Meaning
International standard defining the permissible dimensional variations for injection moulded parts based on material shrinkage and production conditions. DIN 16742 plastic tolerances provide a consistent framework for designers and moulders to agree on the required precision of a part. This standard replaces older guidelines and accounts for the unique properties of polymers, such as their high thermal expansion and moisture absorption.
It defines several tolerance grades, ranging from general purpose to high precision, depending on the application and the material used. The standard ensures that the parts will fit together correctly and function as intended. It applies to all injection moulded components, but it does not cover defects like sink marks or warpage.
Shrinkage Variation
Plastic parts shrink as they cool in the mould, and the amount of shrinkage depends on the type of resin, the specific part geometry, the processing parameters and the mould temperature. Amorphous resins like polycarbonate have low and predictable shrinkage, while semi crystalline resins like polypropylene shrink much more and can be more difficult to control. DIN 16742 plastic tolerances take these differences into account by providing different tolerance values for different groups of materials.
The standard also considers how the moulding process, including the melt temperature and the injection pressure, affects the final dimensions. Moulders use this information to set up their machines and ensure that the parts stay within the required limits. Understanding the relationship between material shrinkage and tolerances is the key to producing consistent parts.
Precision Grade
The standard offers several levels of precision, known as tolerance groups, which allow the designer to choose the right level of accuracy for their application. For parts that are used in non critical applications, a general tolerance grade might be sufficient, while high precision parts like those used in medical devices require a much tighter grade. Choosing a tighter tolerance grade usually increases the cost of the tooling and the production process, as it requires more precise control and more frequent inspection.
The standard provides clear tables and formulas to help the designer calculate the correct tolerance for each dimension. This clarity prevents misunderstandings between the customer and the supplier and ensures that both parties have a realistic expectation of what can be achieved. Every dimension on a drawing should be assigned a tolerance based on the requirements of the standard.
Compliance Verification
To ensure that a part meets the DIN 16742 plastic tolerances, it must be measured using calibrated equipment in a controlled environment. The measurement should be taken after the part has had enough time to fully cool and reach its final dimensions, which can take up to forty eight hours for some materials. If a part is out of tolerance, the moulder must adjust the process parameters or modify the tool to bring it back into compliance.
This verification process is an essential part of the quality control system for any moulding operation. It provides the data needed to prove that the parts meet the customer’s specifications and are fit for use. Regular auditing of the production process ensures that the tolerances are maintained over the entire life of the project.
