|
|
Article - Specific Gravity -
See also material densities
previous page
The Materials Analyst:
Density, bulk density, melt density, and specific gravity
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
dmlab@execpc.com
.
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.
Source:
Injection Molding
Magazine |
