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Electric or Hydraulic?

Originally Published in MPMN May 2001


Electric or Hydraulic?

A comparison of injection molding machines

Electric injection molding machines, like this one from Milacron, are accurate and energy efficient.

When electric injection molding machines were introduced in 1985, they gained immediate popularity because of their low energy consumption and high accuracy. However, these benefits came at a cost: the sticker price of an electric machine was far greater than that of a conventional hydraulic molder. More than 15 years later, the debate on the cost-benefit ratio continues. This article compiles some of the claims made by manufacturers on both sides of the divide. It is based primarily on information provided by Boy Machines (Exton, PA), a supplier of hydraulic equipment, and Milacron Inc. (Batavia, OH), which produces electric injection molding machines.

Price is one of the first things that companies consider when investing in capital equipment. While there are exceptions to every rule, the base price of a hydraulic machine is generally less than that of an electric counterpart. How much less varies, but Boy Machines national sales manager George Dallas says that "in a 14.2–88-tn range of clamping forces, hydraulic machines often offer a 50% saving." Advocates of electric molders are quick to point out, however, that the actual cost of a machine involves more than its purchase price; operational costs must also be taken into account.

Power consumption is one of the major factors in how expensive a molder is to operate. An increasingly salient issue as the price of electricity rises across the country, energy efficiency is often what allows electric injection molders to justify their higher purchase price. In some comparisons performed by Milacron, electric machines consumed up to 90% less power than hydraulic molders. Manufacturers say these savings can be achieved because electric machines do not constantly circulate hydraulic fluid, consuming energy only when motion is required. With generally lower operating temperatures, electric machines also require less power for cooling. According to Milacron general manager Barr Klaus, these factors make it possible to "run two or three all-electric machines on the same amount of electricity as one conventional [hydraulic] unit." He goes on to say that "the larger the [electric] machine and the higher the throughput, the greater the savings." Those in the hydraulic machine camp respond that these figures compare new electric machines to dated hydraulic ones, and that recent technological advances have reduced this deficit. For example, Dallas says that Boy's new hydraulic machines "employ an electronically controlled variable-flow pump that makes their energy consumption only slightly higher than comparable electric machines."

Cleanroom use

This hydraulic molder by Boy Machines and other similar units have low purchase prices and other benefits.

Since many medical parts must be produced under sterile conditions, suitability for cleanroom use is an important consideration for manufacturers, and both machines have their advantages. Electric models don't contain fluids that can leak and taint parts. Hydraulic molders eliminate the belt drives and other power-transmission elements that can abrade and cause particulate contamination. Both sides also have a ready response to these concerns. Boy's new machines localize the use of hydraulic fluids, minimizing the damage caused should a leak develop. Klaus says that Milacron's electric molders do not produce particulates "when the belts are set up correctly and operating at the recommended tension."

When it comes to accuracy and repeatability, electric machine makers claim superiority because electric units are digitally controlled like precision machine tools. According to Klaus, Milacron's "electric machines consistently hold tolerances one-half those of even a Hunkar Class 1 hydraulic machine." The lack of hydraulic fluid and tubes that can expand and valves that can stick contributes to this effectiveness. Because they use less material, electric molding machines also offer the benefits of producing less waste and reducing manufacturing costs. Hydraulic machine manufacturers point out that accuracy is not just a function of the control system. Instead, the ability to accurately produce parts is determined by the control system in conjunction with the entire mechanical system. New hydraulic units, say supporters, have been designed with innovative features and accurate mechanical systems that allow them to achieve similarly accurate outcomes. The variable-flow pump that limits drift on the Boy machines is an example of such technology.

Productivity factors

Precision means little, however, if a molder doesn't also offer good rates of production. While throughput depends upon the complexity of the part, both types of injection molding machines have features that enhance productivity. Both electric and hydraulic machines can be used during unmanned shifts.

Similarly, both units can run instantaneously upon start-up. Waiting for oil to preheat, a concern for hydraulic systems in the past, is no longer an issue, according to Dallas. Boy's machines, he says, are "ready to operate from the moment they are switched on." One advantage of hydraulic machines is the ability to perform certain hydraulic functions, such as core pulls, within the mold. (Integrating a hydraulic unit to an electric molder to perform such tasks can negate some of the benefits cited by manufacturers.)

Hydraulic and electric injection molding machines offer other benefits that don't fall neatly into any of the above categories. For instance, both sides maintain that their units have fewer parts, and thus require less maintenance. It is possible that both assertions are correct, if the comparison is limited to a type of part. Hydraulic machines don't have as many motors, joints, gear mechanisms, and subsystems. Electric models eliminate the hoses, filters, tubes, and valves of a hydraulic system. Manufacturers of electric machines say that their equipment offers a number of other cost-related benefits. Higher accuracy means electric units use less total material and produce less waste, reducing manufacturing costs. The costs associated with storing and disposing of used hydraulic fluid are completely avoided. Lower power requirements can also allow electric molders to lower construction costs for new plants as they can function with smaller electrical hardware, bus bars, and air-conditioning units. Another advantage of electric machines is generally quieter operation. Klaus says that Milacron's electric machines run at "about 70 dB, the same noise level as an office copier." This low noise level allows them to be used where sound is a concern.

Hydraulic machines have their advantages as well, however. Manufacturers say that one of these benefits is the elimination of toggle clamps, parts that build tonnage through mechanical leverage and thus are subject to wear. Hydraulic machines also don't use recirculating ball screws, components that are repeatedly subject to wear at the same point in the cycle. In keeping with their lower purchase price, hydraulic machines "can sometimes offer less-expensive replacement parts and servicing," Dallas says.

There are no clear-cut answers for manufacturers weighing the pros and cons of electric and hydraulic injection molding machines. There are advantages and disadvantages to each option: ultimately the machine that is right for your company must be determined based on the specific application.

Zachary Turke

Copyright ©2001 Medical Product Manufacturing News

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