LSR injection molding equipment built specifically for micro sized applications.

In the field of injection molding, there is a variety of machine suppliers from which a company can choose. However, when it comes to machines that are specifically designed for liquid silicone rubber (LSR) applications, the selection of possible vendors diminishes drastically.

High precision applications

Back in the mid-1980s, Kuntz Mfg. was contracted by a large pharmaceutical company to develop a new style of liquid injection molding machines for their then new line of intraoccular lenses (IOLs). Already familiar with Kuntz' design and development experience with special production systems, the pharmaceutical IOL manufacturer felt that Kuntz could develop a silicone liquid injection machine that could meet their specific needs. The liquid injection machines they found on the market at that time were all industry prevalent barrel screw type systems that were not well suited to deliver extremely low viscosity silicone materials accurately through the injection system. The barrel screw systems were designed for thermoplastic molding applications where hard and highly viscous pellets are driven through the screw in a heated barrel until they melt down under high viscosity. For liquid silicone, although injected into a hot mold for curing, the injection assembly needed to be chilled to inhibit silicone curing and clogging. Trial runs using these machines confirmed that their low viscosity silicone cavitated because it lacked the resistance to push. The liquid injection system Kuntz developed as a result of this contract was capable of handling 1 cps (centipoise) to upwards of 2 million cps. All the available systems offered high flexibility in typically large shot sizes because of the needs of large contract molding markets to which these presses were targeted. This meant large shots that could be adjusted to smaller shots; however, not small or precise enough for the tiny IOLs that represented a fraction of a cc even in multi-cavity molds. To solve these problems, Kuntz engineers developed a small six ton press, the Model 284, which featured a precise injection head sized to fit the unique application. Initially, a 3 cc injection head assembly was developed, and later a 10 cc version. Kuntz' injection designs consisted of only six components, excluding fasteners. The smallest assembly still offers the best precision through the rules of positive displacement, but both sizes provided the shot accuracy that no other system could easily deliver.

FDA considerations

As an IOL manufacturer, there were several requirements necessary to comply with the FDA. First, the injection head design, as well as the rest of the machine system, had to meet GMPs (good manufacturing practices) for implantable devices in clean rooms down to class 100. Second, the nozzle of the injection assembly had to be chilled right to the very tip and had to be protected from direct heat contact while injecting into the hot mold upwards of 450 [degrees] F. The nozzle also had to have a positive shutoff that disallowed drooling or leaving any trace of residue on the tip between shots that could potentially get into the next shot. That silicone residue, though small and literally immeasurable, if injected into the next shot inside the mold would end up clouding the required optically clear lenses. Kuntz engineers decided to use heat treatable stainless steel for the injection head so it would offer substantial production life while satisfying the FDA. The injection assembly was further enhanced for the second generation of machines by combining a cone shaped shut off pin that goes through the centerline of the injection plunger with a cone shaped injection tip that is precision ground and lapped to an exact fit.

The IOL company produced over 80 different powers of lenses with a combination of different haptics (the small curly type wires that are inserted into certain styles of lenses). Because of the delicate nature of the lenses themselves, some being less than 1 mil in thickness, automatic ejection became impractical. Instead of the traditional method of injecting from the top of the mold that is used in most vertical and horizontal machines, the manufacturing engineers opted to inject into the centerline, or parting line, of the mold. This was preferable because of mold handling for part removal and the critical need to clean the mold between each injected lot as required by the FDA. Tiny parts are difficult to remove with gloves required in hot mold handling. So a special mold cooling press was developed to speed up total cycle time otherwise delayed by the cooling requirement. The variety of lenses and silicone formulations for different part families created a need for fast mold changeover and a quickly removable injection head. To deliver the silicone, the team decided to use six ounce disposable plastic cartridges, as the small parts eliminated the need for typical five to 55-gallon in-line mix metering systems. The injection head had to be easily cleaned also, and the plunger type system subsequently developed was far easier and more efficient to clean than barrel screw machines. The use of disposable cartridges eliminated time consuming cleaning procedures as well. Kuntz' engineers developed a cooling jacket subsystem that fit right around the silicone feed cartridge. The design avoided bellowing on the feed cartridge that could create blow-by when forcing the cartridge plunger down. The cartridge conveniently screws onto a check valve or electromechanical ball valve, and the silicone is pushed through the injection head where another force created by vacuum comes into play. By pulling the silicone into the injection head, the plunger cylinder is withdrawn for reloading. This double acting force minimized cycle times. To further satisfy the FDA, the Model 284 injection press was produced as the first ever non-hydraulic molding machine to eliminate the risk of oil leaking on the floor or dripping on unwanted parts of the machine. Today, many of the molding machine suppliers have followed suit even in non-critical medical device and thermoplastic applications by providing the option of oil-free molding machines. The high cost of floor space in clean rooms makes floor space utilization or space savings very important. The Kuntz molding machine took up significantly less floor space than even the smallest molding machines available. Because of FDA requirements regarding implantables, Kuntz placed an integrated chiller inside the frame of each machine (models for central chiller hook-up are now available). The machine also met important safety guarding requirements for machine operator intervention, as well as ergonomic practices and designs. A color operator touchscreen with lockouts which prevents unauthorized changing of specific, pre-programmed run parameters, as well as storable recipes for different set-up routines, have become standard on Kuntz presses. By complying with these high standards, the validation process with the FDA went smoothly. Over the years, Kuntz has built a total of 18 presses for this pharmaceutical manufacturer alone.

