Pellet shape, size impact parts

Add one more item to the hundreds of parameters to consider when molding with powder: variation in feedstock. This subject is currently under investigation by Uwe Haupt, international technical support, and Hartmut Walcher, PIM development department, for Arburg GmbH+Co. in Lossburg, Germany. In a paper the two authored at PM2TEC 2000 in New York, Haupt and Walcher take the first steps to determine which parameter settings help eliminate the influence of pellet size on a molded part's quality. 

Working from the premise that feedstocks are produced on a variety of equipment, the researchers ran tests on a feedstock generated by three different methods. The entire batch, composed of 78 percent powder (­45-µm sieved bone china) and 22 percent binder, was first processed on a shear roller compactor. One batch, receiving no modifications after this initial processing, resulted in homogeneous, cylindrical granulated material with a diameter of 4.5 mm and length of 5 mm. The shear roller compactor milled the second batch into a fine powder consisting of sharp-edged granules with a maximum size of 1 mm. In the third batch, a Sigma Blade Mixer kneaded the material into round feedstock of varying sizes. 

Each feedstock was then molded into an 80-by-20-by-3.5-mm test bar on a 25-metric-ton Arburg. Parameters recorded for each shot include injection time, dosage time, material cushion, material temperature, switchover pressure (hydraulic), peak pressure (hydraulic), and peak cavity pressures 1 and 2. 

The results of the experiment show the influence that pellet shape and size has on some of these recorded parameters. Perhaps most significant are the differences in dosage time for the three batches. In the original mix, this measurement ranged from 13.5 to 8.6 seconds; the authors attribute the variance to the pellet size being larger than the depth of the screw at the feed, requiring more or less time to mill or cut the pellet at the inlet. Dosage time for the milled feedstock was a much lower 5.17 seconds, and the kneaded batch held steady at 13.06 seconds. 

Material cushion also yielded scattered results. That of the original material remained constant at 2.41 cu cm, while the milled feedstock was consistently higher at 2.45 cu cm, and the kneaded batch was unstable, hovering around 2.43 cu cm. Peak cavity pressure showed inconsistency only in the kneaded feedstock, which degraded, gradually dropping pressure below that of the original material. 

Haupt and Walcher say this set of data is the first in a series of tests aimed to optimize the molding process without having to account for pellet size and shape. Their initial findings indicate possible solutions might include modifying backpressure during dosage and adjusting screw rotation speed to provide a more consistent cavity pressure profile. 

For more information, contact Arburg GmbH, Lossburg, Germany; +49 (7446) 33 3432; fax +49 (7446) 33 3852; www.arburg.com.