How to automate LSR molds

A founding father of the U.S. LSR molding business specifies the six key things you should specify in molds for liquid molding and why they’re important.

If you’re a thermoplastics molder interested in molding thermoset LSR, you must, as a wise Jedi master in the Star Wars movies once said, “Unlearn what you have learned,” especially when it comes to tooling. A wise master of LSR moldmaking says that if you want to run LSR molds as automatically as your thermoplastics molds, particular attention must be paid to six key elements: cold runners, ejection, shutoffs, temperature control, materials of construction, and vacuumization.

After more than 35 years of designing, engineering, and building LSR molds, George Kipe, 60, the founder and president of Kipe Molds Inc. (Placentia, CA), says he can’t remember ever seeing the amount of interest he’s seeing these days in molders wanting to mold or multimold LSR parts.

“Why? Well, I think everybody wants to tie up with the medical market and medical companies want to tie up with LSR. But LSR molding is very, very different from molding thermoplastics,” he says. “Don’t let anyone tell you it isn’t.”

Kipe’s business is growing. In fact, he says he’s running out of room at his 13,000-ft2 facility. “We’ve got about 20-30 molds running through the shop right now. We’ve been having a lot of fun,” he adds.

Sensitive cavities

One reason for all the fun, he says, is his discovery of all of the benefits that the temperature- and pressure-sensing, closed-loop process control technologies of Priamus System Technologies LLC (Brunswick, OH) can bring to LSR molds and to LSR molding in general. Temperature (hot temperatures, that is) is critical when running thermoset LSR. LSR cures—not cools, cures—in molds at temperatures greater than 400°F. Temperature sensing is key.

“Knowing the dispensing temperature of silicone as it passes over a given area of a cavity—that’s what fun is. The Priamus temperature sensors at the last place to fill give you the same results as pressure sensing when you’re running silicone,” says Kipe.

“You can ‘see’ the fill pattern and adjust the valve gate controller to make filling patterns all the same, shot after shot. That gives you better control—you can delay the opening of a gate and you can control gate closing, too. Opening gates is more important in a valve-gate mold, though—the higher the velocity, the more stable the process.”

Thermostat switchover

At the very beginning of a shot, the Priamus system can control the opening of any given nozzle. Kipe says that’s another key benefit Priamus’s systems provide. “The Priamus system will tell you if you have one cavity filling faster than the others. But it’s the cavity temperature sensing that’s most important,” he says.

“Now you know what cavity temperature is. It’s usually different from what you can see with a pyrometer or monitoring the molding machine’s temperature—‘radiation’ temperature is different outside the mold than inside the mold. Temperature has a profound effect on how silicone flows. Variations can cause flash, cutoff, short shots—all kinds of problems.”

Packing pressure also can have a significant impact on maintaining part tolerances when molding LSR. Kipe says silicone shrinks .030 inch, but is prone to pack.

“It’s like stuffing a bunch of marshmallows in a box. The more short shots you have, the more shrinkage you have. And the more you pack, the more you flash.”

“You need devices in the tool like the Priamus system to automatically control the inmold environment. That way you have a better chance of holding your tolerances.”

Pressing matters

Kipe also believe that a state-of-the-tech injection press is necessary for carefree silicone molding. He’s got a couple of Engels and “a whole bunch of little Arburgs, including a new, little, electric one” for mold tryouts. He also has a new 100-ton Toshiba all-electric.

“If you’re thinking about molding LSR, you’ve got to know the material and you’ve got to have a good machine to process it on,” he says. “What’s fun about electrics is their quick response time and repeatability. And customers want to know what’s going on in trial runs. They want to see printouts of everything. Electrics certainly can deliver that kind of data.

“You don’t want the bottom end of molding machines. Don’t shop on price. You need the best controls and features you can get to run silicone. You need a new machine.”

LSR Auto-tool Tip #1: Cold runners

Either valve-gated or open-nozzle systems can be used. George Kipe prefers pneumatically driven valve-gated systems with tapered shutoffs. He’s found that the material has a tendency to cure prematurely in transition with open-nozzle systems, which can cause sprue plugs to get into molded parts, owing to the material’s viscosity and the amount of fill. Gate vestige also can be a problem.

“The varying viscosity of silicone is a problem, too,” he adds. “If the hole is too big, there’s a tendency for drool. You’ll lose control of the shot. And as the material cures, as it gains heat, there’s thermal expansion and it tries to force itself back through the hole.”

He’s quick to point out that open-nozzle systems are more economical and less complex. And he adds that a number of European LSR mold manufacturers have developed very good open-nozzle cold runner designs. Material suppliers also are willing to modify filler loadings, which can affect the silicone’s flow rate and viscosity, to provide custom blends to meet open-nozzle requirements.