Liquid injection molding machines today

Since Kuntz' introduction to the LSR molding market, one of the downstream designs has been patented and several improvements to the original machine have been developed, leading to the latest horizontal injection press model, the 384. Customized features have been innovated for the specific needs of mostly implantable manufacturing clients and medical device manufacturers. Products such as catheter and guidewire components, drainage tubes, cochlear hearing products, heart defibrillators and multi-lumen components have been successfully molded using Kuntz injection presses, and world leading companies have standardized on Kuntz horizontal injection machines to meet their critical production needs. However, Kuntz found that while their centerline injection approach was very successful, this style of molding represented only 10% of the total market for silicone molding machines. Kuntz had not sold many machines to contract mold manufacturers who make up the bulk of the LIM market, even though they have shown consistent interest over the years. The reasons? Contract manufacturers wanted flexible machines that could make small precision parts with the versatility of large shot runs for high cavitation or large parts to meet various client needs. As it turned out, one of Kuntz' valued clients that had standardized on Kuntz injection molding machines was in need of a new machine that would work with an in-line mix metering unit for both six ounce cartridges and five gallon pails. The client was St. Jude Medical in Sylmar, CA. The product engineer explained to Kuntz the need to obtain larger shot sizes while maintaining the injection accuracy to which their product team had become accustomed. Kuntz engineers proposed a new system that utilized their proven injection head shut-off feature and integrated mix metering equipment which allowed shot sizes up to the limit of the specific mix metering unit. A bigger 15 cc injection head was designed, and for the first time, 80 gram parts were produced on Kuntz injection presses.

This new element led Kuntz to develop a molding machine that injects from the top of the mold. The new Model LSR 2010 vertical injection press features a bigger platform with bigger clamp force (up to 10 tons), incorporating all the innovative features of the Model 384 parting line injection machines. The streamlined, small footprint LSR 2010 is especially well suited for custom molders who rely on quick mold changeovers, utilize standard mold sizes and prefer a flexible shot size range. The first machine was introduced at the Medical Design and Manufacturing show in Anaheim, CA last January with great success. Primary enhancements integrated in the new design include adaptation to standard molds up to 8" x 8" (typical mold size of the Model 384 machine is 4" x 4"), standard platen mounting hole patterns and adjustable daylight in 1" increments to save mold processing time. Optional automatic door opening provides easy part removal, reducing cycle time. Both standard models are small enough for R&D set-ups and durable enough for continuous production runs. They are ideal for micro- to medium-sized parts, single or multiple cavities, insert and overmolding of medical and non-medical applications alike.

In addition to injection molding machines, Kuntz Mfg. has supplied special automation production equipment to a variety of industries since 1969. The company offers several mechanisms of interest to injection molders wishing to automate their processes, including systems that involve automatic feeding, orienting and placing inserts into injection molds at high speeds. Provisions can be made for part checks prior to mold closing and placement of finished parts in trays for further processing or shipment. Another system removes whole runners with parts from the mold, laser cuts the parts and places parts in discrete magazines for further processing. Kuntz can typically customize any process operation to customers' needs and will review special requirements.

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