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The shot size is the maximum amount of plastic injection mold


1.1. Injection Unit

The injection unit must 1) melt the polymeric material and forms the shot, 2) transfer (inject) the melt into the mold, 2) build up packing and holding pressures, 3) bring the nozzle into contact with the sprue bushing of the mold, and 4) generate contact pressure between the nozzle and sprue bushing. While single stage ram plunger, dual stage ram plunger, and ram assisted-screw plunger machines are available[7] the most machines contain reciprocating screw injection units. As illustrated in Figure 3, these units contain a 1) hopper, 2) screw motor, 3) injection mold mechanism (hydraulic cylinders or motor-driven systems), 4) barrel and screw, and 5) nozzle.

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Figure 3. Reciprocating screw injection unit[8].

Polymer pellets are fed (by gravity) though the hopper, pass through a cooled feed throat, and drop onto the rotating screw. The feed throat is water cooled to prevent bridging (i.e., partial melting) of the resin. When the motor rotates the screw, friction from the rotating screw and conduction from the electric resistance heater bands surrounding the barrel melt the polymeric material. The rotating screw also conveys the polymer toward the nozzle. At the end of the screw, the molten polymer passes through a non-return valve, which prevents the melt from flowing back towards the hopper. Melt is trapped between the blocked nozzle and the non-return valve (Figure 4). This melt forces the screw backwards (i.e., towards the hopper). The screw stops rotating when sufficient melt has become trapped between the nozzle and non-return valve. While this melt is called the ¡°shot size,¡± the measurement is axial travel of the screw or ¡°stroke.¡±

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Figure 4. Barrel, screw, non-return valve (check ring) and nozzle[9].

The injection unit or sled also travels toward and away from the clamping unit. To facilitate purging, change nozzles, or adjust the travel distance, the unit is backed away from the stationary platen. During molding cycles, the injection unit is forward to that the nozzle and sprue bushing make intimate contact. The contact pressure (i.e., pressure exerted between the nozzle and the sprue bushing) ensures proper alignment of the nozzle and sprue bushing and also keeps the two items in contact during injection.

The injection unit of a mold machine is specified by its:

1. Shot size. The shot size is the maximum amount of plastic injection mold that can be injected in one molding cycle and is rated in ounces of general purpose polystyrene (GPPS) for U.S. machines[10] and cm3 for European and Asian machines[11] For best quality, parts must use about 60 to 70% of a machine¡¯s rated shot size[12]. Smaller shot sizes produce greater irregularities and loss of precision, whereas larger shot sizes do not allow sufficient melt cushion for packing and for inefficiencies in plastication.

 

You will be using a micro injection molding machine with a 3-g shot size, but the Department of Plastics Engineering has machines with shot sizes of 3-g to 8 oz.

2. Plasticating capacity and recovery rate. The plasticating capacity is a measure of the amount of plastic that can be melted and homogenized per unit of time (lb/hr or kg/hr), with insufficient plasticating capacity (with respect to the shot size) producing unmelted plastic and too high plasticating capacity causing thermal degradation due to longer dwell time in the barrel. The recovery rate is a measure of the volumetric output of the injection molding machines (expressed in in3/s). Both recovery rate and plasticating capacity are determined by running polystyrene at 50% of maximum capacity.

3. Maximum injection velocity. The maximum injection velocity in a conventional injection molding machine ranges from 150 to 250 mm/s (6 to 10 in/s)[13] and can be as high as 2000 mm/s for thin wall machines[14].

The micro injection molding machine that you will be using has a maximum injection velocity of 160 mm/s.

4. Maximum available injection pressure. In all injection molding machines,

plastic injection mold velocity and injection pressure are linked so that the set plastic injection mold velocity cannot be maintained without sufficient pressure. The maximum available injection pressure for a standard injection molding machine is 138 MPa (20,000 psi) and can be as high as 324 MPa for thin wall machines[15].

1.2. Clamping Unit

The clamping unit supports the mold; opens and closes the mold; holds the mold closed during injection, packing and holding; and holds the ejection unit. The two major types of clamps are hydraulic systems and toggle clamps. A hydraulically-actuated toggle clamp is shown in Figure 5; (note: you will be using an electrically-actuated toggle, but I could not find a good picture of one). As with all clamping units, the toggle unit contain a stationary plane (1) and a moving platen (3) on which is mounted the mold (2). These platens and the tailstock platen (at the end of the machine) are usually supported by tie bars (7). The stationary plane has a hole, which facilitates mounting of the mold and allows the nozzle to contact the sprue bushing. The ejection system (4 and 5) is usually supported by the moving platen.

In hydraulically-actuated toggle clamps, a toggle mechanism (6) provides a mechanical linkage between the moving and tailstock platens and the hydraulic cylinder (9). Forward actuation of the cylinder extends the toggle, thereby moving the moving platen and closing the mold. Full extension of the toggle provides the clamp force required to keep the platen closed during the molding cycle. Reverse actuation of the cylinder retracts the toggle and opens the mold. Clamp adjusting ring gear and a hydraulic motor (9 and 10) adjust the position of the moving platen relative to the tailstock platen. This movement is called the die height adjustment and allows different-sized molds to be fit into the clamping unit. Finally, if material is trapped between the two sides of closing mold, this produces a pressure in the hydraulic cylinder. Therefore, mold protection typically consists of reversing the clamp motion when mold cannot close, but the pressure has reached a preset level. The time of forward motion without closing the mold can also be limited to a preset value. Electrically-actuated toggle clamps are very similar hydraulically-actuated toggles, but electric motors (with various mechanisms) drive the toggle and the ejection mechanism.

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Figure 5. Hydraulically-actuated toggle clamp unit[16].

In hydraulic clamps (Figure 6), there are two hydraulic cylinders and no toggle. The small double acting traverse cylinder is actuated to open and close the mold, but the larger main cylinder helps provide the clamp force. To efficiently fill the latter cylinder, a prefill valve is opened before the traverse cylinder starts to close the clamp. Hydraulic oil is suctioned from the main or an auxiliary tank to the main clamp by the movement of the traverse cylinder. When the mold halves touch, the prefill valve is closed and further movement of traverse cylinder compresses the oil in the main cylinder, thereby producing the clamp force. This process is reversed when the mold is opened and a separate cylinder provides for part ejection. With hydromechanical clamps, the mold is open and closed using a toggle mechanism whereas the clamp force is produced by one or more hydraulic cylinders[17].

Clamping units are specified by the clamp force. In addition, a number of parameters limit the size of the mold that can be mounted in the machine.

The micro injection molding machine that you will be using has a clamp force of 3 tons, but the Department of Plastics Engineering has machines with clamp forces from 3 to 100 tons.

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Figure 6. Hydraulic clamp unit[18].

During mold open, the part remains with the moving side of the mold, usually with the assistance of a sprue puller. An ejection system (Figure 7) usually detachs the part from the mold. To separate the part from the mold, a hydraulically or electrically-actuated ejection platen is forced forward (i.e., toward the stationary platen). Knockout rods, that connect this platen to the ejector platen in the mold, force the ejector plate forward. Thus, the ejector pins mounted to the ejector plate know the part out of the mold[19]. Ejector return pins help return the ejector pins to the retracted position as the mold closes for the next cycle.

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Figure 7. Ejection unit[20].

 



   
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