Plastic injection molding makes parts by melting plastic pellets, forcing the melt under pressure into a steel or aluminum mold, and ejecting the cooled part. A successful project follows eleven steps: confirm the part suits molding, pick the resin, allow for shrinkage, position the gates, fix the quantity, size the mold and press, then design, build, debug and quality-check the mold.
Key Takeaways
- Injection molding is a volume process: Protolabs Network suggests a minimum run of about 500 units, with per-part cost falling to a few cents to $1 at 100,000+ units (as of September 2026).
- The mold is the main cost: aluminum molds run about $3,000 to $5,000 and last 5,000 to 10,000 cycles; steel molds cost far more but are built for 100,000+ cycles.
- Shrinkage depends on the resin: about 0.6% for ABS and polycarbonate, 1.5% to 2.0% for polypropylene and 3.0% for polyethylene, so the cavity is cut oversize.
- Uniform walls, at least 1 degree of draft, ribs no thicker than 60% of the wall and no undercuts prevent most defects before the mold is cut.
- Press size follows clamp force: roughly 1.8 to 7.2 tons per square centimeter of projected area, or 4 to 5 tons per square inch as a rule of thumb.
If you are here right now and want to know how plastic injection molding is done and the steps involved in it, then without a doubt, this is the right place for you.
Plastic Injection Moulding Process (Step By Step)
Today we will be talking about the process and how everything is done in it so that if you want to get low-scale plastic moulding or plastic injection molding done for mass production, you know where to begin with.
Injection molding is a well-established process, but it rewards careful pre-planning because the mold is the largest upfront cost and most defects trace back to design decisions. Molding service providers quote this work, and it is worth comparing several quotes. One such provider is qmolding which, like any supplier, should be compared on tooling cost, mold life, lead time and sample-part quality before you commit.
Now, coming back to plastic injection molding, before you start the design and production part, there are a few steps you need to know about. The eleven steps below are listed in the order a molding project normally follows, from feasibility to quality control.
Determining the moldability
The very first thing is to figure out whether injection molding is the right process for the part at all. Molding pays off at volume: Protolabs Network’s injection molding guide recommends a minimum run of about 500 units, and per-part cost drops from roughly $1 to $5 in pilot runs of 500 to 10,000 units to a few cents to $1 at 100,000+ units (as of September 2026). For a handful of prototypes, CNC machining or 3D printing is usually cheaper because no mold has to be built; the comparison of CNC machining, 3D printing and injection molding explains when each process wins.
Finding the right plastic
If injection molding is required, then the next step is to find the right plastic that will be injected. It’s all based on the use and the end product that you want. Common choices are polypropylene for chemical resistance, ABS for low-cost impact resistance, nylon (PA6) for strength and abrasion resistance, and polycarbonate for the highest impact strength, according to Protolabs Network’s material guide. The resin also fixes the shrinkage rate and workable wall thickness (see the table below), so choose it before the mold is designed.
Determining the shrinkage
When you inject plastic into a mold, it will start to cool immediately, and then it will start shrinking. The mold cavity is therefore cut slightly larger than the finished part, using the resin’s published shrinkage rate. Typical rates listed by the PlastikCity material calculator are about 0.6% for ABS and polycarbonate, 1.5% for polypropylene homopolymer, 2.0% for polypropylene copolymer and 3.0% for HDPE and LDPE. Semi-crystalline plastics such as nylon and polypropylene shrink more than amorphous plastics such as ABS, and glass-filled grades shrink less because the fibers restrict movement during cooling.
Determining the gates
Gates are the small openings where molten plastic enters the mold cavity from the sprue and runner channels; the melt is injected under pressure, not poured. Each gate leaves a mark, so gates go where the mark is acceptable and the melt can fill the cavity evenly. Tunnel (submarine) gates sit below the parting line and are cut from the runner on ejection. Poor gate placement causes jetting, weld lines and gate blush, so it is usually checked with a mold-flow simulation before steel is cut.
Deciding the quantity
The quantity of the production parts that you want will determine the material that you need. The larger the production is, the higher the quality of the mold is supposed to be. Aluminum molds are typically rated for about 5,000 to 10,000 cycles and suit pilot runs, while steel molds are built for 100,000+ cycles; many hardened steel molds are designed to produce well over a million parts. The number of cavities also follows from quantity: a multi-cavity mold costs more to build but produces several parts per shot.
Identifying the size of the mold
The next step is to identify the size of the mold, and it, of course, depends on the end product that you are expecting. Remember that there should always be room for shrinkage if you want to get the end product just as you want it to be.
Determining the press size
The size of the mold and the projected area of the part determine the press that must hold the mold closed during injection. Presses are rated by clamping force in tons: the projected area is multiplied by roughly 1.8 to 7.2 tons per square centimeter, and 4 to 5 tons per square inch is the usual rule of thumb. Stiff resins need more tonnage. If clamp force is too low, the mold opens slightly and the part shows flash along the parting line.
