100 to 1,000 units: bridge tooling, urethane casting or machining?

For 100 to 1,000 plastic parts, a low-cost injection mold, usually aluminum and often called bridge tooling, is usually the cheapest choice from a few hundred parts up. Below that, 3D printing is usually the cheapest for small parts, and urethane casting for larger parts that have to look good, up to about a hundred. In between, it depends on the part, so get quotes for both. Machining fits metal parts, parts with tight tolerances and plastic parts that need a real engineering plastic, but its cost per part stops falling after the first few dozen. Compare the total cost, setup plus every part, not the unit price alone, and do it for each plastic part: every distinct part needs its own mold, so a product's tooling cost is the total across all of them.

Cost isn't the only test. For drop tests and snap fits, machined parts in the same family of plastic are a fair early check, but only molded parts in the exact grade show how production parts will behave. A safety listing, such as UL or ETL, is tied to the listed grade of plastic and its wall thickness, so plan on molded parts for it; FCC emissions testing usually doesn't need the production plastic. And if the design may still change, stay with a process where a change costs little.

How do the options compare?

Process	One-time cost	Cost per part	Where it usually fits	Material	Watch for
3D printing (SLA, SLS, MJF)	None	High; powder printing gets cheaper with volume	One to a few hundred small parts	Printing resins and nylons; some, such as MJF nylon, can be what ships	Layer lines or a grainy surface; strength can differ by direction
Urethane casting	Low: a master part, plus a new silicone mold every 20 or so parts	Medium	About 10 to 100 parts, sometimes a few hundred with more molds	Urethane resins that imitate ABS, PC, rubber or clear plastic	Not the production plastic; parts vary slightly between molds
CNC machining	Low: programming and fixtures	High; falls over the first few dozen, then levels off	A few parts to a few hundred, metal parts, tight tolerances	Engineering plastic or metal, cut from solid	Shapes a cutter can't reach; thin walls and tall ribs are slow to cut
Bridge tooling (aluminum or soft steel mold)	Medium	Low	A few hundred to tens of thousands	The production plastic	Needs a design for molding; changes mean changing the mold
Hardened steel mold (production tooling)	Highest	Lowest	Thousands and up	The production plastic	Pays off at volume

3D printing: no setup, about the same price for every part

Industrial printing, with resin (SLA) or nylon powder (SLS and MJF), needs no tooling, so the first part costs about what the hundredth does. Powder printers can pack many small parts into one build, and services give volume discounts for that, which keeps printing competitive into the hundreds for small parts.

The limits: most printing materials are not the production plastic, though some, such as MJF nylon (PA12), are what some products ship in. The surface shows layers or grain and usually needs sanding or painting for a cosmetic face, and strength and color can vary with direction and with where the part sat in the build. Printing is the fastest way to the first parts and the cheapest way to change the design.

Urethane casting: good-looking parts for the first hundred

Urethane casting, also called vacuum casting, starts from a master part, usually printed or machined and then finished by hand. A silicone mold is cast around the master, and urethane resin is poured into it under vacuum. Each silicone mold makes only about 20 parts before it wears out, so 100 parts means about five molds, and the molds keep costing as the quantity grows.

The resins imitate ABS, polycarbonate, rubber or clear plastic, take color well and copy the master's surface, so cast parts can look close to molded ones. What they aren't is the production plastic: strength, heat resistance and flammability rating differ, which matters for drop tests and for units sent for safety testing. Parts can also vary slightly from one silicone mold to the next.

Machining: real material, at a price per part that levels off

CNC machining cuts the part from a solid block of plastic or metal. Setup is small, mostly programming and fixtures (holders that keep the part in place while it's cut). The cost per part falls over the first few dozen parts as the setup spreads out, then follows machining time and levels off.

Plastic stock doesn't always come in the exact production grade or color, especially flame-rated and glass-filled grades. Machined parts also have none of the weld lines and less of the locked-in stress that molding leaves, so they often test a little better than molded parts will. Some shapes are hard or impossible: deep, thin ribs, undercuts and sharp inside corners, where the cutter always leaves a radius. A case already redesigned for molding, with thin walls, tall ribs and snap hooks, is slow and costly to machine, because thin walls vibrate under the cutter, and a cosmetic face needs bead blasting or paint.

