We work the MIM break-even out in one division: break-even quantity = (MIM tooling − CNC programming and fixtures) ÷ (CNC unit cost − MIM unit cost), then multiply by 1.5 to 2 for the order we quote. On the small parts we have taken through it, the landing point sits between a few thousand pieces and twenty or thirty thousand: below that the tooling cannot be spread, above several hundred thousand pieces MIM steps aside for powder pressing and cold heading. Cavity count and shape complexity move the line; quantity is only the result.
The rule of thumb: for parts of a few tens of grams or less, the economic batch usually sits at 5,000 to 30,000 pieces per order, placed by tooling cost divided by the unit-cost gap. Under 3,000 pieces tooling amortisation wins for CNC or stamping; over 500,000 pieces powder pressing and cold heading are cheaper per part.
1. Split the cost structures before comparing unit prices
The commonest quoting mistake is setting MIM's unit price against CNC's directly; they are not comparable. MIM pays up front for the injection mould, the setters and fixtures for debinding and sintering, and the sizing mould — small-part tooling usually runs from several tens of thousands of yuan to the low hundreds of thousands. Feedstock costs more than bar stock of the same alloy, and unit cost carries injection labour, sintering charged per furnace, gate removal and sizing.
CNC's fixed investment is far lower — programming plus fixtures, a few thousand to just over ten thousand yuan — but its unit price hides material utilisation: on small 316L or titanium parts, removal rates of 60 to 70% are common, and the cut-away is pure loss. MIM is near net shape, so utilisation is far better.
An 8g 316L part with MIM tooling at 80,000 yuan and unit cost 3.5 yuan, against CNC fixtures at 3,000 yuan and unit 18 yuan, gives (80,000 − 3,000) ÷ (18 − 3.5) ≈ 5,300 pieces. That is not the number to quote. Multiply by 1.5 to 2 — the T0 trial usually needs two rounds of tool change, yield-ramp scrap is yours, the sintering batch must fill a furnace, the surplus sits in inventory — and the honest MOQ is 8,000 to 10,000, because the supplier runs its own furnace and rework rate.
2. Cavity count is the only lever you can turn yourself
The same mould in 4 cavities and in 16 is a tier up in tooling and a tier down in unit cost; on small parts 32 cavities is not unusual. More parts per shot spread injection labour and machine time thinner, so unit cost falls. Above tens of thousands a year we recommend multi-cavity, since tooling is one-off; a first batch testing the market is better on a low-cavity mould, dearer per part, than dropping the whole tooling bill at once.
Alongside cavity count sits a variable rarely printed on a quotation: furnace loading. Debinding and sintering are charged per furnace, and how many parts fit depends on weight and tray layout. A 5g part fills a tray at low energy per part; past 100g the furnace takes few, the amortisation climbs and the advantage decays.
3. The more complex the shape, the earlier the break-even
Shape decides the result. Angled holes, profiled slots, internal cavities and multi-directional holes mean re-clamping, dedicated fixtures and slow passes on CNC, while on MIM they grow out of the mould at almost no extra cost.
| Part shape | CNC unit labour time | MIM break-even scale | Verdict |
|---|---|---|---|
| Simple turned or 2.5D part, a few through holes | Short | Tens of thousands | Cold heading or stamping first; MIM is no advantage |
| Medium complexity: angled holes, profiled slots, local curved surfaces | Medium | The 10,000-piece class | MIM's main battleground; negotiable around 10,000 |
| High complexity: multi-directional holes, internal cavities, surfaces hard to clamp | Long | A few thousand | Worth it at a few thousand if the design demoulds |
The upper limit is the same: above several hundred thousand pieces a simple part goes to conventional powder metallurgy pressing, one cost tier below MIM and limited to axial-demould 2.5D shapes, or to cold heading when axisymmetric. MIM sits in between — shape freedom near precision casting, cost near pressing.
The limits that flip MIM from worthwhile to not, from drawings we have changed: wall thickness jumps crack during debinding because thick regions shrink more, so hollow the thick area or add a transition and re-check strength; thin long through-holes bend the core pin, so keep the length-to-diameter ratio within 3; sintering shrinkage runs in the teens up to 20% with general tolerance at ±0.3%, so a critical fit leaves stock or takes a sizing step costed into unit cost.
4. After the division, do three things first
List part weight, maximum wall thickness, minimum hole diameter and whether there are multi-directional features — those four numbers decide whether MIM can do the part and what it costs. Ask for a quote stating cavity count, not one blended unit price; 4-cavity and 16-cavity quotes on the same mould show the real break-even. Then multiply by 1.5 to 2 before quoting, leaving room for T0 tool changes, yield ramp and inventory. Working the sum out early beats reworking the structure later.
FAQ
Q: How many parts before MIM is cheaper than CNC?
There is no fixed quantity; divide the tooling gap by the unit-cost gap. A complex small part pays off at a few thousand pieces, a simple part often needs tens of thousands, and the result is then multiplied by 1.5 to 2.
Q: How large a part can MIM make? Is there a weight ceiling?
A few tens of grams and under is most economical; past 100g per part the advantage visibly decays, because debinding and sintering are charged per furnace and a heavy part lets the furnace hold few. Heavier parts are possible, but the economics fall away: heavy plus modest volume usually gives a better account on CNC or precision casting.
Q: How many cavities should an MIM mould have?
Steady annual volume in the tens of thousands and above can go multi-cavity, with 8 to 16 cavities common on small parts; a first product still proving its market is safer low-cavity. Tooling is one-off and fades over tens of thousands of parts, while a low-cavity mould is cheap to tool and dear per part. The worst move is a high-cavity mould before volume is settled.
Q: What tolerance can MIM parts hold?
Sintering carries linear shrinkage in the teens up to 20%, with general tolerance held to ±0.3%; a critical fit either leaves machining stock or takes a sizing step whose cost belongs in unit cost. Tighter requirements are met by a MIM blank finished on one or two faces by CNC. Judge at design stage which dimensions really need precision.
Originally published at hezidesign.com
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