Question 1

How much does injection molding cost?

Accepted Answer

Injection molding has two costs: a one-time tool (mold) and a per-part price. Tooling ranges from roughly $2,000 to $10,000 for a prototype aluminum mold up to $10,000 to $100,000+ for a hardened multi-cavity steel mold. Per-part prices typically run about $0.50 to $5 depending on part size, resin, and volume. Because the tool is fixed, the more parts you run, the lower the effective cost per part. Upload your part to see real tooling and piece pricing from independent shops.

Question 2

How much does an injection mold cost?

Accepted Answer

A simple single-cavity prototype aluminum mold often runs $2,000 to $10,000; a production-grade hardened steel mold runs $10,000 to $100,000+. Cavity count (each cavity adds roughly 30 to 50 percent), part complexity, undercuts, and cosmetic polish all drive the number. For low volumes, a prototype or bridge tool keeps upfront cost down.

Question 3

What is the minimum order for injection molding?

Accepted Answer

Many shops on OpenSpindle take low-volume and short-run work with no hard MOQ, running as few as a few hundred parts off a prototype tool. This is exactly where a marketplace of independent molders beats the large instant-quote platforms, which favor higher volumes.

Question 4

How fast is prototype injection molding?

Accepted Answer

With rapid aluminum tooling, first-article (T1) samples can come in roughly 2 to 4 weeks, with parts shortly after. Production steel tooling takes longer to cut but lasts for millions of shots. Tell the shop your timeline and volume and they recommend the tool.

Question 5

Can I use my existing mold?

Accepted Answer

Yes. If your tool already exists, you only pay the per-part price to run more parts, not the tooling again. Note that the mold lives at a specific shop; moving it to a new shop is a mold transfer. If you saved a molded part to your OpenSpindle parts library, you can reorder from the existing mold directly.

Question 6

Which plastics can be injection molded?

Accepted Answer

Common thermoplastics include ABS, polypropylene (PP), polyethylene (PE), polycarbonate (PC), nylon (PA), acetal/POM, PC-ABS, TPU/TPE, acrylic (PMMA), and engineering grades like PEEK. Glass- and mineral-filled versions add stiffness and heat resistance. Not sure which resin fits? Say what the part must do and the shop advises.

Question 7

What is the difference between prototype and production tooling?

Accepted Answer

Prototype (aluminum or soft steel) tools are cheaper and faster but wear out sooner, ideal for low volumes and bridge production. Production (hardened steel) tools cost more and take longer to cut but run hundreds of thousands to millions of parts. Many programs start with a prototype tool and move to steel once the design is locked.

Question 8

Who owns the mold?

Accepted Answer

In most custom molding arrangements the customer owns the tool once the tooling cost is paid, even though it physically lives at the molder that runs it. Ownership, storage, and maintenance terms should be spelled out in the quote or purchase order. Confirm the tool is yours to transfer before you commit, especially for a long production program.

Question 9

Can I transfer an existing mold to another shop?

Accepted Answer

Yes. Because a mold is a physical asset you own, it can be shipped to a different molder to run more parts, which is called a mold transfer. Expect a short qualification run to re-sample and verify the tool on the new press. If you saved the part to your OpenSpindle parts library, note that the tool currently lives at another shop when you request the quote.

Question 10

How many parts can one mold produce?

Accepted Answer

It depends on the tool material. A prototype aluminum mold typically lasts on the order of 10,000 to 100,000 shots, while a hardened steel production mold runs hundreds of thousands to millions of parts. Abrasive and glass-filled resins wear tools faster, so tell the shop your resin and annual volume and they size the tool steel to match.

Question 11

Can a mold be repaired or modified?

Accepted Answer

Yes. Molds are cut steel-safe so that changes add material where possible: opening up a feature by adding steel is straightforward, while enlarging a part by removing steel may need welding or a new insert. Worn or damaged tools can often be repaired, re-polished, or have inserts replaced rather than recut from scratch.

Question 12

Can silicone be injection molded?

Accepted Answer

Yes, through liquid silicone rubber (LSR) molding, a separate process from thermoplastic injection molding that injects a two-part silicone into a heated tool where it cures. LSR suits seals, gaskets, medical parts, and soft-touch components that need heat resistance and flexibility. Ask for LSR when you request a quote and shops set up for silicone respond.

Question 13

Can you mold threaded features or metal inserts?

Accepted Answer

Yes to both. Internal and external threads can be molded directly, often with an unscrewing mechanism in the tool, or added as a secondary tapping or insert step. Metal inserts such as threaded bosses, pins, and terminals are placed in the tool and molded in place, which is called insert molding. Call out threads and inserts on the drawing so the tool is built for them.

Question 14

How much draft angle does my part need?

Accepted Answer

A common rule of thumb is 1 to 2 degrees of draft per side, with more on textured surfaces (roughly 1 degree for each 0.001 inch of texture depth). Draft lets the part release from the tool cleanly and reduces drag marks and ejector stress. Deep ribs and tall walls need more, so ask the shop to review draft during design-for-manufacturability.

Question 15

What causes sink marks?

Accepted Answer

Sink marks are shallow depressions that form where thick sections, ribs, or bosses cool and shrink more than the surrounding wall. The fix is usually design: keep walls uniform, core out thick areas, and hold rib thickness to roughly 40 to 60 percent of the adjoining wall. A textured finish can also hide minor sink on cosmetic faces.

