Injection Molding: Process, Plastics, Tolerances & Rules

Injection molding injects molten thermoplastic into a steel mold, where it cools and solidifies into the part. It is the standard process for high-volume plastic parts: tooling cost is high, but per-part cost at volume is very low, which makes injection molding the dominant process for everything from consumer product housings to automotive components produced in the millions. As a process within extended manufacturing, injection molding parallels die casting as a near-net-shape route that trades high tooling cost for low per-part cost, and it competes with 3D printing and machining where volumes are low.

The common materials are ABS, polypropylene (PP), polycarbonate (PC), nylon (PA), polyethylene (PE), and acetal (POM, or Delrin), each chosen for a balance of strength, toughness, temperature resistance, and cost. Part design must account for shrink, draft, uniform walls, and gating, because the physics of molten plastic cooling in a mold set what the process can produce. Understanding that physics, and designing for it, is the key to a molded part that comes out of the mold sound, to tolerance, and at the low per-part cost the process is known for.

How injection molding works

The injection molding cycle has four stages: injection, packing, cooling, and ejection. The cycle stages and the mold itself below cover how the plastic fills the cavity and how the mold is built.

The cycle stages

In injection, plastic pellets are fed into a heated barrel, where a rotating screw melts them and forces the molten plastic through a nozzle into the closed mold cavity, filling it under high pressure. In packing, additional material is forced in to compensate for the shrink that happens as the plastic cools, so the part is dense and free of sink and voids. In cooling, the plastic solidifies as the mold, chilled by cooling channels, draws heat out of it; cooling dominates the cycle time, so efficient mold cooling is central to productivity. In ejection, the mold opens and ejector pins push the solidified part out, and the cycle repeats. A well-run cycle takes seconds to tens of seconds per part, which is why injection molding produces parts so cheaply at volume.

The mold and its features

The mold itself is a precision assembly of hardened tool steel, in two or more halves that close and clamp under high force during injection. The mold contains the part’s cavity, the runners and gates that deliver plastic, the cooling channels that manage solidification, and the cores, slides, and ejectors that form internal features and release the part. The cavity surface can be polished to a high gloss or etched to a texture, which transfers directly to the molded part, giving injection molding its excellent surface finish control. The mold’s design, its cooling, and its gating set both the part quality and the cycle time, and a well-designed mold is what makes injection molding productive and economical.

The materials

The choice of plastic follows the part’s function. ABS and polycarbonate (PC) are the housing materials, tough and rigid, with PC offering higher temperature and impact resistance; PC-ABS blends combine the two. Polypropylene (PP) is the chemical-resistant and living-hinge material, used for caps, containers, and parts that must flex repeatedly. Nylon (PA) is chosen for strength and toughness, in gears, structural parts, and wear components, with glass-filled grades adding stiffness and heat resistance. Polyethylene (PE) is the commodity plastic for packaging and low-cost parts. Acetal (POM, Delrin) is the low-friction, dimensionally stable choice for gears, bushings, and precise mechanisms. Each material has a shrink rate, a melting temperature, and a set of processing characteristics, and the material is chosen early because it sets the mold design and the part’s performance.

Design rules for molded parts

The design rules for molded parts group into the wall and geometry rules that let the part mold soundly, the shrink and gating plan, and the ribs and undercuts that add function without cost.

Walls, draft, and radii

Keep walls uniform, about 1.0 to 1.5mm for common plastics, since uniform thickness avoids sink, warp, and shrink stress, and transitions between thicknesses should be gradual. Provide draft, commonly 1 to 2 degrees, so the part ejects cleanly from the mold without scoring or binding. Add radii to all corners, because sharp corners cause stress concentration and poor plastic flow, while fillets improve both and help the mold fill.

Shrink and gating placement

Account for shrink, about 0.5 to 2.5 percent depending on the material, so confirm the material’s shrink rate before tooling, and design gating and wall transitions to limit warpage. Design gating and ejector placement carefully, since gates leave marks and ejector pins leave witness marks, so place them on non-visible or non-functional surfaces. The gate and ejector plan is part of the mold design, decided with the molder before the steel is cut.

