Sanding profiled edges
MDF is often used for applications where the panel edges are profiled and then finished with stain and clear lacquer or paint. The quality of the finished edges depends largely upon the attention given to the preparation of the profiled edges before finishing.
Companies with only basic machining facilities can achieve good finishes on MDF edges provided sufficient sanding is carried out to remove any machining marks.
The high cutting speeds and controlled feed rates provided by CNC routers can however be used to good advantage on MDF, eliminating cutter marks and burnishing defects which often appear when using hand feed machines.
Subject to good edge machining, sanding materials and processes can be selected solely on the basis of their efficiency in smoothing the fibrous edges of MDF without loss of detail from the profile. The choice between the different methods, ranging from hand sanding using flexible blocks to in line multiple head machines, is largely dictated by the complexity of the mouldings, the number of components to be sanded, and financial constraints.
As a general rule, silicon carbide abrasives are preferred for use on MDF as aluminium oxide abrasives have a tendency to dull with a consequent loss of cutting efficiency.
Hand sanding
Flexible sanding blocks preformed to the shape of the profile can be used as an alternative to the wasteful use of abrasive sheets which have a tendency to square off curved profiles and round off sharp edges. Hard foam blocks are best on flat profiles but a softer grade will conform better to the shape of a curved profile. Blocks surfaced with grit in the range 150 to 240 are recommended for sanding MDF edges prior to finishing. Material costs can be reduced by using foam blocks with "Velcro" surfaces and replaceable brushed nylon backed abrasive strips.
Sanding stars
A sanding star consists of two layers of slashed abrasive cloth pieces, laid spirally in a circle back to back and fixed in position with an open centre bush. The stars can be used singly on narrow profiles or in multiples clamped on a single spindle for wide profiles. The spindle can be fitted to a hand held, air operated grinder with variable speed up to 3000 rpm. Alternatively, sanding stars can be fitted to electric motors in single or multihead, through feed machines for processing large numbers of similar components.
Sanding stars of 100 mm or 150 mm diameter are recommended for use on heads operating in the range 1000 to 3000 rpm. Larger diameter stars giving a longer useable life should be used at slower head speeds. A satisfactory finish on MDF edges can be achieved with a single head machine operating with feed speeds up to 5 m/min. The best results however, are achieved using a two head machine with the first head running against the feed and the second head running with the feed. Higher feed speeds are possible when using multihead machines.
Grit impregnated wheels
Grit impregnated woodworking wheels are widely used for finish sanding MDF edges. They are particularly successful on the more complicated profiles as the profile detail is maintained throughout the life of the wheel, subject only to periodic machine adjustment to take up any wear.
Grit impregnated wheels are manufactured by mixing grit in a polyurethane (PU) or polyvinyl alcohol (PVA) matrix. Wheels up to 200 mm diameter are available with grit particles in the range 40 to 320 but for MDF, the best results are obtained with 60 to 100 grit.
The sanding wheels can be shaped to the required profile by running them against a high density hardwood strip cut to the desired profile with saw cuts spaced at approximately 20 mm intervals along the length. The prepared moulding is pressed forcefully against the rotating wheel, across successive saw cuts, until the negative profile has been completely ground into the edge of the wheel. Sample hardwood profiled mouldings should be retained from each production batch for shaping wheels required for future production.
Although abrasive wheels can be fitted to air operated machines and used by hand, the best results are obtained on through feed machines fitted with a swivel head with both vertical and horizontal adjustment to take up the wear on the wheel. Head speeds up to 1500 rpm are acceptable for wheel diameters up to 200 mm when using power feed machines.
With hand feed, the wheel diameter should not exceed 150 mm.
Small diameter abrasive wheels or points are available for sanding internal mouldings, the face grooves on MDF doors for instance. These wheels can be mounted on a CNC machine or on a custom made internal sander running at speeds up to 3000 rpm. They effectively sand the sides of the profile leaving the base to be finished by hand sanding.
Abrasive belts
Profiled edges in MDF can be sanded using a variation of the flat edge belt sander fitted with a pad made from hard felt or steel, premachined with the inverse of the required profile. A coated abrasive normally with a J weight, flexible cloth backing should be used to ensure that the belt conforms to the pad profile. Frictional effects when using steel pads are minimised by drilling small holes in the pad and maintaining a compressed air cushion between pad and belt.
With relatively simple profiles, one sanding head will be sufficient but an improved finish can be achieved using a two stage machine fitted with an 80 grit belt running against the feed, followed by a 120 grit belt running with the feed to knock down any projecting fibres. These machines operate at feed speeds in the range 5 to 25 m/min depending on the number of heads.
More complicated profiles can be sanded in line using a multistage machine fitted with several heads each operating on a different part of the profile.
The use of coated abrasives in belt form reduces the working surface temperature and, as a consequence, reduces the loading of the abrasive, resulting in a long life between belt replacements. The main limitations of abrasive belts are rounding of sharp edges and difficulties in sanding internal corners.
Profiled sanding heads
Profiled MDF edges can he sanded using rotating sanding heads premachined to the inverse profile and wrapped with coated abrasive. The MDF pieces can he run against the sanding head by hand or through an in line power feed machine. Bullnose and simple curved profiles can be sanded using polyurethane wheel blanks wrapped with a stretch cloth backed abrasive held in place by Velcro.
A commercial profiled head sanding system makes use of customer manufactured, preformed abrasive discs which fit over and are clamped to standard profiled rings. With simple profiles, one ring may be sufficient but a multistage machine would be required for more complicated profiles with each head operating on a different part of the profile.
With other systems, six, eight or ten sprung rubber blocks are fixed round the perimeter of a head attached to the drive shaft of a sanding machine. The rubber blocks are shaped to the required profile by bonding abrasive paper onto a moulding with the profile to be sanded and then pressing onto the rotating head. Abrasive papers are then bonded to the profiled surfaces of each block and these act as sanding shoes for the moulded edges of the MDF components.
Process selection
1. External profiles - simple.
- Flexible sanding blocks used by hand
- Grit impregnated wheels to ensure consistent profiles
- Abrasive belts
- Profiled sanding heads.
2. External profiles – more complicated.
- Flexible sanding blocks used by hand
- Sanding stars
- Grit impregnated wheels - to ensure consistent profiles
- Multistage abrasive belts
- Multistage profiled sanding heads.
3. Internal profiles.
- Flexible sanding blocks used by hand
- Sanding stars using a hand grinder
- Grit impregnated wheels or points mounted on a CNC router or internal sander
As a general rule, edges on short run components can be hand sanded relatively easily but the resulting quality varies. Output can be increased significantly using single head sanding stars or grit impregnated wheels with typical feed speeds of 5 m/min. Proportionally higher speeds are possible when using multiple head machines. Feed speeds of 12-15 m/min are typical for abrasive belt machines fitted with power feed.
