Place the MIM parting line on a non-functional edge or a low-visibility face, keep it clear of sealing, sliding, bearing, datum and inspection surfaces, and tolerance any dimension that crosses the plane separately from one formed inside a single mold half. A planar parting surface is the lower-cost default; stepped or angled surfaces pay off only when they release an undercut or move the seam off a critical face. Reported as-sintered witness lines run about 0.008–0.025 mm, but that range is tool- and process-specific, not a specification.
The split is a tooling fact: every seam, flash path and cross-plane dimension follows from it. This page covers placement, planar versus stepped surfaces, why MIM flash behaves differently after sintering, and how to tolerance and specify the seam.
Where the Parting Line Belongs
The parting line is the interface where mold halves, slides, inserts and cores meet, and it transfers to the part as a witness line that no MIM process removes entirely. Because it is a tooling decision, place it deliberately: along a natural edge, corner radius or step, on the face least seen and least measured.
The faces to keep clear are the ones that do work. A seam on a sealing land is a potential leak path, on a bearing or sliding surface a wear initiation site, and through a datum a corrupt reference for every dimension measured from it. Where geometry forces the seam near one of these, control it explicitly on the drawing.
Placement also fixes draft. When a long feature is divided between the two halves, the seam decides how its draft and shrinkage are split, which is why the split is reviewed together with gate and runner layout on the gate and runner design page.
- Run the seam along an edge, corner radius or step rather than across a flat, a seal groove or a mating face.
- Keep it off surfaces marked as datum, inspection reference, sealing land or bearing bore.
- Confirm the seam does not cross a thread flank, press-fit bore or knurled band that must hold its own form.
- Decide in writing which face may carry a visible seam — a verbal agreement is not inspectable.
The general part-design rules are in the MIM design guide; this page covers only what changes when the split itself is the variable.
Planar vs Stepped Parting Surfaces
A single planar surface is the default because it is the lowest-cost to build and the easiest to keep closed: fewer interfaces, less fitting at assembly, a shorter maintenance list. It concentrates the seam on one continuous line, which is either ideal — the seam runs on a single edge — or the reason the design needs a second option.
A stepped or nonplanar surface earns its cost in two situations: it releases geometry that would otherwise need a slide, or it moves the seam off a visible or functional face while keeping critical surfaces in one tool member. The trade-off is blunt — every step, insert and shutoff is another interface that must fit closely, can flash, and needs inspection.
| Decision dimension | Single planar parting surface | Stepped or nonplanar parting surface |
|---|---|---|
| Tooling complexity | Typically lower: fewer interfaces, simpler construction | Typically higher: extra machining, inserts or coordinated shutoffs |
| Witness-line placement | One continuous seam, which may cross a visible or functional face | Can move the seam to an edge and keep critical faces in one tool member |
| Undercut capability | Limited to features compatible with the main opening direction | Can release geometry that would otherwise need a slide |
| Flash exposure | Fewer interfaces, but the main seam stays the critical one | Every step and shutoff is another gap to fit and inspect |
| Cross-plane risk | Half-to-half alignment and mismatch drive the result directly | Several local interfaces affect different dimensions; validate each feature |
| Surface trade-off | Lower tooling burden, higher risk of a seam on a critical face | Higher tooling burden, better control of visible or sealing faces |
What the table does not provide is a separate published tolerance for stepped surfaces. A nonplanar split can be better or worse than a planar one depending on how many shutoffs it adds and how they are built, so capability is validated feature by feature.
Flash: Why MIM Is Less Forgiving Than Plastic Molding
MIM feedstock flashes more readily than many thermoplastics, so fits at the parting line, slides, cores and inserts need close control. Metal powder at roughly 60% of the volume in a warm binder enters a thin gap far more willingly than a polymer melt, and once it does, the material is inside the part rather than on top of it.
That is what makes flash expensive here. After debinding and sintering it is a metallic burr bonded to the component — it cannot be wiped or cut away like polymer flash. Removal means grinding, filing or machining, which adds an operation, can round a dimensioned edge and can disturb the datum used to inspect the part.
- Mold-half mismatch or wear at the shutoff — the most common cause, and one that worsens with volume.
- Insert and core clearance: every added interface is another gap the feedstock can reach.
- Clamp behavior and tool stiffness: deflection under pressure opens a closed seam.
- Venting, local geometry and injection pressure: contributors, which is why simply lowering pressure rarely fixes a flashing tool.
A witness line and flash are different defects. A witness line is a mark or seam within the intended surface; flash is unwanted material beyond it. A drawing that accepts a visible seam has said nothing about protrusion.
Tolerancing a Dimension That Crosses the Plane
A dimension measured across the parting plane is formed by two tool members, so it accumulates mold-half alignment, insert fit, thermal behavior, sintering shrinkage and any flash step at the seam. It should not automatically get the same tolerance as a dimension contained in one mold half, even on the same drawing and alloy.
