Multishot molding techniques enable many components, such as gaskets or handles, to be molded directly into parts. Photo courtesy of MGS MANUFACTURING GROUP (Germantown, WI)
The reason? “We've had some experience with healthcare companies over the last several years, and they're seeing tremendous payback” on their multishot investment, says Chris Navratil, vice president of TecStar Manufacturing Co. A business unit of MGS Manufacturing Group Inc. (Germantown, WI), TecStar provides molding services and equipment to a number of industries.
Replacing conventional molding and assembly operations, multishot molding can streamline the manufacturing process, reduce part costs, and improve product quality. But multishot molding also has significant limitations and downsides that must be understood before OEMs can make sound decisions on whether the process is right for them.
What is Multishot?
Multishot molding is a process in which a number of separate material shots are used to produce the final product. Movement of the molding tool and the molded part can allow two-, three-, and even four-shot processes. (The term two-shot molding is commonly used to describe processes involving shots of two different materials.)
Multishot molding is widely used to make products that fit in the hand. In the medical device industry, these include small drug-delivery systems and various handheld metering and diagnostic devices, says Dave Thoreson, general manager of medical molding and assembly for Phillips Plastics Corp. (Hudson, WI).
Products made by multishot processes combine the characteristics and properties of two or more different materials. For example, shooting a so-called soft-touch elastomer material over a rigid plastic gives a device a pleasant feel for doctors or patients who handle it. More important, an elastomer outer layer also makes it easier to grip and hold on to a device covered with blood or some other fluid, notes Jim MacDonald, new business development manager for Vaupell Rapid Solutions, a molder in Hudson, NH.
Material deposited by a second shot can perform a number of functions, from shock absorption to noise and vibration dampening. Second-shot materials can also improve waterproof devices such as a handheld meter developed by Thermo Electron Corp. and manufactured by Mack Molding Co. (Arlington, VT). Used in medical facilities to measure pH and dissolved oxygen, the meter originally included inlet and outlet ports that had to be closed by small stand-alone plugs. Operators often lost these plugs after removing them, reports Ron Geis, Thermo Electron's director of operations for water-analysis instruments. So a new meter design includes small tabs that are molded into the device in a two-shot process. Like the old plugs, these tabs are used to close the ports to waterproof the device. But unlike the plugs, the tabs are actually part of the meter, so operators cannot lose them.
Photo courtesy of PHILLIPS PLASTICS CORP. (Prescott, WI)
Using multishot techniques, many common components can be molded right into parts. Some examples include O-rings, gaskets, handles, and hinges. “If we can join two components in the mold, we can eliminate many downstream events” in the production process, notes John Berg, marketing director for MGS.
Two-, three-, or four-shot molding processes can be used to make devices such as surgical instruments, IVD and drug-delivery devices, and orthopedic and testing equipment.
Multishot molding is sometimes called in-mold assembly, because the resulting part consists of two or more pieces made of different materials that would normally be joined in a downstream assembly operation. By eliminating assembly steps, multishot molding eliminates the labor, machinery, adhesives, and mechanical fasteners required for assembly. This simplifies production and lowers manufacturing costs. For example, Geis notes that Thermo Electron's handheld meter went from four parts to one, and production-related savings allowed the company to reduce the price of the meter by about $2.60. It also improves the lot-to-lot consistency of products, according to Navratil.
In addition, the in-mold combination of two or more parts into one reduces the amount of quality-related inspection of individual components. Less assembly and inspection translates into less component handling, which in turn lowers the risk of contamination, notes Rogon Walker, engineering manager for Dallas-based molder Moll Industries Inc.
Multishot molding also can eliminate downstream decoration steps. Text, logos, and other graphic designs can be molded into products, producing more-durable decorations than labels and stickers.
Perhaps no multishot advantage is more important than its effect on product quality. A multishot product “comes out as one product, not two components that are glued or screwed together,” says MacDonald. “So it's a better device mechanically, physically, appearance-wise—in every way.”
To make a device used for in vitro diagnostics, one medical firm was using a press-fitting process to join a polypropylene chimney and a die-cut silicone sleeve. This device was plagued by a number of problems, according to Tim Reis, vice president of healthcare marketing for GW Plastics, a molding firm in Bethel, VT. The issues included a poor seal and the possibility that the two pieces might come apart in the field, he says.
To solve these problems, GW Plastics used a two-shot process to produce a one-piece version of the device made of polypropylene and a thermoplastic elastomer. The new device features an improved seal design and a strong chemical bond between the two materials. “We eliminated an assembly step and ensured that the two pieces won't come apart somewhere down the line,” Reis says.
MGS Manufacturing has created vertically mounted units that allow either simultaneous or consecutive injection of more than one resin during molding.
A number of techniques are used for multishot molding. The choice of a method is normally based on cost and part design. Common multishot methods include overmolding, common core, rotary stripper plate, and core toggle or core back.
