In injection molding, tolerances are the difference between a part that fits perfectly (like a phone case that snaps closed) and one that fails (like a gear that jams). Designers and manufacturers often ask: “How tight can my tolerance be?” or “Why is my part coming out too small?” The answer lies in understanding what drives tolerance variation—and how to control it. This guide breaks down the key factors affecting injection molding tolerances, shares actionable solutions to reduce defects, and uses real-world examples to make complex concepts easy. By the end, you’ll know how to set tolerances that balance performance, cost, and manufacturability.
What Are Injection Molding Tolerances?
First, let’s define the basics: An injection molding tolerance is the allowable range of variation for a part’s dimensions (e.g., length, width, hole size). It ensures that even if a part isn’t exactly the size on your drawing, it still works.
- Typical Tolerances: Most injection-molded parts use ±0.1 mm (a common industry baseline).
- Tight Tolerances: For precision parts (like medical components), tolerances can be as tight as ±0.025 mm—but these cost more and require stricter control.
Why does this matter? A furniture maker once designed a plastic hinge with a ±0.2 mm tolerance. The hinges were too loose, causing cabinet doors to sag. Reducing the tolerance to ±0.1 mm fixed the issue—no more sagging, and customers stopped complaining.
Key Factors That Ruin Injection Molding Tolerances (And How to Fix Them)
Tolerance issues almost always trace back to five root causes: material shrinkage, warping, thermal expansion, poor part design, and mold problems. Let’s tackle each one with solutions you can apply today.
1. Material Shrinkage: The #1 Culprit
All plastics shrink as they cool in the mold—but some shrink more than others. This shrinkage directly affects tolerance: the more a plastic shrinks, the harder it is to hit tight dimensions.
Why It Happens
Plastics fall into two categories, and their shrinkage rates differ drastically:
- Semi-Crystalline Plastics (e.g., PEEK, PA/nylon, PP): These have a structured molecular pattern. When melted, the molecules spread out; when cooled, they pack tightly—causing more shrinkage (and worse tolerances).
- Amorphous Plastics (e.g., PC, PS, PEI): These have random molecular structures. They stay amorphous even when melted, so they shrink less (and hold tighter tolerances).
Shrinkage Rate Comparison Table
| Plastic Type | Material Example | Shrinkage Rate (mm/mm) | Typical Tolerance | Best For |
|---|---|---|---|---|
| Amorphous | Polycarbonate (PC) | 0.005–0.007 | ±0.05–0.1 mm | Appliance parts, windows |
| Amorphous | Polystyrene (PS) | 0.004–0.006 | ±0.05–0.1 mm | Cutlery, cups |
| Semi-Crystalline | Nylon (PA) | 0.015–0.025 | ±0.1–0.15 mm | Auto parts, textiles |
| Semi-Crystalline | Polypropylene (PP) | 0.015–0.020 | ±0.1–0.15 mm | Bottles, crates |
| High-Performance | PEEK (Semi-Crystalline) | 0.012–0.018 | ±0.07–0.12 mm | Medical implants, bearings |
| High-Performance | PEI (Amorphous) | 0.005–0.008 | ±0.05–0.08 mm | Aerospace components |
How to Fix Shrinkage
- Choose the Right Plastic: If you need tight tolerances, pick an amorphous plastic (like PC) instead of a semi-crystalline one (like PP).
- Oversize the Mold: Molds are machined slightly larger to account for shrinkage. For example, a 100 mm PC part needs a mold cavity of 100.6 mm (to account for 0.006 mm/mm shrinkage).
- Control Process Parameters: Increase injection pressure (to pack more plastic into the mold) and slow cooling (to reduce rapid shrinkage). A electronics manufacturer used this trick for PC phone cases—shrinkage dropped by 30%, and tolerances hit ±0.08 mm consistently.
2. Warping: When Parts Bend Out of Shape
Warping occurs when parts cool unevenly—some areas shrink faster than others, pulling the part out of alignment. This ruins tolerances: a warped bracket might be the right length, but it won’t fit because it’s bent.
Why It Happens
Uneven wall thickness is the main cause. A part with a 1 mm wall and a 3 mm rib will cool at different rates: the thin wall cools fast (shrinks quickly), while the thick rib cools slow (shrinks later)—causing warping.
Recommended Wall Thickness Table (To Avoid Warping)
| Material | Recommended Wall Thickness (mm) |
|---|---|
| ABS | 1.1–3.5 |
| Acetal | 0.7–3.0 |
| Acrylic | 0.6–12.0 |
| Liquid Crystal Polymer | 0.7–2.9 |
| Long Fiber Reinforced Plastic | 1.9–27.0 |
| Nylon (PA) | 0.7–2.9 |
| Polycarbonate (PC) | 1.0–3.8 |
| Polyethylene (PE) | 0.7–5.0 |
| Polypropylene (PP) | 0.88–3.8 |
| Polystyrene (PS) | 0.88–3.8 |
How to Fix Warping
- Keep Walls Uniform: If you need a thicker section (like a rib), limit thickness variation to 15% of the nominal wall. For example, a 2 mm wall can have a rib up to 2.3 mm (2 mm + 15% = 2.3 mm).
- Use Tapered Transitions: Avoid sharp steps between thick and thin sections. A gradual taper (1:5 ratio) lets the part cool evenly.
- Case Study: A toy company made a plastic truck with a 1 mm body and 3 mm axle hole. The bodies warped, making the axles not fit. They tapered the transition to 1:5 and reduced the hole thickness to 2.3 mm—warping stopped, and 99% of parts met tolerance.
