Figure 2: The most accurate way to align a twin-screw extruder is with a laser
are some specific situations when it is advisable: a) If the extruder has been moved to a new location
b) If the concrete floor is settling, causing uneven support of the extruder c) If screws or barrels are wearing at an abnormally high rate, or wearing in an abnormal pattern d) If shafts keep breaking for unexplained reasons e) If the machine vibrates more than normal f) If the screws are difficult to slide in/out of couplings g) If somebody changed the barrel support adjusters h) If the machine has been hit with a forklift (yes, it happens!)
There is a simple preventive maintenance procedure
to avoid these problems and keep the extruder cooling efficiency like new. The recommended procedure is to circulate a scale-removal chemical through the barrels periodically. With production extruders, this is recom- mended to be done every three to four months. On a lab extruder seeing much less use, probably once every two years is sufficient. Many of the tower and chiller manufacturers make a
small, inexpensive recirculating de-scaler unit on a roll-around cart, with an integral reservoir and pump. This unit is designed specifically for flushing out mould cooling passages, and is also ideal for extruder barrels.
Figure 3: Gearbox
failures can be expensive and time-consum- ing to repair
1 0 Laser barrel alignment
In the old days, extruder barrels were aligned simply with a machinist’s level. Or, in the case of single-screw extruders, an optical bore-scope. But unlike a single- screw machine, a twin-screw extruder doesn’t have an open bore through the gearbox, so a bore-scope cannot be used.
The most accurate way to align a twin-screw Figure 4:
Switching to synthetic gear oils offers a range of benefits
extruder is with a laser (Figure 2). The laser is mounted concentrically to one of the gearbox output shafts in a holding collar, and the laser light is projected all the way down the barrel to a target mounted on the discharge flange. The end target is made from clear plastic, and has vertical and horizontal crosshairs accurately etched into it. When the extruder is perfectly aligned, the laser dot will appear in the exact intersection of the
crosshairs. If the dot is not in the centre, the barrel supports are adjusted to bring
it into alignment. Extruders do not need to be checked for alignment on a periodic basis, but there 28 COMPOUNDING WORLD | June 2012 1 1 Switch to synthetic gear oil
The gearbox is the heart of a twin-screw extruder. If it fails (Figure 3), it’s likely to be expensive to repair, and it may lead to lengthy downtime. There is one thing everyone can do, easily, to avoid gearbox problems: switch to synthetic gear oil (Figure 4). Synthetic oil is a huge advance in lubrication technology, offering the following benefits[1]
:
a) Synthetic oil is “more slippy”, causing less friction b) Gears, bearings and seals last longer c) Gearboxes run cooler and quieter d) Synthetic oil does not lose viscosity from mechanical shearing e) Maintains higher viscosity at high temperatures f) Improves overall efficiency of the gearbox
1 2 Anti-Seize
Most people in the plastics industry are aware of the need to use anti-seize. There are several different types of anti-seize, such as copper-based, moly, lithium and so on. For something like a bolt thread, any type should work. But there is one application within the extruder where the choice of anti-seize is much more critical: the spline shafts (Figure 5). Screw elements fit onto the spline shafts with a very