Anne Marie Mohan

January 29, 2014

14 Min Read
Stack molding breaks technology barriers

Sixteen years ago, the world's leading suture supplier, ETHICON Products Worldwide, approached custom injection molder Unimark Plastics (, a div. of Jarden Corp. (, Greenville, SC, with a model for a first-of-its-kind plastic suture tray, along with a mold-flow analysis that said the part couldn't be molded, and challenged the company to make it happen. According to Bill Torris, Unimark director of project engineering, due to Unimark's knowledge of processing and tooling, as well as its manufacturing excellence, the company today can lay claim to having molded nearly 2 billion of the "unmoldable" trays.

Along the way, Unimark and Somerville, NJ-based ETHICON Products, a division of ETHICON, INC., a Johnson & Johnson Company, forged a collaborative relationship that resulted in the development of new tray designs that enabled greater and more efficient automation solutions in both manufacturing and packaging processes.

To produce the latest version of the suture tray, the Zipper 3, at the cost and quality levels demanded, Unimark sales representative Steve Setar says that the company has had to go beyond the limits of existing stack-molding technology. "The two-piece Zipper 3 product is molded in a precision family stack mold that has taken current stack molding to a whole new level," he says. "The system leap-frogs technology barriers to allow stack molding of parts that maintain extremely tight tolerances never seen before in the industry from a stack mold."

Unimark Plastics was formed in 1973 in Reedsville, PA, as a private company providing custom injection-molded plastic parts for precision consumer and medical applications. In the late '80s, Jarden Corp., formerly known as Alltrista Corp., purchased Unimark. According to Unimark marketing manager Brad Davis, although the company's business has been more heavily weighted toward medical applications due to its engineering expertise, its expertise is now filtering down to consumer-oriented programs.

Among the other products molded by Unimark are buckets for fertilizer products; shotgun wads for the world's largest ammunition manufacturer; veterinary syringes; caps and other fitments under its Yorker Closures ( product line; and plastic cutlery under its Diamond Brands ( name. These products are manufactured in seven locations in the U.S., the U.K. and Puerto Rico.

Named Processor of the Year in 2003 by Plastics News, Unimark offers quality "that is second to none," says Davis. "Last year, our lot acceptance averaged somewhere in the ninety-ninth percentile range over billions of units, while the industry average is somewhere in the mid-nineties."

When ETHICON Products began working with Unimark in 1990, it was looking for a way to transform its existing suture-tray packaging from a paper folder to a plastic tray that could accommodate a larger number of its stockkeeping units, provide greater safety and sterility features, and allow for increased automation in its suture-winding operations (see complementary article beginning on p. 52 of this issue).

One reason, suggests Robert Cerwin, engineering fellow, advanced packaging development for ETHICON Products, that other injection-molding companies may have claimed the tight-tolerance tray could not be molded is because they were reluctant to invest in the research and development required to produce such a precision piece. "Compared to consumer packaged goods companies, we are a low-volume buyer," Cerwin explains. "Food packagers are more interested in cost, whereas medical device companies also have to be concerned about quality.

"On the key designs and materials today, if you're not committed to process excellence, I won't talk to you. It's just a sign that you're not going to give us the quality we need. To Unimark's credit, they were dedicated to being in the medical products industry."

The original tray, the Door RELAY®* Suture Package for nonabsorbable sutures, was in design for a year and a half and, once produced, consisted of a 31/2-in.-long, 11/4-in.-wide one-piece polypropylene tray, with tabs around the outside that could be folded down after semi-automatic suture winding to hold the suture in place. According to Torris, one of the major requirements of ETHICON Products for the tray design was that it be able to dispense many different types of sutures uniformly. "One of the advantages of injection molding is that it is capable of very high volumes," he explains. "But, if you can't get uniformity within some of your packaging lines, you can't have the high volume, because if you have to make ten different styles, you just cut your volumes by ten."

One of the company's requirements for the tray manufacturing, which Cerwin says pushed Unimark beyond its existing capabilities at the time, was that Unimark maintain control of the trays after molding through the use of automation. "Starting with the Door RELAY® package, Unimark had to embrace bringing new technologies into its facility," Cerwin says. "What we needed them to do was to positively reach into the mold and then take the products out of the mold and stack them."

