To Explore the Effect of Injection Molding Processing Parameters on Crazing in Acrylic Polymers

Abstract

Plastic manufacturing industry is the fastest growing industry as the demand for plastic products is exponentially growing worldwide. Poly (methyl methacrylate) (PMMA), also known as acrylic glass, is a transparent and rigid thermoplastic. PMMA is highly resistant to UV light. Weathering has an excellent light transmission and unlimited coloring options compared to other transparent plastic and has been used in wide applications such as architecture, automotive and transportation, lighting (LED lights), medical and healthcare, and furniture. Injection molding is the widely used plastic manufacturing process to produce plastic products for various applications. The quality of the plastic products depends on the injection molding parameters, viz. melting temperature, injection speed, and pressure, holding and cooling time, and holding pressure. Therefore, it is important to control injection molding parameters to reduce injection molding defects or parts. Hence, the main objective of this research is to optimize the injection molding parameters, including the amount of mold releasing agent, to avoid the crazing marks in PMMA which is a long-standing problem in the production of PMMA products or parts such as motorbike headlight lenses. Three different holding pressures (65, 75, and 85 kg/cm2) were varied against three different injection pressures (70, 80, and 90 kg/cm2). The injection speed (60 %), cooling time (4 s), and barrel zones temperature (185-205: 185-205: 190-210: 195-215) were kept constant not to disturb the production cycle, a constraint from the production industry. The minimum criteria required for the motorbike headlight lens selection was based on the LUX intensity test, density, and crazing demanded by the Japanese standard throughout this research. The optimized injection molding parameters and amount of mold releasing agent (Nabakem mold release R2) were 70 kg/cm2 (injection pressure), 65 kg/cm2 (holding pressure), and 1.18 g, respectively. The motorbike headlight lens produced with optimized injection molding parameters showed no crazing. In addition to the desired LUX intensity, density, and no crazing criteria, the motorbike headlight lens also showed improved impact properties and no substantial changes in mechanical properties when compared to virgin PMMA or literature. Hence, it is concluded that the optimized injection molding parameters (thermo-mechanical history) did not much affect the molecular weight and morphology of the PMMA.