Dental laboratory needs are a specialty of Diversified Chemical Products, Inc. (DCP), Wilmington, Del. Its business includes products for fabricating dental prostheses, cleaning dental instruments, and dissolving dental cement, in addition to other custom powders for dental laboratories, jewelry cleaning, and industrial applications. Since 1974 the company has worked closely with customers, developing products that meet a particular need, manufacturing those products on a private-label or contract basis, and packaging them in the most convenient form for the end user.
For one of its clients, DCP’s development team helped enhance the properties of an enzymatic cleaner for dental instruments. The client wanted a cleaner capable of dissolving dental cement in addition to biological waste such as blood and mucous. “It was a unique mixture that fit a nice niche for one of our customers, and it grew into a rather large product for us,” says Jim Longo, Jr., president of DCP. The innovation was so successful that orders quickly exceeded the capacity of the plant’s two 35-gal (133-liter) planetary mixers. An expansion of plant capacity was desperately needed to avoid losing business.
Complete system solution
The solution is an integrated system to load, mix, and convey the enzymatic cleaner. A Munson ribbon blender incorporates a bag dump station, a receiving hopper, and a pneumatic conveying system (Figure 1). The system was installed in July 2002. All internal finishes meet a 2B FDA requirement; welds were polished with a 120 grit wheel. The construction material is Type 316 stainless steel. Figure 2 shows the internals of the ribbon mixer. A K-tron vacuum pneumatic conveying system transfers the materials from a receiving hopper to a surge hopper feeding the existing filling and packaging line. Munson engineers tested the powders and worked in tandem with the other vendors to assure satisfying all levels of requirements. DCP worked with Jerry Spross of EPI Technical Sales, who put the package together.
Figure 2. Blades of mixer move in opposite directions, promoting solid blending.
Since DCP often deals with powders in the form of low-micron dust particles, dust containment during the manual loading process was essential. The integral bag dump station is mounted atop the blender and includes a dust hood with two PTFE one-micron pleated cartridge filters for dust collection (Figure 3). The bags are laid on top of a grate and slit open, discharging the contents directly into the blender. A hinged, gasketed cover on one side opens to add minor ingredients, which are pre-weighed, or sometimes pre-mixed with small mixers, and added with buckets. Additional improvements were considered but limited by budgetary constraints.
Figure 3. Solids are fed to the mixer with hood cover and exhaust fan for containment.
Blended powder discharges into a transfer hopper that feeds a pneumatic conveying system taking the product to a surge hopper and on to a semi-automatic auger filler. Less free-flowing material is loaded into drums and transferred via handheld pneumatic wand to a fully automatic vertical form-fill-and-seal bagging machine.
Why ribbon blending?
After testing, Munson custom-designed the 52-ft3 (1.5-m3) ribbon blender for process requirements. The ribbon was the most economical mixer able to meet the DCP’s dual objectives of efficiently blending a variety of ingredients with dissimilar properties, including both low-micron-size ingredients and minor ingredients in small amounts, and also handling a range of batch sizes. Ingredients include free-flowing granules, and powders that compact or cake. The ribbon blender provides sufficient shear for non-free flowing materials as well as moderately lumpy materials and can handle friable, shear-sensitive materials.
Ribbon blenders can efficiently mix components in batches smaller than rated capacity, although blending time increases as batch size decreases. For example, at 40% batch capacity, a typical blend can require as much as twice the blend time, compared to full capacity, to effectively disperse minor ingredients such as dyes and enzymes. The mixer was designed to fit DCP’s typical batch size of 2,000 lbs. (907 kg); through experimentation, Longo has evaluated the turndown of the mixing system. DCP has been successful mixing batches from 800 lbs. to 2,200 lbs. (360 kg to1,000 kg).
The counter-flow agitation in the blender mixes materials rapidly and completely, and is particularly efficient at blending ingredients of widely varying quantities, densities, and particle sizes, a good fit for DCP’s requirements. Most products require colors, dyes, or fragrances in very small amounts, as small as 1 to 2 lb. (0.45-0.9 kg) in a 2,000-lb. (900-kg) batch. The ribbon blender efficiently disperses these minor ingredients, according Longo, producing a homogeneous product in five to 15 minutes, depending on batch size. A five-to-10 minute batch would be a free-flowing material batch, ranging from 75% to 100% of the batch capacity with little or no liquid addition. A 15-minute mix time would be for a batch at less than 50% capacity for a non-free flowing material, with, or without, liquid addition.
This is a substantial improvement compared to the old 35-gallon planetary mixers. A typical batch size with the old mixer was 150 to 200 lb., depending on product density.
Uniform blending is a consequence of the mixer’s design and fabrication. A pilot study was completed at Munson’s laboratory to validate the new ribbon blender design. The U-shaped vessel is constructed with large-radius fillet welds, ground smooth inside the body to eliminate rough spots that might trap material and prevent complete blending. An exceptionally small clearance, 1/8-in, or 3 mm, between blade and shell, further enhances complete mixing, with no dead spots, pockets, or corners.