3. Thermal Expansion: Tolerances That Change With Temperature
Plastics expand when heated and contract when cooled—more so than metals. This means a part that meets tolerance in a 25°C factory might be too big in a 40°C car (or too small in a -10°C garage).
Why It Matters
If your part pairs with metal (like a plastic gear on a steel shaft), thermal expansion is a disaster. Metals have low expansion rates—so when the plastic gear expands, it jams; when it contracts, it’s loose.
How to Fix Thermal Expansion
- Pick Heat-Resistant Plastics: For parts used in extreme temps (e.g., engine bays), use ULTEM or PEEK—these have lower thermal expansion rates than ABS or PC.
- Test in Real-World Conditions: Don’t just measure tolerances in a controlled lab. Test parts in the environments they’ll actually be used in. A automotive supplier tested plastic clips in 80°C ovens—they found the clips expanded by 0.15 mm, so they adjusted the mold to 0.15 mm smaller. The clips now fit perfectly in hot engines.
4. Poor Part Design: Tolerances That Were Doomed From the Start
Your part’s design is the foundation of good tolerances. Even the best materials and molds can’t fix a design that ignores basic rules.
Common Design Mistakes
- Thick Walls: Thick sections (over 3.8 mm for most plastics) shrink unevenly, ruining tolerances.
- Large Sizes: A 500 mm part will shrink more (total shrinkage = size × shrinkage rate) than a 50 mm part. Tight tolerances are harder to hit for big parts.
- Lack of Ribs/Gussets: Adding ribs (instead of thickening walls) adds strength without causing shrinkage.
How to Fix Design Issues
- Use Ribs, Not Thick Walls: A 2 mm wall with 1.5 mm ribs is stronger than a 3 mm wall—and shrinks less.
- Keep Critical Features Small: If you need a ±0.05 mm tolerance, put that on a small hole (e.g., 5 mm) instead of a large surface (e.g., 200 mm).
- Example: A drone maker designed a 100 mm plastic frame with a ±0.08 mm tolerance. The frame shrank too much (0.6 mm total), missing the tolerance. They split the frame into two 50 mm parts (each with ±0.08 mm tolerance) and glued them—total shrinkage dropped to 0.3 mm, and both parts hit their tolerances.
5. Mold Problems: When the Tool Is the Issue
Molds are precision tools—if the mold is off, your tolerances will be too. Common mold issues include:
- Poor Cooling: Molds with uneven cooling channels cause parts to shrink inconsistently.
- Worn Cavities: Over time, mold cavities wear down (especially aluminum molds), making parts larger than intended.
- Multi-Cavity Imbalance: In molds with 2+ cavities, some cavities might fill faster than others—leading to size differences between parts.
How to Fix Mold Issues
- Use Steel Molds for High Volume: Steel molds last 100,000+ cycles (vs. 10,000–50,000 for aluminum) and maintain tolerance longer.
- Add Temperature/Pressure Sensors: Sensors in the mold track real-time data, letting you adjust cooling or pressure to fix imbalances.
- Case Study: A bottle manufacturer used a 4-cavity mold for PP bottles. Two cavities made bottles 0.1 mm too small. Adding sensors showed those cavities cooled 5°C faster. Adjusting the cooling channels fixed the issue—all bottles now hit ±0.05 mm.
When to Avoid Tight Tolerances (And Why)
Tight tolerances sound great, but they’re not always necessary. Here’s when to loosen up:
- Non-Critical Features: A decorative part (like a toy’s sticker holder) doesn’t need ±0.025 mm—±0.2 mm is fine.
- Cost Concerns: Tight tolerances add 20–50% to production costs (more precise molds, slower cycles, more inspections). A startup saved $10,000 by loosening a ±0.05 mm tolerance to ±0.1 mm for their plastic handle—no one noticed the difference.
- Large Parts: A 300 mm plastic shelf can’t hold ±0.05 mm—aim for ±0.2 mm instead.
Yigu Technology’s Perspective on Injection Molding Tolerances
At Yigu Technology, we believe injection molding tolerances are about balance—not perfection. Too often, clients overspecify (e.g., ±0.025 mm for a non-critical part) and waste money. We help them align tolerances with function: for example, a medical client switched from PEEK (semi-crystalline, high shrinkage) to PEI (amorphous, low shrinkage) for a diagnostic tool, cutting tolerance-related defects by 40%. We also share mold design tips (like uniform walls) upfront to avoid rework. Tolerances aren’t just numbers—they’re a way to deliver parts that work, on time and on budget.
FAQ
- Can I use ±0.025 mm tolerance for all my injection-molded parts?
No—±0.025 mm is a tight tolerance that only works for small, precision parts (like medical implants) made with low-shrinkage plastics (e.g., PEI). For most parts (like toy components or furniture hinges), ±0.1 mm is enough. Tighter tolerances cost more and require stricter process control. - Why do my parts have different tolerances from batch to batch?
Batch-to-batch variation usually comes from material or process changes: e.g., a new batch of plastic with a higher shrinkage rate, or a mold that’s worn down. Fix this by testing material shrinkage before each batch and inspecting molds for wear every 10,000 cycles. Using steel molds (vs. aluminum) also reduces batch variation. - How do I calculate the right mold size to account for shrinkage?
Use this simple formula: Mold Cavity Size = Desired Part Size × (1 + Shrinkage Rate). For example, if you want a 100 mm PC part (shrinkage rate 0.006), the mold cavity should be 100 × (1 + 0.006) = 100.6 mm. Always check your plastic’s datasheet for the exact shrinkage rate—don’t guess!