(1) Shot size (mass per shot). The total calculated or estimated shot size, that is, the total mass (weight) of the products coming from all cavities, plus the mass of the runner system (in the case of cold runners) should be within 30-90% of the shot capacity of the machine. The shot capacity of a machine is given in g/shot of PS, with a specific gravity of about 1.05. The specific gravity of materials such as PE and PP is less (about 0.90 to 0.95); that is, the same mass will have a greater volume. Since shot size is rated in grams (or ounces) but is actually a volume (cross section of extruder barrel times the stroke of the extruder), the shot size of these materials will be less than for PS, by about 10%. These are only approximate figures; exact values should be checked with materials suppliers. What are the practical implications? If, for example, an 8-cavity mold is required to run in a specific machine, but its shot capacity is not large enough, it would not make sense to build it for this machine. This is especially important with cold runner molds, where the mass of the runner can add considerably to the mass of the sum of all molded parts, per shot. A machine could be well suited for a hot runner mold but be unsuited for a cold runner mold for the same number of cavities. (This is a major advantage of the hot runner system.)
(2) Plasticizing capacity (kilograms per hour). Plasticizing capacity is the amount (mass) of plastic a machine can plasticize per hour, that is, melt the cold plastic pellets into a melt of a specific temperature (and viscosity). Plasticizing capacity is usually given as mass for PS, in kilograms (pounds) per hour. Here, the same applies as with shot capacity. The actual mass of other materials, such as PE, PP, or any other, will be different, mostly smaller, sometimes greater. This should be carefully considered before starting. But, first, the designer must estimate the molding cycle, to find out how much plastic per hour will be required. Dividing 3600 (1 hour equals 3600 seconds) by the number of the seconds of the estimated cycle will give the number of shots per hour (N). Multiplying the total shot weight S (g/shot) calculated in (1) above, with the number of shots N per hour we find the total mass Wx in grams per hour required (Wt = S x N). For best quality of the melt (and the molded piece), it is also suggested to use only between 30 and 90% of the rated plasticizing capacity. If Wt is more than the rated capacity, the machine can still be used but the cycle time will have to be lengthened; in other words, fewer shots per hour can be produced than the mold could yield with a suitable, larger size machine.
(3) Injection speed (grams injected into the mold per second). This is an important consideration when molding thin-walled products. Because of the narrow gap through which the plastic must flow within the cavity space, the injected plastic will cool rapidly when in contact with the cooled cavity and core walls. As the plastic cools, the gap narrows even more, making it more difficult to fill the mold. To overcome this condition, the melt and/or the mold temperatures could be increased so that the plastic will not freeze before filling the mold. However, this increase in temperature will also cause an increase in the cooling cycle (and a lengthening of the molding cycle), resulting in a smaller output from the mold. This points to two areas for possible remedy: (1) The injection speed and (2) the injection pressure must be increased. But these two are interrelated. The higher the pressure, the faster the melt will be pushed through its paths, from the machine nozzle to the farthest corners of the cavity space. The problem is now that the injection speed depends on the speed with which the hydraulic injection cylinder is filled with pressure oil. Therefore, the speed of the injection cylinder depends on the hydraulic pump output^oil volume per second—entering the cylinder, but it also depends on the size of the associated hardware—hoses, valves, and so on—from the pump to the cylinder.
Most machines for conventional (not thin-wall) products are served sufficiently well by the output of the pump (and the motor driving it). However, the injection speeds required for thin-wall production require the cylinder to be filled more rapidly than what the pump alone can provide. To remedy this, the machine could be equipped with a much larger pump and motor, but in many cases this would be uneconomical or impractical. The preferred solution is to provide the machine injection system with an accumulator, which stores high-pressure oil during the time pressure oil is not used. Additional valving and other hardware is required, which is often sold as an “option” with the machine, called an accumulator package. The accumulator releases the stored high-pressure oil together with the pump output into the cylinder when required for injection. The designer will need to recognize when an accumulator package is necessary for the product for which the mold is to be designed, and must discuss this with the molder to make sure the right machine is available to run the mold.