Information on the Feedstock

The performance of an extruder is determined as much by the characteristics of the feedstock as it is by the machine. Feedstock properties that affect the extrusion process inciude buik properties, meit flow properties, and thermal properties. Important buik flow properties are the buik density, compressibility, particle size, particle shape, external and internal coefficient of friction, and agglomeration tendency. Important melt flow properties are the shear and eiongational viscosity as a function of strain rate and temperature. The commonly used melt indexer provides only limited information on the meit viscosity. Important thermal properties include the specific heat, the glass transition temperature, the crystalline melting point, the latent heat of fusion, the thermal conductivity, the density, the degradation temperature, and the induction time as a function of temperature. 

A change in the material can cause a problem in extrusion when it affects one or more polymer properties that determine the extrusion behavior of the material. If a material problem is suspected, one should first examine the quality control (QC) records on incoming material to see if a change in feedstock properties was determined. Unfortunately, often the only QC test on incoming material is a melt index (MI) test. This test is only able to detect a very limited number of material-related extrusion problems. Thus, in many cases, material testing may have to be more extensive than the regular QC testing. 

A practical test for a material-related extrusion problem is to extrude some material from an old batch to see if the problem will disappear. If this is indeed the case, then this provides a very strong indication that the problem is material related. For this reason, it is helpful to retain some material of older batches, which will also provide a reference for more detailed measurements. 

If the problem is material related, there are two possible solutions. The easiest solution from an extrusion point of view is to change the material back to the way it was before the problem developed. However, this may not always be possible for other reasons. Thus, if the change in the material is permanent, then the extrusion process will have to be adjusted to accommodate the material change. At this point, the nature of the problem may change from an upset to a development problem. The chance of solving the problem will depend on the nature and the magnitude of the change in the material. 

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Understanding the process, condition of the equipment, collection of historical data, and information on the feedstock... 

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