What is the difference between plastic injection molding and 3D printing?

The short answer

The physical difference comes first

Everything else follows from how the part is formed. Injection molding melts pellets in a heated barrel, injects the melt into a closed steel cavity in under a second or two, holds it under pressure while it shrinks, then cools and ejects it. The polymer is sheared and packed as one continuous mass, so it freezes into a homogeneous part with no internal boundaries. Printing deposits or fuses material in layers, and the bonds between layers are never as strong as the material inside a layer. That single structural difference explains most of the property gap in the comparison below.

The second structural difference is that a mould is a fixed geometry. Once the steel is cut, the part is what it is; the same cavity produces the same part for hundreds of thousands of cycles. A printer is a general-purpose machine that can start a different part on the next build, which is why it is unbeatable for iteration and hopeless for volume.

Tolerance, surface finish and minimum wall

The tolerance difference is the one that causes the most late surprises, because it shows up at assembly rather than at goods-in. A moulded part holds about plus or minus 0.025 to 0.05 mm on features cut into the steel, and the value is repeatable from shot one to shot 500,000. A printed part is typically quoted at plus or minus 0.1 to 0.3 mm for FDM, falling to around plus or minus 0.05 to 0.1 mm for a well-maintained resin process. Where a snap fit, a boss or a port cut-out has to mate with something else, that difference decides how much hand-fitting the assembly line absorbs.

Surface finish follows the tool. A mould can be polished to an SPI A1 mirror or textured to a specific grain, producing Ra 0.4 to 1.6 micrometres and a face that is finished as it leaves the machine. FDM leaves visible layer lines at Ra 6 to 25 micrometres; resin processes are smoother but still need sanding and priming before a paint finish will look like a production part.

The same resin does not give the same part

This is the most commonly misunderstood row in the whole comparison. A buyer who specifies ABS and receives a printed ABS part has not received the same material performance. Printed parts typically retain 60 to 85 percent of the bulk resin's rated tensile strength because the inter-layer bonds are the weak path, and the effect is directional: a part loaded across the build direction can be materially weaker than the same geometry loaded in plane. Moulded parts achieve 95 to 100 percent of the rating because the material is homogeneous. In side-by-side testing of the same geometry in the same ABS grade, the moulded part is consistently stronger in tension than the printed one.

Repeatability, part size and lead time

Repeatability is a quieter difference but a decisive one for anyone buying production rather than prototypes. Moulding routinely demonstrates process capability indices of 1.33 to 2.0 on critical dimensions, which is what allows a plastic part to enter an automotive, medical or appliance supply chain as a controlled component. Printing typically sits at 0.8 to 1.2, which is adequate for fit checks and fixtures but not for a characteristic that has to be statistically capable over a production life.

Part size and lead time pull in opposite directions, which is the part of the trade most often misread. A moulding machine handles parts up to 1,500 mm and beyond, limited only by clamp tonnage; a typical printer bed is around 300 by 300 by 400 mm, and larger parts must be printed in sections and joined. But a printer delivers first parts in hours while a steel tool takes 4 to 8 weeks, so at the start of a program the slower process per part is the faster process to answer.

Which process for which stage of the program

The two processes are not competitors so much as consecutive stages, and the friction usually comes from skipping a stage rather than from choosing the wrong one.

1. Concept and fit. Print in an inexpensive resin to check form, feel and packaging. Nothing is frozen and change costs nothing.
2. Formal validation. Print in the intended resin family, or a close relative, to test function and assembly. Tolerances are provisional, so build in adjustment.
3. Pre-production. Either cut a single-cavity aluminium tool, which gives real process capability in 2 to 4 weeks, or run a bridge tool while the production mould is being made.
4. Production. Move to a hardened multi-cavity tool once volumes and the design justify it. This is where the unit price collapses and where the tolerance, finish and repeatability advantages finally pay.

Where 3D printing stays the better answer

• Genuinely low volumes. Below a few hundred parts, tooling cannot be amortised and printing wins outright on total spend.
• Geometry a mould cannot open. Internal cooling channels, lattice infill and organic shapes with no parting line are additive-only designs.
• Late customisation. Where each unit is different, a tool is the wrong answer by definition; patient-specific medical devices and custom fixtures belong in printing.
• Spares and obsolescence. Printing a spare on demand removes the inventory and the tooling question entirely.
• Speed at the start. A part in hand this week is worth more than a cheaper part in ten weeks when the program has not yet been released.

Getting the right process for your part

The question is rarely which process is better in general, but which is right for this geometry at this volume with this tolerance. Send the model or drawing with the annual volume, the features that must mate with something else, the surfaces that will be seen and the properties the part actually has to meet. We mould engineering resins and quote the additive equivalents beside them, so the comparison can be made on your part rather than on a general table. See plastic injection molding for the production route, SLA 3D printing for fine detail and smooth surfaces, and SLS 3D printing for functional nylon prototypes.