Worn Reciprocating Extruder Screws – What’s the big deal?
In reciprocating blow molding, most processors fail to identify the signs that the recip extruder screw is beginning to wear. Some such signs is having to run the plastic extruder screw at a faster RPM than normal, beginning to see un-melts in the container, and the tail length of the container’s flash begins to vary – are all certain signs of screw wear.
Processors always want to operate their extruder screws as long as possible before replacing it so they can get the most return for this investment. However, there are a few things that processors forget to consider when they try to get the “biggest bang for the buck”.
As the tail lengths vary, bottle weights will start to fluctuate, which in turn causes quality issues. Typical quality issue with fluctuation is having the container weight fall under the quality product weight spec. To compensate, operators will “bump the weight up.” This action results in some of or all of the containers’ being processed to be out of the quality weight spec range on the high or heavy side. Processors are now giving away plastic and losing profitability. Tighter control of tail length = less shot to shot weight variation = lower target = more money to the bottom line.
Another issue with worn extruder screws is that varying or short tail lengths (short shots) don’t get picked up in the recip’s takeout mechanism. The result is a process “jamming” of the machine and or containers falling to the floor. To compensate, operators find it necessary to process the tails longer than the flash was originally designed for. Thus the previous short shots, will now get picked out of the molds by the take out mechanism. No harm in a little longer tail, right? Extra flash = extra regrind = less money to the bottom line. Every gram that can be taken out of the total shot weight is less plastic you have to heat up, cool down, and grind up. All of which take energy at a cost to the processor’s profitability.
Once the recip machine’s screw begins to wear and the extrusion stability deteriorates, it is impossible to process your way out this wear condition. Processors cannot wait for the” it is really worn enough for me to change it yet” thought process. Don’t wait for an end customer to file a quality complaint as it is by then too late to save a very unhappy customer.
R&B Plastics Machinery provides screw and barrel replacements for all makes and models of reciprocating blow mold machinery. Our service technicians can measure recip screw and barrels to determine if wear has become a serious process issue. R&B Plastics Machinery also offers complete mechanical, electrical and hydraulic repair and upgrade packages for recip blow mold machinery. Make sure you don’t wait until it’s too late, schedule an R&B technician today.
Written by: Al Hodge & Matt Gifford