Rotating stack molds give designers a variety of options for not only molding multicomponent parts but also doing secondary assembly operations inside the injection molding machine.
''What will it cost to get that plastic part out of the plant?" This was once a question relegated to manufacturing engineers on the other side of the design-staff wall. But that's no longer the case as OEMs continue to downsize.
With diminished staffs there's increasing pressure on designers to not only devise the latest most-innovative concepts but to also determine which manufacturing process will produce the best parts most quickly and at the lowest possible cost. This often results in the outsourcing of production to countries with cheap labor when plastic assemblies need multiple secondary operations.
Recent advances to injection molding from the Danish company Gram Technology is one option that may help keep manufacturing of complex plastic parts on U.S. soil. The process is a unique stack-molding concept called Spin Stack Technology that delivers faster cycle times, less part handling, increased productivity, and better part quality for complex plastic parts. The system combines multicomponent injection-molding technologies ( over-molding, multimaterial, and multishot) with several assembly steps or secondary operations.
Conventional stack molds are well known for their ability to outpace their single-mold-plate counterparts when molding small, shallow-draw packaging and other disposable items. They can double/quadruple production rates per injection cycle.
That's because conventional stackmolds have more than one mold opening (i.e., more than one parting line). Compared to single mold plate designs where two mold halves come together to form parts, stack molds have a center core that sandwiches between the two mold halves during molding. This center-core forms parts on either side as it mates to the two mold halves and is basically analogous to having two molds running simultaneously in the injection-molding machine.
Gram's Spin Stack Technology builds upon the throughput of stack molding by linking the center core (or cores) with a 360° turning device. "Turning a part of the mold is the most widely used method of transporting molded preforms in multiple component molds," says Hermann Plank of The Tech Group Inc., Scottsdale, Ariz., a licensee of Gram's Spin Stack Technology. "Until now, turning exclusively used vertical turntables which were mounted on the moving machine platen and which rotated around the horizontal axis."
However, a machine turning a mold around the horizontal axis needs enough turning space to accommodate the diagonals of the mold, says Plank. For large parts, this meant a large space for a machine clamp as well as high clamping force for injection of preform and finished parts in one cycle.
Rotating the cube-shaped center core of the Spin Stack system at 90° intervals around a vertical axis lets the injectionmolding machine produce more parts per injection cycle. Moreover, secondary operations can take place at the nonmolding positions during each injection cycle.
The cube-shaped core creates four "workstations" where molding, cooling, assembly, inspection, or other secondary-operations such as painting or weldingcan run simultaneously with each injection cycle. In-mold assembly coupled with multicomponent molding technologies can drastically reduce part cost, says Plank, and may be a key driver that lets U.S. injection molders compete head to head with their offshore counterparts. The key he says is for designers to "think outside the mold."
SPIN STACK BASICS
"Some of the most time consuming operations for injection-molded parts are secondary operations," says Plank. "The worst thing injection molders do is to eject parts from the mold into a box where they can shrink, warp, or deform as they cool. The parts then go to postmold workstations where they are often reregistered onto assembly fixtures for subsequent secondary operations."
This process is often a time-consuming proposition that can reduce assembly quality. Keeping the molded parts on the mold as in Spin Stack systems is the best way to register a part, reduce part handling and inventory headaches, boost part quality, and minimize scrap, says Plank.
According to Plank, a concept front cover for a mobile phone or medical device illustrates how the Spin Stack Technology could drastically reduce assembly time. The phone's front housing would be molded using the first barrel or injection unit in a double-barrel injectionmolding machine.
The cycle starts by molding the housing in station one where the first core face mates with the movable mold half. After rotating 90°, the housings rest in the second station. They are perfectly aligned in the cavity to take on a lens or window through robot insertion, while a second set of housings are molded at station one.
The Stack rotates another 90° placing at the third station the first set of housings that now contain the lens or window. Here the phone key buttons and perimeter seals are overmolded onto the housings.
During this cycle, a third set of housings mold at station one and the second set gets their lenses installed at station two. The cycle continues as the Stack rotates another 90° bringing the front housing assemblies to the fourth position. Here they eject or are taken away with a robot before the mold closes. The molding sequences repeat at stations one and three, respectively, and lenses install at station two.
The rotation sequence completes as a final 90° rotation brings the first cube face back to station one and the process repeats. According to Plank, the concept of molding and assembling a mobile phone using a Spin Stack system could boost output nearly threefold. Total footprint of the molding and assembly area would drop almost 70%, energy consumption would decline 60%, and assembly equipment as well as the number of operators would be reduced.
Another example illustrates how the system can make drastically more parts per injection cycle. A Spin Stack mold with eight cores per column or side for a medical connector shows tremendous potential. Previous connectors were assembled from separately molded parts (plastic port, rubber septum, and retaining ring for septum glued together in secondary operations). With the Spin Stack in-mold assembly and overmolding, the complete device comes out of the injection-molding machine ready for packaging.
The unit molds the internal part of the port at the first mold parting line. After the mold opens and the stack rotates 90° the TPE septum is over molded onto the components as more internal parts mold at station one.
A third 90° rotation encapsulates the device with the same resin as was used in the first shot. A special runner system is used so that the first and third injections can come from the same barrel.
The TPE is shot from the injectionmolding machine's second barrel. A small area of the septum is left uncovered so that a slit can be cut into it after the stack rotates for a fourth time and prior to the assemblies being removed from the mold cores. The slit in the TPE is self-sealing and lets medical personnel use the tip of a syringe instead of a needle to extract fluids from the inside of the connector. Total cost of manufacturing the device dropped 60%. The single Spin Stack mold produces 46,080parts/day (13 million/yr).
In yet another example, according to Plank, Spin Stack systems have produced finished, scratch-resistant coated parts right out of the mold. The sequence starts with the insertion of in-mold decorating films into cavities on the first Stack cube face from the bottom. After a 90° rotation, the plastic part is molded onto the back of the film while films load onto the adjacent core face at station one.
A third 90° rotation positions the molded plastic part into a spray station where a coating is applied via painting telescope. Meanwhile, the second set of in-mold films get backmolded at the second station and more films are inserted at station one on the cube's third face.
Another 90° rotation exposes the painted plastic parts to UV for a fast cure cycle. Other operations continue at the previous stations. A final 90° rotation lets the finished parts be taken out via a robot at the first station followed by the next insertion of in-mold films into the now empty cavities. This system eliminates a paint line, reduces overspray waste, and improves coating hardness and part quality. The pencil strength, a measure for surface hardness of the lens, increased from 2 to 3 H to more than 9 H.
Do's and don'ts
Hermann Plank of The Tech Group Inc., offers some Do's and Don'ts to help minimize potential pitfalls when evaluating whether or not Spin Stack Technology makes sense for a complex plastic assembly:
Talk piece cost. Design your first in-mold assembled part for a product that is critical to your company's success.
Try to use an existing design and "bend" it into an in-mold assembly. Make the design changes necessary after doing DFMA (design for manufacturing and assembly).
Calculate total cost, not just molding. Know the capabilities and properties of your materials and the functional requirements needed by the finished product.
Build a prototype and choose an experienced toolmaker. Involve your suppliers as early as possible in the design phase.