The duroplastic polyurethane sheets or, alternatively, rigid foams are easy to work with and have very favourable properties for use in architectural and designer model making. Polyurethane with unvarnished surfaces should not be exposed to intense sunlight for a long period of time because the UV radiation will affect the colouration and the material will tend to corrode. Even though the SikaBlock sheets are distinguished by their low expansion in heated conditions, extreme temperature swings should be avoided during the transport and storage of completed models in order to avoid cracking in the material.
Treatment: Before doing any work, this material should be at room temperature. PUR panels and rigid foam can be machine worked with the same tools as those used for wood and metal (saws, drills, lathes, milling cutters). They can, however, also be worked on by hand – this is especially true for lighter weight panels with a density of up to 200 kg/m³ whereby you will be pleasantly surprised how easy the work is. Thermal (hot wire) cutting is not possible because polyurethane is a duroplastic material.
Sawing work should be done with carbon-tipped blades with a blade speed of between 3000 and 6000 rpms. It should be noted that dull saw blades will produce more heating of the material and cause extreme distortion.
Milling work should be done with HSS or, alternatively, carbide mill bits at 2000 to 6000 rpm and a feeder rate from 3 to 5 metres per minute. When doing machine work on this material, be sure to include a good system for dust evacuation. A faster feed rate when sawing, drilling and milling will produce thicker shavings and scrapings and therefore less dust. For more specific information about milling work, please take note of the directions supplied by the tool manufacturer.
The surfaces of all the PUR rigid foam and modelling panels can be smoothed and polished using any customary polishing agents – in the case of the less dense versions, there will of course be a greater removal of material than there is with the heavier ones. These lighter types have courser pores and so must be subsequently filled and sanded a number of times in order to achieve a surface ready for paint. In this regard, the SikaBlock panels are particularly good because filler materials adhere to them so well.
For this work we recommend two component POLYESTER FINISHING FILLER for any unevenness that is particularly course and polyester spray filler for everything else. The full strength spray filler should first be applied twice by brush before any spraying is done (each coat must be sanded before the next is applied). The in-between sanding work is best done with 100 to 240 grit paper while the final sanding should be done with 400 grit paper (600 grit for dark, high gloss colours). Very high quality models will always have a primer filler coat applied prior to the final painting.
Gluing work on clean, dry, dust-free and grease-free surfaces should be done with epoxy or polyurethane based TWO COMPONENT ADHESIVES. Because these glues are harder than the material themselves when fully cured, two blocks should be glued to one another in such a way that the glue will not end up on any of the exposed surfaces as this will make the job of sanding much easier.
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