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Technological secrets on processing of organic glass
Glossary of terms
Cutting plexiglas can be as if the simplest tool (hand saw for metal), as well as more complex (laser). In most cases, a circular saw is used for straight cuts, also a band saw or a cutter for cutting curvilinear shapes. A band saw is more suitable for cutting preforms before forming, as if there is no need to obtain a clear cut in such cutting (a band saw with a speed of rotation of 700-800 m / min is used for cutting casting acrylic). To obtain a clear and smooth surface when cutting out complex shapes, it is recommended to use a milling cutter. Disk saws allow you to produce straight, accurate cuts with a clear cut (when cutting casting acrylic, use a disk saw with carbide-reinforced teeth with a rotation speed of 800-1200 m / min). When carrying out small tasks, saws with a blade for light metals can be used.
It is preferable to use high-speed equipment for more efficient cutting. With a high cutting speed and a minimum feed rate, it is possible to obtain a high-quality edge (when cutting extrusion plexiglass with high-speed circular saws, the recommended speed of rotation of the disc is 4000 rpm. (For finishing it is necessary to use 250 mm diameter discs made of high-speed steel or reinforced with hard To avoid deformation, the saw teeth must not be incised).
Laser cutting has a number of advantages: exceptional cutting precision, low waste quantity, cuts of high quality, requiring minor grinding. Nevertheless, this means contributes to the appearance of a high internal stress of the material, and further gluing of such surfaces is not recommended.
Casting plexiglass can be easily machined, but due to low thermal conductivity it quickly heats up in the cutting zone. Therefore, the treatment should be carried out with the smallest depth of the cut layer, but with impressive cutting speeds.
Attention! To avoid scratching during storage or mechanical finish, do not remove the protective film or paper cover.
Drilling of extrusion plexiglass is performed using a stationary or mobile drilling machine with the use of special drills for light metals from high-speed high-speed steel, specially sharpened for drilling extrusion plexiglas. It is necessary to monitor the smoothness of the edges of the drilled hole in order to avoid the formation of cracks. In the case of bottomless drilling, it is often advisable to lift the bit to remove chips and also to limit the heating of the material.
When drilling casting plexiglas, any metal screw drills are used. The coefficient of correction of the drill diameter is +0.05 mm relative to the diameter of the hole. To avoid internal stresses during drilling, it is necessary to cool the cutting tool and also the material with cooling lubricating fluids or compressed air.
In most cases, it is produced with the help of rippers mounted on pantographs also equipped with small diameter cutters (2-6 mm) of various profiles. Laser cutting machines can also be used, which allow to engrave the route of limiting the bottomless action of the laser beam.
Coarse surfaces or surfaces with defects, such as scratches, are ground with a wet sandpaper of fine corundum. After rough polishing the surface should be treated with special means. (polishing pastes of type VIAM, trepel, crocus, dispersed in wax). All operations are carried out either manually or on a polishing machine.
Cut edges also matte surface can be polished by mechanical means with the help of diamond tools. However, if this is not possible, (for example, a curvilinear cut), then a flame polishing is recommended. This tool consists in finishing the products with a clean "hard" flame of high temperature, but some precautions should be followed. So, for example, the polished surface must be completely clean, and besides this method promotes the appearance of significant stresses, which must be removed by annealing before gluing or painting.
Molding in hot condition
This kind of finish allows you to give the material a variety of shapes. It consists of three stages:
Heating: If the resulting part does not correspond to the desired shape, it can be reheated and then corrected. Cast acrylic has a memory of the shape and is also able to easily return to its original location when it is reheated. With extruded, this is possible only if the material has not been stretched in any way. The protective film must be removed from the cast plastic before heating and can also be left on an extruded sheet.
Before molding, it is necessary to pre-heat the extruded plastic to avoid bubbles in the finished product at the folding sites. The need for drying arises due to the fact that during storage or transport, a small amount of moisture can penetrate the material. Drying is carried out either in a convection oven with hot air circulation (at a temperature of 75-80 ° C for 1-2 hours for each mm of thickness - as if the position, with a high moisture content, enough 24 hours of drying), or by infrared radiation at a temperature of from 75 up to 80 ° C for 1-2 hours for each mm of thickness.
