Meaning
Cosmetic surface irregularity characterized by silver streaks, bubbles, or splash marks aligned with the direction of melt flow on an injection-molded plastic part. In the moulding plant, a splay defect occurs when volatile gases such as moisture, air, or polymer degradation byproducts become trapped in the melt and are dragged along the mold cavity wall during injection. This defect compromises the visual quality of the component and can indicate underlying chemical degradation.
Defect Cause
Vaporization of trapped moisture in the resin during the heating cycle generates steam bubbles that stretch out against the mold surface. When a splay defect appears, the source is frequently inadequate resin drying, particularly in hygroscopic materials such as nylon, polycarbonate, or polyethylene terephthalate. Other causes include excessive melt temperature, which initiates thermal breakdown of the polymer, and excessive screw speed that generates high shear heat.
Production Impact
Rejection of molded parts by quality control due to these cosmetic blemishes increases the overall cycle-to-cycle scrap rate. When a splay defect occurs in optical or medical components, the parts are completely unusable, which increases production costs and consumes machine capacity without yielding sellable product. In addition, the gas pockets that cause the surface marks can also form internal voids, which reduces the structural integrity and toughness of the part.
Moulders must trace the source of the volatiles immediately to prevent large-scale rejects.
Remedial Action
Increasing the resin drying time or temperature removes the residual moisture from the raw pellets before they enter the moulding machine. This process eliminates the primary driver of the surface blemishes.