Through the use of top-entry robots from Wittmann (U.K.) Ltd. ( and custom-made stacking mechanisms, Unimark designed a solution that allowed the trays to be removed from the eight cavities of the injection molder and stacked into columns, which were then placed in medical-grade paperboard sleeves for shipment to ETHICON Products. This robotic automation served the dual purpose of giving ETHICON Products the ability to track individual trays back to an exact mold cavity?or even to an exact batch of raw materials?in case of problems, and it paved the way for enhanced automation in the ETHICON Products suture-winding operations.

With the design of the Door RELAY® package, suture winding evolved from a completely manual process, whereby the needle and sutures?wound manually in a figure eight?were placed by hand inside a paper folder, to a semi-automatic air-winding process, where operators park the needle in the tray, and the machine winds the suture. Air winding increased product quality by ensuring the sutures were wound with the same tension each time. At the same time, it greatly reduced operator issues with carpal tunnel syndrome.

Following on the heels of the Door RELAY® package, the Zipper 1, another one-piece tray design, was introduced to accommodate a line of synthetic absorbable sutures under the VICRYL®* (polyglactin 910) Suture name.

As both Unimark and ETHICON Products will attest, the real leap in molding and suture-winding technology came with the introduction of the Zipper 2, or Z2, for the PROLENE®* Suture line of nonabsorbable PP monofilament sutures in 1997, followed by the Z3 in 2002. The Z2 is a two-piece high-density polyethylene tray, ultrasonically welded in-line by Unimark, that is produced using family stack-mold technology?a process that Cerwin says "wasn't meant to make precision parts."

"Z2 is where the next level of automation was put into place," says Cerwin. "We wanted to expand the tray line, and we wanted to use stack molding to get the cost down."

As Torris explains, Unimark was able to meet the challenge of producing 16 assemblies, or 32 parts/cycle, in a family stack mold while holding the tight tolerances required through the use of high-precision molds engineered by Holbrook Tool & Molding ( "Not only are we making parts on two faces, but we're making two different parts in the same mold," he says. "With standard injection molding, you make the same part in the mold. If you have a family mold, where you have more than one part in the mold being made at the same time, it usually tends to be a low-tolerance part. As you can imagine, trying to hold very tight tolerances on plastic parts in multiple cavities is difficult enough, but when you try to do different parts at the same time, it's that much harder because you have different size parts and different amounts of plastic that need to be used, and all of those variables start to take effect.

"What we have done is taken a very tight-tolerance part, with a top and a bottom, and we put it in a family stack mold, and we still hold those very tight tolerances just by building a higher-precision mold than most companies in injection molding would."

One advantage of the two-piece tray is that it takes some of the materials cost out of the package. With the one-piece design, certain features, such as ribs and gussets, needed to be built into the tray to maintain rigidity. Because the two-piece tray is welded together from two parts, it provides the necessary flatness without the extra plastic.

From the standpoint of ETHICON Products, moving to a two-piece tray enabled the next step in packaging technology, which was automated stylus winding. Among the advantages of automated suture winding are increased productivity, less ergonomic concerns for operators and greater winding accuracy. Explains Cerwin, "One of the things that we never want to have happen is sutures out of the tray getting caught in the seal [of the secondary pouch package]. That was one of our biggest concerns, because it would be a break in sterility. Stylus winding guaranteed that we were placing that thread in that tray."

Not content to rest on their laurels, Unimark and ETHICON Products pushed the limits of automation and design even further with their introduction of the most recent suture tray design, the Z3. The first "universal" tray from ETHICON Products, the Z3 is engineered to hold the largest range of suture materials and thicknesses, accommodating 90 percent of the company's worldwide suture volume.

For Unimark, the challenge was to take cost out of the package by using less plastic per piece and by increasing the production cycle times. The former was achieved by making the part lighter, while the latter was accomplished through the use of new side-entry robots that pick parts out of the mold "faster than gravity can drop them," according to Setar. This technology is being used at Unimark's Reedsville, PA, location, which comprises two plants that continuously run eight to 10 molding machines specifically for suture tray production.

