Struggling to design a plastic injection molded part that meets all necessary specifications and is free of deformations and flaws? The problem is likely stress — or, more accurately, residual stress.
When a plastic resin is melted down prior to injection, the molecular bonds of the material are temporarily broken due to the heat and sheer force of the extruder. This allows the molecules to flow into the mold. The bonds then reform as the material cools, but the stresses on the material remain.
How Residual Stress Affects a Part
Residual stress does not always manifest in the same way. The resin material, melting temperature, and specific injection processes all play a role in how stress occurs, and even the tiniest variable shifts can lead to notably different physical manifestations.
Residual stress can lead several forms of distortion. Warpage, a type of stress caused by relaxation distortion, is especially common. Unfortunately, residual stress warpage is generally undetectable until the part is already in use, when real-world fluctuations in temperature bring the stress to light. Sink marks — part surfaces that have become depressed below their intended plane — are frequently found in warped parts and can present both structural and aesthetic issues.
Cracking and crazing, which produce a series of extremely fine micro cracks, are also common manifestation of residual stress. Parts that are subjected to chemicals, environmental exposure, and temperature extremes may be vulnerable to cracking or crazing.
Residual stress and its related issues can ultimately lead to premature part failure, but this can be easily avoided if armed with the right knowledge.
Residual Stress Solutions
First, select your material carefully. Residual stress in plastic injection molding is unavoidable due to the simple physics of the process, but certain materials are more prone to physically manifesting that stress than others.
You also can prevent residual stress by designing smooth transitions between different features of your part and including rounds, fillets, and other special features in areas likely to experience high stress.
To learn more injection molding basics that can help with your next project, download our free injection molding glossary today.