Choosing a static mixer

Variables can affect performance of two-component adhesive
By David W. Kirsch
December 1, 2007
COMMERCIAL, RETAIL, FABRICATION : ADHESIVES

 

Choosing a static mixer requires more than reading a sales catalog and selecting a part number. Adhesive manufacturers and end users both should investigate many variables when evaluating mixer characteristics for specific applications. This article is a guide to help make the right decision when choosing a static mixer to properly mix a two-component adhesive.

Sometimes called a motionless mixer, a static mixer is a simple device with no moving parts. It consists of a series of internal baffles or elements within a plastic tube. This seemingly elementary product is used to effectively mix two flowable liquids in a complicated process. As adhesive components are forced through the mixer, they are repeatedly divided and recombined, creating a complete and uniform mixture.

Utilizing a static mixer provides many benefits, including consistency of mix and eliminating air in the mixing. The latter is essential, since air represents a source of voids in cured bond lines and the possibility of a bonding failure. Process control in hand mixing is difficult to maintain; it can create serious bonding, waste, cost and safety problems.
Static mixers consist of a series of internal baffles within plastic tubes.
Applications
Static mixer applications can be divided into two categories: for use with a cartridge and a hand-held dispenser; and for use with meter mix and dispense equipment.

Flow rate is a key factor when choosing between hand-held cartridge and MMD systems. In any two-component adhesive operation, the components are kept separate in a pre-proportioned, molded plastic cartridge or in machined steel cylinders before being introduced into a static mixer. The user must calculate the amount of pressure necessary to keep the two liquids flowing at the appropriate rate for a specific application.

For example, with high viscosity components, it might require a significant force to move the liquids at a proper flow rate. Understanding the maximum flow rate that a hand-held cartridge system can generate provides a data point for making a decision for cartridge vs. MMD equipment.

MMD systems are automated and can dispense higher volumes of liquid than hand-held cartridge systems. “Volume is one of the widely used reasons for switching to an MMD solution, although volume alone is not a deciding factor,” says Don Leone, general manager, Ashby Cross Co., Newburyport. Mass. “In many instances, the use of MMD systems is an upgrade from hand-held cartridge systems, particularly when time-controlled adhesive shots are needed. This is frequently the case with assembly line and robotic applications, which often are handled with MMD systems.”

In contrast, airplane manufacturers use hand-held cartridge systems because their bonding operations are spread out across large factory areas. An employee inside the plane must work in an extended area that has many tight locations. Plant maintenance operators also are likely to use hand-held systems for similar reasons.

Complex manufacturing or maintenance operations often use different adhesives in a wide range of applications. Changing adhesives with MMD systems is laborious; some say changing adhesives in a hand-held system is no more difficult than changing a cassette in a VCR. MMD systems are primarily used inside buildings and plants; hand-held systems are used both inside and outside.

Hand-held cartridge systems are manufactured in a limited number of volumetric ratio systems: 1:1, 3:2, 2:1, 4:1, and 10:1; MMD systems always are customized, and yield a nearly unlimited set of volumetric ratios. Many ratios outside of structural applications are not compatible with the discrete ratios offered in cartridge systems. A ratio such as 100:36 or 100:28 requires the use of MMD equipment.

Each specific application frequently represents a simple decision for adhesive manufacturers and end users because flow rate, volume, production and related requirements dictate a straightforward choice between hand-held cartridge and MMD systems. Nonetheless, a hand-held cartridge system is often a good choice to simulate an adhesive application before moving to the more expensive MMD solution. An adhesive manufacturer or end user can experiment with hand-held cartridge systems to test variables that affect static mixer performance for specific applications, such as temperature, humidity, length or diameter of the mixer, and curing time and work life of the adhesives.

Reliability
Reliability in adhesive operations is paramount. Every operation needs to have consistent results.

Although a static mixer typically gives consistent results, variables can affect its performance. For example, temperature and humidity can have a significant impact on chemical and physical reactions and processes; therefore, static mixers should be tested under various climatic conditions.

