The Snowball Principle3 Feb, 2015 By: Art Post
Batch-wise blending can be accomplished in a twin-shell, ribbon blender, tumbling mixer, double cone blender, or Henschel mixer. Problems arise when there are significant differences between the ingredients in particle shape, size or density. These differences cause one powder to sift through another rather than to intimately mix with it. It is not unusual in such cases to actually cause a solid/solid mixture to separate rather than to blend with prolonged mixing!
To correct this problem for external additives, consideration should be given to the snowball principle. This method was described many years ago in technical literature published by Degussa, describing the proper manner in which their fine particle and fumed silica’s should be blended with other powders.
In this procedure, the full amount of silica is first charged into the batch blender and blended for a short period of time. Next, a fraction of the toner is added and blended with it. The mixer is run for a relatively short interval, stopped, and another increment of toner is added. This step is repeated until the proper ratio of toner to additive has been achieved.
The term snowball is used because each particle of toner is rolled in a “snow bank” of additive powder until it becomes uniformly coated. The reservoir of “snow” is gradually depleted by fresh, uncoated toner particles periodically introduced into the mixing chamber.
If, on the other hand, the mixing procedure were reversed (as some toner manufacturers actually do!), the small amount of additive becomes completely enveloped by toner and forms into agglomerates, or clumps. These clumps provide little or no improvement in toner behavior and can become lodged under the cleaning blade causing specs or streaks in the printed image.
We recommend the following procedure be used to achieve a homogeneous blend of additive and toner, with a minimum of clumping or aggregation:Place the total amount of additive into the Henschel mixer.
- Add other micro fine additive particles & blend for a brief period.
- Add a fraction (e.g., a third, a fourth, or a fifth) of the total amount of toner and blend for several minutes. The submicron particles of flow agent and other additives will form around the much larger particles of toner, leaving an excess of additive in the mixer.
- Add the second fraction of toner. These fresh particles will capture their share of additive from the available excess.
- As the remaining fractions of toner are added—with each increment blended for the appropriate time—each toner particle will ultimately be coated with almost a monolayer of flow agent.
Art Diamond is an experienced veteran in the office supplies industry and Senior Consulting Editor for Recycling Times. Visit www.irecyclingtimes.com for detailed info.