Plastic properties and molding conditions
There’s a close internal connection between plastic properties and molding conditions. The properties of a plastic determine its appropriate molding conditions, and appropriate molding conditions can fully utilize the plastic’s excellent properties, thereby producing high-quality plastic parts. Plastic properties primarily include physical, chemical, mechanical, and processing properties, such as density, melting point, thermal stability, strength, hardness, and fluidity. Molding conditions, on the other hand, include molding temperature, molding pressure, molding time, and mold temperature. A deeper understanding of the relationship between plastic properties and molding conditions is crucial for optimizing the injection molding process and improving part quality. For example, plastics with good fluidity can be molded at lower pressures and temperatures, while plastics with poor thermal stability require strict control of molding temperature and time to prevent degradation.
Plastic fluidity is a key property that influences the selection of molding conditions. Fluidity refers to the ability of a plastic melt to fill the mold cavity under a certain temperature and pressure. It is typically expressed by the melt index (MI), with a higher MI indicating better fluidity. Plastics with good fluidity, such as polyethylene (PE) and polypropylene (PP), can be molded using lower injection pressures and temperatures, quickly filling the mold cavity and making them suitable for molding thin-walled, complex parts. Plastics with poor fluidity, such as polycarbonate (PC) and polyoxymethylene (POM), on the other hand, require higher injection pressures and temperatures to ensure the melt fully fills the mold cavity. Furthermore, the fluidity of plastics is affected by temperature and pressure, increasing with increasing temperature or pressure. Therefore, in actual production, the molding temperature and pressure must be appropriately adjusted based on the plastic’s fluidity characteristics to ensure optimal filling.
The thermal properties of plastics have a significant impact on the determination of molding conditions. These properties primarily include melting point, glass transition temperature, and thermal stability. The melting point is the temperature at which a plastic transitions from a solid to a liquid state. The molding temperature must be higher than the melting point to allow the plastic to melt into a melt. The glass transition temperature is the temperature at which an amorphous plastic transitions from a glassy state to a highly elastic state. For crystalline plastics, the glass transition temperature is lower than the melting point. Thermal stability refers to a plastic’s ability to resist degradation at high temperatures. Plastics with poor thermal stability, such as polyvinyl chloride (PVC), readily decompose at high temperatures, producing harmful gases and leading to reduced part performance. Therefore, the molding temperature and the residence time of the melt in the barrel must be strictly controlled during molding. For example, the molding temperature for PVC is typically controlled between 160°C and 190°C. Excessively high temperatures or prolonged residence time can cause decomposition, impacting part quality.
The mechanical properties required of plastics will also affect the choice of molding conditions. The mechanical properties of plastic parts include strength, hardness, toughness, elasticity, etc., and different usage scenarios have different requirements for the mechanical properties of plastic parts. For example, structural parts need to have high strength and hardness, while elastic parts need to have good elasticity and toughness. To ensure that the plastic parts can achieve the expected mechanical properties, parameters such as molding pressure, holding time, and cooling time need to be reasonably controlled during the molding process. Higher molding pressure and longer holding time can make the plastic parts denser and improve their strength and hardness; while appropriate cooling time can reduce internal stress within the plastic part and improve its toughness. For example, when producing high-strength polyamide (PA) plastic parts, higher injection pressure and holding pressure are generally used, and the holding time is appropriately extended to improve the density and strength of the plastic parts.
Mold temperature is another crucial molding parameter closely related to plastic properties. Mold temperature affects the cooling rate, crystallinity, and internal stress of the plastic melt. For crystalline plastics such as polyethylene (PE) and polypropylene (PP), mold temperature affects both the crystallization rate and degree of crystallinity. Higher mold temperatures promote crystallization, increasing the crystallinity and hardness of the part, but also prolonging the cooling time. Lower mold temperatures inhibit crystallization, reduce crystallinity, and improve part toughness, but may also increase internal stress. For amorphous plastics such as polycarbonate (PC) and polystyrene (PS), mold temperature primarily affects the cooling rate and internal stress of the melt. Higher mold temperatures slow cooling, reduce internal stress, and improve part toughness. Lower mold temperatures accelerate cooling and improve production efficiency, but may result in surface defects such as silver streaks and cracks. Therefore, mold temperature should be appropriately set based on the crystallization characteristics of the plastic and the performance requirements of the part. For example, when producing high-toughness polycarbonate plastic parts, a higher mold temperature (80-120°C) is usually used to reduce internal stress and improve the toughness of the plastic parts.
