PREFORM PRODUCTION PETplanet Insider Vol. 24 No. 10/23 www.petpla.net 22 BOTTLE MAK NG time and the pressures to 60, 50, and 40%, respectively of the actual injection pressure. Fig. 8.6 gives a guideline for the total hold time as a function of preform wall thickness. Figure 8.6 Recommended hold time versus preform wall thickness Cooling time After the hold timers have timed out cooling time starts and the screw moves back relieving pressure in the hot runner and cavities. Cooling time should be chosen to allow the preform to shrink away from the cavity for easier preform removal and to control gate crystallinity. Fig. 8.7 shows recommended times depending on wall thickness. Figure 8.7 Recommended cooling time versus preform wall thickness product development caps & closures design high cavitation moulds multi-component moulds closure production systems after sales service competent – professional – inspiring +43 5572 7272-0 | sales@z-moulds.com | www.z-moulds.com z SHOW DETAILS GULFOOD MANUFACTURING Dubai, Nov. 7-9th, Booth S2-C41 PLASTIMAGEN M xico Mexico City, Nov. 7-10th, Booth 219 NEXT SHOWS GULFOOD MANUFACTURING & PLASTIMAGEN Come and see us! Cushion control The screw should not bottom out at the end of the barrel with each stroke; instead there should be what is called a cushion, that is, some material is left to cushion the screw stroke. Typically 5–10mm is chosen but in single-stage processing a larger cushion may be selected to give the material more time to homogenise. A practical example This job is for a 60mm screw machine, running a eight-cavity tool with a preform/bottle weight of 23g. Total shot size is therefore 184g. At a melt density of 1.16g/cm3 the screw stroke is 184g/1.16g/cm3 per (62 × 3.14/4) = 5.6cm or 56mm. This is the stroke the screw should traverse during injection. Since this is only 87% of the total, the stroke to the end of hold is 56/87 × 100 = 64mm. In other words, the screw should traverse 8mm during hold. Choosing a 5mm cushion the total stroke setting should be: Shot size (the point where the screw starts): 69 Transition point: 13 Screw stops at: 5 Speed settings could be 30% from 69 to 60, 50% from 60 to 30, and 25% from 30 to 13. Initial speed settings will result in a certain injection time and need to be adjusted to come close to the abovementioned recommended injection times. These values can be a good starting point but should be verified on the machine once it is running. Other considerations may take precedence as explained later. Assuming a preform wall thickness of 3.2mm and an observed injection pressure of 50 bar (700 psi) hold settings could be as follows: Total hold time (from the graph): 5.1s Hold times: 1.7 s each Hold pressure 1: 30 bar (435 psi) Hold pressure 2: 25 bar (360 psi) Hold pressure 3: 20 bar (280 psi) Sink marks in the neck area may require higher hold pressure 1 and/or time settings. Gate appearance will be controlled with hold pressure 3. 8.4 Interaction between injection and blow It should be obvious at this point that the injection process has an impact on the blow process. Different hold and cooling times change preform temperatures and this in turn will change the way the bottle blows, especially in areas that are close to the upper and lower limit of PET’s ability to stretch.
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