Machine shot capacity

Decrease cushion Raise material temperature by raising cylinder temperature Raise mold temperature Increase overall cycle Check shot size vs rated machine shot capacity if shot size exceeds 75% of rated (styrene) shot capacity, move to larger machine... 

Very few of the installed IMMs run shot sizes anywhere near the full shot size capacity of the injection unit (Chapter 4). Typical usage is from 25 to 60%. Most suppliers of injection machines offer several sizes of injection plasticating units for any given press tonnage. The problem of having too much shot capacity can render some IMM unusable for certain plastics and applications. An example is excessive residence time for the plastic particularly the engineering materials. Any plastic that... 

When a molding machine is purchased, then the buyer is offered a choice of screw sizes three are usually available and these may be referred to as A, B or C. They have different screw diameters and allow an appropriate match of shot size (the feed system volume plus the molding volume) to barrel capacity. However, because they have different screw diameters, not only will the shot capacity vary but so will other things, for example, injection pressure and plasticizing capacity. Normally t e screws have the lowest shot capacity, and the highest injection pressure, whereas C type screws have the highest shot capacity and lowest injection pressure. 

The shot capacity should ideally be between 80 to 95% of the machine s capacity to prevent the material cooking in the barrel. As degradation depends on residence time as well as temperature, mold this material on a machine which has a small a shot capacity as possible. Yellowing of the material occurs (i.e. where the molding has a distict yellow cast, or hue) if the barrel residence time is too long, or the shot capacity of the machine is too large. 

Shot capacity n. The maximum weight of plastic that can be delivered to an injection mold by one stroke of the ram or screw. In the case of screw-injection molding machines that are not equipped with backflow-preventing valves at the end of the screw, slippage of material may occur in the screw flights and may not be reckoned with in calculations of shot capacity that are based on cubic displacement. 

Fig. 4-11 illustrates, the 4-stage cycling of the rotary turntable machine. Rotary turntable machines are available with from two to twelve stations, platen sizes up to 24 in. by 29 in., with vertical clamp design and clamping pressures from 385 - 880 tons. The clamping mechanism is fully hydraulic. Shot capacities range from 26 - 267 oz. Machines are also available with metered shot control that permits injection of a different shot... 

Effective increase in the shot capacity of the machine as, once the hot rurmer is filled, the injection capacity can be fully concentrated into the cavities. 

An ounce is a unit used to represent the capacity of an injection molding machine, showing the polystyrene equivalent weight of polymer for one shot . 

In the injection molding process, setting the temperature involves optimization of the temperature profile of the plasticating unit (extruder barrel), temperatures of the mnners and gates, (aU these determine the molten polymer temperature) as well as the mold temperature. The temperature setpoints depend on the material type (viscosity profile, thermal and shear stability, thermal properties) as well as machine or process considerations (machine capacity to shot size ratio, screw design, mold and part design, cycle time, etc.). Temperatures of the two basic units, the injection system and the mold, should be discussed separately since their selection stems from very different considerations. 

The optimum temperature prohle should be selected according to the ratio of shot size to machine capacity, screw design, mold and part design, as well as cycle time. Barrel temperature prohle should be set so the material melts gradually — lower near the feed, hotter near the die zone. However, for vented barrel machines, a relatively hat temperature prohle is recommended to ensure that the polymer is melted by the time it reaches the vent zone. Reverse temperature prohles are also sometimes used, primarily to compensate either for improper screw design or with short L/D ratios, and to reduce machine amperage or torque requirements. 

ABS/PVC blends (e.g., Triax CBE, Enplex tm) are formulated to provide a broad window of molding conditions. Because of the presence of PVC, shear or thermal degradation must be minimized. Shot size should be about 50% of machine capacity — less than 35% will result in a long residence time, whereas in excess of 65% will increase plasticating time, both of which create conditions for resin degradation. High compression screws should be avoided since they generate excessive shear heat — a linear taper screw with a compression ratio between 1.5 and... 

The optimum temperature profile depends on many variables such as the ratio of shot size to machine capacity, screw design, mold and part design and cycle time. The upper range melt temperatures should be used while molding thin wall parts. Similarly, mold temperatures in the upper range are advisable for thin wall parts, large parts, and parts that require dimensional stability and a low level of molded in stresses. Mold temperature has minimal effect on gloss level of finished part. Uniform heat removal from the... 

For impact-modified blends (Xenoy 1200 and 5720, Stapron E), screw speed should be set at minimum, as low as possible (less than 50 rpm), just sufficient to assure a reasonable cycle time. Melt temperature should be kept as low as possible, without binding the screw. When temperature control is critical, as is the case here, the acmal melt temperature should be checked with a pyrometer or digital thermometer, rather than relying on barrel set points. The material runs best in thick parts with oversized gates. Injection speed of 25 mm/sec is suggested. When running a large shot (>70% of machine capacity), a flat or reverse barrel heat profile may help to convey the melt. 

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. 

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