Rebuilt extruders can put more pounds on the ground for fewer dollars if you think about your process needs.
February 1, 2007 by Jim Anderton, technical editor
Reduce, reuse, recycle” is a popular mantra these days as climate change and environmental consciousness has become a mainstream Canadian issue. If you’re looking for high throughput upgrades to your extrusion processes, however, remanufactured equipment is much more than a way to salvage machinery; it’s an opportunity to gain process efficiency and productivity at a very attractive cost compared to new, in many cases.
START WITH SAFETY
Extruders are unique among plastics processing equipment in the maturity of their core technology. With the right screw/barrel assembly, modern controls and drives along with solid tooling, a thirty year old machine can make money even in high volume applications like pipe and profile extrusion.
An important, but often overlooked component of the rebuild, however, is safety.
Guarding is an obvious upgrade, but electric current is a less obvious risk. Can operators kill unit power from anywhere they’re expected to control or monitor the machine?
How does the extruder handle overpressure? Changing the screw for a new material and/or process could generate higher pressures than those for which the process was originally intended. If there’s a rupture, are there burst plugs to direct the explosive decompression? Similarly, the machine’s electrics should be upgraded to insulation, grounding and fusing standards that match modern retrofit controls.
Documentation is important, since setup, maintenance and repair procedures can be radically different in an electromechanical or first generation PLC machine upgraded to computer control. If your business generates and tracks safety documentation entirely in-house, remember to include a meeting with the rebuilder to define the standards needed before work begins.
In every industry that turns electricity into rotary motion, the debate is the same: AC or DC drives? It’s not a trivial question. Frictional heating can easily provide over 80 per cent of the energy requirement for melting the resin, and that energy comes directly from the drive system. For pipe and profile extruders, however, the need for consistent melt flow is just as important, so upgraded or rebuilt drives need to be more than just big electric motors.
DC drives are simple, have wide constant torque range capability and are compact for their horsepower ratings. AC types have very wide constant torque ranges, no brushes or commutators and the ability to use “vector” or phase control techniques for very close speed/torque control. Advanced systems can even torque compensate without external closed loop sensing systems, simplifying the drive. AC systems are also expensive, although the real cost depends on several factors, the most important of which is return on investment.
The right choice delivers enough resin to the die with surge limits that don’t overwhelm downstream sizing/calibrating equipment’s capability. The cost of a “Cadillac” drive or servo system may not be prohibitive if it frees up capacity that’s trapped behind an underpowered screw.
Some screw designs use a reverse pitch surge suppression stage, a feature that might be unnecessary with a powerful, highly controlled drive. Eliminating the need to deal with surge in the barrel would be equivalent to gaining “free” L/D with a simple screw change.
Injection molders aren’t the only processors who deal with shrinkage; extrusion dies are built to accommodate the correct “draw down” for the sizing technology used downstream. For the widely-used vacuum or water bath sizing technology, for example, the extrudate leaves the die lips a little large. Extrusion engineers frequently speak of “draw balance”, but in older systems, a calibrator downstream reacts to changes in the pipe or profile’s dimensions, like I.D., O.D. and wall thickness, usually with limited or no control authority over the upstream process.
Small changes in the extruder performance can put important parameters outside of the calibration equipment’s capability, so extruders with drives that can deliver consistent mass flow are critical, requiring a good knowledge of the what the calibration equipment can do and the quality specs that the finished product must achieve.
A knowledgeable rebuilder will ask about the application and downstream equipment, and will use “extruder-duty” motors and properly rated, lubricated and cooled gearsets for better control and consistency.
THE LITTLE THINGS AREN’T SO LITTLE
Remanufacturing extrusion equipment isn’t only cost-effective, it’s an excellent chance to change materials, processes and products using equipment you already know and understand. Radical changes in materials or processes, however, will require a learning curve much like the one expected for new equipment.
