Injection stroke

Si — Filling and injection stroke Ft — Press force top pfunger... 

Filling and injection stroke Plunge-in stroke top-punch Tablet th ickness in lower dead centre Return-expansion of pressed article... 

The injection molding machine should be capable to deliver a shot size of 50% or less of the injection stroke. Typically, the machine should be able to inject its rated capacity in less than 2 seconds, except for high viscosity resins or when warping or jetting must be prevented by slower injection rate. Depending on the grade, the cylinder temperature setting should be between 190 and 220°C. 

Injection-molding pressure n. (1) The pressure applied to the plastic by the injection screw (or ram) during the injection stroke. (2) The pressure of the melt within the mold cavity just prior to the freezing of the gate. 

The conventional machine is basically run at a single speed throughout the injection stroke, with the option of a boost in the first section, supplied by a second hydraulic pump or a pressure accumulator. The basic speed is adjustable, but is then fixed during operation. 

Sequence of injection skin material begins to inject first. When the ram hits a limit switch on the machine carriage the core material begins to inject. The exact injection profile of each cylinder can be adjusted by altering the slide position on the injection speed control console - each slide position represents a certain percentage of the injection speed which has previously been digitally set. A small amount of skin material is injected into the cavity at the end of the injection stroke. 

The plastic melt in the injection cylinder is subjected to just enough pressure to force the plunger back until the correct amount of melt has accumulated in front of the plunger. During the injection stroke, gases escape between the plunger and cylinder wall with a bit of melted plastic which is occasionally collected and discarded. 

The Optifoam process is developed by the Institute of Plastics Processing (IKV) in Aachen, Germany. The technology, which is available for injection molding [8] and extrusion [9], introduces the SCF into polymer at the nozzle. For extrusion, this occurs constantly and with injection molding, the SCF is delivered during the injection stroke. The Optifoam nozzle delivers the SCF to the passing polymer via a sintered metal coaxial device (Fig. 11.9). 

Figure 7.6 shows residenee times for a range of cycle times. Injeetion strokes are expressed in terms of D, the screw diameter. If, for example, we wish to limit residence time to five minutes, this means that the injection stroke should not be less than about ID for a fifty-second cycle, or not less than about 0.6Z) for a thirty-second cycle. The two values equate to about... 

Mold Open Fast open Injection stroke Injection Injection speed... 

Run Injection Stroke fniml Injection Speed [mm/s] speed-change stroke [mml... 

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