Belt loading can occur when sanding virtually any wood species, especially if excessive material removal is being attempted, but is especially encountered when sanding soft resinous woods. To minimize belt loading, do not force the cut, keep material removal rates within the recommended maximum parameters for each species being sanded and within the feed speed parameters.
The true end point of any abrasive belt should be when the sharpness of the cutting mineral is worn so that the belts no longer cut, not when they no longer cut because of sanding residue building up in the voids between the mineral particles and masking the abrasive minerals.
Cleaning of abrasive belts can significantly increase belt life and lower sanding costs if belts are currently being discarded because they are loaded and there is still mineral life left. Through the years, I have encountered and examined various types of abrasive belt cleaning.
Air nozzle blow off: Nondestructive to belts, even paper-backe d belts, and will slightly increase belt life. However, a large volume of clean, compressed air is required. It does delay loading and streaking and reduces belt changes. Because of marginal improvements, most systems are not in use on older machines.
Eraser-type load removal: Extends belt life marginally and can be used on paper belts. Belt width is limited by manual reach. There is no cost for equipment. Erasers are consumed. Incomplete load removal reduces sanding quality.
High-pressure water/steam blasting: Extends belt life, but can’t be used on paper belts. Low cost for equipment. Water saturation/shrinkage affects belt backings. Belt removal for cleaning reduces sander production time. It requires a significant amount of labor for belt removal, transport, cleaning, drying, return and reinstallation of belt. Additional floor space for cleaning and drying racks is needed.
Caustic soaking with removal brushes: Extends belt life and can’t be used on paper belts. Low cost for equipment. Water saturation/shrinkage affects belt backing and sanding quality. Belt removal for cleaning reduces sander production time. It also requires significant amount of labor for belt removal, transport, cleaning, drying, return and reinstallation of belt. Additional floor space for cleaning and drying racks is needed.
Wire brushing: Some extension of belt life due to eliminating streaking. Incomplete or excessive brushing reduces sanding quality. Belt cleaned on sander can slightly improve sander production time. However, direct interaction with a running belt is not entirely safe. No additional floor space for cleaning is needed.
Glass bead blasting: Limited extension of belt life since the process is destructive to belts, affecting the belt and sanding quality. There’s a cost for equipment and only fair return on the investment. Belt removal for cleaning does decrease sander production time. Additional floor space required for cleaning equipment.
Dry ice blasting: Removes all loading. Not destructive to belts, even paper-backed belts. Maximizes belt life and reduces quantity of belt changes. Minimizes streaking and burning. It can be used for other cleaning requirements throughout a plant. High initial cost. Small amount of additional space required.
Belt cleaning is not for everyone, but in certain cases, it is a cost-saving and quality improvement tool that should not be ignored.
Multiheaded machines in production lines offer the greatest savings potential. Also, anyone sanding resinous materials such as Ponderosa or Loblolly pine should investigate its use. Applications that have a low tolerance for longitudinal streaks are also prime candidates for this concept.
Small shops are not normal candidates for abrasive belt cleaning except by the eraser concept. Depending on the application, mid-size shops using two or more multiheaded sanders might consider an offline high-pressure water blasting system or a machine-mounted dry-ice-blast system. Depending on the application, large shops using multiheaded sanders in a production line should consider a machine-mounted dry-ice-blast system.
Sanding fuzzy wood
There are many contributing factors to fuzzy or blotchy finishes. Simply stated, fuzzy surfaces are the result of wood fibers being bent instead of cut cleanly. These fibers then stand up after staining. Some species of woods, such as cotton wood, red oak and soft maple, are more prone to having the characteristics that contribute to this end result, but a fuzzy surface can occur in all wood species.
One major contributor to the problem is the moisture content of the wood at the time of sanding. Wood should be dried to the minimum acceptable level of moisture content.
The type of abrasive mineral is another major contributor. There are three basic types of minerals used in abrasive belts: silicon carbide, aluminum oxide and ceramic. Silicon carbide is the sharpest, followed by aluminum oxide and then ceramic. Use the sharpest mineral (silicon carbide) when trying to eliminate fuzz. High abrasive belt speeds (5,000 to 7,000 sfpm) are desired in conjunction with slow through-feed speeds.
Generally speaking, using finer grit minerals does not eliminate fuzzing. You just end up with finer fuzz. Care should be taken not to exceed the cutting capabilities of any given grit belt at the feed speed being run on a given species of wood. In other words, do not force the cut.
If possible, run the finishing sanding heads in opposite directions. This will tend to cut fibers that have been bent or laid down in one direction by a sanding head. Harder durometer contact drums might help, but care must be exercised not to go too hard, which could result in chatter or belt splice marks. Attempting to sand some types of products hot — such as plywood or particleboard — can also contribute to fuzzy sanding. If possible, use a size coat before sanding.
For help with sanding problems, contact: Howard Grivna, Sanding Systems Consulting Inc. Tel: 218-678-2929. www.sandingsystemsinc.com
This article originally appeared in the June 2013 issue.