What is bulk density?

The bulk density of plastic materials influences many different operations within a plastics processing plant.

So what is bulk density, anyway?  Bulk density is the weight of a material based on a given volume.  In the world of plastic resins, bulk density is typically expressed in one of several ways:

• Pounds per cubic foot (lb/cu ft)
• Grams per cubic centimeter (g/cc)
• Kilograms per liter (kg/l)

Because the bulk densities of various plastic materials differ, plastics processors measure productivity based on weight:  the pounds, kilograms, or tons of plastic material processed through their production equipment.

Why is the bulk density of plastic materials important?

Bulk density affects material storage.  Material is often packaged or stored in containers of a fixed size (e.g., a Gaylord, a silo, a hopper, a railcar). However, because resin pellets of different materials have different bulk densities, identically sized containers of different materials will hold dramatically different weights of material.  In the example shown below, it processing a ton of virgin PET pellets would consume only one Gaylord of material. However, if the process was consuming PET sheet regrind, it would take three Gaylords of this material to provide the same weight of output.

Bulk density affects the sizing of drying equipment. Drying operations require that the required weight of material be dried at the right temperature for the correct residence time. To ensure that you get the right output of dried material for processing at the right time, the sizing of the material hopper is a critically important factor.

Bulk density affects material flow.  Here are a few ways that material bulk density affects material flow:

• Materials with different bulk densities can move at different rates through material handling systems. For example, if a processor blended a 50/50 mix (by weight) of recycled PET flakes and virgin pellets into a container, but then used a vacuum conveyor to move that mix a relatively long distance (100 feet) to another production machine, the conveying process would likely pick up more low-bulk-density recycled PET flakes and fewer high-bulk-density virgin PET pellets.  What started as a 50/50 mix could be knocked out of balance simply because of bulk-density differences.
• Materials with lower bulk densities affect conveying throughput. Lighter materials will require more receiver cycles to achieve a higher throughputs, based on weight.
• Materials with very low bulk densities (>15-18 lbs/cu ft, e.g., regrind sheet PET, PE, or PP) don’t flow very well. These light, flaky materials tend to bunch together, cling to equipment, or “bridge” instead of flowing smoothly. You should always test the flow characteristics of light materials to see whether they need some “flow enhancement” such as:

1. Poking around with a stick or a hammer to break up bridges in material bins, receivers. (Not a good long-term solution!)
2. Amending a percentage of virgin pellets (± 20-25%) into light bulk-density materials like regrind flake. When mixed, this small amendment increases the bulk density of the entire lot of material and makes a big difference in how it flows.
3. Adding mechanical flow enhancements, such as mechanical agitation or compressed-air “pucks” that use blasts of air to break to prevent bridging at pick-up or discharge points (i.e., the cone-shaped areas at the bottoms of receivers or hoppers). These flow enhancers should be triggered “on” at the beginning of every conveying cycle, or whenever a receiver is full and needs to empty itself out to receive more material.

These are just a few of the reasons why differences in the bulk densities of materials are so important.