The baseline here is as-sintered ±0.3–0.5% of nominal, about ±0.025 mm on features under 3 mm, tightening to roughly ±0.01 mm after sizing or CNC. Published MIM design tables cite bands of approximately 0.2%, 0.5% and 1% depending on feature and process control, and MIM association guidance commonly cites about ±0.3% average as-sintered capability — roughly ±0.075 mm on a 25 mm dimension. These are process references, not guarantees, and none is a parting-plane tolerance; capability on a specific feature is quoted on request.
For the seam itself, published guidance reports witness lines of roughly 0.008–0.025 mm. General injection-molding benchmarks place flash below about 0.10 mm with tighter targets near 0.05 mm, but those describe molding practice, not MIM acceptance limits — copying one onto a drawing turns a benchmark into an unvalidated claim.
- Move the inspection datum off the seam so the dimension can be measured on one tool member.
- Treat cross-plane dimensions as their own characteristic with their own limit, not as part of the general tolerance block.
- Where function needs a tight limit across the seam, plan a finishing operation rather than asking the tool to hold it.
Tolerance bands and the operations that tighten them are covered on the MIM tolerance page and in MIM + CNC secondary operations.
Specifying and Inspecting the Seam
The drawing has to separate three things that are easy to conflate: which faces may carry a visible witness line, which must be free of it, and what protrusion, if any, is acceptable. Critical dimensions should be identified individually, with tighter limits only where assembly, sealing, motion or inspection requires them — a blanket tightening note usually buys tooling cost and inspection argument rather than a better part.
Inspection needs the same discipline. State the datum, the instrument and the condition — as-molded, as-sintered or after finishing — because a seam measured with a caliper across its crests and the same seam measured on a comparator will not give the same number. Where the seam serves as a process check, read it with the identification method in how to identify MIM parts.
If flash must come off, choose the route deliberately: tumbling for general deburring, grinding or machining for a controlled edge, polishing only where appearance matters — each carries a different risk of moving a dimension. Routes are catalogued on the MIM surface finish page.
- Mark permitted and forbidden witness-line faces, and say which datum the part is inspected from.
- State allowable protrusion at the seam as a number or a functional condition, not as the word sealed or clean.
- Flag every dimension that crosses the plane and give it its own limit.
- Ask for DFM feedback on the split before the tool is cut; a seam moved at drawing stage is free, one moved after T1 is a tool change.
Frequently Asked Questions
Q: Where should the parting line be placed on a MIM part?
On a non-functional edge or low-visibility face, following a natural corner, step or radius. Keep it off sealing lands, sliding and bearing surfaces, mating pilots, datums and inspection reference faces, where a seam becomes a leak path, a wear site or a measurement error rather than a cosmetic mark. Then confirm draft on both sides of the split is compatible with the opening direction.
Q: How much flash is acceptable at a MIM parting line?
There is no universal MIM flash limit, and any single figure presented as one is supplier- and tool-specific. Published injection-molding benchmarks put flash below about 0.10 mm with tighter targets near 0.05 mm, but those describe molding practice, not MIM acceptance criteria. Specify allowable protrusion against the function of the surface, then agree the number with the tool builder; seam-specific limits are quoted on request.
Q: How visible is the witness line on an as-sintered MIM part?
Published MIM guidance reports witness lines of roughly 0.008–0.025 mm, depending on tool condition, geometry, seam location and finishing. Sintering shrinks the mark with the part rather than erasing it, and blasting, tumbling or polishing changes how visible it is without changing whether it exists. Specify visibility as an appearance requirement on named faces rather than assuming it from a generic finish callout.
Q: Can a dimension crossing the parting plane hold the same tolerance as one inside a mold half?
Not automatically. A cross-plane dimension adds half-to-half alignment, insert fit, thermal behavior, shrinkage and any flash step to normal variation, so it deserves its own limit and often its own datum. Where function demands tight control across the seam, plan a finishing operation: sizing or CNC brings treated features to roughly ±0.01 mm, against as-sintered capability of ±0.3–0.5% of nominal.
Q: When is a stepped parting line not worth it?
When a planar surface already puts the seam on an edge and clear of functional faces. A stepped split is not recommended just to avoid minor tool work: every added step, insert or shutoff must be fitted, can flash, needs maintenance and adds an inspection point, so the extra cost only pays when it releases an undercut or rescues a sealing or visible surface. Moving a seam for tidiness alone is a trade-off that usually loses on total cost.
Parting-line quality is decided at the drawing, not at the press: the split fixes draft, flash exposure and the tolerance of every dimension that crosses it, and none of those are easy to recover once the tool is cut. Where a feature must be divided across the plane, plan its limit and finishing route together.
For a part where the split matters — a housing with a sealing land, a lever with a bearing bore, a datum close to the seam — send the drawing through the contact page. Emitech reviews placement against draft, ejection and flash exposure and returns a quotation within 24 hours, drawing on 17 injection machines and 22 sintering furnaces listed on the MIM capabilities page.
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