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Good Vibrations: Ultrasonic Technology Applied to Micro Molding

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25. June 2014

Back in 2007, researchers at the Ascamm Technology Centre in Barcelona, Spain started investigating melting thermoplastics via ultrasonic energy. After proving out the process, the researchers considered possible commercial applications, according to Enric Sirera, who became sales director at Ultrasion, the commercial venture spun off in 2010 from Ascamm’s to commercialize the invention. 

 

“[The researchers] saw a market need for small parts, micro parts, including ones with higher aspect ratios,” Sirera explained. “They saw a commercial opportunity.” Ultrasion was created in 2010 as means of “designing, developing, and industrializing a machine surrounding this ultrasonic molding process.”

 

Ultrasion’s vision is to use ultrasonic waves to melt plastics prior to molding, as opposed to the shear and radiant heating used in the heater-band, reciprocating-screw, and barrel set up of traditional injection molding. By doing so, the researchers believed they could prepare only the required amount of material for each part versus bringing an entire barrel of material up to temperature, with the subsequent residence time and potential for degradation.

 

In their first crack at a ultrasonic-centered machine, the researchers constructed a prototype press by taking a standard injection molding machine, removing the entire injection unit and substituting one of their design.

 

“It worked perfectly,” Sirera recalled, “it was great step forward. At that point, however, we realized that the hydraulics and the clamping force were over sized and over dimension for what we needed. So, we said, ‘Hey, let’s think about redesigning a new machine according to this process.’”

 

With the first prototype machine completed in 2010, the company hit the show circuit to begin promoting the technology, including stops in Germany at Fakuma and Orlando at NPE2012. Last year, Ultrasion participated in the K Show in Germany, as commercial sales began in earnest.

 

Today, Sirera notes there are 12 machines in the field, with seven running production and the rest involved in further research at universities and R&D centers. Those machines are spread throughout the U.S., U.K., Poland, the Netherlands, and Spain, working in medical, aerospace, and precision mechanics applications.

 

Key difference
Sirera notes that one key differentiator for Ultrasion’s molding technology (they drop the injection, more on that later), is how ultrasonic melting of the pellets lowers the material’s viscosity.

 

“This means at the same melting temperatures,” Sirera says, “the viscosity by ultrasonic heating drops down, leading to the possibility of molding at much lower pressure, with less stresses internally, as well as the ability to make the material flow into thinner, tinier geometries that previously had not been able to be filled.”

 

Instead of a traditional hopper-fed barrel and screw, Ultrasion machines feature a dosing unit, dispensing only the amount of material needed to be melted for each cycle. Once inside the dosing chamber, the resin is heated via ultrasonic waves, vibrating the plastic and creating spaces within its molecular structure. “When you create more space around the molecules,” Sirera explains, “you lower the viscosity. As the free volume increases, the viscosity drops down.”

 

In micro injection molding, Sirera notes that pressures can easily rise to 1200 bar and higher. With ultrasonic melting, however, those pressures drop down to the 300 to 500 bar range.

 

The Ultrasion machine is technically rated with a clamping force of 3 m.t., but even that description is overkill, according to Sirera. In production, he notes that  the Ultrasion machine typically uses from 1.5 to 2.2 m.t. of clamping force. As an added bonus, the elimination of heater bands, as well as hydraulic pumps and motors normally used to keep the clamp shut under high pressure, means that energy consumption for the Ultrasion is reduced by 85 to 90 percent compared to a standard injection molding machine.

 

Residence time
In a standard micromolding setup up, where a part might utilize a .1g shot and the machine has a 100g capacity barrel, a molder would have to go through 1000 shots to clear the barrel. “This can lead to big problems,” Sirera notes. In the Ultrasion design, the dosing unit handles the material at room temperature, and only as needed.

 

“Imagine a hopper with material at room temperature,” Sirera explains. “The machine stays at room temperature. As soon as we want to mold a part, we close the mold, dose raw material as pellets into the mold—using just the amount of material for that shot—and then the horn comes down, vibrates, and melts only the amount of material dosed into that shot.” Once melted, a plunger pushes the molten plastic into the tool cavity at much lower pressures.

 

“There’s no residence time at all, which means the machine can be started and stopped at any time,” Sirera says, adding that there are no purging operations either. If a material change is needed, the hopper is simply emptied and refilled.

 

Sirera says parts still have a runner and sprue, which can become outsized in micro molding, but here he notes Ultrasion still saves between 40 to 70 percent of the equivalent cold runner compared to traditional micro injection molding.

For the material, eliminating the dual stresses of thermal degradation caused by long residence times as well as injection under high pressure has had some interesting results.

 

Ultrasion has seen less change in the polymer’s molecular weight, helping materials retain mechanical properties, while the process also means the polymer chains “refreeze nicely”, according to Sirera, resulting in a stronger, more homogenous melt and part.

 

Apart from silicones, Sirera notes that the technology is suitable for all types of thermoplastics, including high-temperature materials like PEEK, PSU, LCP, and POM. In filled materials, or ones with additives, Ultrasion has also seen better dispersion and more homogeneity in the finished compounds and parts. At this time, maximum overall shot sizes are around 1.5 to 2 g, but could go bigger, to a point.

 

“If you ask me if some day will we make a bumper fascia using ultrasonic molding, I don’t think so,” Sirera says, before adding. “It’s too soon to tell.”

 

That doesn’t mean there aren’t big opportunities in small parts, however. “Mold geometries that had previously proven impossible are now possible,” Sirera says. “When we talk about design for manufacturing, now you have a new manufacturing technique that will allow you to try new geometries. We don’t know what the limits are yet, but we envision a huge opportunity.”

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