TECHNICAL ARTICLE:

As a world leader in the delivery of equipment for the pipe and film industries, Maillfer is strengthening its technology partnership with Dreistern in supplying global solutions in the manufacturing of metallic composite pipes.

Dr Theo Krückels founded Dreistern-Werk Mashinenbau GmbH & Co, located in Schopfheim, Germany, in 1949. Starting with the delivery of groove and bending machines, its successful focus on high quality led the company to rapid growth. Dreistern has since achieved international status and has 250 employees with a yearly turnover of DM 60 million. Over the years, it has built up a solid experience in turnkey roll-forming lines and tube mills for every kind of profile and tube form.

With its first pipe mill in 1962, Dreistern has gained a comprehensive knowledge in the field of forming and welding longitudinal welded pipes. Maillefer, formally known as Nextrom, began the collaboration in 1983 when equipment was delivered for the first revolutionary line making polythene aluminium composite pipe. This product is now better known under the MEPLA™ trade name.

Growth in Composite Pipe Metallic composite pipe is a proven product that has gained wide acceptance and found its place in the market. Its properties combine the advantages of metal with the longevity of plastic. The ease of handling with the appropriate fitting system, has convinced fitters that composite pipe is a replacement to copper. In Europe, the installed base and market for metal composite pipes is in constant growth. The main applications are sanity, radiator heating and floor heating (see figure 1). Figure 1

The standards (ASTM & others) principally define the maximum or minimum wall thickness (PE/Al) and the admissible internal pressure for a desired service water temperature. But it does not fix those values. Therefore, the pipe manufacturers have a wide window to play with in determining the necessary thickness for the desired application. This freedom becomes a real challenge for the machine design, as typical wall dimensions (PE/Al) can rarely be repeated from case to case.

A real competence is required to design and manufacture the right extrusion, calibration and forming tools to rapidly meet the high requirement of the finished product while maintaining high productivity. The line uptime depends very much on the frequency of tool changeovers. Changing a production line to a different pipe size takes up to several hours. It is obvious that lines used for a limited number of pipes achieve higher productivity than lines used for a wide range of pipe sizes.

Maillefer and Dreistern bring together the needed set of skills and competencies for the customer's benefit in engineering different pipe constructions within a short time. Excellence and experience in extrusion, forming and welding technologies allow production speeds up to 40m/min. and even more.

The composite pipe manufacturing process is made in one single step and all operations like first layer extrusion, bonding, aluminium tape forming, welding, jacket bonding, jacket extrusion and final product conditioning are made in tandem. The combination of process steps requires shop floor area but special layout solutions are available that reduce the total line length.

Inner Tube Extrusion The type of material used for the inner layer depends of the application and can include compounds like HDPE, MDPE (Dowlex, Solvay and others), PEX-b (Sioplas like Pandanaplast, Monosil) and PEX-c (by irradiation). Maillefer is recognised for its long experience is crosslinking technologies using Sioplas and Monosil PEX-b processes. Because each material has its unique properties, Maillefer offers the optimum solution for crosslinked PE extrusion. The extrusion process stability plays a key role to ensure the bonding quality of the first layer with the aluminium one. Maillfer uses a spiral grooved extruder that works like a volumetric pump (see figure 2). High pressure is generated at the cylinder entrance facilitating the melting process and ensures a stable output independently of the pressure generated by the extrusion tooling. Other additional devices like volumetric or gravimetric dosing, or gear pumps, can be added when necessary.

Special care must be given to guarantee that during to guarantee that during long runs the pipe wall and diameter remain within tight tolerances. Besides requiring a stable extruder output, vacuum control with regulation loops as well as optimal calibrator design are the key elements to assure the pipe dimension stability.

Composite pipes like they are produced on Maillefer/Dreistern lines are metal pipes under the brand name "composite pipes" on the market, which provide a diffusion barrier rather than the full characteristics of metal pipes. Compared to plastic pipes the production of composition pipes requires skills in metal forming and welding technologies. Dreistern supplies pipe mills for a large variety of applications and the most important pipe welding methods, which are Tungsten-Inert-Gas (TIG), Laser and High Frequency welding.

Today's pipe producers are facing two major challenges - securing superior product quality and maintaining high productivity of the capital investment. Long term product liability and a highly competitive market in the building sector are the reasons for that. The speed, the available uptime and the scrap rate of the line mainly influence productivity. The actual speed of production lines in operation today is in the range of 20 - 40 m/min. A higher speed of 50 m/min and more seems to be a possible in the near future.

