PETpla.net Insider 05 / 2021

BOTTLE MAKING 19 PETplanet Insider Vol. 22 No. 05/21 www.petpla.net in one continuous motion, whereas linear ovens may feature a U-shaped track system with no oven module in the turnaround section. The latter arrangement leads to two increments of equilibration time as shown later. The possibility of overheating the outer preform wall is minimised with the addition of the second equilibra- tion period. 7.2 Blowing bottles Fig. 7.2 is a cycle diagram of a typical rotary stretch blow mould- ing machine. Black arrows indicate the duration of the machine func- tion. In rotary machines, all times are actual degrees of rotation of the blow wheel rather than time increments. This has the advantage that cer- tain process characteristics can be described without referring to varying cycle times. A disadvantage of cam- controlled movement comes into play when the machine has to be adjusted for throughput. This might be the case when a thicker-than-usual pre- form wall has to be blown and the existing oven banks are not able to reheat these at the faster cycle time. For optimum performance it becomes necessary here to adjust the cams for mould opening, closing, and so on since the same number of degrees now means longer times and there is no process reason to slow down the mould closing simply because the machine is running slower. Cam adjustment is more time-consuming and so adds to the changeover time of the machine. Figure 7.2 Rotary blow mould- ing machine cycle diagram. Rotary machines use increments of one rotation of the blow wheel to control machine functions. There are two important points that require careful mechanical adjust- ment:  The point where the stretch rod just touches the bottom of the preform without stretching it, the so-called “0” point.  The end position of the stretch rod, the “10” point. This position must leave approximately 1–2mm (0.040–0.080 in.) between stretch rod and base insert for the pre- form gate depending on the wall thickness in the gate area of the preform. Both adjustments must be made with cold preforms while the machine is operated manually. Exact adjustments are important for proper processing. For the process it is irrelevant whether the pre- form is neck up (as in the following drawings) or upside down. Mould closed Figure 7.3 An O-ring may seal the preform from the environ- ment. Use and placement of O-rings are particular to each machine manufacturer. At the start of the blowing cycle is a temperature-conditioned preform located on a mandrel or placed in the blow mould via grippers (Fig. 7.3) . An O-ring situated at the inside of the neck or at the top thread surface inhibits air exchange between the inside of the preform and the environ- ment. On machines where the preform is taken off the mandrel for blowing, a blow nozzle is pushed into the neck of the preform for this purpose. The mould cooling water temperature will be between 8 °C and 65 °C (45 °F and 150 °F) with higher temperatures being more favorable to the process with the caveat that they may increase cycle time. At 12 °C in the cycle the base insert starts lifting. It has to be in position to interlock with the blow mould. To save time, the mould starts closing at 14 degrees and is clos- ing around the base insert at 39 °C. Once the blow mould has closed at 49 degrees and locked at 57 degrees, air pressure inside and outside the preform is equal and the stretch rod DNC boosters Turnkey all-in-one system • Compact powerhouse with a footprint of only 2.35 m² ‡ ,QLWLDO SUHVVXUH XS WR EDU ² ÀQDO SUHVVXUH XS WR EDU • Plug and play: ready for immediate operation ‡ 3UHPLXP HˣFLHQF\ ,( PRWRU GHOLYHUV LPSUHVVLYH SHUIRUPDQFH DQG HQHUJ\ VDYLQJV • Sigma Control 2 controller: RSWLPXP HˣFLHQF\ DQG QHWZRUN FDSDELOLW\ ,QGXVWULH www.kaeser.com