To what extent can the tolerance of plastic parts in injection molds be controlled?

 

Plastic part tolerance refers to the allowable dimensional variation of a plastic part. Plastic housing injection molding tolerances determine the quality and value of the final product. We are often asked by customers: "To what extent can the tolerance be controlled?" The standard for plastic part tolerances in injection molds is often influenced by multiple factors, and there is no single, absolutely uniform value. However, the following aspects can provide general guidance on determining their basis:

 

I. International Standards and Industry Specifications

 

1. ISO (International Organization for Standardization) Standards: Standards such as ISO 294-4 provide guiding principles and ranges for dimensional tolerances of Plastic injection molded parts. These standards typically classify tolerance levels based on factors such as part size, material type, and complexity.

 

2. Industry-Specific Standards: Different industries have different tolerance requirements for injection Molded Plastic Parts. For example, in the automotive industry, quality standards such as QS-9000 have strict regulations on the dimensional tolerances of automotive parts to ensure interchangeability and reliability during vehicle assembly. In the electronics industry, some electronic device manufacturers may establish specific tolerance standards based on their product requirements to ensure a tight fit between electronic components and plastic housings.

 

II. Material Factors

 

1. Plastic Material Shrinkage: Different plastic materials have different shrinkage rates, which directly affect the dimensional accuracy of injection molded parts. For example, crystalline plastics typically shrink more than amorphous plastics, so this factor needs to be considered when designing molds and determining tolerances. Generally speaking, materials with higher shrinkage rates also have wider tolerance ranges.

 

2. Material Flowability: The better the material's flowability, the easier it will fill the mold cavity during the injection molding process, potentially achieving higher dimensional accuracy. For materials with good flowability, the tolerance range can be appropriately reduced. Conversely, for materials with poor flowability, the tolerance range may need to be relaxed to avoid underfilling or excessive dimensional deviation.

 

III. Part Size and Complexity

 

1. Part Size: Generally speaking, smaller plastic parts require higher tolerances, as even slight dimensional deviations can significantly impact their function and assembly. For example, for a small plastic gear with a diameter of several millimeters, tolerances may need to be controlled within tens of microns. For larger plastic parts, such as car bumpers, the tolerance range can be relatively wide, potentially several millimeters or even larger.

 

2. Part Complexity: Injection molded parts with complex structures typically have tighter tolerance requirements than parts with simpler shapes. Complex parts may contain multiple small features, thin walls, or undercuts, all of which increase the difficulty of injection molding and require more precise mold design and stricter tolerance control. For example, a plastic housing with internal snaps and complex curved surfaces may require tighter tolerances than a simple flat plastic part.

 

 

1. Usage Environment: If injection molded plastic parts are used in harsh environmental conditions, such as high temperature, high pressure, and high humidity, even tighter tolerance control may be required to ensure they maintain good performance and dimensional stability under these extreme conditions. For example, plastic parts used in aerospace applications often have very tight tolerance requirements due to the significant pressure and temperature fluctuations they experience.

 

2. Assembly Relationship: If a plastic part needs to fit tightly with other parts, the tolerance requirements will also be increased accordingly. For example, if a plastic gear needs to mate with a metal shaft, to ensure transmission accuracy and stability, the gear's aperture tolerance and tooth profile tolerance must be strictly controlled to ensure the clearance with the shaft is within a reasonable range.

 

According to the precision industry standard, toys meet Class 3 standards, while cosmetics can meet Class 1 standards.

 

In summary, the tolerance standards for plastic parts in injection molds are the result of a comprehensive consideration of multiple factors and need to be determined based on specific product requirements, material properties, and production processes. In actual production, injection mold manufacturers typically ensure that plastic parts meet dimensional tolerance requirements through mold design optimization, process parameter adjustments, and quality inspections. Our HUIWEN precision molds can achieve Class 1 tolerances. Please contact us for more information if you require them.