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The Injection Molding Process
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The Injection Molding Process
During injection molding, rubber compounds are subjected to more severe processing conditions than during compression or transfer molding. Temperatures, pressures, and shear stresses are higher, though cure times are shorter. Control over process variables can be more precise.
The cycle time can be minimized by independently controlling barrel temperature, screw speed, mold temperature, cure time, and injection pressure. Compounds with widely differing flow and cure characteristics can be molded into a variety of complex shapes. The skill lies in the optimization of the process; this depends on close interaction and understanding between rubber compounder, mold designer, and processor.
The highest productivity is achieved when the compound is injected at a temperature close to the curing temperature, into a mold at a slightly higher temperature, because these conditions minimize both injection and cure time. Scorch safety of the compound is the limiting factor in the process, and the effect of machine variables on compound temperature needs to be understood if high injection temperatures and short injection times are to be achieved without scorch. Equally, it allows adjustments for batch-to-batch variations in viscosity, cure rate and scorch safety to be made, if necessary.
Thus, to a large extent, controlling the injection molding process reduces to a question of homogeneity of temperature and heat history of the compound at each stage in the process. The temperature of the mix in the injection chamber, prior to injection, is determined by the temperatures of the extruder and injection chamber, by the screw speed, screw design, and by the applied back pressure.
The injection, or mold filling, time and temperature depend on the temperature of the mix, as determined by the above factors, the injection pressure, the dimensions of nozzle, runners and gates, and the viscosity response of the compound. The cure time depends on the mold temperature and the temperature of the compound as it enters the mold. A full analysis of the process is given by Whelans for an out-of-line reciprocating machine.
Cold feed extruder designs have been optimized to enable control of the amount of frictional heat generated, and to maximize the overall heat-transfer rate by constantly exposing fresh rubber surfaces to both barrel and screw. Proper flow also ensures thermal homogeneity of the entire volume of compound in the injection chamber, whose temperature is kept as high as possible without scorching.
Screw speed and design can both significantly affect heat generation in the extruder. In a given machine, screw design is fixed, and thus screw speed is the primary control factor. Plasticization time also depends on screw speed, and a high screw speed can be used to minimize heat history by having the screw rotate only for the time required to fill the injection chamber.
Back pressure is controlled by the pressure against which the screw must work whilst filling the injection chamber. Elevated back pressure raises the temperature of the mix and is only normally needed for low viscosity compounds that might not otherwise generate sufficient shear heat in their passage through the extruder.
The work done by the ram in injecting the compound through the nozzle and runners is dissipated as heat, and this can boost the material temperature by over 40 °C above that in the injection chamber. Thus as high an injection pressure is used as possible, consistent with freedom from scorch.
In summary, for safe, that is scorch free, injection molding the barrel and injection chamber temperatures, screw speed and back pressure are raised until the mix in the injection chamber is at the maximum safe temperature for the time it must remain there waiting to be injected.
A nozzle diameter is then chosen that will permit a 5 to 10 s injection time. Injection pressures and speeds as high as possible are then used consistent with freedom from scorch in nozzle, runners, gates, and mold together with minimum mold filling times. Finally, mold temperatures can be raised to achieve minimum cure times. Thus it is clear, as stated earlier, that scorch safety of the compound is the limiting factor in injection molding. Compounds generally need to be scorch safe to 130 °C.