Extrusion Control Options for Saving Costs & Improving Accuracy

Closer to target coating thickness, savings in polymer materials, faster startups, quicker material changeovers and more.
Increased product uniformity and savings in raw materials are two of the chief benefits that wire and cable manufacturers obtain by installing an extrusion control system. Many variables affect the rate at which extruders consume compound including the source of the material, storage conditions, bulk density, particle geometry, die and heater conditions, screen packing, fluctuations in electrical power and line speed. An extrusion control system provides its benefits by compensating for the process variations caused by factors such as these.

A Look at the Hardware
The heart of the LineMaster system is a loss-in-weight (LIW) hopper, which is mounted on the throat of the extruder. The hopper is suspended on a pair of load cells that continually monitor the weight of the material in the hopper. Once each second when in automatic mode, the LineMaster controller uses this data to calculate the amount of material being metered into the extruder.

Another core component of the LineMaster system is a drive control package that provides the interface between the LineMaster controller and the extruder drive, the takeaway drive or both. For wire and cable installations where line speed needs to be determined, Maguire supplies a digital encoder that is mounted on the take-away rollers downstream of the extruder. It monitors line speed so that the weight per length of product can be calculated.

Methods of Extrusion Control
In wire and cable installations, the LineMaster system can be deployed to provide two types of extrusion control.

Weight-per-length control: monolayer. Most frequently, this control is exerted via the speed of the extruder drive. Using throughput rate data from the LIW hopper and line speed data from the digital encoder, the system automatically calculates the actual product yield, compares it with the target and makes any necessary adjustment by regulating the extruder drive. Alternatively, when there are no secondary processes that would be compromised by changes in line speed, it is possible to exert control via the speed of the take-off drive, using throughput data from the LIW hopper and line speed data from the digital encoder.

In reel or bobbin changeovers, the LineMaster system immediately reacts to the resulting changes in line speed by slowing down the extruder in a controlled manner. This makes it possible to maintain gpm or gpf even with changes in haul-off speed.

Weight-per-length control: multi-layer. The goal here is to maintain the target layer ratio between the insulation and sheathing materials used in a wire construction. This is achieved by mounting separate LIW hoppers on each of the extruders and slaving their controllers to Maguire's G2™ networking software, installed on a computerized operator interface. The operator enters the total gpm or gpf target for the combined coextrusion and sets a ratio of that target for each extruder-for example, 80:20. As a result, in the finished product the weight of each compound per unit of length is constant.

These are generalized descriptions of how LineMaster technology can be deployed in wire and cable operations. In actual practice, no two installations have been alike. The relative simplicity and versatility of the technology accommodates a range of customized deployments.

Older types of extrusion control, particularly yield-control systems, involve elaborate and often cumbersome equipment setups and cost more than the relatively simple LineMaster system. The LineMaster system can be set up in a short time and does not require specially trained operators. The material savings resulting from the accuracy of the system and the reduction in scrap can often repay the original investment in well under a year.