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March 1, 1999

11 Min Read
Large, Complex Technical Moulding

It's a frequently debated issue: should a custom moulder specialize, whether it be in a certain process, market, or material, or is a generalist more likely to succeed? IMI recently visited Coko Kunststoffwerk in Bad Salzuflen, Germany, to see a company that has most definitely specialized in a type of injection moulded product, and in doing so, has become a solid success.

Coko decided to specialize in large parts: housings, panels, cabinets, and other shapes where the shot is measured in kilograms, the flow paths are long, and the moulds weigh multiple tons. When it comes to technology, however, Coko is less of a specialist. Because of the large variety among what we can generally call large parts, Coko has had to master a number of injection moulding technologies. It uses convention-al injection moulding, structural foam, two-component moulding including foam-core sandwich moulding, and most recently, gas-assisted injection moulding. The company builds its own moulds in a tool shop designed around producing large and what is increasingly becoming very complicated tooling.

Coko is not a newcomer. It began 60 years ago in the plastics business as an extruder. The company grew, expanded into injection moulding, opened the tool shop, and gradually began developing the expertise in larger parts. Injection moulding is the company's only plastics processing focus now. Thomas Schröder, who is Coko's manager of customer support, started our tour in the mouldmaking shop. And he did that very deliberately.

Coko strongly believes in being a single source for its clients. The company slogan, loosely translated, says "We give form to your products." Schröder says the customer can come in with an idea, a sketch, or a rough design, and Coko will take it from there. The overwhelming majority come in with solid product designs, but in any case, Coko feels that the mould is the foundation of the entire production process, so it must be perfect. They do send up to 25 percent of their mouldmaking work outside, but only the least complicated moulds, and even then only to toolmakers close-by and well-trusted. Fortunately, Bad Salzuflen is one of those not-so-uncommon islands of the plastics business in Germany, and a number of skilled toolmakers are in the neighbourhood.

Making Big Moulds

The mouldmaking operation is in a building of 1,400 sq m separate from the injection moulding operations. Two shifts are staffed by 60 craftsmen five and sometimes six days per week. The machining equipment, almost all CNC driven, includes straightforward large-piece rough-ing stations, small finishing machines, and EDM systems, and culminates in an automated machining station from Makino whose programming includes automated selection and mounting of the proper tool from among 59 in its ready rack. Some 70 percent of the work is done by erosion.

Workpieces can be as heavy as 13 tons and as large as 2,000 x 1,500 x 1,000 cm (780 x 585 x 390 inches). A large machine using a very long coring tool was acquired so cooling channels through large mould pieces could be made in a single continuous operation from one side. Eliminating the rotation of the workpiece needed to bore from two sides improved channel accuracy and reduced machining time. Noting the various years that the machines were acquired, Schröder mentioned that Coko accepts continuing investment in new machining technologies as normal. "You absolutely must be current in this business."

Mould Design

At the back of the machine shop is a door leading to a product display room packed with a wide variety of large parts. Past that is a staircase up to the "Konstruktion" department. In German that word also means design, and this mould design department is as up-to-date as the machining facilities. Coko switched to CAD/CAM systems eight years ago, and today designers are busily working at Silicon Graphics workstations and smaller PCs using both 3-D (Intergraph) and 2-D CAD software. Coko accepts CAD files in all the standard formats and says client files are supplied mostly in 2-D.

Schröder noted that for reasons of speed, 2-D is almost always employed at Coko for the shape of the product prior to actually designing the mould. The reason — 2-D files are considerably smaller than 3-D. The computer can incorporate modifications and redraw a design more quickly, so the work goes faster. 3-D, he continued, has become indispensable for today's mould designing because so many of the parts are very complicated. Thick and thin sections with little transition, lots of undercuts and rib patterns — with and without gas channels — all making for complex moulds with many core pulls, slides, and unscrewing mechanisms.

In addition to the computer hardware, Coko's design group has a number of large drafting tables holding design plots of large moulds in progress. When the designs are complicated and the moulds are big, which is most of the time at Coko, it is difficult to see the whole design and the details simultaneously on a computer monitor. Standing, or more accurately, walking in front of life-size renderings, gives Coko's designers another way to use their experience (they have built more than 7,000 moulds) to catch potential problems.

Big Injection Machines

Coko has roughly 300 working moulds and 45 injection moulding machines with clamp forces ranging from 60 to 2,000 metric tons. Most are larger than 500 tons and most, along with their robots, are from Battenfeld. Six machines between 200 and 400 tons are from Krauss-Maffei, also equipped with robots. Because Coko changes 12 to 15 of those large moulds each day, the Stäubli quick mould change system was adopted a few years ago. It reduces the average mould changing time by 80 percent.

The company's reason for using a 60-ton machine can be summed up in one word: assemblies. Coko has substantially added to its business by providing clients with complete assemblies. The small machines support that capability by making the small parts needed.

All machines are gas-assist-ready, and all but the smallest have already run gas-assisted moulds. Coko has been using gas assist for more than six years, but Schröder says product designers are still learning the options gas assist gives and the restrictions it removes. As they learn they naturally create more products that use those advantages. Because of that learning curve and the unique design and function possibilities, not to mention process cost reduction offered by gas assist, Coko sees continued growth in this area.