Ribs and undercuts

Use ribs for stiffness, not thick walls, because ribs stiffen a part without the sink and cycle-time cost of a thick wall, so keep rib thickness below the wall to avoid sink on the opposite face. Design undercuts deliberately, since undercuts need side actions or lifter cores that add cost, so include them only where function requires.

Tooling and mold types

Injection-mold tooling is the largest cost and longest-lead-time item, and its design sets the part quality and the process economy. Molds are built from hardened tool steel for production, with the cavity polished or textured, and fitted with cores, slides, lifters, ejectors, runners, and cooling channels. For development and low volume, softer prototype molds (in aluminum or soft steel) reduce cost and lead time at the expense of mold life. Mold types include two-plate molds (the simplest), three-plate molds (which allow gating at optimal points), and hot-runner molds (which keep the runner molten, eliminating runner waste and improving cycle time for high-volume work). The choice of mold type follows the part, the volume, and the gating needs, and a well-designed mold produces parts to spec at the lowest cycle time, which is where the per-part cost is won.

Shrink and warpage

Shrink and warpage are the characteristic challenges of injection molding, and they drive much of the part and mold design. The contraction behavior and the design remedies below cover what causes distortion and how it is kept in check.

Contraction and the compenated mold

As molten plastic cools, it contracts, typically 0.5 to 2.5 percent depending on the material, and the mold is cut oversize to compensate so the finished part is to dimension. The shrink rate is a property of the material, set by its chemistry and whether it is filled, and it is confirmed before tooling so the cavity is cut to the right oversize. Getting the rate right is what lets a part come out of the mold to dimension rather than uniformly small.

Warpage and design remedies

Warpage is the harder problem: if the part shrinks unevenly, because of varying wall thickness, differential cooling, or the orientation of fibers in glass-filled material, it bends and twists out of shape. The remedies are design-driven: uniform wall thickness, generous radii, ribs and gussets placed to balance stiffness, gating that promotes even flow, and cooling channels that chill the mold evenly. Material choice also matters, with glass-filled materials stiffer but more prone to anisotropic shrink. Managing shrink and warpage is the central craft of molded-part design, and it is what separates a part that molds cleanly from one that distorts.

Tolerances

Injection molding holds tolerances that are good for a high-volume process but looser than machining. The dimensional ranges and the shrink challenge below cover what the mold alone can hold and what design must manage.

Dimensional tolerance ranges

Typical molded tolerances run about ±0.1 to 0.3mm, tighter on dimensions within one mold half and looser across the parting line or between moving mold sections, which include the variation in mold closure and core position. Tight features are held either by tight mold tolerancing, cutting the cavity to high precision, or by machining after molding. The feature’s location in the mold, as much as the mold’s overall precision, sets what the process can hold.

Shrink and warpage as a tolerance challenge

The biggest tolerance challenge in molding is not the cavity precision but the material’s shrink and warpage, which can move dimensions and shape after the part leaves the mold. So tolerance in molded parts is as much about controlling shrink and warpage through design as about the mold’s accuracy, with uniform walls, balanced gating, and even cooling working together to keep the part to dimension.

Gate types and runner systems

The gate is where the molten plastic enters the cavity, and the gate type and placement affect the part’s appearance, its weld lines, and its flow. An edge gate, on the parting line at the edge of the part, is the simplest and most common, but it leaves a visible gate mark at the edge. A sub-gate, which tunnels under the parting line, shears off automatically at ejection, leaving a smaller mark and suiting automated production. A fan or tab gate spreads the flow for parts that need even filling. Valve gates, used in hot-runner molds, open and close mechanically to control the flow precisely and leave minimal mark, suited to cosmetic and high-volume parts. The gate type is chosen for the part’s appearance, the mold type, and the automation, and it is part of the mold design.

