PETpla.net Insider 10 / 2014

PREFORM PRODUCTION 43 PET planet insider Vol. 15 No. 10/14 www.petpla.net In the feeding section, resin is only transported and not meant to melt. Usually the surrounding area in the extruder housing is water-cooled with tower water to avoid “bridging”, a situation where partly melted mate- rial clumps together and prohibits new material from entering the extruder. In the feeding zone the root diameter of the screw is the smallest and the material is given room to enter and to be pushed forward (figs 3.3 and 3.4). Resin transport is accomplished by manufacturing the barrel surface slightly rougher than the screw sur- face. This allows the screw to turn under the material (slipping), whereas the outer layer of the material sticks to the barrel. As a result the material is pushed forward. When burned mate- rial accumulates on the screw the screw may become “sticky”, and this process does not work as well any more. This shows up as decreased screw output, and the screw needs to be cleaned. In the compression or transition section the root diameter of the screw increases, leaving less and less room for the material, thereby forcing the pel- lets against each other and the barrel and screw. At the end of this section most of the material has been melted. The metering section’s task is to melt any remaining unmelted mate- rial and force it to become a homoge- neous mix. The root diameter of the screw is the largest here but does not change any more. A screw may differ in how the screw ends are shaped with some screws having special adaptors that are especially useful to mix color or other additives (fig. 3.5). In a standard type, screw melt- ing and mixing happen gradually and there is a chance that not all material has undergone the complete process by the time it leaves the extruder. PET is shear sensitive, i.e. it starts to burn when too much shear heat is gen- erated by too steep a compression zone or when screw revolutions per minute (rpm) or injection speed is too high. Therefore, screws must melt the material gently but this may lead to un-melts. To overcome these problems, designers have developed the so- called barrier screws (see fig. 3.6). There are a number of designs in the market that deliver outstanding results not just for PET. In this design there are two threads going along the screw diameter at different depths. Material flows back and forth between the two channels, thereby mixing more vigor- ously. A disadvantage of this design is that improved mixing can lead to higher shear heat. Increased clearance between the channels seems to be the most successful way of reducing this. Machine control of the operator is mainly through adjustment of the  Heats  Screw rpm  Back pressure Although heater screens differ from machine to machine, they all convey the same information. They may show:  The location of the heater band (usually going right to left)  Degrees centigrade or fahrenheit  The set point  The actual temperature Figure 3.3 All screws feature a small root diameter in the feed section. (Diagram courtesy of Barr Inc). Figure 3.4 Clean screws are important to allow material to be conveyed forward. (Diagram courtesy of Barr Inc). Figure 3.5 Standard screw designs may lead to incomplete melting for shear-sensitive materials like PET. (Diagram courtesy of Barr Inc).