Electric vs. hydraulic—a scientific approachElectric vs. hydraulic—a scientific approach

As plants intermingle all-electric machines with hydraulics, problems can arise when molds are moved from one press to the other. A scientific perspective can help.

Given the current state of the industry, molding consultant John Bozzelli figured his speaking schedule would have slowed down recently. But over lunch during his latest stop at the Plastics Technology Center (PTC) in Angola, IN, the prophet of what he calls scientific molding admits that he’s been busier than ever, precisely because of the current business atmosphere.

As molders search for answers to ever-looming competitive questions, many have turned to Bozzelli, who preaches tenets that center on “the plastic’s point of view.”

Repeatability is the goal of any molding program, and by creating detailed parameters for a mold, variables like shot size, fill time, and plastic pressure can be adjusted for any machine to produce good-quality parts. On this occasion, the PTC’s facility provides a unique opportunity for Bozzelli to apply the parameters from a 90-ton hydraulic Boy to a 75-ton electric Sumitomo.

On the final day of the three-day seminar, Bozzelli did just that, and using the setup sheet from the Boy to define the process’s baselines, the Sumitomo was creating good-quality parts after only 10 shots and approximately 7 minutes. The only changes: position cutoff by .1 inch and a higher hold pressure. IMM examines the steps Bozzelli took for a seamless mold transfer from a hydraulic to an electric, with scientific precepts.

A Ratio for Success

Bozzelli started by determining the Boy’s parameters, but not necessarily those on the spec sheet. He was most interested in the intensification ratio, which defines the actual plastic pressure at the nozzle.

In simplest terms, the intensification ratio is found by dividing the maximum injection pressure by the maximum hydraulic pressure. For the Boy, the intensification ratio is calculated by dividing the area of the intensifier or ram in the injection cylinder by the area of the nonreturn valve. As an example, if the intensifier area is 10 sq in and the valve is 1 sq in, the ratio is 10:1. So if the hydraulic pressure behind the ram is 268 psi, at the nozzle the plastic pressure would be 2680 psi.

After finding the ratio for the Boy (7.33:1) Bozzelli began to mold parts, all the while increasing the fill time, and subsequently the viscosity, as he went. The PP part he was molding came from a Glenn Beall training tool and weighed approximately 28g. Over 12 shots, Bozzelli took from .36 second to 13.28 seconds to fill the single cavity.

Following his scientific maxims, Bozzelli analyzed the rheology curve to obtain the lowest viscosity/most repeatable/fastest process. Molded with an injection time of .36 second at a plastic pressure of 5530 psi, the test part was made with a relative viscosity of only 1991 psi/sec. Conversely, the slowest injection with a fill time of 13.28 seconds used a machine velocity of 1.58 mm/sec, which created a lower plastic pressure of 4203 psi but an incredibly high relative viscosity of 55,814 psi/sec.

The series of shots is used to create a viscosity vs. flow rate curve. Using this, molders can target a section of the curve that follows Newtonian principles to determine the ideal fill and pressure scenario, since the non-Newtonian section of the curve is less stable in the face of process variations.

Universal Setup?

On the final day of the seminar, Bozzelli applied the lessons learned on the hydraulic Boy to the all-electric Sumitomo. The machine was smaller in terms of size, 75 vs. 90 tons, and screw diameter, 32 vs. 48 mm. To accommodate this difference, the stroke length, which was 25.4 mm on the Boy, was increased to 57.1 mm on the Sumitomo to achieve the desired shot volume. Bozzelli also had to account for the lack of hydraulic pressure on the electric and how that would affect the intensification ratio.

Normally, the pressure at transfer (650 psi in the case of the Boy) is multiplied by the intensification ratio (7.33 on the Boy) to determine the plastic pressure (4678 psi for the Boy). Using this number as a benchmark, Bozzelli set a fill time of .48 second, which created a baseline for process parameters (shot size, fill time, transfer position). But with the all-electric, the entered hydraulic pressure must be the actual plastic pressure, since the ratio is 1:1.

Those misunderstandings cause unnecessarily long setup times for transferring molds from hydraulic to electric machines. Bozzelli says molders often run electrics with low backpressures, assuming they can carry over hydraulic backpressures.

“It’s a big deal today,” Bozzelli says. “A lot of people are doing it , and a lot of people are getting confused. A lot of people will put 140 psi for backpressure [on a hydraulic], and then they’ll go to an electric and put 140 psi, where you should put something like 1000 psi.”Using the intensification ratio of 1:1 and therefore dealing with plastic pressures for first-stage limit and transfer, hold, and backpressures, Bozzelli accurately predicted shot size for the test mold’s parts when he transferred the tool to the electric. The predicted cutoff position was only .1 inch off—due to the electric’s brake—and pressures were virtually identical.

The only other difference of note was in the machines’ mold temperature controllers and cooling. In the Boy’s cooling system, water was diverted to cool the hydraulic oil. This, of course, wasn’t an issue for the Sumitomo, where cavity pressure dropped faster since the part shrunk away from the walls due to better cooling. Bozzelli says that most shops can easily account for this discrepancy, giving both machines the same cycle time.

Initial part weight was within .2g; that was easily equalized by adding 100 psi of plastic pressure to the second-stage or holding pressure. As mentioned before, all these adjustments were made in the first 10 shots and took about 7 minutes, showing how Bozzelli’s Universal Setup and scientific approach allow you to drop a mold into any press with predictable results, even if one is hydraulic and one is electric.