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September 1, 2001

8 Min Read
Concept to cell in less than 15 months

Anyone who has ever handled—make that wrestled with—a traditional liquid propane gas (LPG) container has wished for a better way. The steel container is heavy, particularly when you are transporting it to a cabin in the woods, or installing it on a boat or camper. 

Raufoss Composites AS of Norway saw that problem as an opportunity. The company knew the market and the specific drawbacks of the present solution, which is generally disliked. Raufoss's experience in nonmetallic containment units for various types of vehicles provided the know-how to design and produce a better product. Therefore, in a 50/50 partnership with Gas Group Statoil, Norway's state-owned energy company, Raufoss set about constructing a production facility to bring the new container to market. 

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Figure 1. The completely assembled pressure container (right) made by Raufoss for 10 kg of propane consists of a blowmolded and fiber-wrapped inner container and a three-part injection molded HDPE outer case. This case consists of a gas container carrier, the container top, and the top ring (all shown at left), which are welded together after the inner container has been inserted.



Market Intelligence 
Raufoss read the market well, and the new LPG container, called Complet (Figure 1), quickly became a success. The container is sold through Statoil's outlets and currently is finding markets in other countries and continents. Manufacturing started in March 2000 with a year's production goal of 100,000 containers. A recent count of units sold exceeds 250,000 and is going strong. 

The container's key advantages are its light weight and transparency, complemented by a sleek design and complete attention to safety. An empty Complet container weighs 6.5 kg, half the weight of steel, and holds the same 10 kg of LPG. Besides helping the user avoid back problems, the transparent container means he or she can actually see how much propane is remaining. 

Apart from the obligatory standardized valve, Raufoss's Complet contains no metal at all. The inner cylinder is a blowmolded 23.6-liter shell that holds 10 kg of LPG. The shell is wrapped with resin-soaked continuous glass fibers using a proprietary, patented process. Oven curing yields high mechanical strength, yet leaves enough transparency to check the contents from the outside. IMM cannot show you this proprietary technology, but the injection molding and assembly technology is world class. 

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Figure 2. Three injection molding machines are linked by robots, handling systems, and joining and assembly stations—all automated. The two machines in the foreground feed the welding station on the left with the two parts of the container top. The large machine in the background produces the outer shell of the carrier.

A to Z Workcell 
Raufoss wanted a state-of-the-art production facility to make a state-of-the-art product, and it has one. Adjacent to the cylinder production area and connected by robotic handling stands a fully automated three-machine injection molding cell with a welding station that feeds a six-axis articulated robot producing the final assembly (Figure 2). 

Three parts are molded in HDPE. First is the latticed external jacket that completely surrounds and protects the gas container. Second is the top of the container, which protects the valve and serves as a handle, and the third part is a ring that attaches to the top (see sidebar on workcell below). 

With only 15 months to assemble the system, Raufoss appointed Demag Ergotech to perform overall system engineering, including responsibility for mold construction, automation, material supply, and cooling water supply. 

Demag quickly brought together a team of partner companies with specific expertise. Klaus Hänsch Konstruktionsbüro GmbH handled design engineering. Molds were entrusted to Siebenwurst Modell und Formenbau. Material feeding was by Werner Koch Maschinentechnik, and GWK (Gesellschaft Wärme Kältetechnik) engineered the cooling water supply and controls. Demag coordinated the 3-D development of part geometry using Catia software. STL models were made for functional testing. Demag also scheduled and monitored key dates for all suppliers and provided monthly status reports to Raufoss, which held ultimate and controlling authority. 

The three molds were constructed in parallel and optimized using CAE simulations and Moldflow analysis software. FEM analysis of the main outer jacket revealed a potential problem that was resolved by redesigning for asymmetrical filling. 

Overlapping the mold design was system planning. This included alternative machine and production layouts, appropriate automation, and robot selection. The plans included linking the three molding machines, assembly, mold support, material supply, and cooling water and temperature control systems. Parts were molded by Demag using prototype molds to help select the best welding process and optimize the parts for that process. Plate welding was chosen. 

Koch's material supply system consists of outdoor silos; a dryer; blending, mixing, and conveying equipment; conveyance tubing for the entire site; a sprue regrinder; and regrind return. One microprocessor controls the entire system. 

The cooling water system, besides complete piping infrastructure, includes a cooling container with double pump sets for cooling molds and machine hydraulics through separate circuits. The GWK-designed system provides individually adapted multicircuit temperature controls for each machine. 

Machine Range 
The outer shell is molded on an 800-ton Ergotech machine equipped with a Demag DR 432 CNC robot. The mold from Siebenwurst is impressive to see in action. It weighs 10 tons and incorporates a complex collapsible core, outer valves, and a Mold-Masters hot runner system. The gripper system takes the carrier from the mold to a manipulating station, stands it vertically, and puts it on a conveyor. 

The 420- and 80-ton Ergotechs are linked in a subcell to make the two-part top of the case. A Demag DR 433 CNC robot removes the main top part of the tank from a hot runner mold in the 420-ton press. The top ring is removed from the smaller machine by a DR 420 CNC robot. Both parts arrive at the welding station simultaneously where the ring is placed in a heating element and welded to the top part. Any flash is milled off. 

The inner gas container, the carrier shells, and the tops continuously move on conveyors and ultimately converge at the final assembly cell. There, a six-axis ABB 4400 robot first inserts the inner shell into the gas container carrier. An automatic tolerance adjustment system compensates for height variations of the inner shell before the robot mounts a cover and welds it to the carrier. The finished Complet containers are aligned, stacked, and palletized to go to Statoil for filling. 

Safety First 
Traditional LPG containers are generally designed to withstand normal operating pressure of 7 bar and exceptional pressure to 20 bar. For security, Raufoss tests every inner container to 30 bar. Every 500th container takes a burst test at 100 to 120 bar. Norwegian government inspectors, in the plant several times a week, check all results and participate in testing while they are there. Each container is individually and indelibly marked. Process data and parameters are recorded online and documented. Should there be a problem later, all process parameters for that specific unit can be traced. 

The full molding cell was assembled and tested at Demag Ergotech's plant in Schwaig, Germany before it was shipped to Norway and installed in the new plant. Raufoss personnel were trained in Schwaig on the Ergocontrol system and general machine operation. A busy 15 months after winning the contract, Demag handed over the system. 

With the molding cell linked to all upstream and downstream operations, the nominal operating cycle time is 58 seconds. Running seven days a week, the cell can produce up to 1488 containers a day. Raufoss invested 15 million euros in this new plant, a good portion of which went toward machinery, auxiliaries, and materials handling systems. There are 55 employees and Raufoss forecasts profitability for 2001, its first full year of production. 

The workcell at work 

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The outer shell is removed from the 800-ton Ergotech machine by a DR 432 CNC robot.

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The container top is removed from the 420-ton Ergotech by a DR 433 CNC robot.

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At the top's welding station, the left robot positions the ring and the right robot handles the main part.



Advantages vs. steel 

• Half the container weight.
• Transparent for fuel viewing.
• Excellent behavior in fire.
• Corrosion resistant.
• Attractive design.
• Integrated casing protects vessel and valve.
• Improved grip.
• Stackable.
• Easy steam cleaning.
• Tough UV-resistant surface.
• Good evaporation capacity.
• Overall safety same as or higher than steel. 



Contact information
Raufoss Composites AS
Raufoss, Norway
Hans-Magne Gardsrud
+47 (611) 52201
www.raufoss-composites.com
[email protected]

Demag Ergotech
Nürnberg, Germany
Gerd Liebig
+49 (911) 5061 232
www.demag-ergotech.com

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