Doing more with stack and multi-cavity molds

Anyone who grew up in the 1970s will remember Steve Austin, the $6,000,000 man, who gained special powers through the application of new technology — in his case, “bionic” limbs. The interesting thing was that he still looked the same. In that sense, he shares something with stack mold and multi-cavity mold technologies, both of which improve the performance of injection molding machines without altering either their outward appearance or footprint.

STACK MOLDS STEP UP

For molders looking to maximize productivity and profitability — particularly with such low-margin parts as lids, closures and dairy containers — stack molds are an increasingly attractive option. “A stack mold can virtually double the output of an injection molding machine,” said Martin Baumann, marketing manager, hot runners, at Husky Injection Molding Systems. “Importantly, the stack system achieves this higher productivity without additional machinery, which means less utility cost, less maintenance cost and smaller footprint because all molding is accomplished in a single press.”

Having long since proven its worth for small parts run on smaller tonnage machines, stack mold technology is now being adapted for larger parts molded on press applications of 1,000 tons or more. Last year, for example, the Plastcoat division of Decoma International, based in Mississauga, Ont., used a two-level stack mold manufactured by Husky with one cavity in each face to simultaneously produce front and rear wheel flares for Ford Motor Company’s XLT trucks. Molded on Husky’s 2,200-ton EWLL two-platen machine, each part is several feet long, and uses about 300 grams of thermoplastic olefins (TPO).

And as the part sizes continue to get bigger, the melt channels get smaller. For example, Mold-Masters Ltd. offers multi-material stack systems that provide a manifold with three levels of melt channels within a thickness of 45 mm. “This technology results in a more compact system while still maintaining full rheological balance,” the company said.

STACKING UP PRODUCTIVITY

In retrospect, it seems inevitable that the pressures of high-speed production would push stack molds beyond two single-face molds. StackTeck offers a four-level stack mold, made possible through the use of a twin valveless melt transfer system (VMTS) that allows the transfer of melt to both hot runner manifolds. According to the company, the system is capable of producing over 75,000 parts per hour at cycles close to five seconds. “As the available maximum shot height of bigger injection molding machines increased with a bigger machine layout, it opened up the possibility of four-level molds for a wide range of products,” said Jordan Robertson, StackTeck’s general manager. “We can now look at a four-level mold with a plastic part height of up to four inches.”

The development of side-entry robots from the automation suppliers has also offered new opportunities for larger parts than are simply free-dropped onto a conveyor belt, Robertson added.

Production of stack molded parts are also being facilitated by easier access into the injection molding machine itself. For example, Husky recently introduced a new split sprue bar designed to keep the molding area clear for either free drop of parts or easy robot access. Capable of handling throughputs up to 1,500 grams per second, the split sprue bar system can be used on two-, three- and four-level machine-based carriers.

“Stack molds are an area in which we are definitely investing,” said Mark Fitzpatrick, business manager, closures, at Husky. “In the area of packaging in particular, where the volume demands are high, we anticipate growing demand for high cavitation stack molds.”

MULTI-CAVITY MEANS BUSINESS

The trend to high cavitation molds for medical parts reflects the large and intensely competitive nature of the sector. With a U.S. market valued at approximately US$80 billion in 2007, annual growth of eight to ten per cent, and a viable export business, there are plenty of opportunities for interested companies.

As with stack molding, multi-cavity technology continues to evolve in new directions, and is driven by the same need: producing the maximum possible number of parts per cycle. As the name implies, the process involves using a mold with two or more mold impressions capable of producing more than one molding per molding cycle.

Toolmakers are helping to achieve high part tolerances demanded by medical applications. For example, the Melt-Disk from Mold-Masters is a circular manifold for edge-gating parts. Each disk molds eight parts, the company said, and when mounted in a mold, a series of of Melt-Disks can be used to create up to 192 cavities.

In today’s fiercely competitive climate, simply having more than one mold impression sometimes is not enough. The most obvious evolution, then, is in regard to manufacturing molds with the greatest possible number of cavities. The results are impressive. For example, at the National Plastics Exhibition (NPE) 2006 in Chicago, StackTeck displayed a 128-cavity mold. “The 128-cavity mold is the largest that StackTeck has manufactured to this point, and is available in four by 32 and two by 64 cavity formats,” Jordan Robertson said.