Machining suits metal parts, plastic parts that need tight tolerances, and the few parts where a printing material won't do. For a metal part that will later be die cast (made by injecting molten metal into a steel die), the first few hundred are usually machined, or made by investment or sand casting; sheet metal works only if the part is redesigned for it.

Bridge tooling: real molded parts from a few hundred up

Bridge tooling is an interim injection mold, usually aluminum and sometimes a softer steel, that makes real molded parts in the production plastic before, or instead of, a hardened steel production mold. It costs less and is ready sooner than a hardened steel mold, and at a few hundred to a few thousand units it is often the production tool itself.

Aluminum molds commonly last thousands to tens of thousands of shots (one fill of the mold each), fewer with glass-filled plastics, against hundreds of thousands or more for hardened steel. The line between bridge and production tooling is less about the metal than about cost and life. In some countries, molders quote a simple mold in pre-hardened steel by default even for a few hundred parts, at a price close to an aluminum mold elsewhere. Mold cost depends more on the part's size, the side actions, the number of cavities and the country than on aluminum versus steel. The names vary too: some suppliers say rapid tooling or soft tooling for aluminum molds, and some use soft tooling for silicone molds, so ask what the mold is made of.

The catch is that the part has to be designed for molding first: draft (a slight taper so the part slides out of the mold), even walls, and as few undercuts (hooks or holes that would lock the part in the mold) as possible, as described in Moving from a 3D-printed prototype to injection molding. Quick-turn molders can deliver first parts from a simple aluminum mold in one to three weeks; complex molds and overseas shops take longer, and the redesign and sample approval add time.

Changes and ownership. The first parts from a new mold usually need adjusting. A change that removes metal from the mold, such as making a wall thicker or a hole smaller, is cheap; one that needs metal added back can mean a new insert. So leave fit-critical features steel-safe, and budget for a round or two of small changes after the first samples. Ask, too, whether you can take the mold to another molder: some quick-turn services build molds as inserts for their own frames and keep them. Ask about storage fees and how long they keep a mold that isn't running.

How do you compare the cost?

Total cost is the one-time cost plus the cost per part times the number of parts. Divide it by the number of parts for the real cost of each one. Do this for each plastic part: a product with four molded parts usually needs four molds, or fewer if one family mold holds several, and the best process can differ from part to part.

Get quotes for at least two processes at your real quantity, and at one lower and one higher, because the cheapest process changes as the quantity grows. A mold becomes cheaper than printing once the quantity is more than the mold cost divided by the difference in cost per part. Then add what quotes often leave out: finishing for printed parts, the redesign a mold needs first, and changes after the first samples.

The numbers below are made up to show the arithmetic; they are not typical costs. Say you need one small case part:

3D printing: no setup and $35 a part. 100 parts cost $3,500, 300 cost $10,500, and 1,000 cost $35,000.
Urethane casting: an $800 master, $1,000 for each silicone mold of 20 parts, and $25 a part to cast. 100 parts cost $8,300, 300 cost $23,300, and 1,000 cost $75,800.
Machining: $300 of setup and $60 a part. 100 parts cost $6,300, and 300 cost $18,300.
Aluminum mold: a $7,000 mold and $3 a part. 100 parts cost $7,300, 300 cost $7,900, and 1,000 cost $10,000.

In this made-up example only, printing is the cheapest at 100 parts, and the aluminum mold is the cheapest from about 220 parts on ($7,000 ÷ ($35 − $3) ≈ 219). At 1,000 parts it costs less than a third of printing. Add $3,000 for the redesign and a round of mold changes, and the crossover moves to about 310 parts ($10,000 ÷ $32 ≈ 313). Urethane casting never wins here, because the part is small enough to print cheaply. For a large part that has to look good, where printing costs more and so does the mold, urethane casting can be the cheapest choice up to a hundred or so.

What decides it besides cost?