But Kipe believes that valve gates are more versatile. His company’s systems reportedly are capable of running runnerless parts as small as .01g to parts weighing more than 200g. The company supplies its own PLC cold runner system control, which can even commandeer sequential filling applications.

LSR Auto-tool Tip #2: Ejection

Kipe reminds LSR newcomers that silicone parts are soft. When the mold opens, they might get stuck, requiring a three-plate mold, a slide rack, or a robot with properly designed EOAT for removal. But he says TP molders have a leg up on their counterparts running conventional thermoset rubbers when it comes to understanding the costs and complexities of high-quality tooling and the automation associated with it.

Part design is key. A flat strip, or a round, disk-shaped part can simply be blown off. But parts with undercuts or those requiring sidepins have to be removed in a very special manner. LSR parts are flexible. Kipe says it’s only logical to get a robot when you get your LSR molding machine, and pay very special attention to what will be required to get your parts out of the mold.

LSR Auto-tool Tip #3: Shutoffs

If you’re thinking about shutoffs in an LSR tool, Kipe says you’re thinking about shutoff tolerances down to two tenths. He calls it an area of overexpectation.

“You can achieve your shutoffs through grinding, lapping, jig grinding, EDM, whatever—but it still boils down to two tenths. LSR will flash at two tenths,” he says. “You’ve got to have a Mr. Wizard toolmaker and you’ve got to have air conditioning.

“If you have one plate, or a set of inserts that are 25 deg F hotter or cooler than another, you’ve already burned your two tenths and have created a potential for temperature fluctuations, and that equals flash. You have to study the part and know the abilities of silicone—what you can and cannot do.

“You’ve also got to know the fill pattern, paying special attention to areas where you’ll have the best control over the pressure of silicone. You don’t want your shutoffs next to the gate where the pressure’s highest—you’re more likely to flash if you do. Molding silicone is like trying to make a waffle in a waffle press.”

LSR Auto-tool Tip #4: Temperature control

Kipe calls temperature control in an LSR tool a “double whammy.”

“You have to heat up your mold, but then, all of a sudden, you may find one cavity’s running cooler—there’s more sticking and parts may be more prone to flash. So then you heat up the mold, but find you’re running it too hot.“You have to find the sweet spot. Temperature control is extremely critical. You’ve got to take your best shot when it comes to heaters and wattage, and it may involve a lot of trial and error. Platen thickness also must be optimized.”

Kipe says that calrod-type heaters work well, but they’re more expensive to manufacture. Kipe Molds has settled on more conventional ram-type cartridge heaters. “If the heaters are too close to the cavity, you’ll have hot spots. If they’re too far away, the material won’t cure. With the Priamus system automatically controlling switchover, you’re killing two birds with one stone.”

LSR Auto-tool Tip #5: Materials of construction

Although medical customers like stainless steel because it’s not prone to corrosion, Kipe says stainless doesn’t conduct heat very well. He’s found that powdered metal cores and cavities are more uniform conductors of the heat required to cure LSR, while also providing good compressible strength (50-55 RC) and good ventability.

“I like cavity inserts,” says Kipe. “They may be a more expensive method, but the issue gets back to shutoff quality and ease of service. I like to make inserts wherever possible, even though I’m sometimes criticized for the cost of my molds.

“European moldmakers trial their tools out before their customers take delivery, and that’s what we do here. Cavity inserts make trialing easier.

“Nowadays we have special grades of silicone, we have speed goals, we have reject requirements—we have to know how it’s all going to work! I can’t afford to make a mold and just keep my fingers crossed.”

LSR Auto-tool Tip #6: Vacuumization

Silicone’s flowability isn’t a problem. But getting the air out of the cavity is, according to Kipe, especially in complex cavities where proper filling is an issue.

“Silicone has a tendency to run over, or to find the path of least resistance. Usually you’ll have a seal on the outside of the cavity, and have seals on other mechanisms, like ejectors,” Kipe says, “and right before you fill the mold, you’ve got to leave it open about 25-30 thousandths, just enough to engage the seals.”

Fortunately, most modern injection machines have vacuum sequencing capabilities. Once proper cavity vacuumization is reached, the press switches to clamp-up and injects silicone, somewhat like the coining sequence in molding TPs.

Vacuum sequencing can remove air from the cavity, sure, but Kipe says there are many opportunities for air to get into the material: during materials formulation, batch or continuous mixing, or pumping the material into the barrel. Too much air can cause blistering, and will stop the curing process.