Plastic Pellets...How Much Do I Need?

Recently, an educated customer who was making cornhole bags asked me about the bulk density of the materials we sell. While this customer did not know that they were actually asking about "bulk density", they asked me how much one cup of pellets will weigh! In essence, this is the same question. When making items like cornhole bags, weighted blankets, stuffed dolls, reborn dolls, vase stuffing, and other crafty items, it is very important to consider the bulk density of the plastic pellets. Let me explain in layman's terms.

There are two "weight" properties of plastic pellets that will be important to most applications of plastic pellets...even in the traditional plastics processing world. The first property is the actual "density" of the plastic material itself. Density is a measure of mass per unit area. It is commonly reported in grams per cubic centimeter, but can also be reported in lbs per cubic inch and in many other units. It is essentially the mass (you can think of this as weight) of a specific volume of the plastic material. This is important because different plastic materials have different densities. This means that some polymers will simply weigh more than others as a result off their chemical composition and morphology (the way the polymers line up with each other). The second property, which is often times more important in crafting applications is the "bulk density" of the pellets. The bulk density has the same units of measurement (g/cm3 or lbs/in3, etc), however, the definition is a bit different. This property refers the mass (weight) of a specific volume of the pellets in their spherical form. In other words, the bulk density (weight) will always be LESS than the actual density of the plastic material due to the air spaces created by the inefficient stacking and packing of the plastic pellets!  As you can imagine, there are several things which can impact bulk density including the shape of the pellets, foaming of the pellets, additives in the pellets, etc.  

In order to illustrate, lets consider a solid cubic foot of plastic material. The density of this material might be 10 lbs per cubic foot (just as an example). If you were to weigh a full cubic foot block of plastic it would weigh 10 lbs! On the other hand, if you cut this block up in to small round plastic pellets, placed the pellets into a one cubic foot container, the weight of the pellets in the container would be significantly less due to the poor packing efficiency of the pellets. This is what we mean by "bulk density".

One cup of our virgin plastic pellets weighs about 5 oz per cup or 142 grams. Hence, the bulk density can be reported to be 142 grams per cup! On the other hand, one cup of our "Heavy Pellets" weighs in at a whopping 11 oz per cup! 

Let me give you a very practical example of a mis-match of the material for a market application. One poor woman was trying to use plastic pellets to fill a bean bag chair when she should have been using a product called expanded polystyrene. This is essentially a foamed plastic material with a MUCH lower bulk density! As a result, she ended up with a VERY heavy bean bag chair. Impractical by most standards. So...if you are in the market for pellets, you will want to match the bulk density of the pellets to your application! If you are buying by the pound and want to maximize the weight of a cup of pellets (weighted blankets might be a good illustration), you will want to buy heavy pellets as you get more weight per dollar spent. If you are buying by the volume and you want the most volume of pellets per dollar spent, you will want to buy pellets which have a lower bulk density. Also, you can mix pellets to get the bulk density you desire.  

So remember...consider the bulk density of the plastic pellet BEFORE you buy your material. Email us with any specific questions and we will be happy to help!