Coko has a central 2 Airmould gas system, consisting of a gas generation unit that extracts nitrogen from the ambient air and a gas pressure control system. It can supply gas to as many as five machines doing rod-type parts or 10 doing flat parts, and the system is expandable as demand grows. To increase flexibility now, Coko uses Battenfeld's Airmould Mono-Module. Compact and wheel-mounted, this gas pressure control unit can be rolled alongside any press in the shop and connected to the preinstalled interface in the machine's controller.

Handling the Materials Mix

More than 7,000 tons of thermoplastic material are used each year, mostly PP, PS, and PE (stored in five 30-ton-capacity silos) but there is also a high percentage of technical materials including PC, ABS, blends, nylon, PPO, and others, but no thermosets or elastomers. Everything is checked for melt flow index and dryness on arrival. The central material handling system, designed by Coko and supplied principally by Azo with some components from Motan, was installed several years ago. Material blending and feeding is gravimetrically controlled and the system, located logically between the materials storage area and the production floor, has contributed measurably to Coko's overall productivity.

Pointing to a stacked collection of full, reusable Gecon containers from GE Plastics, Schröder noted how such seemingly simple things can contribute to productivity. The pallet-mounted containers are rigid and high-capacity, meaning faster, yet more secure handling into and out of storage. When empty, the container structure collapses onto the pallet for return to the manufacturer, eliminating much of the time and money formerly spent on disposal of one-trip containers.

Customer Support Technology

Walking between two long rows of injection moulding machines, one sees solid moulded shapes, skin-over-foam sandwich moulded panels, two-colour parts, structural foam housings to be plated or painted, and gas-assisted structures that ignore many of the old rules on part shape, flow length, and wall thickness. Is that a bit baffling to a client? Absolutely not, says Schröder. It's not the client's job to select mould and moulding technology, even though the more experienced often have very clear ideas about production from the start.

Coko's first step with the customer is to look at the performance specifications for the part or assembly. The important thing here is the application of know-how. The choice of injection technologies depends on far more things than the physical dimensions of the part. How does the part function? What are the critical performance requirements? What is its competition in its market? What are the projected quantities, and the cost parameters? They search for anything to give the client an advantage.

Gas-Assist Advantage

Coko began using gas-assist technology six years ago. Having decided to enter that field, they found expertise at the Institut für Kunststoffverarbeitung (IKV), The Institute for Plastics Processing that is affiliated to RWTH, the University of Aachen, Germany. That plastics think-tank, some 500 persons strong, was already well into the subject and had a large knowledge base. Schröder, who is a University of Aachen graduate, joined Coko at about the same time. The affiliation with IKV continues as a working relationship for Coko.

Gas assist is an area where expertise acquired through hands-on experience is critical. IKV, for example, has produced a remarkable video that shows the gas-assist process as it is occurring inside the mould. It first had to make a mould with a transparent window. What one learns in the video can be summed up simply: gas does not always do what you logically think it would. Very often it does not travel symmetrically through a part, even a symmetrical part, and the video clearly shows this. Coko was also involved in the production of the video.

Schröder says that Coko now almost always injects gas directly into the mould rather than through the nozzle. If gas flow is not symmetrical and you send it through the nozzle, the gate will most likely have to be off center. The mould would have to be mounted off center on the platen, bringing you face-to-face with loss of platen parallelism. Coko frequently uses multiple gas injection ports to precisely control distribution of the gas pressure, taking advantage of hot runner technology to have custom-positioned gates. Also, the timing of the gas into those ports is often sequenced. There is, however, a way to decide where to position the gas ports and gates and how to sequence and pressurize them

Prototyping and Simulation

Most of us agree that prototyping and mould simulation is good policy before cutting metal on any mould. When a mould is going to be as large as Coko's normally are, with the correspondingly large investment, Coko feels there can't be too much prototyping and simulation. If the client doesn't provide a product prototype, Coko turns to one of several nearby companies with stereolithography (STL) facilities. Prototype moulds can be made internally in a shop that regularly produces aluminum production moulds for its structural foam parts.

When it comes to moulding simulation, Coko again uses its affiliation with IKV to advantage. IKV has developed and commercialized its own mould design software package, Cadmould-3D, which includes extensive mould simulation. IKV and Coko create as many simulations as they need to be totally sure of the design. The payoff from the simulation is a lot more than positioning gas ports and nozzles. Using Cadmould, IKV provides Coko with a full set of baseline processing parameters for the machine: temperatures for the mould and the material, injection pressure, holding temperature, pressure and time, and more.

As much as Coko benefits from this extensive preparation, its clients benefit more. The flat panel for a Canon copier shown above proves the point. Formerly, the part had been made in Japan of painted structural foam. There were sink marks opposite the mounting bosses and other thick sections, and the overall quality level was not what Canon wanted. So Coko changed it to a solid part with gas assist. The painting cost was immediately eliminated. Average thickness was reduced by half. So was the overall weight, and along with it, the material cost. The cycle time went from more than three minutes in foam to less than 90 seconds for the part shown. And those quality problems are history.

These "more than just plain moulding" projects don't appear by chance. To secure them, Coko has built an extensive assembly operation that includes inserts and outserts, ultrasonic and vibration welding, screen and pad printing, hot stamping, and a complete painting facility that includes EMI shielding. What doesn't take place in house, such as metallizing, can be done by someone in the neighbourhood. Coko is offering its customers a single point of responsibility for large, complex parts and supplying the know-how to give the customer significant benefits. The formula is working.

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