Overmolding. Not considered true two-shot molding by everyone, this technique involves two separate molds in two different machines. After plastic components are molded in one machine, they are brought to a second machine that adds a rubber layer or part to the components. A relatively low-cost two-shot alternative, overmolding is suitable for low-volume jobs that do not justify the purchase of expensive two-shot machines and molds, MacDonald says.
Common Core. In this process, parts stay on a common core throughout the manufacturing process. After the first shot, Berg explains, a platen or turntable rotates 180Þ to move parts to different mold cavities that create space for the second shot.
Rotary Stripper Plate. When second-shot material must completely envelop the first shot, molders change both core and cavity steel for the second shot. This can be done with a rotary stripper plate, which lifts first-shot material off its core steel, rotates 180Þ in most cases, and then sets the material on new core steel. New cavity steel is also waiting at the second location. The new core and cavity create the space for second-shot material to run in, around, and through the first shot, Berg notes.
Core Toggle or Core Back. Unlike common-core and rotary-stripper-plate techniques, this method involves no tool opening or rotation between the first and second shots. Instead, Berg explains, slides inside the tool move to create space for the second shot. By eliminating rotation in the mold, this method lowers two-shot costs. But not all part geometries can be produced this way. Sometimes, however, a molder can make design changes to initially unsuitable parts that will allow the use of core toggle.
Limitations and Downsides
In many cases, multishot molding lowers production costs by reducing labor and eliminating second and sometimes third sets of tooling used to create pieces of parts that are assembled outside conventional molds. On the other hand, multishot tooling is more complex than conventional tooling because it includes rotating platens or other features that allow it to create multipiece parts in one mold. Therefore, companies shopping for multishot tooling can usually expect to pay at least twice as much for it as they would for a regular injection mold, according to Stefan Rasch, director of application development for Mack Molding.
Is it worth the extra cost? The answer depends on the application. Thoreson advises OEMs that, absent a specific need for multishot capabilities, they should opt for conventional molding. Even though multishot molding eliminates assembly, he notes, manufacturers may find that it is less expensive to mold two low-cost conventional parts and assemble them using a high-speed automated cell.
With its high up-front costs, multishot molding is normally used in high-volume manufacturing operations that will provide an adequate return on investment. Generally speaking, Thoreson believes manufacturers should consider multishot molding only if they are producing more than 500,000 units per year. But there are exceptions to this rule. For example, consider a case where a two-shot process eliminates a failure mode in a part made in a conventional molding operation. In a case like this, a manufacturer might decide that two-shot molding is worth the added cost even for a relatively low-volume job.
Multishot molding has other downsides besides high upfront costs. For example, complex issues such as proper gate placement and cavity fill may prevent manufacturers from using the process, according to Rudolf Pavlik, new product development manager for Advanced Scientifics Inc., a contract manufacturer in Millersburg, PA. In addition, Pavlik notes, multishot presses and tools require more maintenance than their conventional counterparts.
Keys to Success
Orthopedic instruments from Stryker feature soft-grip handles created by insert molding with a double overmold, done by Mack Molding Co. (Arlington, VT).
When multishot molding is chosen for a job, there are a number of keys to getting the best results from the process. One of these is proper material selection. In many cases, the choice is two compatible materials that will chemically bond to each other. But if for some reason incompatible materials that won't stick together are chosen, the part design must include interlocks that create a mechanical bond between the materials. For example, Navratil says, a flat first-shot part design might include a hole that allows the second-shot material to flow through it and create a lug on the back side of the part. Once the second-shot material cools, the lug can't be pulled through the hole, so the incompatible materials are held together by a combination of part features.
Sometimes designers deliberately choose two materials that will not chemically bond because they want to use the multishot process to create a part with two pieces that move relative to each other. For instance, a two-shot process might be used to create a joint consisting of a ball and socket made of different materials that won't stick together and therefore will allow the relative motion necessary for the operation of the joint.
Although designing for bondability is not always necessary, designing for manufacturability should be a part of every multishot process. As early as possible in the process, Walker recommends, the part designer, molder, and tool manufacturer should meet to come up with a manufacturable design. According to Reis, such a design may include wells or channels that allow material flow. Other part features may allow the mold to seal off or contain the second-shot material in its intended shape, Navratil says. In addition, Thoreson believes multishot parts should be designed with an understanding of issues such as how the tool will be manufactured and how the part will be gated and ejected from the mold. In case all key issues aren't addressed up front, Thoreson advises people developing multishot timelines to allow time for at least a minor redesign for manufacturability.
Sometimes, manufacturable designs evolve during extended periods of dialogue between OEM and molder. At Mack, Rasch has seen some initial product designs that simply could not be produced in a multishot process. Customers “can't have absolutely everything they want,” he says. “So usually there is a lot of back-and-forth” to get the design into a more manufacturable state while still meeting the customer's requirements for the product.
If manufacturability is not taken into account in the product design process, molds can become so complex that multishot molding becomes cost-prohibitive or even impossible. So OEMs that can't or won't make some design concessions for the sake of manufacturability may have to look for another way to make their parts, Thoreson says.
