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The Materials Analyst, Part 69: Density, bulk density, melt density, and specific gravity (Web-exclusive)

January 1, 2006

9 Min Read
The Materials Analyst, Part 69: Density, bulk density, melt density, and specific gravity (Web-exclusive)

This series of articles is designed to help molders understand how a few analytical tools can help diagnose a part failure. Michael Sepe is our analyst and author. He is the technical director at Dickten & Masch Mfg., a molder of thermoset and thermoplastic materials in Nashotah, WI. Mike has provided analytical services to material suppliers, molders, and end users for 15-plus years and can be reached at [email protected].

Used correctly, these measurements can help with troubleshooting and quality control.

There are a lot of numbers on a material property data sheet. A truly complete property sheet may have more than 60 entries for a broad range of mechanical, thermal, and electrical properties while others may list values for only four or five. But even the most cursory treatment will include a value for density or specific gravity. These are very often treated as the same measurement, primarily because specific gravity compares the density of the material to that of water and as it happens, the density of water is so close to 1 that it has a negligible effect on the obtained value.

However, a conscientious experimenter, when measuring specific gravity, will always note the temperature at which the determination was performed. This is because the density of water varies as a function of temperature, and an accurate measurement of density must adjust for the actual density of the water used.

The table gives some of the values for the density of water as a function of temperature. A little manipulation of the numbers shows that the effect of converting specific gravity to density is fairly trivial on a practical level until you start to calculate to the third decimal place. (If you run the tests in water at 4°C, you don't even have to worry about discrepancies until you get out to the fifth decimal place.)

The other detail associated with the comparison between specific gravity and density is that of units. Density is fundamentally a measure of weight per unit volume. Despite the general disdain of the metric system in the United States, we are quite comfortable with expressing this measurement for our plastic materials in terms of grams per cubic centimeter (g/cc). Specific gravity is a ratio and therefore is unitless.

Bulk density

Bulk density is just what it sounds like-a measure of the density of the bulk material. This property determines how many pounds of material will fit in the silo, the rail car, the gaylord, or whatever other container is used to transport and store the material. It is not a fundamental property of the material because it takes into account the space between the pellets.

In general, the bulk density is approximately 50% of the solid-state density. In other words, if you filled a hopper with pellets and then raised the temperature of the material high enough to make it melt and flow, you would collapse the material to about half of its original volume by the time it solidified and returned to room temperature. It is difficult to capture this relationship intuitively because when it comes to bulk density we revert back to our bias for so-called “English” units and express bulk density in terms of pounds per cubic foot (lb/ft3).

While the bulk density may seem far removed from the practical considerations of fundamental material properties, it is actually a very good and simple quality control test for incoming raw materials. This is because bulk density changes as a function of pellet size and shape. It can therefore detect quality problems such as long strands, fines, and the presence of a large amount of volatile material trapped in the pellets.

For this reason most material suppliers use bulk density as a quality control parameter, although they seldom list it either on a data sheet or in a release certification. Anyone with a reasonably accurate scale and a container of fixed volume has the capability of measuring bulk density. It is a good first line of defense for detecting variations in pellet quality.

At one time we worked with a processor of flame-retardant, high-impact polystyrene who suddenly encountered splay. Drying the material did not remedy the problem, but a visual inspection of pellets from a good lot and the bad lot showed a significant difference in pellet size and shape. Under magnification, it was evident that the larger size of the pellets from the bad lot could be attributed to a fine distribution of very small air pockets. These had arisen from a failure of the vacuum pump on the extruder during the compounding process, which trapped volatile material inside the pellets. Once melted, this gaseous material was released, causing the splay. A measurement of bulk density for the two lots showed a considerable difference.

Fallible density ratios

Specific gravity or density is used all the time by processors during the quoting process. Years ago the quoting process began with a laborious estimate of part volume based on the drawing. This was then converted to part weight by applying the appropriate value for the density of the specified molding compound. Now software calculates the volume, saving valuable time and improving accuracy. In evaluating alternate materials for possible cost reduction, it is important to factor in the density because while molders buy material by weight, end users buy it by volume. An alternate material may produce a lower-cost part even if it is more expensive by the pound; this increase can be offset by a lower density.

