Process Capabilities. Injection molding is a high-rate production process arid per¬mits good dimensional control. Typical cycle times range from 5 to 60 seconds, but they can be several minutes for thermosetung materials The molds arc generally made of tool steels, beryllium-copper, or aluminum, and they may have multiple cavities, so that more than one part can be made in one cycle. (See also Fig. 11.25.) Mold costs can be as high as ร 100,000; mold life may be on the order of 2 million cycles for Steel molds, but it IS only about 10,000 cycles for aluminum molds.
Mold design and the control of material flow in the die cavities are important factors in the quality of the product. Much progress has been made in the analysis and design of molds and of material flow in injection molding. (Sec also Section 18.15.) Modeling techniques have been developed for studying optimum gating systems, mold filling, mold cooling, and part distortion; they take into account such factors as injection pressure, temperature, and the condition of the resin. Software programs are now available to expedite the design process for molding parts with good dimensions and characteristics.
Machines, injection-molding machines are usually horizontal (Fig. 18.7) Verti¬cal machines arc used lor making small close-tolerance parts and for insert molding. The c.amping force on the dies is generally supplied by hydraulic means, although electrical means are now available. Electrically driven models weigh less and arc quieter than hydraulic machines.
Injection-molding machines arc rated according to the capacity of the mold and the clamping force. [ท most machines, this force ranges from 0.9 to 2.2 MN (100 to 250 tons). The largest machine in operation has a capacity of 45 MN (5000 tons), and it can produce parts weighing 25 kg (55 lb) (most parts weigh from 100 to 600 g (from 3 to 20 02)) The