射出成形のコスト解説：金型、キャビティ数、そして1個あたりの実際のコスト

Every injection molding quote contains two numbers that behave in opposite ways. The tooling cost is paid once, before a single part exists. The per-part cost repeats on every unit for the life of the programme. Getting a good price means understanding what drives each one separately, because the design decisions that lower tooling cost are not always the ones that lower cost per part.

This guide breaks down where the money goes in both, how aluminum and hardened steel tooling really compare, when adding cavities pays for itself, and the design changes that reliably cut a quote before any steel is cut.

The seven things that set your tooling price

Tool builders price from geometry, not from a catalogue. These are the factors that move the number most, roughly in order of impact.

The pattern to notice is that most tooling cost is machining hours on the cavity and core, plus the mechanisms needed to get the part out of the mold. Anything in your design that adds a mechanism adds real money. Anything that removes one saves it.

Aluminum against hardened steel

This is the first fork in any tooling decision, and it is decided by quantity and resin rather than by preference.

Aluminum has one advantage people rarely account for. Its thermal conductivity is far higher than tool steel, so heat leaves the part faster and cycle times can be shorter. On a low-volume programme that can partly offset its shorter life.

When aluminum is the right call

• The design is not fully frozen and a change is plausible.
• You need real molded parts in the production resin for testing, certification or a pilot launch.
• Annual volume sits in the low thousands or below.
• A launch date arrives before a steel tool can realistically be built.

When hardened steel is the right call

• The programme will run for years at meaningful volume.
• The resin is glass filled, mineral filled or otherwise abrasive.
• The part needs a high polish or a fine texture that must stay consistent over a long run.
• The resin is corrosive, such as PVC or certain flame-retardant grades, in which case a corrosion-resistant steel grade is specified rather than a general tool steel.

If you are still deciding whether a mold makes sense at all, our breakdown of 射出成形とCNC加工および3Dプリントの比較 covers the break-even calculation before tooling enters the picture.

Cavity count and when extra cavities pay off

A single-cavity mold produces one part per cycle and costs the least to build. A multi-cavity mold produces several identical parts in the same cycle, so the machine time per part drops sharply.

The important detail is that cavities are cheaper in bulk. Doubling the cavity count does not double the tool price, because the mold base, the ejection system and much of the engineering are shared. But it does roughly halve the machine time attributable to each part, since a cycle producing four parts takes only marginally longer than a cycle producing one. That asymmetry is why cavitation pays off.

Family molds carry a caveat worth stating plainly. They only make sense when the parts are consumed in the same ratio in which the mold produces them. If one part in the family is used twice as often as the others, you either overproduce the rest or run the tool inefficiently. They also complicate fill balance, because cavities of different sizes fill at different rates and need careful gate and runner design.

Cavitation should be sized against realistic annual volume rather than the first purchase order. A tool built for a quantity you never reach is a permanent cost, and one built too small becomes a capacity ceiling that is expensive to fix later.

What actually makes up the cost per part

The recurring number has fewer components than the tooling number, but one of them dominates.

1. Machine time. Press hourly rate divided by the number of parts per cycle. This is usually the largest share.
2. Resin. Part weight plus runner weight, at the current price of that grade. Filled and engineering grades cost considerably more than commodity resins.
3. Runner waste. Cold runners produce scrap on every shot. Some of it can be reground and reused where the material and application allow, but not all applications permit regrind.
4. Secondary operations. Trimming, tapping, texture finishing, labelling, ultrasonic welding and assembly all add per-part labour.
5. Inspection and packaging. Sampling, documentation and protective packaging scale with volume rather than disappearing into the tooling cost.

Cycle time is where most per-part cost is decided

A molding cycle is fill, pack, cool and eject. Cooling normally accounts for the majority of the total, and it does not scale gently with wall thickness. Cooling time rises roughly with the square of wall thickness, so a part with a 3 mm wall takes in the region of four times as long to cool as the same part at 1.5 mm.

That single relationship explains why wall thickness is the most powerful cost lever in part design. Thinning a wall does not just save resin, it shortens every cycle for the life of the tool. Practical wall sections for most thermoplastics fall between roughly 0.6 mm and 4.5 mm, with uniformity mattering more than the absolute figure.

Design changes that reduce injection mold cost

Most savings come from the model, not the negotiation. These changes are worth reviewing before tooling is quoted.

