What tolerances can injection molding hold? Many molded dimensions hold about ±0.003 to ±0.005 inches. Small, well-designed features can go tighter, often to ±0.001 inches. The real answer depends on material, part geometry, tooling, and process control.
We have molded precision plastic parts in Riverside, California, since 1984. Our customers are in medical, aerospace, defense, automotive, electronics, and industrial markets across the U.S. Many of their parts need tight, repeatable dimensions.
Injection molding can hold tight tolerances, but not every feature needs them. Tolerance capability depends on what you mold and how you mold it. A small acetal gear behaves very differently from a large polypropylene housing.
When customers ask what tolerances can injection molding hold, we start with a general range. Standard commercial tolerances often fall around ±0.004 to ±0.005 inches for a one-inch dimension. Larger dimensions usually need wider tolerances because total shrinkage grows with size.
These figures are general guidelines, not guarantees. Each part needs its own review.
Commercial tolerances cover normal production with standard process control. They keep costs reasonable. Fine tolerances need more precise tooling, tighter process windows, and more inspection.
Industry guidelines, including ISO 20457, define tolerance classes for molded plastic parts. Most drawings use commercial tolerances in general and fine tolerances only on critical features.
With the right resin and tooling, small features can hold around ±0.001 inches. Some very small features can go tighter still. Each step tighter adds cost through tooling precision, cycle time, and inspection.
Our plastic injection molding work includes tight-tolerance parts for regulated industries. We set tolerance targets with each customer during design review.
Material has a large effect on dimensional accuracy. Every resin shrinks as it cools, and each one shrinks differently. The table below shows typical guideline tolerances for a one-inch dimension.
| Material | Typical Shrink Behavior | Commercial Tolerance (approx.) | Fine Tolerance (approx.) |
| ABS | Low, even shrinkage | ±0.004 in. | ±0.002 in. |
| Polycarbonate (PC) | Low, even shrinkage | ±0.004 in. | ±0.002 in. |
| PEI | Low, even shrinkage | ±0.004 in. | ±0.002 in. |
| PEEK | Moderate shrinkage | ±0.004 in. | ±0.002 in. |
| Nylon (PA66) | Higher shrinkage; absorbs moisture | ±0.005 in. | ±0.003 in. |
| Acetal (POM) | Higher shrinkage | ±0.005 in. | ±0.003 in. |
| Polypropylene (PP) | High shrinkage | ±0.006 in. | ±0.004 in. |
| TPE | Flexible; hard to measure | ±0.008 in. | ±0.005 in. |
Actual results depend on geometry, wall thickness, and tooling. Use these figures for early planning only.
Toolmakers build the mold cavity slightly larger than the final part. They size it to match the resin’s expected shrink rate. If the actual shrinkage differs, the part ends up out of tolerance.
Amorphous resins like ABS and polycarbonate shrink less and more evenly. Semi-crystalline resins like nylon, acetal, and polypropylene shrink more. They can also shrink differently along and across the flow direction.
Yes. Glass fibers reduce overall shrinkage but make it uneven. Parts may shrink less along the fiber direction than across it, which causes warp. Mineral fillers usually shrink more evenly.
Moisture matters too. Nylon absorbs water after molding, and its dimensions grow slightly. We account for this when we set measurement conditions and tolerances.
Several tolerance factors work together. Material is only one of them. Part design, mold design, and process control all play major roles.
Larger parts shrink more in total, so their tolerances need to be wider. Thick walls cool slowly and shrink unevenly. That can cause sink marks, voids, and warp.
Uniform wall thickness helps the most. When walls cool at the same rate, the part holds its shape more reliably.
The mold sets the starting point for every part. Gate location controls how plastic fills and packs the cavity. Cooling channel layout controls how evenly the part cools.
Cavity count matters too. Each cavity in a multi-cavity mold varies slightly. Features across the parting line or formed by slides often need wider tolerances. We build our tooling in-house, so mold design and molding stay closely linked.
Melt temperature, mold temperature, injection pressure, and hold time all affect final dimensions. Small changes can shift a critical feature. Stable, documented process settings keep every lot consistent.
Regulated parts often need validated processes. Our cleanroom medical injection molding runs under controlled, documented conditions in an ISO 7 (Class 10,000) cleanroom.
Most tolerance problems start on the drawing. Good design decisions make tight tolerances easier and less costly to hold.
Critical features can often hold fine tolerances when the design supports them. Keep those features away from parting lines and slides when possible. Place them in areas with uniform walls and steady cooling.
Mark only truly critical dimensions with tight tolerances. Use commercial tolerances everywhere else. This approach controls cost without hurting function.
Tolerance stack-up happens when several parts fit together. Small variations on each part add up across the assembly. The final fit can fail even when every part is in tolerance.
Review the full assembly, not single parts. Mating parts made from different resins can also shift as temperatures change. Plan for that movement in the design.
Design for manufacturability (DFM) review checks whether the part can hold its tolerances in production. It looks at wall thickness, draft angles, gate location, and rib design. We review every new design before we cut steel.
Small design changes at this stage cost far less than tool changes later. They also reduce scrap during production.
Holding tolerances requires measuring them correctly. The inspection method and timing both matter.
Common tools include coordinate measuring machines (CMMs), optical vision systems, and calibrated gauges. CMMs measure complex geometry in three dimensions. Vision systems check small features quickly.
Timing matters as well. Many resins keep shrinking slightly for hours after molding. We measure critical dimensions after parts stabilize at controlled conditions.
Medical programs often need validated processes and detailed lot records. Suppliers typically follow a quality system aligned with ISO 13485. We hold ISO 13485:2016 certification.
Aerospace programs need first article inspection and full traceability from resin lot to finished part. Most aerospace customers require AS9100 certification. We hold AS9100 Rev D, along with ISO 9001:2015.
Yes. Ultrasonic welding, heat staking, and annealing can all shift dimensions slightly. Assembly steps can also add to stack-up. Plan secondary operations during design so the final assembly stays in tolerance.
We have built molds and molded precision parts for more than 40 years. That experience shapes how we plan, mold, and inspect tight-tolerance parts.
What tolerances can injection molding hold for small medical parts?
Small, well-designed features can often hold about ±0.001 to ±0.002 inches with stable resins and precise tooling. Results depend on material, geometry, and process validation. Confirm critical dimensions during DFM review.
Do tighter tolerances increase cost?
Yes. Tighter tolerances need more precise tooling, closer process control, and more inspection. They can also raise scrap rates. Apply tight tolerances only where the part’s function requires them.
Can a multi-cavity mold hold the same tolerances as a single-cavity mold?
Not always. Each cavity varies slightly in fill and cooling, which adds cavity-to-cavity variation. Single-cavity molds often hold tighter tolerances. Multi-cavity tools need careful balancing and per-cavity inspection.
Which materials hold the tightest tolerances?
Amorphous resins such as polycarbonate, ABS, and PEI usually shrink less and more evenly. Semi-crystalline resins like nylon, acetal, and polypropylene shrink more, which makes tight tolerances harder to hold.
What inspection reports should I expect for tight-tolerance parts?
Expect a first article inspection report on critical dimensions, plus ongoing in-process checks. Aerospace and medical programs often add capability studies, lot records, and full traceability under their quality systems.
So, what tolerances can injection molding hold for your part? The answer depends on material, design, tooling, and process control working together. Keep these takeaways in mind:
If you are planning a tight-tolerance part, our team can review your drawings with you. Contact us or visit Hi-Rel Plastics & Molding Inc. to learn more.