The need to achieve a consistent mix underscores the importance of experimenting with both manual and pneumatic dispensers, because the type of dispensing gun and how it is used can influence adhesive outcomes. Each person operating a manual gun might have a different dispensing style and hand strength.

Hand-held system solutions are the most prevalent, but for a given application a pneumatic gun might be needed for static mixer usage. Pneumatic guns can provide constant pressure on the cartridge and mixer and minimize problems caused by constant flexing. “Problems arising from such flexing can be mitigated to some degree by using plastic cartridges with thick walls, which are more rigid and stronger,” says Rich Wilson, business manager, ConProTec, Salem. N.H. “This introduces another variable in the decision process for manufacturers and end users.

“A user can take an integrated approach and select dispensing tools and static mixers made specifically for the cartridge being dispensed,” Wilson says. “This can help avoid a situation in which a company utilizes dispensers that are too powerful. In such situations, the dispenser’s mechanical advantage may be offset by flexing and subsequent mixing problems.”

When using a hand-held plastic cartridge, an important step is to experiment with a manual gun to measure the effects of flexing and the individual characteristics of the different employees who apply the adhesives. Combining this approach with testing various static mixer configurations can provide a comprehensive testing model.

Selection process
Testing or experimenting should be viewed as a reliable process to help choose the right static mixer, rather than a trial-and-error approach. This decision will determine the success of adhesive manufacturers’ and end users’ applications. Both technical and financial implications must be included in this decision making process.

Ken Lambert, product manager, ITW Plexus, Danvers, Mass., emphasizes the importance of proper selection: “We view the static mixer as a central part of the adhesive system,” Lambert says. “Choosing a device with the proper configuration should not be an afterthought. Our approach is to recommend static mixer combinations that have been successful in our laboratory testing and field experience. It is an important support we provide as an adhesive manufacturer.”

Adhesive manufacturers often run tests and experiments more extensive and rigorous than end users would implement. An adhesives manufacturer might chart work life, gel and curing times for various adhesives. This data is distributed to end users with disclaimers outlining chemical properties and operating limits based on specific applications of the adhesives, and the conditions under which they are dispensed.  An end user can take the specifications and test various dispensing systems under different operating conditions, such as varying temperature or humidity, or with different lengths or special attachments.

Another useful experiment involves evaluating how the curing rates of adhesives interact with various static mixers. If there are pauses in the dispensing phase, over time any curing that occurs in the tube could alter the effective diameter of the mixer. Changes in the effective diameter can negatively impact the chemical and physical properties of the adhesive. Useful experiments must consider the consequences of hardening of the materials in the tube.

If the application requires frequent replacement of the static mixer due to hardening, the end user might have to weigh the benefits of a low cost static mixer vs. a more expensive mixer configuration that would eliminate the need for frequent replacements. Such a choice might mean a higher materials cost, but ultimately a lower operating cost.

By measuring content volume, cost of the static mixer, and different dispensing routines, various process models can be evaluated and the results used to select a static mixer. For example, contrasting the costs and efficiencies associated with (a) running the mix and purging, versus (b) stopping a dispensing process with some frequency and throwing away the mixer, represents what could be a valuable financial test.

Finally, financial considerations can influence end users to adopt different testing approaches. Some end users report that initial testing that incorporates off-the-shelf, hand-held cartridges is the most cost-effective method. Users that rely on MMD equipment manufacturers to perform a range of tests and experiments sometimes conclude that added costs make outside testing less attractive. To be sure, all outside testing represents an additional cost, but also includes significant non-recurring expenses, such as charges for parts used in the testing operation itself, as well as the basic cost differentials when using customized MMD equipment.

There is no formula that provides consistent answers. Simple guidelines like “all epoxies need 24 elements” can be dangerous. An acceptable mixer configuration for one use might not be appropriate for a different application, even if the same adhesive is being dispensed.

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The author is sales & marketing manager, Sulzer Mixpac USA, Salem, N.H., 603/893-2727 www.sulzermixpacusa.com.