Machinery's Handbook, 31st Edition
DESIGNING PLASTIC PARTS 595 also helpful, is more subtle: plastic melts have non-Newtonian physical properties. That is, viscosity depends not only on temperature, as with ordinary liquids, but also on the flow rate and pressure drop D P . Doubling the injection pressure may triple or quadruple the flow rate through the mold passages, but the pressure itself squeezes molecules closer together and tends to increase viscosity and freezing point. Unpredictable results can be obtained if the melt temperature is too near the freezing range. If the product designer is calling for thin wall sections, Melt Flow Rate ( MFR ), formerly known as Melt Flow Index ( MFI ), can be used as a selection property characteristic among grades of candidate plastics. The higher the flow rate, the easier it will likely be to fill thin sections during molding. Flow rate values are obtained by testing (ASTM Test D 1238) in a simple extrusion plastometer. In this device, a small sample of resin is loaded into a 9.55-mm diameter well in a heated cylinder and brought to a test temperature above the plastic’s melting range. At the bottom of the well is a die 2.1 mm in bore diameter and 8 mm long. A weighted piston drives the melt through the die while a timer runs. Extrudate is collected and weighed, and the flow rate is reported in grams per 10-minute interval. To accommodate the wide range of plastics flow behaviors, 36 standard conditions specifying both the melt temperature in ° C and the total piston weight in kg have been adopted. MFR values are meaningless without the condition numbers. Most polypropylenes (PP), for example, are tested at condition 230⁄2.16, meaning a PP resin reported to have an MFR of 8 (230⁄2.16 assumed) would be one that at 230 ° C under a piston weighing 2.16 kg f , produced a flow rate of 8 g⁄10 min. Resin and compound suppliers can supply the MFR values for their grades. Product designers should obtain all three numbers. A somewhat more practical way of judging resin flows under actual molding conditions uses a spiral-flow mold. This injection molding method was in common use by the early 1950s. A similar mold is the basis for testing the flow of transfer-molding compounds (ASTM D 3123). A half-round or shallow-trapezoidal groove, 1 ⁄ 8 inch (3.2 mm) wide and beginning at the center, circles outward with the spiral radius increasing about 3 ⁄ 8 inch (9.5 mm) per turn, to a total channel length of 70 to 100 inches (178 to 254 cm). Inch numerals and 0.1-inch marks are engraved in the flat upper half of the mold. The injection nozzle fills the mold from its center and, under specified melt and mold temperatures and injection pressure, holds pressure until flow stops. The mold is opened, and the flow distance is read off the molded spiral and recorded. With careful control of molding conditions, flow distances of 40 inches (102 cm) and more can be closely reproduced in repeated shots. Many resin suppliers have spiral-flow data available for product designers and processors. Design for Assembly.— An advantage of the flexibility of plastics parts compared to other materials is that they can often be designed for assembly by means of molded-in snap-fit, press-fit, pop-on, and thread fasteners. With careful design, no additional fasten ers, adhesives, solvents, or special equipment is required, significantly reducing assembly time and costs. Improper assembly can be minimized, but tooling is often made more complex and disassembly may be difficult with these methods. Mechanical fasteners designed for metals are generally usable with plastics, and there are many other fasteners designed specifically for plastics. Typical are bolts, self-tapping and thread-forming screws, rivets, threaded inserts, and spring clips. Care must be taken to avoid overstressing the parts. Creep can result in loss of preload in poorly designed systems. Snap-fit designs are widely used, a typical application being to battery compartment covers. All snap-fit designs have a molded part that must flex like a spring, usually past a designed-in interference, then return to its unflexed position to hold the parts together. The design must have sufficient holding power without exceeding the elastic or fatigue limits of the material. With the typical snap-fit designs in Fig. 20, beam equations can be used to calculate the maximum strain during assembly. If the stress is kept below the yield point of the material, the flexing finger returns to its original position.
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