Shot size capacity

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... 

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... 

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. 

Shot Size (oz) Plasticizing Capacity (Ib/h) Approximate Heat Input (kW) Screw Power Input (hp)... 

Select a machine such that the shot size is between 30 to 80% of machine capacity and for higher temperature resins 60 to 80% of machine capacity. Try to keep the residence time as short as possible. 

Consider a screw that is a 2-in. diameter, 20/1 L/D with 20 oz melt screw capacity. With a 15 s cycle and a shot size of 2 oz, it operates as follows ... 

Whereas the projected area determines the clamping force, the weight or volume of a shot determines the capacity of the IM machine required. For the hot runners of TPs, the shot size includes the gate and runners. Capacities of machines are generally rated in ounces of general purpose PS with other resins, convert to the correct capacity by relating the resin densities to that of PE. If the shot size is based on volume, densities are not involved. 

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. 

Injection units are specified by shot size, maximum injection pressure, plasticating capacity and recovery rate, maximum injection velocity, and other less important factors. The shot size is the maximum weight or volume of plastic that can be injected in one shot. Injection... 

When processing the resin at, or near, the upper limit of the recommended melt temperature range, the shot size should approach 70 to 80% of the machine s barrel capacity. If the cylinder temperature exceeds the upper limit of the suggested melt range, thermal degradation of the resin and loss of physical properties may result. 

When one is injection-molding polycarbonate, it is important not to subject the melt to long residence times as this can cause material degradation and a subsequent drop in properties. The ideal maximum residence time is between 6 and 12 min, depending on the selected melt temperature. When one is processing standard polycarbonate grades on the upper limit of the melt temperature range, it is recommended that the shot size be 60 to 80% of the barrel capacity to minimize residence time. 

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... 

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