• Keep walls uniform and thin. Uniform sections cool evenly, reducing warp and sink while shortening the cycle. This helps both numbers at once.
• Core out thick sections. Replace a solid boss or block with a cored shape supported by ribs. Ribs should generally sit between about 0.6 mm and 2.5 mm thick so they do not create a sink mark on the opposite face.
• Add generous draft. One to three degrees on faces perpendicular to the pull direction lets the part release cleanly. Insufficient draft causes drag marks and can force a redesign after T1 samples.
• Remove undercuts wherever possible. Each slide, lifter or collapsible core adds build cost, adds a wear point and adds maintenance. Sometimes a small geometry change or a repositioned parting line eliminates one entirely.
• Simplify the parting line. A flat parting line is far cheaper to machine and fit than a stepped or contoured one.
• Specify finish only where it is visible. Polishing every surface to a cosmetic standard costs skilled hours. Non-visible faces do not need it.
• Choose the resin honestly. A commodity grade that meets the requirement will beat an engineering grade on both material cost and, often, cycle time.

Gate design deserves separate attention because it affects tool cost, cosmetic appearance and fill quality at the same time. Our guide to injection molding gate types and placement covers how that choice plays out on a real part.

Costs that appear after the tooling quote

A tooling number is not the total cost of getting to production. Several items sit outside it.

• T1 samples and revisions. First trial shots almost always reveal something. Ask how many sample rounds and minor tool adjustments are included before changes become chargeable.
• Engineering changes. Once steel is cut, a dimensional change may mean welding and re-machining, or in some cases a new insert. This is where an unfrozen design becomes expensive.
• Tool maintenance. Molds need periodic cleaning, polishing and replacement of wear components. On long programmes this is a real line item.
• Storage and ownership. Agree in writing who owns the tool, where it is kept and what happens if you move production. Settle it before the tool exists rather than after.
• Resin price movement. On multi-year programmes, material pricing moves. Understand whether your per-part price is fixed or indexed.

How to compare two mold quotes properly

Two tooling quotes with different numbers are often quoting different scopes. Before comparing them, confirm each one covers the same ground.

1. Mold material and grade, not just the word steel or aluminum.
2. Cavity count and whether the tool can be expanded later.
3. Guaranteed or expected shot life, stated as a number.
4. Whether mold flow analysis is performed before cutting.
5. How many T1 sample rounds and tool adjustments are included.
6. Which surface finish or texture standard is being applied.
7. Runner type, since a hot runner shifts cost from per-part to upfront.
8. Tool ownership, storage terms and lead time to first samples.

A tool quoted without these details is not cheaper, it is less specified. Our mold tool making service covers both aluminum and hardened steel construction, with tooling typically completed in 15 to 25 working days, and ラピッドツーリング available when a pilot run needs to start before a production tool can be built.

Getting an accurate number for your own part

Tooling cost cannot be estimated reliably from a description. It comes from geometry: the size of the part, the number of undercuts, the wall sections, the finish and the volume you need. A quote built from the actual model will be both more accurate and more useful than a range pulled from an online calculator.

Send the CAD model with your target annual quantity and the resin you have in mind, and XY加工 returns an itemised quote separating tooling from per-part price, along with design-for-manufacturability feedback inside 24 hours. If a feature is driving cost without adding function, it is far cheaper to hear that before the tool is cut. You can see the full scope of our 射出成形サービス or read more about our approach to プラスチック射出成形.

よくある質問

Q: Why is the mold so expensive compared to the parts?

A: A mold is a precision machined steel or aluminum assembly with cooling channels, ejection mechanisms and often moving slides. It is effectively a custom machine tool. Its cost spreads across every part, which is why per-part price falls sharply with volume.

Q: How long does an aluminum mold last?

A: Commonly up to several thousand parts, though the real figure depends on the resin and geometry. Glass-filled grades wear aluminum far faster than unfilled commodity resins, so abrasive materials shorten tool life considerably.

Q: Does a four cavity mold cost four times a single cavity mold?

A: No. The mold base, ejection system and much of the engineering are shared, so the increase is well below four times. Machine time per part, however, drops close to a quarter, which is why cavitation pays back.

Q: What is the fastest way to reduce cost per part?

A: Reduce and even out wall thickness. Cooling time rises roughly with the square of wall thickness and makes up most of the cycle, so thinner uniform walls shorten every cycle for the life of the tool.

Q: Who owns the mold after it is built? A: It depends entirely on your agreement, so put it in writing before the tool is cut. Cover ownership, storage location, maintenance responsibility and