PHOTO: A recent installation of dosing equipment by Sysmetric at an international plastics company
IN THE beginning, the extrusion process was fairly straightforward: a single layer, a simple mixture of resin and perhaps a few ingredients to add slip or colour. Today, the science of extrusion is complex. The market demands more technical performance from film and pipes, which means improved control on both single and multiple layers. There is little room for guess work. Accuracy, precision, throughput and the most efficient use of raw materials is essential.
Dosing and blending interacts with the extrusion process by conveying the materials, weighing the various materials according to a recipe, mixing the materials and controlling the extruder yield to achieve a specific result. A superior extrusion process vastly improves downstream performance. Technology and methods for performing these steps vary greatly.
The doser
The most accurate dosing systems convey materials via vibratory feeders and should feature a pneumatic channel which conveys the main material to the scale by way of a metering valve. Vibration feeders excel in their ability to convey almost all types of raw materials, including most kinds of regrind, masterbatches and re-pelletized materials. They have an extremely wide dosing range of 0.5-100%. The vibration feeders create uniform and continuous material flow and are very easy to empty and clean. They do not contain any moving parts and therefore do not 'cut' or 'disturb' pellet flow.
The fact that the raw materials flow through a simple channel means that materials of varying size and materials with angel hair can be accurately and reliably conveyed. The vibration feeders can be easily and quickly emptied and cleaned without the need for the system to be stopped. No maintenance is required.
Weighing unit
The weighing unit or scale is extremely important in accurate dosing and blending of resin and other materials used in the extrusion process. Look for a standard single load cell whose unique installation protects it from mechanical injury and from foreign objects restricting its movement. The weighing unit should require only minimal maintenance over the years.
The mixer
The mixing process provides the opportunity to ensure smooth material flow to enhance extrusion efficiency and product consistency.
Vertical augers feature a mixing chamber designed to contain and mix weighing batches. The mixing chamber's interior ensures smooth material flow throughout the entire chamber without any 'blind corners'. During upward mixing, angel hair has no effect. The auger screw mixes the entire contents of the chamber. Look for a mixing chamber with convenient service doors that provide fast and easy access.
Pneumatic components
It is important to understand how the system manufacturer sources components so you know what you are in for when it comes time to replace common wear parts.
Some companies use specialized components that can only be purchased from the manufacturer. Find a manufacturer that uses only standard pneumatic components (ISO standard pistons and valves). You want parts that can be purchased via many worldwide dealers, so as to minimize downtime and save both time and money. Easily-sourced components means the customer can stock fewer spare parts.
Control components
Proprietary control equipment that is private and protected is a popular choice for some manufacturers. The problem for the customer is that there is no access to any parts of the control system. All service is provided by the manufacturer. The control equipment is manufactured using developing technologies, so every few years the control equipment becomes obsolete with no available spare parts. This means that the customer has to invest a large sum of money in replacing the system’s central components.
It is to the customer’s advantage to own equipment with standard control components that allow complete access to the PLC program, including programme backup. This allows the customer to replace all system parts, including critical system parts, which saves a great deal of money on both service and replacement parts and leads to a significant shortening of system stoppage time. Look for an industrial, easy-to-source PLC where the manufacturer provides total support for the entire control platform throughout the years as well as ensuring reverse compatibility.
The use of industrial PLCs allows flexibility in creating interfaces with additional components on the line. The data base should be openly available for any external system with no extra charge.
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