Take the screw, for example. A change to a volatile-producing resin will likely need barrel venting, but consider the compression zone. It’s effectively longer, since the melt must be de-compressed, then re-compressed before the screen pack and die. Since the barrel length is the same, is the metering zone now shorter? Probably, and if mixing was marginal before, it will be a definite issue now.
Change the screw design and the learning curve starts again. In fact, a straight rebuild can still send extrusion processors back to the drawing board. Why? Because a good rebuild produces an essentially new machine, with tight screw/barrel clearances and better control of screw speed and barrel heat.
A well-maintained extruder can last for decades, and if maintenance/setup logs aren’t comprehensive, it’s very likely that the current operators and engineering staff have no idea how the machine performed when new. Less heat may be needed, for example, after the rebuild, and faster speed may be possible while maintaining good mixing of the melt.
If add-on static mixers or barrier screw retrofits have been used in the past, for example, the higher performance of a remanufactured machine may allow good mixing (distributive and dispersive) without the add-ons.
HOW FAR CAN YOU GO?
How far can extruder technology be upgraded for pipe and profile production? There are few limits to what can be done to improve throughput.
Barrels can be replaced for larger screws and lined with hard wearing carbide surfaces for abrasive resins. L/D can be boosted with barrel extensions and cooling systems can be retrofitted to increase or decrease capacity as material changes. Feed issues can be addressed with grooved feed section screws or with crammers to force material into the throat.
Screw designs have proliferated in the last five years and with advanced CNC machining technologies, custom designs are more affordable than ever for retrofit applications. Tooling can be upgraded to match, with quick change or multi-layer systems, while downstream sizing and calibration equipment can be integrated with an upgraded machine for true closed-loop control.
The only restriction in upgrade possibilities is the price point where the machine retrofit matches the cost of new equipment. Single screw extruders today cost almost the same as they did ten years ago. Twin screw extruders, while more productive in many instances, are more expensive. However, for custom rebuilds for special capability, it may still make sense to upgrade due to shorter lead times and the ease with which the existing extruder can be re-integrated into an existing line.
Some processors in the industry, often injection molders, believe that extrusion is simple, but for applications where “quality” is about safety and regulatory approval, pipe and profile extrusion is as tough as any OEM molding operation. With cost pressures driving the same margin squeeze as other forms of resin processing, getting the most equipment “bang for the buck” is as important to the maker of sewer pipe as it is for the molder of artificial heart valves, and maybe more.
With remanufactured and upgraded
equipment offering new or better-than-new performance at prices that can be two thirds to three quarters the price of new machines, “renew” versus “new” is a more cost effective option than ever.
SPIRAL DIES SAVE MONEY
One way to enhance extrusion productivity is with proper die design to ensure accurate flow of melt to the tooling. Spiral flow dies for pipe and tubing came on the market in the mid-1990s, as an outgrowth of technology for the blown film industry.
In a spiral die, the rotation of the melt balances its velocity when it hits the tip, thus ensuring that the velocity is equal all the way around the die. Another benefit is the elimination of knit lines down the length of the tube.
Perhaps the biggest benefit is cost savings in material. With spiral die design, you can increase concentricity of the pipe or tube and reduce wall thickness variation.
However, one spiral die design does not fit all materials, according to experts from die manufacturer Guill Tool. Guill’s engineering department uses software called computational fluid dynamics to calculate optimal flow and pressure rates to come up with the best spiral die design for the application — with a four- eight- or sixteen-fluted spiral.
Guill Tool & Engineering Co. Inc. is located in West Warwick, R.I. and is represented in Canada by Extrusion Systems, Markham, Ont.
Both new extruder manufacturers and specialty firms have capabilities that can breathe new life into older extruders and downstream equipment.
For suppliers with upgrade and refurbishment capabilities, check the Canadian Plastics Buyers’ Guide. There’s a list under “Remanufacturing: Extruders” on page 156 of the 2007 print edition. The Buyers’ Guide is also available on-line at www.canplastics.com. Select “Buyers Guide”, “Browse by Product/Service Categories”, “Machinery & Equipment”, “Processing Machinery”, “Remanufacturing: Extruders”.