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Views: 0 Author: Site Editor Publish Time: 2025-11-04 Origin: Site
In injection molding, tolerance is not just a number on a drawing. It determines whether two parts snap together smoothly, whether a car lamp seals without water leakage, or whether a product fails during assembly. Sometimes, a difference of just 0.01 mm changes everything.
Unlike metal machining—where material is removed and dimensions remain stable—plastic parts are born from heat, pressure, cooling, and shrinkage. This makes plastic “alive” in a way metal never is. It expands, shrinks, warps, and reacts to temperature and processing conditions. That’s why tolerance in injection molding isn't only about design—it’s about the entire journey from mold manufacturing to final production.
This guide explains injection molding tolerance from a practical, real-world perspective—especially from the viewpoint of mold makers and automotive lighting engineers, where tolerances like ±0.01 mm are not theoretical, but daily reality.
Injection molding tolerance is the allowable size variation between the CAD design of a plastic part and the actual molded part. It tells us how much deviation is acceptable while the part still functions correctly.
Actual Dimension = Designed Dimension ± Tolerance
So, if a dimension is 50.00 mm ± 0.05 mm, any part between 49.95 mm and 50.05 mm is considered acceptable.
This sounds simple, but in plastics, it gets tricky. Unlike metal, plastic is not cut to size—it cools, shrinks, and changes shape depending on the mold, material, and processing.
Plastic behaves more like a living material—it reacts to heat, pressure, humidity, and time. That’s why achieving tight tolerance is not just “machining accuracy.” It’s process accuracy.
Assembly Fit – Do parts align and connect smoothly, or do they creak, misalign, or require force?
Functionality – Poor tolerance can cause leaks, light gaps in lamps, moving parts jamming, or rattling in vehicles.
Aesthetics – Gaps, uneven surfaces, and light leakage are all tolerance issues.
Production Cost – The tighter the tolerance, the higher the cost in mold manufacturing, machining, testing, and scrap.
Mass Production Stability – If tolerance cannot be repeated across batches, quality becomes unpredictable.
In industries like automotive, precision is not a luxury—it’s safety, reliability, and brand reputation.
Understanding tolerance starts with knowing the types:
Variation allowed in linear dimensions such as length, width, thickness, hole diameter.
Example: 30.00 mm ± 0.05 mm.
Controls the shape and position of features:
Flatness – Controls how flat a surface must stay.
Parallelism & Perpendicularity – Ensures surfaces are correctly aligned.
True Position – Ensures holes or features are in the right place.
Concentricity – Used when two circular parts must share the same center.
Mold tolerance must be tighter than part tolerance because plastic will shrink and deform after leaving the mold.
Several standards define acceptable tolerances for plastic parts:
F (Fine) – Tight tolerance, precision parts.
M (Medium) – Standard consumer parts.
C (Coarse) – Large parts or low-cost products.
Tolerance is not determined only by the machine. It depends on:
Different plastics shrink at different rates.
Semi-crystalline plastics (PP, PA, POM) shrink more and are harder to control. Amorphous plastics (ABS, PC, PMMA) are more dimensionally stable.
Precise cavity and core manufacturing.
Balanced gate design to ensure even plastic flow.
Optimized cooling layout to avoid warpage.
Use of high-quality steel (H13, S136, 1.2343) to prevent deformation.
5-axis CNC machines achieve ±0.005–0.01 mm precision.
Fine EDM machining can reach ±0.005 mm on small features.
Polishing and fitting affect final dimensions—especially on optical parts.
Melt temperature, injection pressure, and cooling time influence shrinkage.
Mold temperature must be stable—automotive molds often control within ±1°C.
Inconsistent settings = inconsistent tolerance.
CMM (Coordinate Measuring Machine), 2D projectors, and laser scanners measure critical features.
SPC (Statistical Process Control) ensures deviation stays within tolerance.
Regular checking of electrodes, mold inserts, and molded parts prevents errors from stacking.
To achieve stable and tight tolerances, especially for automotive lighting molds, professionals follow three main steps:
Proper parting line, draft angle, and gate position.
Moldflow analysis to predict shrinkage and warpage.
Avoid large flat surfaces to reduce deformation risk.
5-axis CNC, mirror EDM, grinding, and fitting by experienced toolmakers.
Cavity and core accuracy controlled to within ±0.005–0.01 mm.
Steel heat-treated to 48–52 HRC to prevent distortion.
Mold temperature machines controlling ±1°C.
Injection pressure and holding pressure stabilized.
Cooling and ejection carefully controlled to avoid part deformation.
In automotive lighting, even a hair-thin deviation causes light leakage or optical distortion. At Guangdian Technology, we routinely build molds with ±0.01 mm tolerance. How?
Optical areas machined with mirror EDM and diamond polishing.
Inserts and sliders fitted to within 0.005 mm.
Full CMM measurement before mold assembly.
Mold temperature kept within ±1°C during sampling.
✅ Specialized in automotive lamp molds and optical parts.
✅ Two-shot and multi-cavity mold experience.
✅ In-house 5-axis CNC, mirror EDM, deep-hole drilling.
✅ CMM, projector, and SPC quality systems.
✅ Expertise with H13, S136, and optical-grade steels.
Need more details? Explore our pages on Auto Lamp Mold, Two-Shot Mold, Mold Design, and Injection Molding—or reach out directly.
Plastic tolerance is not just about numbers—it decides whether a part fits, functions, and lasts. In injection molding, tolerance comes from the mold, the material, the process, and the skill behind all of it.
At Guangdian Technology, we combine precision machining, engineering experience, and strict quality control to deliver molds that meet tight tolerances—especially for automotive lighting and high-standard plastic parts.