GETTING FASTER

Injection machine manufacturers have also been at work improving the speed at which the process can operate. Also at NPE 2006, Sumitomo showcased its SE7M, a 7.7-ton all-electric unit for micro molding that molded 0.035 gram connectors in four cavities with a cycle time of 3.3 seconds. At the recent ChinaPlas show in May 2007, Demag Plastics Group demonstrated an El-Exis S 200/560-1450 for production of 500-ml yogurt cups using a four-cavity mould with a cycle time of 3.4 seconds.

In the past, multi-cavity molders have often been bedevilled by difficulties controlling the molding process in regards to fill velocity, pressure, position and hold time of each cavity. This issue, too, is being tackled. The new SCS (sequential cavity separation) process introduced by Mitsubishi Injection Molding manufactures multiple part cavities one part at a time. According to the company, the SCS process allows the molding of parts of different shapes and sizes in the same machine by opening and closing hot runner valve gates to fill first one part and then another. “The SCS provides the molder with the ability to run up to four cavities independently within a single mold,” said Dave Voisard, special projects manager at Mitsubishi.

Machine suppliers are also zeroing in on such trouble areas as reducing colour change time in such common multi-cavity applications as closures. In 2006, Husky introduced the Ultra 750 HT-S6 nozzle, designed specifically for the closures market. “The goal was to improve colour change time in multi-cavity closure molds,” said Husky’s Fitzpatrick. The nozzle’s design includes three melt channel sizes for optimal rheology, and comes standard with six ports in the tip, instead of the traditional three ports. “The six ports allows us to improve colour change time by 40 to 50 per cent in multi-cavity high cavitation HDPE closure applications,” Fitzpatrick said.

Is your machine stack ready?

Just because a plastics processing shop wants to run a stack mold doesn’t necessarily mean that it can. An injection molding machine must be able to accept a mold that typically has twice the weight and shut height of a single-face mold that is molding the same parts. In determining whether a stack mold operation is right for you, the following issues should be considered:

* Clamp Opening. Stack mold systems with two mold openings require approximately double the shut height and opening stroke relative to single-face applications.

* Injection Capability. The machine must provide double the shot weight, plasticating ability, and injection rate than for a comparable single-face mold.

* Tie Bar Suppor
t for Centre Section. Machine tie bars must provide sufficient support for the hot runner and cavity plates as well as accurate, repeatable alignment during mold open and close to prevent damage to cores and cavities.

* Sprue Break Capability. The injection unit of the machine must retract from the sprue bar enough to clear it in order to prevent collision and possible damage between the injection nozzle and the sprue bar upon mold close.

* Injection Unit Shut-off Nozzle. This allows recovery and prevents resin drool while the mold is open. Although it is possible to run stack molds without it, the shut-off nozzle will help improve cycle time (recommended).

Source: Martin Baumann, Husky Injection Molding Systems

Resource List

Arburg Inc. (Newington, Conn.); www.arburg.com; 800-221-0712

Patton Plastics (Mississauga, Ont.); www.pattonplastics.com; 905-568-1133, ext. 106

Dcube (Montreal); www.dcube.ca; 514-272-0500

Demag Plastics Group (Strongsville, Ohio); www.dpg.com; 866-491-1045

Stephen Sales Group (Markham, Ont.); www.stephensales.com; 905-940-5577

Engel Canada Inc. (Guelph, Ont.); www.engelglobal.com/na; 519-836-0220

Husky Injection Molding Systems Ltd. (Bolton, Ont.); www.husky.ca; 905-951-5000

Mitsubishi Injection Molding Machinery Inc. (Bensenville, Ill.);

www.mhiinj.com; 630-693-4880

Barway Plastic Equipment Inc. (Vaudreuil-Dorion, Que.);www.barway.ca; 450-455-1396

Mold-Masters Ltd. (Georgetown, Ont.); www.moldmasters.com; 1-800-387-2483

Stackteck (Brampton, Ont.); www.stackteck.com; 416-749-1698

Sumitomo Plastics Machinery (Norcross, Ga.); www.sumitomoPM.com; 770-447-5430

Plastics Machinery Inc. (Newmarket, Ont.); www.pmiplastics.com; 905-895-5054

Resource Polytec Inc. (Vancouver, B.C.); www.resourcepolytec.com; 604-454-1295