A North American manufacturer was looking to reduce material handling and operator fatigue, while maintaining consistency of product on an existing production line. In the current process, an ingredient was encapsulated into a heated liquid by filling a series of large and heavy square rubber trays containing multiple cavities that formed “pucks” when the product solidified. The trays travelled along a cooling belt, where operators struggled to lift and shake the pucks free. In doing so, there was a high degree of operator strain and some breakage of product due to the handling. Extra time was also spent cleaning the trays and returning them to the start of the line.
To address this, the customer planned to eliminate the trays in favor of a process where a continuous sheet of liquefied product was “slip formed” across the cooling belt. The customer required a way to “flake” the solidified sheet once it reached the end of the belt. To ensure product consistency, the flaked material would be screened into three sizes, where the oversized and undersized (fine) fractions would be reworked. Flake size requirements were specific to achieving an optimal bulk density and high yield for packaging.
Prater made an on-site visit to observe the customer’s existing process, after which a set of objectives for the size reduction equipment was identified: 1) produce a high yield of material in the target particle size range; 2) minimize the reduction in material thickness; 3) select a size reduction machine that was gentle enough not to melt the product. Based on these goals, Prater recommended a lump breaker for the application.
Principal of Impact
Lump breakers are designed to work on the principal of impact. Typically used at the end of a belt conveyor or at the discharge of bulk storage devices (i.e. silo, FIBC station, or bag dump hopper), they are capable of crushing and breaking up a wide variety of brittle materials. Product entering the lump breaker is impacted along the edge of rotating blades, with size reduction enhanced by imparting bending stress and shear of partially fractured material that is accelerated across the face of stationary breaker bars (Figure 1).
Many lump breakers also include provisions for utilizing a sizing screen at their discharge. Screens come in a variety of apertures and can be interchanged to tailor the particle size output. In comparison to hammer mills and micronizing mills, lump breakers use a lower impact force and larger operating clearances between rotor blades and the breaker bars/screen. This makes them an ideal choice in many applications where a gentle granulation, coarse size reduction, or conditioning of bulk solids is required.
In follow-up to the on-site visit, a detailed plan was developed with the customer to test two thicknesses of material. The variation would help identify potential differences in machine performance and particle size under a given set of operating parameters, while providing the customer more options to configure the speed of their cooling belt. Slabs of material were hand formed to match the 12x12 in. opening of Prater’s test machine, then sent to Prater. These slabs were pre-broken into several large pieces, replicating how material would snap when projected off the end of the belt, then fed to the lump breaker at a defined volumetric flow rate that matched the planned production line speed.
With the customer in witness, a series of test runs were completed, where machine speed was varied to alter the amount of impact force on the product. As expected, a slower tip speed produced a coarser, more desirable flake with less fines. With the ideal speed identified, both product thicknesses were crushed across two different styles of blades: blunt edge (squared face) and knife point. Blunt edge blades are the standard style, favored for their wide contact area across the striking surface. On the other hand, knife edge are traditionally used to provide a cutting action rather than blunt impact. It was believed that the knife edge would make a cleaner cut with less fines, but this proved true only with the thinner material. Due to the coarser flake requested, sizing screens were omitted from all test runs.
Based on results of the trial, the customer purchased a machine that was scaled up to a custom 12 in. deep x 36 in. wide machine designed to fit the width of the cooling belt (Figure 2). The wider design required several modifications to withstand shock loading from impacting the product. First, a large diameter hex shaft machined from high grade steel was utilized for constructing the rotor. The six sides of the hex provided ample bearing surface to drive the rotating blades, while the larger diameter shaft stock reduced deflections across the wider bearing centers. Next, rotor blades and blade spacers were milled on opposing faces to provide exact spacing between the rotor blades and breaker bars over the entire length of the machine (Figure 3). Finally, the bearings were a specialized pillow block designed for a greater degree of angular runout and included grease fittings for re-lubrication. The lump breaker body was reinforced by using thick plate steel precision cut using a CNC water jet process. Side and end pieces utilized “mortise and tenon style” joinery, making them self-fixturing for exact alignment during the welding process. This alignment was critical for locating the bearings to achieve consistent blade tip to body clearances. Thicker top and bottom flanges ensured a flat surface for mount-up in the customer’s process while reducing torsional stress when fastening.
In anticipation of potential changes to how the product would be packaged, the customer requested additional versatility to tailor the lump breaker’s operating parameters. Therefore, it was equipped with an inverter-duty motor capable of speed regulation using a VFD. An additional set of breaker bars with wider spacing was also supplied to provide less shear for manufacture of coarser product.
Once installed, the lump breaker will provide a consistent flake size delivered on a continuous basis to the customer’s screening and packaging system, eliminating extra handling and operator strain while improving product quality.
For more information on Prater Industries, Bolingbrook, IL, call 630-759-9595 or visit www.praterindustries.com.