Q: I see dry blend color has an advantage of quick color change without waste vs. compounding. Is this significant in your view? Can dry blend be “good enough” for roto?
Dr Nick: Dry blend is still common in North America, mainly because of cost and convenience. It can be “good enough” for some non-critical applications.
The main disadvantage is a significant loss of impact properties. The effects are highly pigment-specific. It appears that pigment coats the outside of the powder particles and interferes with the bond between sintered adjacent polymer particles. As well as loss of impact strength, failure mode is invariably brittle.
There was somewhat of an “urban myth” prevailing in our industry, for many years, concerning the best mixing method for dry blending. It was claimed that superior impact properties could be achieved by using a high-intensity mixer rather than a slow-speed ribbon blender. Several studies have been unable to demonstrate any reliable correlation; once again the effects tend to be pigment-specific.
The maximum pigment addition rate possible with dry blending is much lower than with compounding (dry blending is probably less than 20% of compounding levels). Consequently, thin-walled dry blended parts can be relatively translucent.
The other main disadvantage of dry blending can be wipe-off of pigment, especially from the inside surface of the part.
I am happy to welcome input from others in the comments.
Dr. Nick Henwood serves as the Technical Director for the Association of Rotational Molders. He has 30 years-plus experience in rotomolding, specializing in the fields of materials development and process control. He operates as a consultant, researcher and educator through his own company, Rotomotive Limited, based in UK.