Designing the mold
Then comes the part where you have to design the mold, which is where most later defects are either prevented or locked in. A standard mold has an A plate (injection side) and a B plate (ejector side) with the sprue, runners, gates, cooling channels, vents and ejector pins laid out so the part stays on the ejector side and falls free. The design-rule section below gives the figures to apply.
Building the mold
Once your design is complete, and once you know the mold that you have to use, you can now build your mold and meet the specifications of your final product accordingly. Molds are cut mainly by CNC machining, with electrical discharge machining (EDM) used for shapes that are hard to mill and for shaping pre-hardened steel without a second heat treatment. Pre-hardened steel molds are typically 38 to 45 Rockwell C; hardened steel molds are heat treated after machining to 50 to 60 Rockwell C for the longest life. Protolabs Network quotes roughly 4 to 6 weeks to build an aluminum mold and 4 to 6 months for a steel production tool (as of September 2026). See also CNC controls for moldmaking.
Debugging the mold
Once you are done with the initial running of the plastic injection, the next step is to test the prototype that you have created. Here you are supposed to check the measurements, the errors, and some unexpected problems if there were some. First-shot samples are measured against the drawing and inspected for short shots, flash, sink marks, warping, weld lines and splay; the defect table below lists the usual causes. Many molders use scientific (decoupled) molding, which fills the cavity to about 98% under velocity control and then switches to pressure control for packing, to keep dimensions consistent shot to shot.
Quality Control
Just like how it’s done in all the other manufacturing processes, you need to get the product to a place where it’s checked for quality control. Typical checks are dimensional inspection against the drawing tolerance, visual inspection for surface defects, part weight and color matching against an approved sample (see getting the correct color for a plastic part). Wikipedia cites a standard tolerance of about plus or minus 0.008 inch (0.2 mm) on a 1-inch dimension for an LDPE part with a 0.125-inch wall.
Conclusion
These are the simple steps involved in successful plastic injection molding. The process is well understood, but it is not trivial: the mold is expensive, lead times run from weeks to months, and most defects trace back to decisions made in the design stage, so the early steps deserve the most attention.
Now that you know everything about it, it’s time for you to start looking for a good service provider for plastic injection molding for your products.
What Is Plastic Injection Molding?
Plastic injection molding is a manufacturing process in which molten plastic is injected under pressure into a mold cavity, where it cools and hardens into the shape of the cavity. It is used for everything from bottle caps to automotive body panels. According to Wikipedia, John Wesley Hyatt and his brother Isaiah patented one of the first injection molding machines in 1872, and James Watson Hendry built the first screw injection machine in 1946; the reciprocating screw remains the standard design today.
How Does the Injection Molding Cycle Work?
The injection molding cycle is the repeating sequence that produces one shot of parts. Pre-dried pellets are fed from a hopper into a heated barrel, where a rotating screw melts them and pushes the melt forward. The cycle then runs as follows:
- The mold closes and the press applies clamping force.
- The screw moves forward and injects the melt through the nozzle, sprue, runners and gates into the cavity; the injection itself often takes well under one second.
- Holding (packing) pressure is maintained to feed more material as the part shrinks.
- The screw rotates to prepare the next shot while the part cools in the water-cooled mold.
- The mold opens and ejector pins push the part out.
Protolabs Network puts a typical full cycle at 15 to 60 seconds, most of which is cooling time. Hydraulic presses are the most common type worldwide; all-electric machines are quieter, faster and more accurate but cost more, and hybrid (servo-hydraulic) machines combine the two.
Which Plastic Should You Use? Shrinkage and Wall Thickness by Material
The resin decides shrinkage, workable wall thickness, cost and part performance. The table combines shrinkage rates from the PlastikCity material shrinkage calculator with wall thickness ranges published by Protolabs (both as of September 2026); a specific grade’s datasheet takes precedence.
| Material | Typical shrinkage | Recommended wall thickness | Typical uses |
|---|---|---|---|
| ABS | 0.6% (range 0.4 to 0.7%) | 0.045 to 0.140 in | Housings, consumer goods; low cost, impact resistant |
| Polypropylene (PP) | 1.5% homopolymer, 2.0% copolymer | 0.025 to 0.150 in | Packaging, living hinges; excellent chemical resistance |
| Polycarbonate (PC) | 0.6% | 0.040 to 0.150 in | Lenses, guards; highest impact strength |
| Nylon (PA6 / PA66) | 1.2% / 1.5% | 0.030 to 0.115 in | Gears, bearings; strong, abrasion resistant |
| Polyethylene (HDPE / LDPE) | 3.0% | 0.030 to 0.200 in | Containers, lids; flexible, chemical resistant |
| Acetal (POM) | 1.8% | 0.030 to 0.120 in | Precision gears, clips; low friction |
| Polystyrene (PS) | 0.5% | 0.035 to 0.150 in | Disposable goods, packaging |
| Acrylic (PMMA) | 0.6% | 0.025 to 0.500 in | Clear covers, light guides |
Amorphous plastics (ABS, PC, PS, PMMA) shrink less than semi-crystalline plastics (PP, PE, nylon, acetal). Glass-filled grades shrink much less: about 0.4% for 30% glass-filled nylon 6 against 1.2% unfilled. Nylon must be dried before molding or moisture causes splay marks.