To heat the injection Plexiglas before molding, use equipment that creates a uniform temperature field with a difference in the plane and also the thickness of the sheet does not exceed + 3C. The molded glass is heated 3-4 minutes / mm thick up to a temperature of 165-190 ° C. Block plexiglass is heated softly, but its strength is reduced insignificantly compared with extrusion. The cast also extruded acrylic plastics react differently to heating. A cast can have a uniform shrinkage of at most 2%, the same in all directions (the thickness of the sheet increases accordingly). Extruded - from 3 to 6% in the extrusion direction also 1-2% in the transverse course (for the sheet 2m x 3m, the extrusion course is a side of 3 m). Cast acrylic easily tolerates the heating non-uniformity at 10-15 ° C (superheated cast-glass Plexiglass, unlike extrusion, does not crack at all and does not break at deformation), in the case of extruded, the difference in the heating temperature can lead to considerable stresses in the material.
Forming: The behavior of these materials during hot forming is also different. To deform the cast acrylic, you need to apply considerable pressure, the extruded mold is easily formed also without much effort (the recommended temperature range is 160 - 170 C, the heating time is 3 minutes per 1 mm of material thickness).
Forms (matrices also punches) are made of various materials: wood, gypsum, aluminum, steel. Molding in the hot state can be carried out in simple ways, for example by placing the plastic after heating to a convex or concave shape, where it takes its shape under its weight. Molding can be carried out similarly by free retraction in a vacuum-tight chamber and also by free blowing with a compressed atmosphere, both with the help of molds and also without them.
If it is necessary to bend a flat sheet in a straight line, it is sufficient to conduct the local heating of the plastic along the axis of the bend with one or more rectilinear heating elements, for example a nichrome string.
Attention! During hot processing and / or molding, the protective coating must be removed and thoroughly cleaned, as if the remnants of the adhesive or protective film, when the sheet was softened, could damage its surface.
Cooling: In order to avoid the occurrence of internal stress, the cooling should be as if it were as long as possible also homogeneous. For best preservation of the printed form, also to avoid deformation, the product should be left on the matrix until it is cooled down to a temperature of 60 to 70 ° C. The molded articles must undergo thermal conditioning before their interaction with solvents, paint or self-adhesive film to reduce stresses.
Plexiglas, both cast and extruded, can easily be bent cold, but the minimum bending radius should be observed in order not to subject the cooker to excessive stress. The minimum radius of flexion should be 230 times the thickness of the plastic.
Welding and bonding
In the case of injection plexiglas it is recommended to use (as well as for extrusion plexiglass) "related" adhesives, which makes it possible to achieve the most durable connection also of its weather resistance. To obtain transparent colorless joints when gluing together or overlap with a one-sided overlay, it is preferable to use a PMM based on polymethylmethacrylate adhesive.
Before gluing a part from injection Plexiglas it is necessary to anneal to relieve internal stresses. The recommended annealing temperature is 65-80C, depending on the glass softening temperature (90-120C), after which natural cooling is preferred.
Welding is also used to produce blanks for the shaping of non-standard products, for fixing the lining also for nurturing the thickenings. Welding of injection plexiglas is carried out at higher temperatures (up to 300С). The finished part is subjected to a thermal finish after the mechanical finish of the joint to relieve internal stresses.
Sheets of extrusion Plexiglas can be glued together with themselves also with other materials, while obtaining a high-strength also durable suture. Before gluing it is desirable to conduct heat treatment.
Materials for bonding extrusion plexiglas
Film type of glue: solution of polymethylmethacrylate in a volatile solvent.
Polymerizable type of glue: PMMA syrup with dimethylaniline (CMM), at which moment the polymerization initiator (benzene peroxide) is added at the moment of gluing.
Other types of glue: epoxy, polyester, polyurethane also of other brands AK-90, MP-88 (81), BF-2 (4.6).
Solvents: dichloroethane, methylene chloride.
After drying the glue hardening at room temperature, it is recommended to perform annealing for 2-5 hours at a temperature of 60 ° C.