Short Answer:

Tolerancing in plastic part design is applied by considering the material behavior, manufacturing method (like injection molding), and functional requirements of the part. Unlike metals, plastics can shrink, warp, or vary more due to temperature and pressure, so wider and more flexible tolerances are usually applied.

Engineers apply tolerances by specifying critical dimensions tightly and non-critical features loosely, ensuring the part fits and works correctly without increasing manufacturing cost. Proper tolerancing avoids problems like part misfit, excessive stress, or poor assembly performance in plastic products.

Detailed Explanation:

Applying tolerancing in plastic part design

Tolerancing is the process of specifying acceptable variation in part dimensions. In plastic part design, it becomes more complex because plastics behave differently from metals. They are softer, more flexible, and react to temperature and humidity. Also, plastic parts are often made through injection molding, which includes natural variations in size due to cooling, mold filling, and shrinkage.

Therefore, applying proper tolerances in plastic design is about balancing accuracy with manufacturability. If tolerances are too tight, it increases mold cost, cycle time, and rejection rate. If they are too loose, it may lead to poor fit or malfunction of the assembly.

Key considerations for tolerancing plastic parts

1. Understand material properties

• Plastics expand and contract more than metals. Thermal expansion must be considered during design.
• Shrinkage occurs when the plastic cools inside the mold. Each material has a known shrink rate (e.g., ABS: 0.5–0.7%, PP: 1–2%).
• Hygroscopic plastics like nylon absorb moisture and swell over time.

Designers must use material datasheets and mold flow simulation tools to predict shrinkage and expansion during molding.

2. Consider molding process effects

• Injection molding involves melted plastic filling a cavity and cooling inside.
• Part dimensions depend on:
  • Mold temperature
  • Injection pressure
  • Cooling time
  • Part geometry
• Features like ribs, bosses, or thin walls can cause warping or uneven shrinkage.

Tolerances must account for these molding variables. For example, holes may need looser tolerances than external dimensions.

3. Use tolerance standards for plastics

• Industry standards like ISO 20457 or SPI tolerances provide general guidelines for plastic parts.
• They offer recommended tolerance ranges based on dimension size and material type.
• For example:
  • ±0.1 mm may be realistic for small, critical features.
  • ±0.3 mm or more may be used for larger, non-critical areas.

These guidelines help designers avoid over-tolerancing, which can be expensive.

4. Identify critical and non-critical features

• Critical features are those that affect function, assembly, or safety (e.g., snap fits, holes for fasteners, mating edges).
• Apply tighter tolerances (e.g., ±0.05 to ±0.1 mm) on these dimensions.
• For non-critical features (e.g., cosmetic surfaces, internal ribs), use looser tolerances (e.g., ±0.3 to ±0.5 mm).

This approach helps maintain product quality without increasing cost unnecessarily.

5. Use GD&T wisely

• Geometric Dimensioning and Tolerancing (GD&T) can be applied to control form, orientation, and location of plastic features.
• Examples:
  • Position tolerance for holes.
  • Flatness control for sealing surfaces.
  • Profile tolerance for complex contours.
• GD&T helps ensure consistent fit and function even when size varies slightly.

However, overuse of GD&T in plastic parts should be avoided due to natural part flexibility.

6. Add assembly allowances

• Tolerances should consider how parts will fit or snap together.
• Use draft angles and clearance gaps to allow easy assembly.
• Add interference or snap-fit tolerances carefully, as too tight a fit can cause stress or failure.

Designers may also add features like alignment pins or ribs to aid assembly even with minor tolerance shifts.

Best practices

• Always consult with mold makers and manufacturers before finalizing tolerances.
• Use prototyping to test part fit and adjust tolerances accordingly.
• Use design guidelines from material suppliers.
• Avoid unrealistic tolerances that require expensive precision tooling.

Conclusion:

Applying tolerancing in plastic part design requires a good understanding of material behavior, molding process, and part function. It involves using standards, identifying critical features, and balancing cost with performance. With proper tolerancing, designers can ensure that plastic parts fit well, function properly, and are easy to manufacture without unnecessary cost or defects.