Compression and injection molding wrestle for closures

About 50 billion plastic screw caps are made annually, and demand is rising all the time. In the game between the two rival production processes, neither holds all the aces.

When it comes to choosing technology for making caps, it''s rather like computers. PC or Mac? Compression or injection? Processors say both have their benefits, with neither the answer for every situation. But just as with computers, choosing between the two is getting more difficult as developers on both sides improve their processes.

Advocates of injection molding generally argue that the process allows for better precision, making it suitable for higher-value caps. They admit that compression molding is good for high volumes but say this is offset by the inability to do much in the compression molding tool, which limits the designs possible and forces compression molders to compete at the low-value range of the closure market. Some also say that injection molding can be just as fast, if not faster, than compression molding.

Despite what they identify as compression molding''s drawbacks, injection molding machine makers have a healthy respect for it. "There is definitely significant competition from compression molding," says Reto Morger, marketing manager at Netstal Machinery (Naefels, Switzerland). "Closures are a typical part for machines with very fast cycle times, since you don''t want to spend much time on a throw-away part. And certainly, compression molding is very fast."

Processors using compression molding machinery say the technology has developed rapidly in the last two years, enabling them to better compete with injection molding on closure design while still holding an output advantage. Among compression molders of closures, the largest is reckoned to be Alcoa CSI, based in Indianapolis, IN. It acquired H-C Industries in 1986, gaining its rotary compression molding technology for closures, said to be the first. The firm also injection molds closures, says Erica Gibson, marketing manager at its West Midlands, U.K. facility.

Alcoa acquired injection molding technology by purchasing a molder in the U.K. in the 1990s, followed by Southern Plastics Inc. in Kilgore, TX, in August 2000. Gibson agrees that compression molding''s reputation was made with high volumes of simple caps, but she says progress at the firm enables it now to successfully compression mold almost any design. "Plus we''ve about 300 compression molding units around the world, so obviously we''re not going to leave that process anytime soon," she says. Also, she points out that gate marks continue to be an issue with injection molded caps.

One of the most significant issues facing closure molders is the transition from two-piece to one-piece designs, she says. "The marketplace has gotten very sensitive to liners, especially the water bottlers who worry that a liner could give water an off-taste or smell," she explains. One-piece caps are typically HDPE, while the two-piece are typically PP. Two-piece closures have an extra seal which is either inserted into the cap in a secondary process when compression molding, or comolded if using injection.

Reducing costs is of course always an issue, Gibson says, and the secondary step required to place liners in compression molded caps was seen by many end users as an expense that they did not want to fund. Two-piece closures have in the past usually cost 10% to 15% more than one-piece ones.

To improve competitiveness, the firm last year developed the means to compression mold one-piece closures designed so that the inside of the cap forms a seal with a bottle neck, a role normally fulfilled by the liner. "Private label brands, where costs are paramount, are quickly becoming good customers for these one-piece PE closures," she says.

Yet there is still a substantial market for two-piece closures, Gibson says, especially in hot countries. Bottle necks shrink over time in heat, and the insert in the closure is able to account for this shrinkage. "Our HR two-piece PP closures offer a good seal in these hot lands," she says.

At compression molder Alternative Systeme (Worms, Germany), Marketing and Logistics Manager Thomas Weiss says the firm does not intend to add injection molding to its arsenal. "We''ve always been compression molders," he notes, explaining that the firm''s founder—his father—was a long-time Alcoa CSI employee and one of the leaders of that firm''s entry into compression molding in the 1980s. He cites large European mineral water bottlers as the firm''s top customers.

Lower tooling costs a big plus for compression

As benefits over injection he cites reduced operating costs. His firm uses equipment manufactured by Sacmi Imola (Imola, Italy). There are few manufacturers of these machines; another supplier is Oberburg Engineering (Oberburg, Switzerland). "When we founded the firm [in the mid-1990s] compression molding was the process more suited to high-volume production of low-cost caps," says Weiss.

