Blow molding is one of the most widely used plastic forming processes, and is used in the production of bottles, large drums, tanks, toys, and countless automotive applications. But, even in light of the advances made in the industry, many molders are still using virgin resin or a homemade concoction such as resin mixed with laundry detergent to purge screws and barrels during color or resin changes. These purging methods may help you eventually get from point A to point B, but after how many pounds of scrap are dumped on the floor? It is not uncommon for a color change, from dark to light, to take 4 or more hours in an extrusion blow molding process. However, mechanical purges with good material affinity and good detergent power can offer drastic reductions in changeover times by scrubbing the first material/color off screw flights and head tooling. Chemical purges can be used to reach hard-to-clean areas in the accumulator or flow head, especially if they are not optimally designed. Since mechanical and chemical purges clean in different manners, it makes sense to look at them separately where applicable. Commercial purging compounds (CPCs) are making inroads in the blow molding community for a variety of reasons.
Since blow molding is a term applied to a number of unique molding processes, it makes sense to look at each of the major blow molding processes, and disucss tips and best practices, by category.
The major subsets of blow molding are: