Mechanical properties of PS injected parts under surface-active substances
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MECHANICAL PROPERTIES OF PS INJECTED PARTS UNDER SURFACEACTIVE SUBSTANCES
The main objective of this work was to develop the influence of surface-active substances in the mechanical properties of injected plastic parts in presence of cracks, a tension test and a constant load. The material used was Polystyrene and the surface-active substances were olive oil and corporal sweat. This study was conducted because it was observed that the ...view middle of the document...
Nowadays, it is necessary to know and understand why the defects occur and to find the way to avoid them. The polymers are particularly sensitive to its processing. They are affected by environment, time, and temperature storage, shipping and service. Additionally, the plastic parts with geometric changes, holes and cracks in their structures have many stress concentrators that may induce them to an early failure during its use. Polymers failures may occur at low stress levels caused by a creep test, fatigue and liquid environmental agents. When a polymer is exposed to a chemical environment and simultaneously to a stress, it is observed a drastic fracture time reduction. Such agents do not attack the polymers molecules chemistry. They are not also solvents able to dissolve, however, they penetrate superficially the part, allowing the release of freeze stress and superficial rupture. This kind of failure has been called Environmental Stress Cracking (ESC) and it should be under consideration, since 20% of the plastic parts failures are caused by them. That is the reason why, the main purpose of this work is to develop a methodology that allows studying the influence of the surface-active substances over mechanical properties of injected PS parts. Previous works, for example, Kramer
Results and Discussion
This study came from a discovery made by Candal et al , where it was found that corporal sweat may act as an surface-active substance when interact with PS, affecting the results of the mechanical tests. Nowadays, there are several plastic parts (CD boxes, table settings, glasses, trays, tool boxes) that may be in contact with corporal sweat. Since there are not previous studies regarding this subject, the following methodology was design: 1. It was chosen a type of specimen to use: it was important to have a plaque able to modify the crack length in order to study the crack’s effect over the mechanical properties of plastic parts. Likewise, it was decided to use a normalized tensile test specimen type IV to study the effect of a surface-active substance without incorporating the crack influence.
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2. Material selection: the material was selected to complete the study and to know the mechanical and rheological properties. In this case it was chosen a PS injection grade. 3. It was established an injection process window of the polymer: this window indicates the temperature and the pressure ranges in which the material may be process, in accordance with the needed design and aesthetic requirements (good surface appeal, complete transparency and no presence of flashes or sink marks). Starting from here, an optimal process condition was chosen. 4. Plaques injection: the most important effect of injection molding over the PS parts was the imposed orientation, causing mechanical properties changes. Moreover, in the post-molding stage, there is a tendency to accumulate residual stress that are the consequence under low cooling...