The runner system delivers plastic from the nozzle to the gates, and its design affects material use and cycle time. A cold runner, in a two-plate or three-plate mold, solidifies with the part and is removed and reground, which wastes material and adds a step, though it suits low-cost molds and some materials that hot runners handle poorly. A hot runner keeps the runner molten in a heated manifold, eliminating runner waste and shortening the cycle, which suits high-volume production where the runner waste and regrind cost would be significant. The runner system, the gate type, and the mold type are chosen together for the part, the material, and the volume, and they set both the per-part material cost and the cycle time that dominate the economics of injection molding.

Cycle time, cooling, and productivity

Cycle time dominates the per-part cost of injection molding, because the machine and the mold are paid for by the hour, and cooling is usually the longest part of the cycle. After the plastic is injected and packed, it must cool enough to solidify and eject without distorting, and that cooling time grows with the wall thickness, roughly with the square of the thickness, so a thicker part takes disproportionately longer to cool. This is a large part of why uniform, thinner walls are so valued in molded-part design: they cool faster, which shortens the cycle and lowers the per-part cost. Efficient mold cooling, with cooling channels placed close to the thickest sections and balanced to chill the mold evenly, also shortens the cycle and improves part quality by reducing warpage.

Productivity comes from a short cycle, a high cavitation mold (one with many cavities producing many parts per cycle), and reliable automation. A multi-cavity mold produces several parts per cycle, which raises throughput but raises mold cost and complexity. A mold with dozens of cavities, producing dozens of parts every cycle of a few seconds, can turn out thousands of parts per hour, which is how consumer products reach their low per-part cost at high volume. Automation, with robotic part removal and handling, keeps the cycle consistent and removes labor from each part. Together, a fast cycle, high cavitation, and automation make injection molding one of the lowest-per-part-cost processes at scale, which is why it dominates high-volume plastic production.

Worked examples

The examples below show how the material choice, shrink behavior, and tolerance ranges on this page play out on real plastic parts.

Example: consumer housing in ABS

A consumer electronics housing needs a tough, rigid part with a cosmetic surface at high volume. ABS suits it, since ABS is the standard housing material, tough and rigid, and the molded surface takes a gloss or texture from the cavity. Walls are held at a uniform 1.0 to 1.5mm with 1 to 2 degrees of draft so the part ejects cleanly, and the cavity is cut oversize to compensate for ABS shrink (within the 0.5 to 2.5 percent range). The bulk of the housing holds the ±0.1 to 0.3mm molded tolerance, while any threaded inserts are added after molding to keep the mount precise.

Example: polypropylene container with a living hinge

A container needs a lid that flexes open and shut repeatedly without failing, at high volume. Polypropylene (PP) suits it, since PP is the living-hinge material that flexes without cracking, and it resists the chemicals the container may hold. The hinge is molded as a thin web integral to the part, the walls are kept uniform to limit warp, and gating is placed to promote even flow across the hinge so it forms soundly. The mold is cut oversize for the PP shrink rate, and the part comes out ready to use with no assembly.

When not to use injection molding

Injection molding is the wrong choice at low volume, because the tooling cost does not amortize over a small number of parts. For hundreds of parts or fewer, additive manufacturing or CNC machining is cheaper per part, since they carry no tooling cost. Injection molding wins once the quantity reaches the thousands, where the tooling cost spreads thinly across the batch and the low per-part material and cycle cost dominates. It is also wrong for parts that exceed the size or tonnage limits of available machines, or for materials that do not injection mold well. For prototypes and low volumes, additive or machining is the route; for high volumes of plastic parts with complex geometry, injection molding is the standard, and its low per-part cost at scale is why it produces most of the plastic parts in the world.

Applications

Injection-molded parts include consumer product housings for electronics and appliances; automotive interior and under-hood components; packaging, caps, and containers; medical device components and disposables; toys and consumer goods; and the housings, gears, and structural parts inside countless products. The common thread is a plastic part with complex geometry needed at high volume, in a material injection molding handles well, at a tolerance and surface finish the process delivers. For these applications injection molding is the standard high-volume process, and its combination of complex geometry, excellent surface, material range, and low per-part cost is why it produces most of the plastic parts in everyday use.