What are the best methods and equipment to use when thermoforming ACRYLITE® rods and tubes?
Free bending ACRYLITE® cast (GP) acrylic tubes and ACRYLITE® extruded (FF) acrylic tubes is performed without support, (i.e. when an angle bend is made in a tube, its walls are not supported either internally or externally; and they are not filled or held by bending jigs). For heating in the oven, the fabricator can choose between three possible fixing methods for the tube when it becomes rubbery-elastic:
- Placing the tube upright on a flat support or centering plug, provided the tube length does not noticeably exceed its diameter and the wall thickness is adequate (see Fig. 38)
- Suspending the tube by means of centering discs at both ends, provided the tube is thick-walled and its length does not exceed three times its diameter (see Fig. 39)
- Suspending the tube with a bellshaped holding collar at the upper end, if the tube is very long and thinwalled (see Fig. 40)
Fig. 38. Heating in an upright position: support (1), ACRYLITE® tube (2), centering plug(3).
Fig. 39. Suspended heating of tubes by means of centering discs: drilled disc, loose (1), ACRYLITE tube (2), centering disc with suspension attached (3).
Fig. 40. Suspended heating of tubes with holding collar: threaded eye-bolt (1), ACRYLITE® tube (2), bell-shaped holding collar (3), rubber block glued to clamping disc (4), threaded clamping disc (5), fastening (6), loose (7), in clamped position (8).
The smallest possible bending radius when heated depends on the tube diameter (d) and also partly on the wall thickness. The table lists the standard minimum permissible bending radii typical for free bending of ACRYLITE® tubes with outside diameters of 10 to 60 mm. As long as these radii are observed, a negligible change of the circular cross-section to oval will occur. Given larger radii, this change is caused by tensile stress generated along the outer curvature, which counteracts the compressive stress along the inside curvature. When a certain stress limit is exceeded, the tube may actually buckle. For tubes made of ACRYLITE® FF, these empirical values signify that heating has to take place in a very narrow temperature range, which must be determined under the respective heating conditions.
|Tube Diameter |
|Min. permissible |
For work requiring very accurate angles, a bending jig should be used. Allowance must be made for the fact that the tube angle will expand slightly on cooling.
For tube angles with a smaller radius, thermoforming of two half-shells from sheet material and bonding them together is recommended. This is especially true for larger dimensions.
For high-definition thermoforming with an exterior wall support, a bending jig must be used that supports the internal and external curvature of the tube and counteracts the tendency to oval deformation of its cross-section (see Fig. 41).
Fig. 41: Thermoforming a tube of ACRYLITE® cast (GP) or ACRYLITE® extruded (FF) in a bending jig. Lateral support (1), section A-A (2).
In order to prevent the tube from buckling, as noted above, an interior support may be used for bending. The use of sand, gypsum, prepared chalk and other materials in powder form, as commonly employed for bending metal pipes, is not possible with transparent plastic tubes because they would become dull on the inside surface. Suitable interior-support materials that do not noticeably affect the brilliancy of the tubing are:
- Rubber rings
- Metal spirals in a rubber sleeve
- Nested rubber tubes
These elastic interior supports can normally be pulled out of the cooled bend with ease after hot curving and cooling of the ACRYLITE® tube if they are sprinkled with talcum before insertion. An essential prerequisite in this case is that the supports match the inside diameter of the tube very accurately. If this is so, the minimum permissible bending radius for free bending (see table above) can normally be further reduced by one third. Pronounced marking on the inside of the tube bend may, however, be unavoidable in this case.
Finally, it may be possible to use a tube with a smaller diameter than is actually required, but with a somewhat thicker wall, and to form it with air pressure in a two-piece hollow mold of the desired bend dimensions (see also Expansion with air pressure below). An uncommon but nevertheless feasible forming method is scarfing, whereby the end of one tube is expanded at forming temperature with the aid of a mandrel and then pushed over and shrunk onto the end of another tube. This "splicing" method is suitable mainly for ACRYLITE® cast (GP) and to a lesser degree for ACRYLITE® extruded (FF). The mandrel can be made of hardwood, metal, or plastic. Its removal after coating may be difficult, but it becomes easier if the mandrel is heated as well prior to forming. The considerable amount of friction between the interior wall of the tube and the mandrel limits the possible enlargement of the diameter, which is about three times the wall thickness, however. The expansion depth (scarf length) reaches its maximum at 1.5 times the tube diameter. A very accurate fit is achieved by subsequently pushing the expanded tube over another tube with the original cross-section and heating it locally by means of hot air so that it shrinks on. The same applies to drawing onto other materials. However, the core circumference must not be smaller than the original internal circumference of the tube. Since the shrunk-on tubes are internally stressed, the influence of aggressive media may result in crazing. Stress-relieving annealing is therefore essential.
For some applications, tubular shapes with a cornered cross-section are required. They can be formed using an expander. For this purpose, tubes of ACRYLITE® cast (GP), which are mechanically expanded to a cornered shape with parallel sides or conical form once they have been heated to forming temperature, will normally be selected. Expanding tubes with compressed air corresponds to stretchforming of sheets with female molds and is mainly used for the manufacture of conical tubes and tubes with changing or non-circular cross-sections. Here too, the wall thickness decreases as the degree of stretching increases. If the cross-section remains the same over the entire length of the tube, the latter can be expanded up to two or three times the original diameter. The molds must be able to withstand the forming pressures that arise. For complex parts, the molds should be heated. Depending on the tube diameters, special clamping or sealing devices must be used. When the heated tubes are placed in the molds, they should be under slight tensile stress in the axial direction, so as to prevent the tube walls from caving in or the tubes from sagging. Molds for long tubes should be designed in such a way that the heated tubes can be vertically suspended in them. Otherwise, defective moldings may be obtained, or--especially with ACRYLITE® extruded (FF)--the tube walls will stick together.
Round and square rods of ACRYLITE® cast (GP) and ACRYLITE® extruded (FF) are thermoformed similarly to tubes with the aid of exterior supports. Forming in this case usually means curving or straight-line bending. For molded rods with rectangular or square cross-sections, our recommendations are the same as for sheets. Circular cross-sections behave like tubes supported on the inside. Generally molds for drape forming without undue mechanical stress, of wood or plastic. The lifetime of a mold depends on the number of moldings to be produced and the mechanical and thermal stress involved. The forming method-into female molds or over male molds-determines the required dimensional tolerance of the mold. Due to the high coefficient of thermal expansion of acrylic glass, ACRYLITE® cast (GP) and ACRYLITE® extruded (FF) shrink noticeably on cooling. Therefore the molds must have a material in-between speaking, the bending radii should be as large as possible in order to avoid pronounced changes in the bend. If square ACRYLITE® rods have to be turned/twisted on the longitudinal axis while warm, this can often be advantageously performed between the chuck and tailstock on a lathe.
Every now and then, stretched round rods of ACRYLITE® cast (GP) are required, such as for the manufacture of shrink rivets. To this end, a round rod is first heated to the forming temperature and then clamped into a lathe, for example, where the rod, with chuck and support, is uniaxially stretched by up to 70%.