Injection Mold Fitting Dimension Tolerances

When fitting dimensions involve purchased components, the fitting relationship should consider both mold requirements and the dimensional tolerances provided by the supplier of the purchased components. The tolerances for self-manufactured parts should be reasonably determined within the scope of cost and manufacturing feasibility. This balanced approach ensures that the final injection molded part meets all quality requirements while maintaining production efficiency.

A common example of this type of fit is between ejector pins and their corresponding holes. The ejector pin is typically a purchased component with specified tolerances, while the ejector hole is manufactured in-house. The tolerance for the ejector hole must be carefully calculated to ensure proper functioning with the standard ejector pin, allowing for smooth movement while maintaining the necessary precision for the injection molded part.

Figure 1-35: Ejector Pin Fit Details

This diagram illustrates the recommended tolerance ranges for ejector pins and their mating holes, critical for proper ejection of the injection molded part without causing damage or excessive wear.

Figure 1-36: Gating System Tolerance Requirements

Proper tolerances in the gating system ensure consistent filling of the mold cavity, directly affecting the quality and consistency of each injection molded part produced.

When determining these fits, engineers must consider the thermal expansion characteristics of both the purchased and custom components, as temperature variations during the injection molding process can affect dimensional stability. The goal is to maintain the correct clearance or interference throughout the production cycle to ensure consistent quality of the injection molded part.

Fitting dimensions between molded components, such as the fit between a core and an insert, require precise tolerance specifications to ensure proper alignment, functionality, and longevity of the mold. These tolerances directly impact the dimensional accuracy and surface finish of the injection molded part, making them critical considerations in mold design.

Figure 1-37: Insert & Core Fit

ФDH7 /m6 - Tolerance specifications for insert and core assembly, ensuring proper alignment that directly affects the injection molded part's dimensional stability.

Figure 1-38: Air Pin Fit

ФdJS6/m5 - Precision tolerances for air pins and exhaust holes, critical for proper venting during the injection molding process and preventing defects in the injection molded part.

Figure 1-39: Molded Part Tolerances

Comprehensive tolerance specifications for various molded components, ensuring consistent quality across all features of the injection molded part.

The selection of appropriate fits for molded components depends on several factors, including the material of the injection molded part, production volume, and functional requirements. For example, tighter tolerances may be necessary for parts that require hermetic sealing or precise mechanical interaction, while looser tolerances might be acceptable for non-critical features. It's essential to balance precision requirements with manufacturing costs, as tighter tolerances typically increase production expenses.

When implementing these tolerances, mold makers must consider the cumulative effect of all dimensional variations across the entire mold assembly. Even small variations in individual components can compound, affecting the final dimensions of the injection molded part. Proper documentation and inspection procedures are therefore crucial to ensure that all components meet their specified tolerance ranges.

Sliding fits are critical in mold design for components that require movement relative to each other, such as inserts with angle pins, slide cores, straight ejectors, and ejector blocks. These fits must provide sufficient clearance to allow smooth movement while maintaining the precision necessary to produce consistent, high-quality injection molded parts. The selection of appropriate sliding fit tolerances directly impacts mold performance, longevity, and the quality of the final injection molded part.

Figure 1-40: Side Core-Pulling Mechanism Tolerance Requirements

General Tolerances

Regular molds: ±0.02mm
Precision molds: ±0.01mm

These tolerances ensure proper alignment of moving parts, directly affecting the dimensional accuracy of the injection molded part.

Side Core-Pulling

Regular molds: ±0.1mm
Precision molds: ±0.05mm

Critical for maintaining proper part geometry when forming undercuts in the injection molded part.

Wear Plates

Regular molds: ±0.03mm (with shim adjustment)
Precision molds: ±0.02mm (with shim adjustment)

Ensures proper alignment and wear resistance, maintaining consistent performance and injection molded part quality over time.

When designing sliding fits, it's important to consider the operating conditions of the mold. Factors such as temperature fluctuations, injection pressure, and cycle frequency can all affect the performance of sliding components. Proper lubrication and material selection, combined with appropriate tolerances, are essential to prevent excessive wear and ensure consistent operation throughout the mold's lifespan.

The clearance specified in sliding fits must be sufficient to accommodate thermal expansion of components during the injection molding process. Insufficient clearance can lead to binding or galling, which can damage both the mold components and the injection molded part. Conversely, excessive clearance may result in flash, dimensional inaccuracies, or poor surface finish on the injection molded part.

Regular maintenance and inspection of sliding components are crucial to ensure they remain within their specified tolerance ranges. Wear can gradually increase clearances over time, affecting the quality of the injection molded part. Implementing a proactive maintenance schedule helps identify and address tolerance issues before they impact production quality or cause mold damage.