PETpla.net Insider 03 / 2015

PREFORM PRODUCTION 43 PET planet insider Vol. 16 No. 03/15 www.petpla.net the water-cooled take-out plate car- ries three times the number of cavities and stops in three different positions to receive preforms. These stay in the plate for another 2.5 cycles before the plate turns and releases them onto a conveyor. The takeout can also be done by transfer into an intermedi- ate plate. In a second step, preforms are then handed over into the actual cooling station on top of or beside the machine. The intermediate plate has the advantage that it can be very light and fast as it carries fewer cavities and/ or has no water cooling. The advantage of the direct approach is that it uses one handshake, i.e., transfer from one plate into another (Fig. 3.15). Figure 3.15 Blow tubes are used to fur- ther reduce cooling times by air cooling preforms simultaneously with water cool- ing. (Photograph courtesy of MHT AG). Patented devices also use tubes of one kind or another to blow air onto the preforms in various ways, further reducing cooling times. All mod¬ern machines feature servo motors that drive the take-out or intermediate plates, and postmold cooling sta- tions can reduce overall cycle times by 20–40% and pay for themselves very quickly. Machines without these sta¬tions must use much longer cooling times, which increases the stress inside the preform walls. This is because during hold the shrinking material is replaced but during cool- ing time the preform shrinks onto the cores and encapsulates, thus incur- ring stresses. Figure 3.16 To better tailor the injec- tion profile to the preform it is cut into sections. 3.2.9 Machine Cycle Improvements In order to produce more preforms in less time, machine manufactur- ers have changed tool actions from sequential to concurrent. An example is to start injection after the mold is closed but before it is clamped, i.e., under full clamp pressure. This is pos- sible because the extruder injects into an empty cavity, thereby not apply- ing any significant amount of pres- sure. This pressure is exerted during hold time when the cavity is full of the material. Similar improvements can be made during unclamping and in the timing of the movements of mold and take-out robot. Operators must be aware of the condition the machine is in as certain functions cannot be accessed when the machine is in “rapid” mode (Fig. 3.16). 3.3 Optimizing the Injection Settings Preforms can be divided into four sections (see Fig. 3.16):  Gate area covering the hemispheri- cal portion, also called end cap  Body area  Transition area where the thicker body merges into the thinner neck  Neck area To keep shear constant and achieve a uniform preform tem- perature, the melt should move at a constant speed inside the cavity. However, the amount of material that fills each cavity section is not uniform because of the differences in wall thickness and diameter. Therefore, the fill speed should be adjusted to account for these differences. Here is how this is done. The first step is to cut the preform at the three places indicated in the draw- ing and weigh each piece, also noting its wall thickness. For the end cap and the transition zone, the wall thickness in the middle of each part should be noted. If only three speed and position settings are available, only two cuts are made, cap and body are weighed together, and the body wall thickness is used for the calculations. Next, each weight is calculated as a percentage of the total weight. The weight percentages are then transferred into position settings from shot-size to transition point and sub- tracted from the shotsize to yield the num-bers to enter on the screen. Let us look at an example:  Shotsize: 120mm  Transition point: 23 mm  Distance between them: 97 mm Here are the weights and calcu- lated results: Figure 3.16.1 Preform Weight (g) Percentage of Weight (%) Screw Distance (mm) Screen Values (mm) Cap 7.5 19.7 (7.5/38) 19.1 (97× 19.7%) 101 (120 − 19) Body 23.8 62.6 (23.8/38) 60.7 (97× 62.6%) 40 (101 − 61) Transition 3.2 8.4 (3.2/38) 8.1 (97× 8.4%) 32 (40 − 8) Neck 3.5 9.2 (3.5/38) 8.9 (97× 9.2%) 23 (32 − 9) Total 38 99.9 96.8 Figure 3.16.1