PREFORM PRODUCTION 21 BOTTLE MAK NG PETplanet Insider Vol. 24 In order to get to these times there are usually three or more speed settings that the processor can use. These are mostly open-loop controls that can be set depending on the screw position. On the machine a proportional valve receives speed percentages as variations of milliamps and allows a proportional amount of oil flow into the injection device. Recommended is a slow start, a fast middle, and then a slowdown before the screw reaches the transition point. Choosing the transition point As explained earlier the transition point should be chosen as that point when the injection cavity is completely filled with material. There are several ways to calculate it. The first takes advantage of our knowledge of melt density versus solid density. For PET these values are 1.16 and 1.335g/ cm3, respectively. We assume that the material is at the melt density during injection and at solid density after hold and cooling, even though this may not be that correct as explained later. Because melt density is about 87% of solid density our initial shot calculation should allow 87% of the stroke to happen in injection and 13% of it during hold. The 13% hold phase stroke allows the material to grow in density, that is, shrink to the solid density. The reason this is not as accurate is that the material, especially the melt front, cools down during injection and increases in density as a result. This is especially true for thin preforms that cool down quickly. The 13% density difference may be as low as 8% or even 6% thereby changing the necessary settings. Another way of finding the transition point is to observe hydraulic injection pressure. During injection the cavity is empty (except for air) and there is little resistance to the flow of plastic into it. Once the cavity is filled with material injection pressure will increase immediately as the material now blocks further injection. An experienced processor can thus determine at which point the cavity is filled. A third way is to calculate the screw stroke with the screw diameter, the melt viscosity and the shot weight as parameters. Hold pressure and time During hold time the material shrinks to solid density as explained and the main purpose is to allow some material to enter the cavity to compensate for this shrinkage. Most machines have three hold pressure and time settings. The first is responsible for the neck area of the preform, the second for the body, and the third for the gate. A general guideline would be to set the three timers to equal
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