Question 16

What causes warping?

Accepted Answer

Warping is distortion from uneven cooling and internal stress, and it is worse with non-uniform walls, glass-filled resins, and poor gate placement. Uniform wall thickness, balanced cooling, adequate draft, and the right gate location all reduce it. Flag any flatness-critical features so the shop can design the tool and process around them.

Question 17

What is flash, and what causes it?

Accepted Answer

Flash is thin excess plastic that squeezes out along the parting line or around ejector pins, usually from too much injection pressure, a worn tool, or not enough clamp force. It is trimmed off, but persistent flash points to a process or tooling issue. A well-maintained tool and a dialed-in process keep parts flash-free.

Question 18

How are cosmetic defects prevented?

Accepted Answer

Cosmetic quality comes from the tool and the process together: the SPI surface finish machined into the steel, the gate location and type (gate vestige is the small witness mark left where the part is de-gated), and process control over flash, sink, and flow lines. Decide cosmetic surfaces before the tool is cut, keep gates off show faces where you can, and specify the finish and any critical A-surfaces on the drawing.

Question 19

What is mold flow (DFM) analysis?

Accepted Answer

Mold flow analysis simulates how molten plastic fills the cavity, predicting fill balance, weld lines, air traps, sink, and warpage before any steel is cut. It guides gate placement, wall thickness, and cooling so the first tool runs good parts. Many shops on OpenSpindle offer mold flow or a free design-for-manufacturability review on request.

Feature	With a drawing	No drawing supplied
General dimensional	±0.005 in	DIN 16742 general
Tight / critical feature	±0.002 in	Call out on the drawing
Wall thickness	0.040 to 0.140 in typical	Uniform walls preferred
Draft angle	1 to 2 deg per side	More on textured faces
Shrinkage	Resin-dependent	Tool cut steel-safe

Design rule	Typical spec	Why
Nominal wall thickness	0.040 to 0.140 in (material-dependent)	Uniform walls fill evenly and avoid sink and warp.
Draft angle	>= 1 to 2 degrees per side	Lets the part release cleanly from the mold; textured surfaces need more.
Rib thickness	<= 0.5 to 0.6x wall	Thicker ribs pull a visible sink mark on the opposite face.
Internal corner radius	>= 0.5x wall	Sharp corners concentrate stress and restrict material flow.
Boss outer diameter	~2x the hole diameter	Supports the boss for a screw without causing a sink mark.
Wall uniformity	avoid thick-to-thin steps	Thick sections cool slower than thin ones and warp the part.

Finish	Materials	Adds	Can be applied with
SPI A-1 / A-2 (mirror / high gloss)	Clear and cosmetic resins	Tool cost up	Optical, lenses
SPI B / C (semi-gloss to matte)	Most resins	Standard	General parts
SPI D / MoldTech (textured)	Most resins	Hides sink and flow	Grip, cosmetic housings
As-molded	All	+0	Internal / non-cosmetic

Process	Best for	Typical volume	Choose it over Plastic Injection Molding when
CNC Machining	Low-volume precision	1 to 1,000	You need a few to a few hundred precise parts with no tooling cost, the tightest tolerances, or metal. Molding takes over once volumes reach the thousands and the tool amortizes.
3D Printing	Rapid prototypes	1 to 100	You want fast design iterations, complex geometry, or one-offs with zero tooling. Switch to molding for repeatable, low-cost parts at production volume.
Vacuum Casting	Small production runs	10 to 100	You need tens to a few hundred parts that look and feel molded, cast from a low-cost silicone tool copied off a master. Molding is the step up for higher volume and end-use resins.
Compression Molding	Rubber and composites	Low to high	Your material is rubber, silicone, or a thermoset composite that injection molding cannot process. Injection molding is for thermoplastics.
Blow Molding	Hollow parts	High	The part is hollow and thin-walled, like a bottle, tank, or container. Injection molding makes solid and structural parts.
Thermoforming	Large thin parts	Low to high	The part is large and thin-walled (trays, panels, packaging) and can be formed from sheet at low tooling cost. Molding suits smaller, thicker, higher-detail parts.
Rotational Molding	Large hollow products	Low to mid	You need a large, seamless hollow product such as a tank, kayak, or bin. Injection molding handles smaller, higher-precision parts at higher volume.

Instead of	Send this
"Mold this part in black plastic"	"30% glass-filled nylon, black, SPI B-1, 5,000 pcs/yr, new prototype tool"
"Need injection molded parts"	"Polycarbonate, natural, SPI A-2 cosmetic front face, 2,000 pcs first run"
"Reorder my molded part"	"Existing steel tool at your shop, run 10,000 more, same PP natural"

Plastic injection molding from independent shops

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How to design for Plastic Injection Molding

Inserts, hardware and finishing

Threads and inserts

Finishing

Mold and tooling options

Tooling class and lead time

Cavity layout

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Complex and multi-material molds

Injection molding vs other processes

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What actually changes your quote

Pick the tool to the volume

Keep walls uniform and add draft

Decide cosmetics before the tool is cut

Amortize tooling across the run

Frequently asked questions

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