Polycarbonate (PC) is a high-performance engineering thermoplastic valued for its strength, transparency, and thermal stability. It is used in applications ranging from eyeglass lenses and protective barriers to automotive components and electronic housings. For designers and manufacturers, specific gravity (SG) is a foundational property that influences design, cost, and processing decisions, and is necessary for calculating part weight and optimizing production efficiency.
Defining Specific Gravity in Material Science
Specific gravity (SG) is a fundamental measure in material science that expresses a substance’s density relative to a standard, typically water. SG is calculated as a dimensionless ratio by dividing the material’s density by the density of water (approximately 1.0 \(\text{g/cm}^3\) at 4°C). This ratio is preferred over absolute density because it provides a universally consistent reference point. An SG greater than 1.0 means the material is denser than water and will sink, while an SG less than 1.0 means it will float. This value allows engineers to quickly compare the relative weight of materials.
The Specific Gravity Value of Polycarbonate
The specific gravity for standard, unfilled polycarbonate typically falls within a narrow range of 1.20 to 1.22. This means PC is 20 to 22 percent denser than water, placing it in the heavier range compared to common commodity plastics like polyethylene or polypropylene. This density contributes to PC’s robust mechanical properties.
The exact SG value can shift slightly depending on the specific grade and formulation of the polycarbonate resin. For instance, optical-grade resins, which require high clarity and purity, will maintain the lower end of this range. The inclusion of common additives will increase the specific gravity of the compound.
When polycarbonate is compounded with reinforcing agents, such as glass fibers or mineral fillers, the SG increases significantly due to the density of the additive material. For example, a 30% glass-fiber reinforced PC grade can have a specific gravity as high as 1.43. Manufacturers must consult the supplier’s technical data sheet for the precise SG, as this variation directly affects final product characteristics and cost calculations.
Practical Implications of PC’s Density in Manufacturing
The specific gravity of polycarbonate has substantial consequences for manufacturing economics and technical processes. Since raw materials are purchased by weight but parts are produced by volume, SG dictates the material yield. A lower SG produces a greater number of parts from the same weight of resin compared to a higher SG, which is a significant factor in cost analysis.
This relationship is relevant for calculating the final weight of a component, a primary concern in the automotive and aerospace sectors. Designers use the SG value to accurately predict the weight of a finished part from its volume, impacting fuel efficiency and structural load considerations. Density also influences how much material processing equipment, such as an injection molding machine, must feed by weight to fill a fixed mold cavity.
In injection molding, specific gravity determines the material’s shrinkage characteristics. Like all thermoplastics, PC changes density between its molten and final solid states. This volume-to-mass change must be accounted for in mold design to ensure the final part meets dimensional tolerances.
Polycarbonate’s SG of approximately 1.20 provides a beneficial balance when compared to other materials. It is significantly lighter than structural metals, such as Aluminum (SG 2.70), allowing PC to be utilized as a lightweight alternative in many applications. However, it is also substantially denser than plastics like polypropylene (SG 0.91), providing superior strength and impact resistance. This density profile makes PC suitable for demanding applications that require high performance without the excessive mass of metal components.