Blistering is a common defect during plastic injection molding. It appears as bubbled or raised areas on the molded surface parts. Since the part is not wooden , this excess material or blobs like structure cant be peeled off and remain a permanant defect affecting product aesthetics and structural integrity and even your reputation with your client. These are generally gas bubbles that form beneath the surface of the molded parts. It affects the compactness of the molded part. They can range from small discrepancy in the product quality to massive structural deformation.
Causes of blistering
Excessive moisture in the raw material
Moisture in the plastic resins or additives which often vaporises during the molding process and gets trapped as vapor gases while the molten liquid is injected in the mold, leading to blister formation in the final product.
Material Overheating
Excessive heating can degrade the plastic material massively altering chemical properties and burning to some extent causing gas formation and blistering.
Polypropylene in high temperature degrades to hydrocarbon molecules and some of these molecules like methane and carbondie oxide are gaseous in nature. Methane if it gets trapped in the mold cavity causes porosity in the product (gases expand rapidly at high temperatures), making it unsuitable in load bearing operations, and also surface deformation.
Excessive injection pressure
To much injection pressure increases the rate at which the molten liquid is forced into the mold. This sudden rapid movement of the fluid plastic material traps gases inside the mold as the gases dont get enough space or time to move out through the vents. High injection pressure causes shear stress in the molten plastic breaking down the molecular structure and emits gases while the plastic degradation takes place.
Improper cooling
When the mold’s cooling system is not uniform, , it creates thermal gradients within the mold, with different layers of plastic being soldified to different levels. The interaction between these layers causes blistering. The heat from the core of the part expands the gases trapped within the molten plastic. In case the outer surface has already solidified to some extent, while the inner layers being molten, the expanding gas creates a bulge or blister.
Contamination of material
Foreign particles and the degraded materials combined with raw resins can cause blister formation
Ensuring that raw materials for plastic injection molding are not contaminated
