Meaning
Stress level at which a plastic material ceases to behave elastically and begins to undergo permanent, plastic deformation under a tensile load. In part design, tensile yield strength represents the maximum stress that a component can withstand before it deforms irreversibly, which would compromise its structural function. This value is typically measured on standard injection-molded test specimens according to international standards such as ASTM D638 or ISO 527.
Designers use this datasheet value to establish safe load limits for the part, although the actual yield point can vary significantly based on the moulding process and environmental conditions.
Process Variable
Moulding conditions can modify this strength value by altering the orientation of the polymer chains and the degree of crystallinity. High injection speeds and low melt temperatures increase the alignment of polymer molecules along the flow direction, which raises the yield strength in that direction but reduces it in the transverse direction. This anisotropy can cause the part to fail prematurely if it is subjected to multi-axial stresses during service.
Moulders must balance these parameters to ensure that the yield strength is sufficient in all directions across the finished component.
Material Composition
The presence of fillers and the ratio of virgin to regrind material also have a significant impact on this mechanical property. Glass fibers increase the yield strength by transferring the tensile load from the polymer matrix to the stiffer fibers, provided there is good interfacial adhesion. However, the incorporation of recycled regrind material can lower the yield strength due to thermal degradation and chain scission that occurs during successive processing cycles.
This degradation reduces the molecular weight of the polymer, making it easier for the chains to slip past one another and yield under load.
Environmental Factor
Temperature and moisture exposure can drastically reduce the stress at which the material begins to deform. For example, hygroscopic polymers like polyamide absorb moisture from the environment, which acts as a plasticizer that increases chain mobility and lowers the yield strength. This reduction means that a part that performs adequately in dry conditions may fail in humid environments, necessitating the use of design safety factors that account for these environmental variations.