As for two-shot tooling, one key to successful molding is the design of the perimeter shutoffs. These features ensure that second-shot material entering the mold “goes where you want it to go and doesn't produce any surprises,” says John Pfaff, vice president of The MedTech Group Inc. (South Plainfield, NJ), a contract manufacturer of medical devices and components.
Sometimes, Rasch notes, second-shot material entering the mold works its way underneath the intended substrate instead of covering the top of it. This can be prevented by proper gating techniques that cause the second-shot material to press the substrate onto the core. This helps to ensure that material doesn't get under the substrate and instead fills the proper cavity.
During multishot molding, rotations must be so precise that the tool can “absolutely accurately fit second-shot cores and cavities together,” Berg says. “There's no give or take there.” Such accuracy requires precise control of the tool's rotational movement, as well as extremely tight tolerances and accuracy in the mold itself, Navratil says.
Featuring various movements and operations, multishot tools are more complicated than conventional molds. So it's unwise to make a complex tool even more so by adding the extra features needed to make complex products, MacDonald says. Besides driving up the price of already-expensive tools, extra features make the tools more difficult to fix. “When one of these molds goes out for maintenance, it's very expensive to keep it sitting on the bench,” he says. To minimize downtime for maintenance, MacDonald recommends simpler multishot part designs with simpler tooling.
Because some multishot toolmakers offer prototyping services, Walker recommends that OEMs choose one of these shops because they let customers try out different material possibilities. “If you have questions about a certain material—like how well it will adhere to another material—these shops have tools that can run samples, so you know how the material is going to perform in your application,” he says.
Mistakes and Pitfalls
The pH meters (top) are injection molded of polycarbonate resin and have TPE input-output covers. An automated external defibrillator (bottom) uses three injection molding processes and three materials. Photos courtesy of MACK MOLDING CO. (Arlington,VT)
Sometimes, multishot molding produces less-than-satisfactory results because of a number of common mistakes and pitfalls. For example, Reis says, some manufacturers select materials that meet part performance requirements but are not good choices for the multishot molding process. Others use regrind, material that comes from remelting scrap or old parts. Remelting causes regrind to degrade faster than the new rubber normally used for overmolding.
During the overmolding process, the substrate is sometimes remelted or damaged by the application of hot second-shot material. According to Reis, this can be prevented by proper selection of first-shot material as well as by design features, such as ribbing to help cool the second-shot material by agitating its flow over the substrate.
In other cases, the two-shot process is adversely affected by system components that are not the right size for the job, according to Pfaff of MedTech Group, whose firm does two-shot molding and makes molds for the process. Many two-shot jobs require amounts of overmolded material that are 80–90% less by weight and volume than that of the substrate material. For jobs like these, Pfaff says, manufacturers should resist the temptation to use machines, barrels, and other system components that have far more material capacity than is necessary for the second shot.
“A lot of people think that they should just get the biggest machine they can, because they'll be able to fit anything into it,” he says. “That sounds good, but you actually lose a lot of [process] control if you don't have a properly sized barrel and machine.” The result can be an overly hot process that degrades materials or an overly cold process that slows material flow through the system.
Other suboptimal two-shot processes produce parts marred by plastic protrusions left by the gating technique. For two-shot manufacturers concerned about product appearance, Pfaff recommends the use of hot-manifold molds with valve gating, which eliminates unsightly gate vestiges. Such systems do leave small round marks on parts, but skillful molders can make these marks look like part features, according to Pfaff.
Thanks to developments in a number of areas, multishot molding is becoming more attractive to OEMs, offering more choices, lower costs, better technology, and new capabilities. As an example, Thoreson points to today's lineup of materials, which includes more compatible options that allow chemical bonding. In addition, versions of some of today's incompatible materials may one day be compatible. One notoriously bond-averse material is silicone, but Navratil cites work being done on grades of silicone that will bond with other materials in multishot applications.
As for the molding industry, OEMs that decide on multishot will find more mold makers in the field than ever before. “Some of the best mold makers are now making two- and three-shot molds,” MacDonald reports. Besides more and better mold-making options, molders cite improved mold technology as well as lower tool-making costs that are bringing multishot molding into the price range of a growing number of companies.
In addition, most if not all major machine manufacturers now offer multishot machines, according to MacDonald. Available in a variety of types and configurations, today's machines feature “phenomenal” process control, resulting in tighter tolerances and improved shot-to-shot repeatability, he says.
Completely new molding technology is the key to a promising process now under development at MGS: multishot micromolding of extremely small parts. Navratil and his colleagues are working on systems that will produce multishot parts weighing less than a gram. Needless to say, such parts will require systems that can deliver very small amounts of second-shot material. They also call for new techniques for depositing material in the mold. “It's not just a matter of reducing [existing] equipment in scale,” Navratil says.
“Traditionally, micro parts have always been made in single-shot processes,” he notes. “But we see a real opportunity for two-shot micromolding as well. And eventually I think you'll see some pretty exciting things coming down the pike.”
William Leventon is a freelance writer based in Somers Point, NJ, who contributes to MD&DI frequently.