Recycled Plastic Compounds

Reference Information

Sieve Testing

Sieve testing is a means to determine the particle size distribution in powdered or granular materials.  The method uses standard sieves (screens) which are arrayed in a stack of progressively finer screens, with a pan at the bottom.  The material is placed into the top sieve and shaken in a specific way, and then the amount of material resting on each screen is measured, thereby determining the particle size distribution.  The amount passing the finest screen is shown as "pan".  The test method is described in ASTM Standard D-1921.

Mesh to Inch and Micron Conversion

Mesh refers to the number of openings per linear inch in a screen.  Generally, "35 mesh powder" would all pass through a 35 mesh screen.  However, a more exacting specification that covers the distribution of the various particle sizes is normal in many applications, such as in  rotational molding.  This table provides the screen opening of each mesh in inches and microns.  

US Mesh Inches Microns
3.5 .222 5600
4 .187 4760
5 .157 4000
6 .132 3360
8 .0937 2380
10 .0787 2000
12 .0661 1680
14 .0555 1410
16 .0469 1190
18 .0394 1000
20 .0331 841
25 .0280 710
30 .0232 600
35 .0197 500
40 .0165 420
45 .0138 354
50 .0117 297
60 .0098 250
70 .0083 210
80 .0070 178
100 .0059 150
120 .0049 125
140 .0041 104
170 .0035 89
200 .0029 74
230 .0024 63
270 .0021 53
325 .0017 45
400 .0015 38

Melt Index

Melt index refers to a specific test that measures the rate of extrusion of a thermoplastic, in grams per 10 minutes, through a specified orifice under specific temperature and pressure conditions.  In layman's terms, the test measures the viscosity of the melted plastic, which is directly related to its molecular weight.  The test is performed on a specialized apparatus called an extrusion plastometer or less formally, a melt indexer.  The test is described in ASTM Standard D-1238.  


Density, as the term is used with polyethylene (PE), actually refers to specific gravity.  Specific gravity is the ratio of the weight of a given volume of material to the weight of an equal volume of water.  Thus, PE with a density of 0.935 weighs 93.5% as much as an equal volume of water; i.e. it floats.  Density in PE tells a great deal about the physical properties that the material exhibits.  A fairly accurate measure is easy to perform using alcohol water solutions (for PE and PP) and a hydrometer.  More accurate testing is more involved.  There are several methods described in ASTM Standards D-2839, D-792, and D-1505.

Bulk Density

Bulk density is the weight of a given volume of material, including the void spaces inherent in it.  Scientists call this "apparent density."  This is an important quality in bulk material handling applications, such as pneumatic conveying.  It is also very important in rotational molding powders, since bulk density directly indicates the fluffiness of a powder.  When pulverized, the particles of PE tend to form hairs.  Under a microscope, they can look more like dust-bunnies than like grains of sand.  This will effect how they mold in rotational molding, since rotational molding is (normally) done at ambient pressure, and the flow of the particles inside the mold is critical.  ASTM Standard D-1895 describes the test method.  


Pourability describes the same fuzziness property of plastic powder that bulk density does, determined by a different method.  It is commonly used with rotational molding powders.  A given quantity is allowed to pour from the bottom of a standard funnel and the result reported in seconds to empty the funnel.  Generally, a faster time means greater bulk density.  

Other Sources of Reference Information

Some other sources of plastics reference information are: