The 2.5-Minute Monument

Goldmann has been in the sandblasting business for more than a century and, for the most part, works with customized projects. The technical challenge of this installation was meeting the customer’s very stringent specifications, which were for each workpiece to be sandblasted in three minutes or less.
Since Goldmann had devised a similar solution several years ago for sandblasting lettering onto monuments – albeit with a seven-minute throughput – so the company was able to leverage its experience from that project for the new one. What’s more, the entire design and development process took a mere six weeks, and fabrication took the normal amount of time for a customized machine.

A MATTER OF THREE
The main technical problem in developing the system was not so much that 7mm (.27”) of sandblasting depth had to be achieved in the specified three minutes of throughput time, but rather that the standard stencil covering the non-sandblasted areas (and its elasticity that “nullifies” the sandblasting energy in these areas) is not rated for the high energy of the target process. The customer’s lettering and ornamentation sheeting for the blasting plant was manufactured in the U.S.
 Another key efficiency parameter for the system was compressed air and blasting-medium consumption for various types of lettering. This prompted Goldmann to develop special color-recognition sensors, which enable the machine to operate at peak efficiency by capturing and passing, to the control system, sandblast beginning, end and width.
The service life of the stencil was an acceptable nine minutes; but, in order to meet the customer’s throughput specifications, the Goldmann solution had to be able to sandblast three monuments concurrently. Goldmann solved this problem by having the workpieces pass through the system on conveyor belts, where the sandblasting fields are marked by the operator. 
After being raised pneumatically and turned sideways, the workpieces are lowered onto the feed conveyor belts and/or roller tracks for the three lines and conveyed to their sandblasting stations, where a color reader automatically determines where and how the sandblaster is to be applied. The entire process is realized continuously and automatically.
The system can handle workpieces 1100mm/43.3”) x 340mm/13.8”, with a thickness of approximately 100mm/3.9”. The operation is controlled by a Siemens S7 controller with custom programming by Goldmann, from the three feed-in procedures to individual control of the three lettering and ornamentation nozzle jets.

JET AND FILTER
The Goldmann continuous sandblasting plant uses nozzle jets, which operate on the Venturi principle, for supersonic blast speeds; jets are made of boron carbide (BC4) sintered to a high degree of airtightness. Hence, this is not a centrifugal system used on classic tumble-belt or wire-mesh-belt machines, since centrifugal blasting would have been unsuitable for the target application. 
This method can only be used to modify surfaces; the energy level is too low to perform deep sandblasting. The blast speed of this new nozzle jet machine is 50- to 100-percent higher than that provided by centrifugal blasting.
In addition, the method employs corundum (aluminum oxide), a synthetic mineral abrasive that would be destroyed by the mode of acceleration of conventional centrifugal blasting, and would subject the blast wheel blades to undue wear.
The mixture of stone dut and blasting medium generated by the granite and marble sandblasting process is suctioned out and separated by a high-performance filtering system, with the separation of the blasting medium into a closed system for recovery and reuse. In addition, the machine continuously adds a small amount of abrasive to the blasting medium to ensure that a uniform carving depth is maintained.
The high-performance, optimized continuous-flow sandblasting plant, now in operation, cleanly sandblasts 7mm-deep lettering or ornamentation into three stones in six-and-a-half minutes. It’s also done more than help the monument fabricator work faster – the increased production capacity also helped the company gain new business and market share.