Mold Micro Parts with Ultrasonic Technology

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

“The researchers saw a market need for small parts, micro parts, including ones with higher aspect ratios,” Sirera explains. 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 conductive heating used in the normal combination of heater bands and reciprocating screw for 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 an 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 recalls. “It was great step forward. At that point, however, we realized that the hydraulics and the clamping force were oversized 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, Fla., at NPE2012. Last year, Ultrasion participated in the K 2013 show in Germany as commercial sales began in earnest.

Today, 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 DIFFERENCES
Sirera notes that one key differentiator for Ultrasion’s molding technology (they drop the term “injection”), is how ultrasonic melting of the pellets lowers the material’s viscosity. “This means that at the same melting temperatures, the viscosity by ultrasonic heating drops down, leading to the possibility of molding at much lower pressure, with fewer internal stresses, 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 hopper-fed barrel and screw, Ultrasion machines feature a dosing unit that dispenses only the amount of material 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 rise easily to 1200 bar and over. 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 metric tons, but even that would be overkill, according to Sirera. In production, he says 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 85-90% compared with a standard injection machine.

NO RESIDENCE TIME
In a standard micromolding setup, where a part might utilize a 0.1-g shot and the machine has a barrel with 100-g capacity, 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 material 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 micromolding, but here he notes that Ultrasion still saves 40-70% of the equivalent cold-runner volume compared with standard micro injection molding. For the material, eliminating the dual stresses of thermal degradation caused by long residence times and by 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.

Sirera notes that the technology is suitable for all types of thermoplastics, including high-temperature materials like PEEK, polysulfone, and liquid-crystal polymers (LCP). In filled materials, or ones with additives, Ultrasion has also seen better dispersion and more homogeneity in the finished compounds and parts. Maximum overall shot size currently is around 1.5 to 2 g, but could go somewhat larger.

“If you ask me if someday 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. “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.”