Material. For drop tests and snap fits, machined parts in the same family of plastic are a fair early check, and molded parts in the exact grade are the real test. A safety listing evaluates the enclosure plastic, including its flammability rating, so units with cast or machined parts may not count if you will ship molded ones. Ask the test lab before you build test units.
Looks. For cosmetic exterior parts, urethane casting from a polished master or an injection mold gives the best surface; printed and machined parts need finishing.
How settled the design is. With printing, casting or machining, a change costs a new file or a new master. With a mold, it costs work on the mold. If changes are still likely, wait before cutting the mold, or leave the fits steel-safe if you can't wait.
Time. Printing is the fastest, then machining and casting. A simple aluminum mold takes one to three weeks at a quick-turn molder, and longer for complex molds or overseas.
What comes next. If you will mold the part later, design it for molding now, so the bridge parts and the molded parts match.

A common path combines them: printed or cast parts for the first few dozen units, such as beta units or early backers, while the aluminum mold is made; the aluminum mold for the first run; and a hardened steel mold once the volume justifies it, as in An ODM quoted a 2,000-unit MOQ, but we need 500.

Example: 300 units of a handheld device

This is an illustration, not a customer project.

A team needs 300 units of a handheld device with a two-part plastic case and a metal bracket inside. Fifty units go to beta testers first, and the testers will hold the case and judge how it looks and feels.

The case, first 50 units. Urethane cast, not printed, because the testers judge the look and feel. The masters are printed from the CAD after it was redesigned for molding, and casting 50 sets takes about three silicone molds for each half, six in all, plus a few spare castings for rejects.
The case, the other 250 units. Molded in the production PC/ABS from aluminum molds, one for each half or one family mold for both. The team starts the molds before the beta feedback is in, as a deliberate trade-off for time, and leaves the fit-critical features steel-safe so small fixes only remove metal. Molded parts also go into the units sent for certification testing. Cast and molded halves differ slightly in shrinkage, texture and color, so they are never mixed in one unit.
The bracket. Laser-cut and bent sheet metal, which needs no dedicated tooling at this quantity and usually costs less than machining it from solid.
The test fixture for the line. The holders are 3D printed, and the plate that holds the spring-loaded test pins is machined, for accuracy and wear.
The comparison. Quotes for casting and aluminum molds at 50 and 300 units show casting is cheaper for the first 50 sets, and the molds for the rest.

Where we come in

This is one of the situations Anamor takes on. We start from your target unit cost and quantity. If a steel production mold pays off at your volume, we line up a partner factory for it. If it doesn't, we suggest a bridge process, such as 3D printing, urethane casting, machining or an aluminum mold.

Anamor takes US hardware teams with a working prototype into their first production run: we redesign the product for production, line up partner factories and suppliers in the US and overseas, price it at your quantities, and run the build through to shipping.

Send what you have: photos of the prototype, CAD if you have it, the parts list, and how many you plan to make. Need an NDA? We'll send our mutual NDA, or review yours, before you send files. Within two business days, a real person on our team will reply with a free first look at your production run and a few times to talk.

Frequently asked questions

Is urethane casting cheaper than injection molding for a few hundred parts?

Usually not. Each silicone mold makes only about 20 parts before it wears out, so the molds keep costing as the quantity grows: 100 parts already need about five. Urethane casting usually fits about 10 to 100 parts. From a few hundred up, a bridge mold, usually aluminum, is the usual choice. Quote both at your real quantity and compare the total cost.

What is bridge tooling?

Bridge tooling is an interim injection mold, usually aluminum and sometimes a softer steel, that makes real molded parts in the production plastic before, or instead of, a hardened steel mold. It costs less and is ready sooner. Aluminum molds commonly last thousands to tens of thousands of shots, so at a few hundred to a few thousand units it is often the production tool itself.

How do I find the break-even point between 3D printing and an injection mold?

A mold becomes cheaper than printing once the quantity is more than the mold cost divided by the difference in cost per part. Work it out for each plastic part, since each usually needs its own mold, and add what quotes often leave out: the redesign a mold needs first and changes after the first samples. Get quotes at your quantity, one lower and one higher.

Can I use cast or machined parts for certification testing?

It depends on the test. A safety listing such as UL or ETL is tied to the listed grade of plastic and its wall thickness, so plan on molded parts in that grade. FCC emissions testing usually doesn't need the production plastic. For drop tests and snap fits, machined parts in the same family of plastic are a fair early check. Ask the test lab first.