Stress Whitening as an Observation Method of Residual Stress in MABS Polymer Material through the Example of Holding Pressure in an Injection Molding Process

Abstract

The effects such as warpage, dimensional instability and environmental stress corrosion, due to the presence of residual stresses in polymeric products, are strongly dependent on injection molding conditions. The holding time and holding pressure belongs to most important processing parameters, determining the dimensional stability and properties of injected goods. A new procedure based on a visualization technique was applied, where the levels of residual stresses of the samples were estimated. The experiments were performed for samples produced of translucent methacrylate acrylonitrile butadiene styrene (MABS), a commodity polymer with a high transparency, necessary for the optical visualization of the stress whitening. The samples produced by injecting molding were deformed to a constant elongation, to observe the dependent stress whitening effect subsequently used to evaluate the stress distribution. It was found that depending on the value of the injection holding pressure, various levels of residual stress and its distribution may be observed in MABS samples. These measurements conformed that the applied optical method is an easy-to-perform technique. The possibility to detect the residual stresses over the whole cross-section of the transparent product, without the necessity for local stress determination, is another significant advantage of this investigation procedure.

Determination of residual stresses in amorphous thermoplastic polymers by DMA

“…However, the rapid production creates stresses in the products. The level of residual stresses depends on processing conditions such as mold temperature, injection rate, holding pressure, time, and so forth 9–12 …”

“…There are two basic mechanisms of stress formation during injection molding, namely flow‐induced stresses and thermally induced stresses 9,13 . Flow‐induced stresses occur during filling, but also in a post‐filling phase.…”

“…However, metals and plastics differ greatly in their structure and properties, so some adaptations were necessary to introduce them 22 . Destructive (layer‐removal, 23 hole drilling, 24 stress whitening 9 ) and nondestructive (X‐ray diffraction [XRD], 25 indentation, 26 time‐resolved fluorescence, 27 photoelastic, 28 etc.) methods have been developed 1,4 .…”

“…The photoelastic method sums stress in the direction of light transmission, while the chemical probe is limited to the surface only. It is therefore important to develop new test methods that can be used universally 9 …”

“…The level of residual stresses depends on processing conditions such as mold temperature, injection rate, holding pressure, time, and so forth. [9][10][11][12] There are two basic mechanisms of stress formation during injection molding, namely flow-induced stresses and thermally induced stresses. 9,13 Flow-induced stresses occur during filling, but also in a post-filling phase.…”