The largest of Unimark's molding facilities, the Reedsville plants, under the direction of J. Alan LeBihan, director of manufacturing excellence and plant manager, cover 76,000 sq ft and are equipped with 44 injection-molding presses. To accommodate requirements for medical applications, the plants have a white room and a 100K cleanroom. Unimark's Davis says that two of the company's other plants?in Greenville, SC, and Springfield, MO?are designed with very similar layouts and are equipped with many of the same makes and models of machinery. "It's a contingency plan for our customers," he explains. "If we have a catastrophe in one of these buildings, we can't stop production. We have fireproof vaults for the tools, so they would survive, and we could relocate production to the other facilities."

During a PD visit to Reedsville, Unimark was running three Netstal SynErgy 4200 injection-molding machines, acquired in 2001 and 2002 from Netstal Machinery, Inc. (, to produce the Z3. The machines offer a clamping force of 4200 kN (4200 kilo newtons, equivalent to approximately 420 tons) and were chosen for this application due to their high repeatability in regard to clamp position and clamp tonnage, and their high injection fill rate and dosage, says Torris. "For our critical medical applications, we are making difficult parts, and we need things like higher injection pressures, faster fill rates and very repeatable molding, shot-after-shot," he explains.

The machine is controlled via Netstal's proprietary DSP (Digital Signal Processing) control system, which runs on a Pentium PC and provides closed-loop control of the mold-open position in standard execution.

Each machine is enclosed in a fully automatic fabrication and assembly cell. Once the trays are molded, side-entry robots provided by CBW Automation ( remove them from the molds and hand them off to a transfer robot, which then hands the trays off to a stacking robot. "In effect," says Torris, we now have three robots doing the job that one used to do, so you're never sitting and waiting for a robot."

Tray tops and bottoms are ultrasonically welded together four at a time through the use of four 2000 Series ultrasonic welders from Branson Ultrasonics Corp. ( After welding, the trays can be offloaded for inspection or are sent on to be stacked and placed into packaging sleeves.

Throughout the process, quality is maintained through the use of tools such as the injection-molding machine's Statistical Process Control (SPC) system, which performs quality checks and tracks process parameters. Torris says that before molding, Unimark validates the high and low process parameters, so that if during production the equipment runs outside of those parameters, it is immediately shut down by the SPC. The system is also equipped with standard tools, such as part presence sensors, says Torris.

The net result of Unimark's efforts to continually stretch the limits of its technological know-how to accommodate the cost and performance requirements for the Z3 is a 20-percent to 30-percent reduction in cycle time from the Z2, as well as 50-percent reduction in cost over the original Door RELAY® tray. But credit for these achievements is not due to Unimark alone, note project participants; the results were achieved through the very strong collaboration between the molder and suture maker.

Chuck Villa, president of Jarden Plastics, says, "We value our alliance with ETHICON Products. The combined drive to succeed and break new ground pushes the respective teams to the leading edge of new technologies in our industry. It is confirmation that positive, open business relationships yield great success."

Adds Mike Zaagman, vp of sales and marketing for Unimark, "It is all about identifying and meeting the customer needs. Any supplier will say that, but it is rare that the supplier/customer relationship is mutually beneficial and the success of the whole team is considered from the inception to the end of each state. That is the attitude we nurture, not only with ETHICON Products, but also strive to achieve with all of our customers." "We greatly appreciate the forward thinking of these types of customers and the open communications they share with us because it fosters vision, creativity and need-meeting."

Concurs Cerwin, "When you talk about the team that formed between ETHICON Products and Unimark, you couldn't recognize who belonged to what company. The members were totally focused on project success. There was no such thing as finger-pointing. There was a complete understanding that the relationship between materials and design and equipment is a marriage.

"There was total involvement," he adds. "They talked to us, they knew what our core customers wanted and what core manufacturing wanted, and our people went through training at Unimark on molding. There was another little basic that we really respected, and that was to let the experts be experts."

"It was a substantial financial investment by both companies. When we had an unknown, we had to generate the information for that. If we couldn't find it in the literature, we had to sort the variables out to solve the problem. Anywhere along the line, the whole system could have fallen apart. Out of sheer determination, we weren't going to let that happen."

*Trademark of ETHICON, INC.

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