Binder Dispersion/Mixing


Proper dispersion of the liquid binder components on sand surfaces is necessary to produce high-quality cores and molds. Mixers that were prevalent in the early to mid-1970s often provided relatively poor blending of binders and coating of sand grain surfaces.

Although high-speed, high-efficiency sand mixers and advanced resin-metering systems were developed in the 1990s resulting in dramatically improved mixing, maintaining and cleaning the equipment is as important as it was in the 70s.

Cores were prepared in a high-intensity batch mixer—each component was mixed for 5, 10, 20, 30 and 60 sec (double for actual total mix cycle). All cores were prepared with silica sand at balanced ratios (50:50).

Cores mixed for 10, 20 and 30 sec exhibited pronounced non-uniform binder dispersion and looked "spotty." Mixing times of 80, 120 and 240 sec provided uniform results. Physical properties, such as scratch and tensile strengths of sand mixed for 40 sec and longer were not impaired, even though traces of inadequate mixing were apparent on the 40-sec mix.

Short mix cycles of 10-40 sec promoted surface and subsurface porosity. Trace subsurface microporosity was found in the remaining castings made with cores mixed for 60-80 sec. In castings with pronounced defects, porosity was formed where the solidifying casting was in contact with binder-rich areas and, particularly, those containing excess polyisocyanate. Sound castings were obtained when total mixing times ranged from 2-4 min.

Table of contents
[Paper Header]
[Porosity Study]
[Binder Ratio] [Binder Level] [Casting Temperature] [Section Size]
[Sand Effect] [Iron Oxide Additions] [ Binder Dispersion/Mixing]
[Metal Composition] [Core Age]
[Eliminating Porosity]
[Ti and Zr Additions] [In-the-Mold Additions] [ Core Washes]
[Core Post-Baking]
[Discussion]