Closure Know-How

To effectively meet the demands of high output beverage closure production, manufacturing systems need to produce quality parts with repeatability, high yields, low scrap and tight tolerances. High speed systems optimized for closure applications are also needed to maximize productivity.

Rising resin and transportation costs, growing environmental awareness and consumer demand are creating the need for lighter closures, with one-piece ones being popular in the industry as they are more cost-effective and environmentally sustainable than conventional ones. They are replacing their two-piece counterparts in a majority of applications. Unlike a two-piece closure, a one-piece version does not require a liner - fewer materials are therefore needed for manufacture, resulting to lower conversion costs. They provide a 10-15% cost benefit to manufacturers in comparison to producing two-piece ones due to faster cycle times, higher efficiency and material savings.

This has led to a growing demand for highly productive systems that are able to produce closures in high volumes. As closures become lighter, much care is needed in the design of a manufacturing system in order to achieve tighter tolerances for high quality parts and less scrap.

Closure molding technologies

Injection and compression molding are the two primary technologies for molding closures. The former was used in the early 1970s to create the first plastic beverage closures and compression molding was introduced in the mid-1980s. With the development of these systems, manufacturers are able to create better precision tooling for more complex part designs and faster cycles. However, there are differences among the two molding methods and here are some of them.

Compression molding uses 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 molding stacks (cavities) are arranged on a rotary turret and each mold cavity is filled with the material. A mold is then closed and pressed onto the plastic, causing the plastic to flow throughout the mold and to solidify. The amount of pressure, temperature and time applied to the closed mold varies with the part design and the material being molded.

The injection molding process begins in a similar way as compression molding - resin pellets are fed into a hopper and then melted using a screw and barrel. With injection molding, however, the screw also moves back and forth.

As molten resin is delivered to the front of the screw, the screw turns and moves backward. When the plastic melts, the screw stops turning and 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 that is being injected into the mold. Once the plastic has cooled, it is removed from the mold.

Tighter tolerances for increased flexibility

While the two molding technologies can manufacture one-piece designs, injection molding can achieve tighter part tolerances on more complex parts, resulting in a greater consistency in part dimensions. It introduces resin in its liquid state into the mold, rather than a semi-solid state as in compression molding. This provides for more technical designs, allowing virtually unlimited flexibility in part designs and shapes and a lower risk of capital investment.

During manufacturing, imperfections and flow lines of plug seals can cause closures to leak. With injection molding, it is possible to achieve tolerances for one-piece plug seals that are better than +/- 0.1mm. By injecting molten material into the mold, it allows the plug seal to form precisely and by applying pressure to the molten material, it allows the material to be 'packed out'.

As compression molding maintains low resin temperatures, the material must be squeezed into the cavity under semi-solid flow, which can impact the surface finish and dimensional consistency. Injection molding however allows the resin to crystallize after it has been shaped in the cavity, which leads to greater stability and a lesser risk of leakage.

Improving productivity

When comparing the productivity of beverage packaging systems, consider the procedure of changing the color of closures. While it is faster to switch from one color to another using compression molding, there is more flexibility with the injection process to fine-tune part dimensions and make adjustments in processes that can compensate for shrinkage behaviors. With today's colorants, injection molding is able to maintain cycle times within a few tenths of a second from one color to the next.

With either process, it is possible to optimally sequence color changes to reduce the time needed. In the event of downtime, injection systems can shut one cavity down to minimize material wastage. This is also possible with compression molding, but the pellet must be cut and scrapped, leading to wastage.

While changing a tool stack is relatively quick using compression molding, injection molding is however significantly faster in performing a complete mold product change. Typically, the tooling sub-components in an injection molding system are conveniently held within two assemblies, which expedite the removal and installation of the mold.

Compression molding tends to consume less energy than injection molding because of lower processing temperatures and cooling. While both molding technologies are able to produce one-piece closures, compression molding generally requires a silted tamper-evident band that adds a step in the production process. Injection molding however produces a finished 'molded-in' tamper band that reduces weight and eliminates the downstream slitting process. When comparing sheer output, highly productive work cells are possible with injection molding with up to 144-cavity systems to optimize the use of capital and floor space.

Meeting industry demands

Today, manufacturers demand fast closure manufacturing systems that produce lightweight, high quality parts with superior repeatability, higher yields, less scrap and tighter tolerances. By working with experienced partners, manufacturers can successfully achieve their goals.

www.husky.ca