The bulk density of plastic is used to describe the material’s mass per unit volume. Bulk density is most commonly expressed in grams per cubic centimeter and calculated by dividing the mass of a material by its volume. Depending on the material, however, bulk density may also be written as kilograms per liter (kg/l) or pounds per cubic foot (lb/cu ft).
While many plastic processors measure total production based on weight (the total number of pounds, kilograms, or tons of a specific material that moves through their equipment), bulk density is highly essential and has a significant impact on nearly all phases of processing and production. Processors use bulk density to determine the best way to treat specific material throughout the production process. From blending and conveying to drying and storage, it is a critical metric that helps the employees at plastics plants optimize the efficiency with which they process different materials. Below are some of the main ways that processors use the bulk density metric to guide their operations and maximize efficiency:
Material Drying
When manufacturing plastic material, removing excess moisture is a major concern, and in many cases, can have a profound impact on the material’s ultimate performance. For example, in PET plastics, too much moisture can result in a much lower tensile strength when the material is in use.
Proper drying can help prevent structural issues like these from becoming a problem, and bulk density plays a major role in determining how specific material gets dried. To ensure proper output, the amount of material in question needs to be dried for the right time at the appropriate temperature. Bulk density is essential for determining how long a material will take to be optimally dried and what the size of the material hopper used should be.
Material Flow
One of the main impacts that bulk density has on a material has to do with the pace and speed of its flow through the different handling systems. Many conveying systems tend to pick up lower-bulk-density pellets at a higher rate, affecting the producer’s desired ratio of materials. If bulk density is too low, the material will tend to bunch and cling together instead of moving smoothly through the machine. Special measures often need to be taken to keep the material flowing smoothly. Additionally, lighter materials often need to be put through more total receiver cycles to achieve a similar throughput based on weight.
Material Storage
In plastics processing, resin pellets of materials are commonly stored in containers (silos, hoppers, railcars, etc.) of a fixed size. Workers use bulk density to calculate the amount of a given material that will fit inside a container and how much storage space is required for predetermined amounts of materials. As a general rule, materials with lower bulk density require more storage space than higher bulk density materials to reach the same output weight.
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