However, there is one area where we use the solid density of materials incorrectly, and it has a bearing on how we calculate shot capacities on our molding machines. When we purchase a molding machine, one of the key decisions that we make is selection of the barrel size. This barrel size is expressed in terms of shot capacity-the maximum amount of material that can be delivered with a single full stroke of the screw. Typically expressed in terms of ounces, it is indexed to polystyrene.

When evaluating the amount of material other than polystyrene that can be delivered, we tend to make the calculation using a ratio of densities. For example, a barrel rated for 70 oz of polystyrene (density = 1.04 g/cc) can be expected to deliver 80.77 oz of polycarbonate based on a multiplier of the density of polycarbonate divided by that of polystyrene (1.20/1.04).

The problem with this logic is that material in the injection unit is not in the solid state, or at least we hope not. Rather, the resin is molten. We know that the density of molten material is not the same as that of the solid material; otherwise we would not have that wonderfully unpredictable property known as mold shrinkage. All polymers shrink as they cool in the mold, which means that the density increases with the conversion from melt to solid.

Of even greater importance is the fact that the magnitude of this change is dependent on the structure of the polymer. Amorphous materials undergo a smaller volumetric change as they solidify. This is why mold shrinkage values for amorphous materials are relatively low. Semicrystalline materials go through a much larger volumetric change because the organized crystal structure that forms takes up much less space than the disorganized amorphous regions. This is why ABS shrinks approximately .006 in/in while polypropylene can shrink as much as .020 in/in.

Values for the density of water as a function of temperature

Missing melt density

If the volumetric contraction of these two classes of materials is that different, then it follows that the volumetric expansion will also be quite different. To obtain an accurate conversion from one material to the other we need to know a property that never appears on the property sheet: the melt density. Most unfilled amorphous resins exhibit approximately the same mold shrinkage; therefore, we can expect that they will also display similar relationships between melt density and solid density.

This turns out to be true. The reason the conversion from polystyrene to polycarbonate comes out very close to being correct is that the melt density of polystyrene is 90.8% of the solid density, while that of polycarbonate is 90%. A rigorous calculation using the correct melt densities of the two materials shows that a 70-oz barrel delivers 80 oz of polycarbonate rather than 80.77 oz. Most of us will not notice the difference.

But consider the same comparison with a semicrystalline material like high-density polyethylene (HDPE), the most crystalline of the semicrystalline thermoplastics. The melt density of an HDPE with a solid density of .957 g/cc is only .759 g/cc; the melt density of this material is less than 80% of the solid density.

Traditionally, if a molder wanted to estimate the shot capacity of a given barrel, he would take the ratio of .957/1.04 and come up with a multiplier of .92. As an example, a barrel rated for 95 oz of polystyrene would be expected to deliver 87.4 oz of this particular grade of HDPE. But in reality, the melt density of polystyrene is .945 g/cc and the ratio of .759/.945 is only .803. When multiplied by 95 we can only expect a maximum delivery of 76.3 oz. We are off by more than 11 oz!

When software saves

This may seem like a trivial matter. However, recently we encountered a problem where a processor needed to mold a part in HDPE weighing 76 oz using a barrel rated for 95 oz of polystyrene. The part would not fill and the injection unit would not hold a cushion. The conclusion was that the screw and barrel were worn and that the extra material was leaking back over the nonreturn valve and into the screw flights.

But replacing the screw and barrel did not remedy the problem. A considerable amount of time was spent evaluating the hardware and trying to figure out where the material was going or whether the machine specifications were wrong. Until the barrel capacity calculations were made using melt density values, the problem remained a mystery and consumed a lot of troubleshooting time. Now, if you look at a property data sheet you will not find any mention of melt density. So where do you find the information? Melt density is a fundamental value required for flow simulation. Therefore, if you have the software for performing flow simulations, this value should be part of the database for those materials that are in the library. Some material suppliers have started to measure the flow rates for their materials in terms of melt volume rate as well as melt-flow rate. Melt-flow rate (MFR) results are expressed in g/10 min while melt volume rate (MVR) results are given in cc/10 min, if you are lucky.

Dividing the mass from the MFR test by the volume from the MVR test gives you the melt density. Alternately, you can use an MFR tester to measure the melt density of a material. Simply allow the piston forcing the material through the orifice to travel a prescribed distance. Knowing this distance and the diameter of the bore in the tester allows for a precise calculation of the volume of the extruded material. Then weigh the extrudate. Resist the temptation to express the results in pounds per cubic foot.

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