Core and cavity from a part
The mould workflow end to end — shrink the part, check it releases, find the parting line, build the sheet, then split a block into core, cavity and part with one command that checks its own arithmetic.
Everything here lives on the Mold tab, left to right in the order the job is actually done. You can do all of it with the general surface and body tools instead; the tab exists because nobody in a mould shop wants to run that seven-step dance by hand.
1 · Shrink the part
A moulding comes out of the tool smaller than the steel that made it. Scale carries a shrinkage preset per material — 0.5 % for ABS, around 1.5 % for PP — so you scale the part up before any tooling is built.
Those figures are typical published ranges for unfilled grades. Real shrinkage moves with wall thickness, gate size, hold pressure and glass fill, so check the supplier's datasheet before cutting steel. A material that is not normally injection moulded gets no preset at all rather than a confident wrong one.
2 · Check it can come out
Draft Analysis colours every face by which half of the mould it releases with — green with the cavity, red with the core, amber where the draft is under your threshold, and violet where a wall is exactly parallel to the pull. That last one is a defect, not a near miss, which is why it gets its own colour.
Undercut Analysis is the same overlay entered on its problems-only view, with the face counts already showing: it answers how many, where Draft Analysis answers where.
It is a way of looking at the part. It never enters the document and changes nothing.
Set the required angle to what your process needs (3° is a common minimum) and flip the pull direction to see the picture mirror.
3 · Find the parting line
Parting Line is the loop where the two halves meet. You do not trace it: you say which way the mould opens, and the loop is the boundary between the faces that release each way.
- Zero-draft walls belong to decides which half a wall parallel to the pull joins. It moves the loop — on a box, core puts it at the top rim and cavity at the bottom — so it is a control rather than a hidden convention.
- It adds a reference, never geometry. The body is untouched.
- A face that crosses the pull and has no exact silhouette is refused by name. Add a Split Line across it and run again. A guessed silhouette becomes a parting surface that does not close, and you would not find out until the split failed three features later.
4 · Close the openings
Shut-off Surfaces caps every window the parting line found, so the sheet you are about to build is watertight across them. Pick the parting line and it takes the rims from there — a moulding with nine windows is one pick, not nine.
- Contact puts a face on the rim's own plane. It is exact, and it is what a bore through a flat wall wants.
- Tangent blends into the faces around the opening — for a bore through a curved wall, where there is no plane to sit in and a flat patch would stand proud of the surface.
- Untick a rim and it is left open for you to close by hand with Filled Surface. The row says how many you left.
The outer silhouette is not a shut-off, and picking it is refused: that one is the next step.
5 · Build the parting sheet
The split needs a sheet to cut with, and it has to reach past the block walls.
Parting Surface radiates outward from the parting line, perpendicular to the pull, with the corners closed. Pick the line — it carries the pull too, which is why the result is a parting surface rather than a guess — and set the distance so the sheet clears the block wall. There is no direction to choose, deliberately.
A planar parting line is offset in its own plane, so a rounded silhouette keeps its radii. A non-planar one is flattened onto the plane perpendicular to the pull, and the row says so: a flat rim is what the block cut wants, but it is a choice rather than the only reading.
Then Knit the parting surface and the shut-offs into one sheet. That knitted sheet is what the split cuts with.
Ruled Surface is a different tool. It grows a straight strip along one direction — along the pull that is a curtain, which is right for a shut-off or a side core and wrong for a parting surface. Used as one it divides the block inside-out: the core comes out as a hollow frame with no floor, and the split's volume arithmetic still balances, because it balances for any sheet that crosses the block. That is why the split now checks the floor and the lid separately.
For a genuinely flat parting plane, a Planar Surface from a rectangle larger than the block is still the simplest thing that works.
6 · Tooling Split
One command, one undo, three bodies: Cavity, Core and Part.
Pick the moulded part and the parting sheet, then set the block: margin is the clearance from the part to the block wall, and the two depths are the plates above and below the parting plane. They are separate because a mould is rarely symmetric about it.
The block is derived from the part's bounding box, so it stays parametric — change the part and the block follows.
It checks its own arithmetic, twice. Core + cavity + part must equal the block exactly, and the row reports that it did. But that sum balances for any sheet that crosses the block, including one that divides it inside-out — so the split also checks that each half keeps its share of the plate beyond the part: the core its floor, the cavity its lid. A sheet that cuts into either says so, and names the feature that fixes it.
Send the halves out as their own documents with Save Bodies.
Using a block you already have
Set Mould block to an existing body and the split uses that instead of deriving one — a bought-in mould base, a plate from a supplier STEP, any pre-made block. The margin and the two depths then no longer apply and the panel says so rather than leaving three fields that quietly do nothing.
The block must be a solid and must contain the part; both are refused by name if not.
Just an impression, without splitting
For a one-sided cavity — sink a moulding's shape into a block and stop — you do not need a parting sheet at all:
- Insert ▸ Part — bring the moulded part in. Its material comes with it, so the shrinkage preset below already knows what it is.
- Scale — type the shrink allowance.
- Indent — target the block, tool the moulding, and tick keep tool so the moulding survives as its own body. Leave the clearance at 0: a cavity is the exact impression, and a clearance is for a press fit.
That is the same result as SolidWorks' Cavity, as three rows you can each edit afterwards.
In an assembly, when the block is its own part
The three rows above work inside one part document. When the block and the moulding are separate components — a bought-in base, a plate you will machine, a moulding you also use elsewhere — use Assembly ▸ Cavity instead:
- Select the moulded component.
- Cavity. Every other top-level part becomes block.
- Set Shrink in the panel. Use Feature scope if only some of the plates should be cored.
The shrink lives on the cavity, not on the moulding. That is deliberate, and it is the reason to prefer this over scaling the moulding: the moulding is a shared library part, so shrinking it in its own history would resize it in every other assembly that uses it. Here the block is cored to the shrunk size and the moulding stays exactly as drawn.
Neither library part is modified — the cavity is owned by the assembly, the same way an assembly cut is. Save Bodies turns the cored block into a part when you want one.
7 · Undercuts
A feature that cannot come out along the mould axis — a side hole, a snap-fit lip — needs its own core, pulled its own way.
Side Core finds those regions and the exact direction each must be withdrawn in, then publishes it as an axis. Run Tooling Split again with its pull set to that axis and the core is cut by the same command that cut the mould.
It refuses two things rather than guessing: an undercut that no single direction frees (it needs two cores, or a design change) and a region that would have released with the mould anyway, which would send you building tooling for nothing.
Zero draft is reported, not hidden — a bore withdraws but rubs, which is normal and worth knowing.
What is not here
Cooling channels, runners and gates are not built. Neither is mould-flow simulation — that is a specialist package's job, and pretending otherwise would be worse than the gap.