In-Mold Assembly Eliminates Secondary Operations

Specialized tools perform snap-fit and material bonding as part of a continuous in-machine process

Doug Smock, Contributing Editor -- Design News, September 18, 2008

Increasingly, injection molding is being used as an assembly system in which components are combined without use of adhesives or other fastening methods.

Most often, this concept is referred to as in-mold assembly in which more than one injection barrel is used to form two or more materials in an injection mold that rotates or indexes to form different parts of a component. It's an assembly operation because it eliminates a secondary operation of welding, or adhesive gluing, or some other fastening method.

One example is a mold called the twin cube, which is a mold comprising two rotating cubes between its two mold halves. They function like two different molds. Different materials can be injected into each cube, and different processes take place in each cube, eliminating the need for several assembly operations that would normally occur following the molding cycle.

"Where previously two machines, two molds and assembly facilities were required, now one twin cube machine equipped with new, patented twin cube stack turning mold does the job," says CEO Rainer Armbruster of Foboha, a German mold manufacturing company that is now part of Advaltech.

There are now 12 different twin-cube systems operating, including one with three injection machines. "We have a couple of applications that are round flip-top disc-top closures in the US, and we also have a much larger oval disc top closure system," says David Nolan, a US-based sales engineer for Foboha. "We are running an automotive speedometer needle-and hub assembly in Europe.

The disc-top closures are molded in two colors and are used for body care product packaging like shampoo, shower gel, or body lotions. Assembly operations in the process can include placement of inserts, labels, or decorative foils, as well as inspection or part removal without any increase to the overall cycle time.

Snapped Together

Two components are snapped together automatically in the middle parting line between the two twin cubes. In-mold assembly is achieved by a precise core pull movement-not by the closing movement of the machine, as it is sometimes done. The exact positioning of the parts is done by the mold. This step eliminates a secondary assembly operation.

Each cube has 4 x 48 cavities. In one of the cubes the body is molded, and in the other the cover of the polypropylene closure is molded.

The substrate material is produced in the first parting line and the second component is injected after a 180 degree rotation of the center plate on a vertical axis. As the second material is being injected, the substrate for the next shot takes place.. In addition to incorporating assembly steps, the turning mold technology provides double the cavitation, and double the productivity, with one molding machine.  Viewed another way, the twin cube approach provides the same production quantities with half the clamping force. Cost of the twin cube tooling  is up to $1 million.

The next step will be a four-sided cube mold with 90 degree rotations.

 "We see tremendous potential in the packaging industry for high volume production of ‘convenience packaging' articles," says Jörg Dassow, head of applications engineering at Ferromatik Milacron, a machinery OEM that works on in-mold assembly projects. "Consider the push-pull closures which are replacing conventional screw-top caps for sports drinks, or the many types of closure mechanisms used for body care or cleaning products." To reinforce the point, Ferromatik Milacron, owned by Milacron of Cincinnati, OH, established an Applications & Systems Business Unit to develop more business that moves assembly operations into the molding process.

Several other mold builders, such as MGS Mfg. Group, are teaming with machinery builders, such as Engel, to also develop systems that can dramatically cut costs and improve component part-to-part repeatability. The big current buzz among machinery OEMs at events such as Fakuma in Germany is about energy savings because of the rapid rise in hydrocarbon prices this year. But the long-term technology that will really impact productivity undoubtedly is this new emphasis on in-mold assembly.

At the Fakuma exhibition in German this month, Arburg  demonstrated electric machines with a high-speed in-mold labelling (IML) application.  IML eliminates the requirement to adhesively adhere packaging labels in a secondary operation. Six yogurt pots are produced-with labels- in an overall cycle time of four seconds. In order to meet the requirements, the Arburg machine is equipped with integrated hot runner control, a high-performance plasticizing cylinder and a mold featuring a pneumatic needle shut-off system. The complete IML automation equipment - feed, separation and placement of the labels, as well as removal and cavity-specific stacking of the pots - are provided by Waldorf Technik.

Metal Matrix Composites

A different approach to an assembly technology is a metal matrix composite developed by CPS Technologies of Norton, MA, in which a  silicon carbide powder is molded as a net-shape porous component that is subsequently infiltrated with aluminum for use in packaging and electronic thermal management applications.

The aluminum-silicon carbide (AlSiC) assembly competes against traditional thermal management hermetic packaging materials, such as copper paired with molybdenum (CuMo) and  copper/ tungsten (CuW).  Components made from silicon carbide infused with aluminum have a significantly lower density, and are 1/5 to 1/6 lighter in weight than CuMo and CuW, respectively, yet have similar thermal conductivity and expansion coefficient (TCE).  AlSiC is also lower in cost.

Another cost element of CuMo and CuW is the need to machine all but the simplest shapes.  AlSiC designs can be cast to the final shape, requiring no secondary machining - a significant cost improvement.  The net-shape casting capability also allows the direct creation of functional design features, such as pockets for circulators as well as cavities or pedestals for the die. 

Mark A. Occhionero, vice president of marketing and technical sales at CPS, likens the final structure to a molded Jell-O with fruit.  "It's essentially like the grapes being the silicon carbide and the Jell-O being the aluminum. In typical applications the silicon carbide is at 63 percent (by volume) and the aluminum is at 37%."

CPS competes against Japanese OEMs such as Sumitomo and Denka for three types of applications: hermetic packaging, based plates for high power silicon switching devices, and application specific integrated circuits.

"What's important about our materials is that in comparison to competitive materials, they have very good controlled thermal expansion, they have a reasonable thermal conductivity, but most importantly they are lightweight," says Occhionero.  They have the weight of aluminum and the strength and stiffness of steel."