the mould meet. It decides which faces each tool half forms, which features are undercuts, and where the witness mark falls on the part.
Can the parting line be moved?
In design, easily. After the tool is cut, rarely and expensively. Raise cosmetic requirements before the tool design is frozen.
Does the parting line have to be flat?
No. Stepped and contoured parting lines follow the part shape and can remove undercuts that a flat line would create, at the cost of harder tool machining.
Why has my tool started flashing?
Usually wear or damage at the shutoff, sometimes insufficient clamp force or over packing. It points at the tool or the process rather than the part.