To effectively meet all the demands of high output beverage closure production, manufacturing systems must produce high quality parts with repeatability, high yields, low scrap rates and tight tolerances. To maximise productivity, manufacturers also require high-speed systems that can consistently endure the rigours of running aggressive cycle times.
Rising resin and transportation costs, growing environmental awareness and consumer demand are creating an ever-increasing need for lighter-weight closures. It’s out of this need that the popularity of one-piece closures has developed.
One-piece closures are typically lightweight, making them not only more cost-effective, but also more environmentally sustainable. Unlike a two-piece closure, a one-piece closure doesn’t use a liner, so it requires less material to manufacture and has reduced conversion costs because of the elimination of the lining process. In fact, because of faster cycle times, higher efficiency and sheer material savings, one-piece closures have a 10-15% cost benefit.
There’s also a growing demand for highly productive systems that are able to produce high volumes with no compromise on part performance or quality. With closures becoming increasingly lighter and moving into one-piece designs, even more stringent care must be taken in the design of the manufacturing system to achieve tighter tolerances for high quality parts and less scrap.
These market demands have spurred a need for complete integrated systems provided by a single supplier to specifically optimise them to handle the requirements of high output beverage closure manufacturing.
There are two primary technologies for molding closures: injection molding and compression molding.
Compression molding involves using heat and pressure to squeeze a material within a mold to obtain a desired shape. Resin is extruded, cut and then placed directly into an open mold cavity. Multiple individual mold halves are arranged on a rotary turret and each mold cavity is filled individually. The molds are then closed, pressing down onto the plastic, causing it to flow throughout the mold. While the mold is closed, the plastic solidifies. The amount of pressure, temperature and time that’s applied while the mold is closed varies with the design of the part and the material being molded.
The injection molding process begins similarly with resin pellets being fed into a hopper and then melted using a screw and barrel. With injection molding, the screw not only melts the material, but reciprocates back and forth. As molten resin is delivered to the front of the turning screw, the screw moves backward. When the precise amount of plastic has been melted, the screw stops turning and then advances to inject the plastic into the mold, filling multiple cavities simultaneously. During the filling process, the mold is clamped shut to counter the force caused by the pressure of the plastic being injected into the mold. Once the plastic has cooled, the part is removed from the mold.
While injection and compression molding technologies can manufacture one-piece designs, injection molding’s ability to achieve tighter part tolerances on more complex parts means greater consistency in part dimensions. Injection molding technology introduces resin into the mold in the liquid phase rather than semi-solid, as with compression molding. This makes more technical designs possible, providing virtually unlimited flexibility with part design and shape.
An example of the quality level required in closure molding is the closure’s plug seal area, where imperfections and flow lines can cause closures to leak. With injection molding, it’s possible to achieve tolerances for one-piece plug seals that are significantly better than +/- 0.1 mm because injecting molten material into the mold allows the plug seal to form precisely, and applying pressure to the molten material allows it to be ‘packed out’.
Because compression molding maintains lower resin temperatures, the material must be squeezed into the cavity under semi-solid flow. This can impact surface finish and dimensional consistency. Injection molding also allows resin to crystallise after it has been shaped through the cavity, which leads to greater dimensional stability of the part and less risk of leakage.
Colour change is another major consideration when comparing the productivity of beverage packaging systems. With advancements in hot runner and machine design, colour change times for injection molding are quite competitive with compression molding. Even in cases where it’s faster to switch colours with compression molding, the injection process allows for more flexibility in fine-tuning part dimensions and making process adjustments that can compensate for different shrinkage behaviours. Also, with today’s colourants, injection molding is able to maintain cycle times within a few tenths of a second from one colour to the next.
With either process, it’s also possible to optimally sequence colour changes to reduce colour change time. For example, it’s generally preferable to change from one light colour to another or from one dark colour to another. In the event of downtime, injection systems can shut down a single cavity without wasting material. This is also possible with compression molding, but the pellet must be cut and scrapped.
While changing a single tool stack is relatively quick for compression molding, in the case of a complete mold product change, injection molding is significantly faster. Typically, all of the tooling subcomponents in an injection molding system are conveniently held within two assemblies. This expedites the removal and installation of the complete mold.
While compression molding typically has an energy consumption advantage because of lower processing temperatures, this is just one contributor to part cost. In addition, many opportunities exist with injection molding to bring energy consumption below that of compression molding, including more efficient actuation and overall system level enhancement.
While both injection and compression molding are capable of producing one-piece closures, the compression molding process generally requires a slitted tamper-evident band that adds a step to the production process. With injection molding, it’s possible to produce a finished ‘molded-in’ tamper band, reducing weight and eliminating the downstream slitting process. When comparing sheer output, injection molding is capable of output rates over 2,500 parts per minute in a single system for optimal use of capital and floor space.
Many new entrants, as well as experienced manufacturers, are looking to work with an experienced supplier who can provide the best overall system performance. Rather than purchase a collection of discrete pieces of equipment, manufacturers are increasingly demanding a molding workcell that’s optimised as a whole to maximise performance, increase quality and reduce waste and energy consumption. They are also seeking suppliers who have the after-sales services to keep these systems running at peak efficiency.
This is the case for beverage closure manufacturing. A complete system that’s specifically designed for closure manufacturing, including machine, hot runner and temperature controller, provides complete melt stream control. Coordination of all elements as a complete package not only facilitates significant weight reduction and faster cycle times, but an optimised system provides consistent clamping force, melt and temperature at every cavity.
Having a complete system approach also has the potential to improve ease of start-up by simplifying and standardising interfaces among all workcell components, as well as providing a level of streamlined integration that reduces a workcell’s physical footprint.
Some suppliers are responding to this demand by offering complete molding systems specifically designed for beverage closure manufacturing. Husky Injection Molding System’s HyCAP is one example of a system-level approach that’s specifically optimised to meet the challenging demands of manufacturing lightweight beverage closures.
Using HyCAP as the foundation, all aspects of the workcell have been seamlessly integrated as a complete solution to deliver aggressive results – from resin handling and leading-edge tooling to optimised melt stream control and full inspection – all coordinated through Shotscope NX, Husky’s process and production monitoring software.
Regardless of supplier, today’s manufacturers demand fast systems that are specifically optimised for the unique needs of closure manufacturing. The systems that will successfully meet these demands are built to produce lightweight, high quality parts with superior repeatability, higher yields, less scrap and tighter tolerances. For manufacturers, working with an experienced partner is the key to achieving all of these goals successfully.
Mark Fitzpatrick is business manager, closures, Husky Injection Molding Systems.
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