Design Rules That Prevent Injection Molding Defects
Most injection molding defects are designed in before the mold is cut. The following figures come from Protolabs’ wall-thickness design tip and the Wikipedia summary of plastic component design guidelines.
- Uniform walls. Keep wall thickness consistent; any wall should be no less than 40 to 60 percent of the adjacent walls. Thick sections cause sink marks, voids and warping.
- Draft angles. Faces parallel to the direction of draw need draft so the part releases. Protolabs advises 1 degree of draft per inch of cavity depth; general guidelines say 1 degree per side is sufficient and 2 to 5 degrees is preferable, with textured surfaces needing more.
- Ribs. Use ribs instead of thicker walls to add stiffness. Rib base thickness should be about 0.4 to 0.6 times the nominal wall, rib height no more than 2.5 to 3 times the wall, rib spacing at least 2 times the wall, and rib draft around 1 to 1.5 degrees.
- Bosses. Boss walls around 0.6 times the nominal wall, height under 3 times the outer diameter, with a base radius of 0.25 to 0.5 times the wall.
- Radii. Inside corners should carry a radius of at least half the wall thickness; sharp inside corners concentrate stress and restrict flow.
- Undercuts. Avoid them where possible; each undercut needs a side action or slide, which Protolabs Network says adds about 15% to 30% to the tooling cost.
How Much Does Plastic Injection Molding Cost?
Injection molding cost has two parts: the one-off mold and the recurring price per part. The figures below are published by Protolabs Network and Wikipedia as of September 2026; actual quotes depend on size, cavities, complexity, finish and tolerances.
| Item | Typical figure (as of September 2026) | Notes |
|---|---|---|
| Aluminum mold | About $3,000 to $5,000 | Rated for roughly 5,000 to 10,000 cycles; 4 to 6 weeks to build |
| Steel production mold | Higher; large tools can cost hundreds of thousands of dollars | Built for 100,000+ cycles, many for over a million parts; about 4 to 6 months |
| Side action / slide | Adds about 15% to 30% to tooling cost | Needed for each undercut |
| Per part, pilot run (500 to 10,000 units) | About $1 to $5 | Depends on geometry and material |
| Per part, full production (100,000+ units) | A few cents to $1 | Mold cost amortized across the run |
| Lead time, pilot run | About 6 to 8 weeks | Mold build plus 2 to 4 weeks of production |
Fewer cavities and simpler geometry lower the mold price; more cavities raise it but cut the cost per part at volume. See also common manufacturing mistakes and how to avoid them.
Common Injection Molding Defects and Their Causes
Debugging a mold means matching each defect to its cause. The table condenses Wikipedia’s defect list. Molds themselves are usually cut by CNC machining, as covered in how CNC machining benefits the prototyping industry.
| Defect | What it looks like | Usual causes |
|---|---|---|
| Short shot | Incomplete part | Too little material, injection speed or pressure too low, mold too cold, no vents |
| Flash | Thin excess skin along the parting line | Over-packing, damaged parting line, clamp force too low |
| Sink marks | Depressions over thick sections | Holding pressure or time too low, cooling too short, walls too thick |
| Voids | Air pockets inside the part | Insufficient packing pressure, thick ribs or bosses |
| Warping | Twisted or bowed part | Cooling too short, uneven shrinkage, wrong water temperatures |
| Weld (knit) lines | Discolored line where two flow fronts meet | Mold or melt temperature too low, flow around cores or holes |
| Splay (silver streaks) | Silver streaks along the flow | Moisture in the resin, material too hot or sheared too much |
| Burn marks | Black or brown areas far from the gate | Poor venting, injection speed too high |
Frequently Asked Questions
What are the main steps in plastic injection molding?
The steps are: confirm the part suits injection molding, choose the resin, allow for its shrinkage, place the gates, decide the production quantity, size the mold, size the press by clamp force, design the mold, build it by CNC machining and EDM, debug it with sample shots, and set up quality control on the finished parts.
How long does an injection molding cycle take?
Protolabs Network puts a typical injection molding cycle at 15 to 60 seconds. Most of the cycle is cooling time, which depends on wall thickness and resin.
How much does an injection mold cost?
As of September 2026, Protolabs Network quotes about $3,000 to $5,000 for an aluminum mold suited to pilot runs, with side actions adding 15% to 30%. Steel production molds cost considerably more, and Wikipedia notes that large steel molds can run to hundreds of thousands of dollars while producing over a million parts.
What is a good draft angle for injection molding?
Protolabs advises about 1 degree of draft per inch of cavity depth, and general design guidelines say 1 degree per side is sufficient while 2 to 5 degrees is preferable. Too little draft causes the part to stick or deform on ejection.
Why do molded plastic parts shrink?
Plastic contracts as it cools from the melt, and semi-crystalline resins such as polypropylene and nylon shrink more than amorphous resins such as ABS and polycarbonate. Typical rates are about 0.6% for ABS and PC and 1.5% to 3.0% for PP and polyethylene, so the mold cavity is cut oversize by the resin’s published shrinkage rate.
