Purging Tip of the Week: #27

Purging compounds are available in several basic classes – chemical, mechanical (sometimes referred to as physical) and hybrid. Chemical purging compounds actually affect the polymer molecules of the production resin, reducing its viscosity so it can be purged out more readily. Mechanical purging compounds rely on differences in “stiffness” to purge out production resin. Hybrid purging compounds blend aspects of these two technologies.

In most cases, mechanical purging compounds are simplest to use. They're appropriate for color and material changes in injection molding systems or small, simple extrusion systems. In contrast, chemical purging compounds are usually a better choice for challenging materials or for large, complex extrusion systems or blow molding systems. Hybrid purging compounds fall between the chemical and mechanical types in both simplicity of use and effectiveness.

Your purging compound supplier can help you to choose the right type of purging product based on the purging task at hand (provided that they offer a full-spectrum product line).

Purging Tip of the Week: #26

On small extruders or molding machines it is usually easiest to load purging material directly into the throat after unbolting and setting aside the hopper. For larger systems it will often be impractical to move the hopper. In these cases access to the throat may be available via a side feeder or a hopper magnet drawer.

Plastic Plantations?

There would seem to be no reason for the polymer industry to discriminate among feedstocks. Hydrocarbons are hydrocarbons, and any viable source of same will serve our purposes. It is as yet "early days", but with increasing frequency we see hints that the day of the dead dinosaur as the source of our polymers is drawing to a close.

The very basic research is underway, and it's interesting! Brookhaven National Laboratory has released information on recent proof-of-principle experiments conducted by biochemist John Shanklin and his team (and funded by the Department of Energy and Dow Chemical). They re-engineered a common laboratory plant (sort of the botanical equivalent of fruit flies and white mice) to produce commercially interesting concentrations of omega-7 fatty acids.

Without going into detail regarding fatty acids it suffices to say that the molecules in question are long chain hydrocarbons, and the path from them to (at least) polyolefins ought to be just chemical engineering.

Of course, myriad open questions remain with respect to reduction to commercial practice and the implications of diverting arable land from food production to plastic plantations. (Of course, it wasn't that long ago that all of our elastomeric materials (i.e., rubber) came from plantations.) But to me, the take-away here is that the march toward an (inevitable?) shift away from fossil hydrocarbon feedstocks toward botanical feedstocks for polymers has begun. And serious commercial enterprises (like Dow) are working the problem.

Purging Tip of the Week: #25

Plunger-type molding machines are old designs and have become quite rare but a few are still in service. Purging procedures for these machines are similar to those for reciprocating screw machines except that the plunger should be operated ¼” to ½” back from its normal set point and at the conclusion of the purge the end of the plunger should be thoroughly cleaned by hand.