Equipment considerations for dynamic mixing of two component materials

In two-component dispensing applications that require dynamic mixing, making a precision chemical reaction with the correct amount of energy to create uniform density foam is challenging enough. To do it at the scale of industrial manufacturing brings the challenge to an even higher level. There are many variables to evaluate to find an harmonious balance between dispensing parameters such as flow rate, ratio, RPMs, and the static mixer.  

The flow rate influences achievement of the proper mix and often dictates other dispensing parameter decisions. If the flow rate is higher than what the mixer can blend, the material chemistry might not react appropriately, whereas too low a flow rate can lead to material curing prematurely within the mixer, causing clogging or poor curing. Such factors can lead to costly equipment downtime and maintenance.

Application type is another consideration for determining the most suitable flow rate. In potting applications, an excessive flow rate may cause the material to trap air as it flows into the part; an inadequate flow rate could cause premature curing before the assembly part is completely full. For gasketing, too high could exceed a realistic robot speed; too low could cause wasting of valuable production time.

Having the proper ratio ensures a consistent cell structure of the foam and meeting of your expectations for performance. Because the A component and B component commonly have different viscosities, it is crucial to balance your A and B pressure to maintain the ratio. Off-ratio dispense can cause striations in the material or voids/bubbles in the cell structure, ultimately impacting performance and application reliability. For chemically formed blown-foam, this is even more important since the chemical reaction creates the foam structure.

When using a dynamic mix valve for foam gasketing and foam encapsulation, it is essential to test and evaluate the rotary element's RPMs to achieve a uniform blend. Depending on the material chemistry, over-rotating or agitating the mix can create excess friction or heat, causing the material to cure quickly and not allowing enough time to work the material onto the substrate. An RPM which is too low will not put enough blending energy into the foam to achieve an homogeneous mix and initiate the desired chemical reaction.

A static mixer can influence material consistency and uniformity, and there are variables to consider in the selection process, including length, elements, diameter, and orifice. Difficult to mix foams often require longer static mixers or increasing of the number of elements to ensure a thorough blend. Materials with a shorter gel time should use a shorter mixer to avoid premature hardening.

Dynamically mixed chemistries can be challenging to dispense. It takes the proper pre-production testing and validation in order to determine the optimal process window for your application. With extensive experience dispensing foams, a library of validated materials, and dispensing machines that simplify these challenges by controlling  ratio, flow rate, and mixer RPM, Graco can assist in perfecting your production process. Contact our experts to learn more about our equipment and process parameters configured for your specific application.