It is a very costly experience for pipe producers to discover pipe defects not detected by their quality control system. The only sure option is to install a reliable process. A crucial issue for the weld quality is the condition of the aluminium strip edges right before welding. There are many parameters that influence weld quality. First of all the edges have to be parallel without any horizontal mismatch. The weld angle has to be small enough to keep the spring-back effect after the weld point low. Any burr, wrinkles or waves in the edges must be avoided. They have to be free of oxides and clean without any impurities. These demands have important consequences on the equipment. All along, from the payoff to the weld box, the tension in the edges of the aluminium strip must be kept below the plastic deformation level. The strip has to be always suitably guided to avoid any waves and wrinkles. The strip accumulator and the edge-trimming device can especially be a source of problems when these principles are ignored.

Among all these important issues, the forming process of the strip into the round pipe has a dominant role. In order to form the strip it is necessary to achieve a plastic deformation of the aluminium. On the other hand any deformation of the edges must be avoided. This means the tension in the edges must remain below the yield strength of the material at any point of the forming process. Only a suitable forming method can fully secure this condition. The sufficient number of forming stands, suitable distance of the stands, right design of the roll tools are some of the parameters which the equipment supplier has to look at. Once the formed pipe reaches the weld box, the equipment has to maintain proper conditions for the pipe. First of all, the edges must be precisely kept in constant position. This task can only be reached when the strip is already suitably guided in the whole forming section (see figure 3).

After the welding process the pipe runs through the calibrating section. The task of calibrating is to bring the metal pipe into a close and even contact to the adhesive layer. Some equipment suppliers use the calibrating for eliminating forming defects. This may cause serious quality problems. If the forming equipment does not ensure a perfect round shape of the pipe the adhesive layer will be pushed away locally during calibrating. This may cause collapsing of the inner plastic pipe during later use. Figure 3: Forming Section

Ultrasonic welding for composite pipes was introduced at the end of the seventies. A weld is made along the longitudinal seam coming from the overlap geometry of the relatively thin aluminium sheet (0.15-0.20 mm thick). The type of pipe made is fine for floor heating applications, where neither mechanical nor thermal stresses are important.

Ultrasonic welding belongs to the group of "coldwelding"- or "press welding" processes, Its main applications are in the very thin sheet range (such as 0.05mm), not so much in the range of 0.40-0.60mmm and not at all beyond. Unfortunately, this technology is somehow difficult to control. On has to first realise the weld seam before the inner plastic layer can be extruder into the metallic tube. Also the weld security factor is rather low.

Since then, the original "one application pipe" of the small diameter range developed into a family of pies covering a range of diameters. Radiator heating and plumbing complemented floor heating. Lately gas distribution systems and others have come into consideration. With this evolution has also come the arrival of new welding techniques. Instead of the ultrasonic weld seam, butt weld seam configurations are preferred. Two processes are specifically suitable for butt welding - TIG and laser. Both technologies are used for sheet thickness under 0.45mm. However, laser technology faces its thickness limitation when exceeding 0.45mm, while TIG can go well beyond.

Maillefer composite standard lines are equipped with a computerised line control that pilots the different regulation loops of control of vacuum, layer diameter and thickness. Product recipe management, production scheduling and product length selection are standard integrated functions. All operator commands are communicated through two touch screen interfaces located by the inner and outer layer extruders. Options like statistical Process Control, trends, remote trouble-shooting and integrated shop floor extension are available.

Pipe Condition   Composite pipe are packaged according to diameter sizes - in coils for DN 32mm or less and straight bars for DN above 32mm. The quality requirement of coils imposes further conditioning operations (i.e. carton box). Maillefer fully automatic coilers (up to 1300mm diameter) assure structural coils dimensions with up to 6 straps per coil and conditioning quality for pipe sizes up to 32mm (see figure 4). The line can also cut bars at selected lengths (typically for sizes over 32mm).

Figure 4: Fully Automatic Coiler 1300mm

Maillefer and Dreistern are global companies offering solutions world-wide to the composite pipe industry. The level of professionalism brought together with the synergy of metal forming and extrusion skills means one-stop shopping. Maillefer and Dreistern understand the need to act quickly and efficiently when engineering pipe constructions not clearly defined by the standards. They answer with their combined solutions and know-how. Especially for starters in the business, consulting services help raise the level of expertise in manufacturing a quality product.

Dreistern-Werk, PO Box 1140, D-79641 Schopfheim, Germany
Fax: +49 7622 391205
E-mail: heinrich.weber@dreisteru.com
Website: www.dreistern.com