During compression molding, an extruder delivers material to the cavities, one at a time, of a tool rotating at high speed on a vertical axis. Now, faster injection molding machines and higher cavitation have made injection machine output "very close" to that of compression molding equipment, he allows. But though a compression unit has a higher up-front cost than injection, the cost of high-cavitation tools required on the latter to compete with compression more than offset machine cost alone, he says. Tool maintenance on compression units is much easier, too, he claims.

One common criticism of compression molding is that tamper-evident bands have to be created in a second step, adding to the cost. But Weiss says that Alternative Systeme, which has a sister firm that develops compression mold tooling and new closure designs, has patented a method of compression molding closures with tamper evident bands in a single step.

Alternative Systeme uses Sacmi''s 32-cavity CCM 001 models; the manufacturer also offers a 64-cavity model, the CCM 002. Outputs are 600 and 1200 caps/min. For comparison, Netstal estimates processors can produce 49,000 2.6 to 3.5g HDPE screw closures/hr using a Synergy machine with 64-cavity mold. That''s just under 817/min.

But much of the technology in injection molding is in the tooling, and moldmakers say they are improving their wares to meet compression molding''s output/cavity. One, Gefit SpA (Alessandria, Italy), has designed a 64-cavity mold for 28-mm HDPE caps for still water bottles that it says can run at 3.4-second cycle times (around 68,000 caps/hr) or even less.

(The highest output ever seen on a single injection machine was performed at NPE 2003 last June. Husky Injection Molding Systems, based in Bolton, ON demonstrated a Hylectric 400, a 4000-kN machine, outfitted with a 2x96 cavity stack mold from Marland Mold Inc. (Pittsfield, MA). It makes 1800 caps/min. Husky says the system "meets the cost target of our customers and provides leading cycle times, gate quality, color change performance, part-to-part balance, and ease of operation." However, it may not necessarily be the most cost-effective way to do it. Gefit''s Sales and Marketing Manager Enrico Bo says two smaller injection machines with single-level molds may prove less expensive.)

Alessandro Sereni, marketing service coordinator at Sacmi, claims a number of advantages over injection molding, including easier die changeover; the requirement of only 15 kN of clamp force, so less energy is consumed; and the ability to process materials with lower melt flow indexes (MFI), which cost less. Additionally, he points out that compression molded closures are ejected in an oriented fashion, which facilitates inline quality inspection.

Processors such as Alternative link their compression molding units with counting and stacking equipment for complete inline processing and packing.

Although about 70% of compression molded caps are HDPE, Sereni says the trend is to the more costly polypropylene due to its higher creep resistance, higher modulus, and lighter weight/cap. Additionally, PP offers the heat resistance necessary for hot filled beverages.

Compression "nothing but trouble"

Not all are won over, of course. Closure molder Bericap (Budenheim, Germany) acquired two compression molding units some years ago but plans no further purchases, says one executive who asks not to be identified. The firm has more than 600 injection molding machines. He says the lower precision, higher initial investment cost, and maintenance costs all speak against compression molding. Problems stem from the stamps in the cavities. Plus, he says the improvements in injection molding machinery are such that the unit cost of molded closures is no higher than that for compression molded ones.

And even as compression molders add new tricks to their process bag, so too is injection molding proving itself the best means to make the high value-added screw caps demanded for top-of-the-line beverages. Beverage market observers say this is significant as the market moves quickly to extremes, with the mid-priced beverage market disappearing in favor of high- and low-cost ones. At processor SIG allCAP (Neuhausen, Switzerland), Marketing/Sales Director Fritz Seelhofer says injection molding allows the incorporation of higher functionality into a cap than is possible with compression and lets molders serve the high-end markets—the processor''s combiTwist caps are one example. They enable a consumer to open a new carton with a single twist; the movement forces a sharp wedge at the bottom of the closure into the carton seal.

Matthew Defosse [email protected]

Contact information
Netstal Machinery www.netstal.com
Sacmi Imola www.sacmi.it
Oberburg Engineering www.packsysglobal.com
Gefit SpA www.gefit.com