Thermoplastic Wire and Cable Extrusion Line Engineering Basics
The 8 Stages of Wire & Cable Production
A cable extrusion line is an industrial production line which coats metal wire in a plastic insulating layer, but when it comes to cable extrusion customization is the name of the game. Depending on your exact needs, production space, and speed requirements, there are many options that can enable your extrusion line to not only meet your precise specifications but also optimize production rates and reduce operating costs.
There are several elements to think about when designing an extrusion line, and each of these elements will impact the final product. Feeding, tensioning, extruding, and cooling stages each require careful consideration to produce an evenly coated, defect-free spool of perfectly insulated wire. In addition, the engineer must take into account inspection of the wire as it comes out of the machine as well as the different processes that may be needed to mark, print, or color coat the product.
Breaking Down The Components of A Wire & Cable Extrusion Line
The main components in an Extrusion Line can be separated into eight sections:
The process starts with the product feeding the extrusion line. Bare copper wire, aluminum wire, or multiple insulated wires cabled together to be jacketed are loaded into the payoff.
Proper tensioning is critical to prevent stretching or breaking the bare wire. To maintain optimal tension during the extrusion process the payoff can have a tension brake or be driven and controlled by a dancer accumulator.
Thermoplastic resin pellets are loaded into a gravity-fed hopper supplying the feed area of the extruder. The feed throat area is cooled to prevent bridging, which occurs when material melts in the opening and clogs the extruder barrel.
The extruder applies the coating to the wire by forcing the melted polymer to envelop the wire as it passes through a small hole or die. The extruder screw augers the pellets into the barrel where they are melted and mixed into a homogeneous liquid. As the screw revolves inside the extruder barrel, it pushes out a constant stream of liquified thermoplastic resin matched to the speed of the wire as it moves through the crosshead. The result: a thin, even layer of insulating material around the wire. The die size, extruder speed, and temperature of the material all affect the resulting wall thickness.
On Thermoplastic Wire Extruders: Vertical vs Horizontal
A horizontal extruder is typically the primary application method, however a vertical extruder may be incorporated for coextrusion capabilities. Vertical extruders are often used to supply a secondary coating into the crosshead, for example applying a color coating, stripe, or alternative exterior coating material (usually for improved heat or moisture resistance.)
One major benefit of vertical coextrusion is that color changes are easier to facilitate. For OEMs producing a range of color-coded materials, co-extruding a white PVC coating and a cosmetic layer via vertical extruder expedites change from one color to another.
After passing through the crosshead, the molten plastic coating must be cooled quickly to form a smooth, defect-free surface that covers the wires evenly. Cooling not only solidifies the liquid plastic tube, it also causes the material to contract snugly around the wire at a consistent diameter and wall thickness.
As the coated wire emerges from the forming die, a capstan draws it through a chilled water trough. The length of the cooling trough is dictated by the amount of material and extrusion speed, and in many cases may require multiple cooling stages to solidify.
As the coated wire exits the trough it passes through an air wipe to remove any residual moisture before it proceeds to the takeup capstan. This moisture removal step is vital to prevent interference with any finishing processes.
This may be achieved with a belt wrap, caterpillar, or dual-wheel capstan, depending on the speed of the extruder and the diameter of the product. The linear speed of the puller follows the extruder speed to control the diameter of the product.
A laser diameter control system can be added to trim the speed of the capstan to more accurately control the diameter of the product.
Wire Marking & Inspection
At this stage, the wire is coated in solid thermoplastic coating and printing or marking can be applied. Inspection protocols vary and are typically dictated by industry and application. A laser micrometer can inspect the diameter and the integrity of the coating can be checked. If irregularities are detected the inspection system can adjust the capstan speed to ensure the correct diameter is achieved consistently.
The printing and inspection systems are often specified separately, and a detailed conversation with your extrusion line supplier’s engineer is critical to optimizing a smooth transition from the end of the trough to the capstan where this equipment will operate. Laser diameter, flaw, high voltage spark, and length measuring systems are all common inspection requirements for copper wire/cables.
The tension of the product must be controlled between the capstan and the Takeup. A horizontal accumulator, vertical dancer-accumulator, or catenary arm dancer can be used to do this. Sometimes a torque control can be integrated into the takeup.
Regulating the tension is critical at this stage. Tension control keeps the coated wire or cables taught as they roll onto the reel without causing damage to the product. Excessive tension can break or stretch the wire, which will impact the integrity and diameter of the coating.
In the final production stage, the takeup winds the coated wire onto the reel. Takeups are supplied to match the production speed and diameter requirements, however the reel size and speed will be key in determining the appropriate coiling machine setup for your application.
A simple shaft type machine can be used for limited speed or laboratory applications, while pintle machines handle larger heavier reels running at higher speeds. Two machines can be set up next to each other with an accumulator of sufficient capacity to switch from a full to empty reel. Dual parallel-axis takeups can be supplied to automatically cut and switch from reel to reel at speeds up to 3,500 feet per minute. In industrial cable production scenarios, walk-through takeups up to 120 inches in diameter can be used for large diameter cables.
Expert Engineering Support To Optimize Your Wire Extrusion Line
There are a lot of components to wire extrusion lines, but the most critical part is how well your supplier’s engineers can understand and help you achieve your objectives. At TEC, we apply over 40 years of industry expertise to provide world-class wire and cable extrusion line engineering, fabrication, and installation across the country and around the globe. This includes designing custom wire coating systems, innovative tension control solutions, automated control and inspection – just about anything you can imagine!
Whether you’re looking for help optimizing an existing wire or cable production setup or need engineering support from start to finish on new projects, we’re ready to work with you. Talk with our engineers today to start designing a custom thermoplastic wire or cable extrusion line tailored to your unique needs.