Technology of Meter-Mix-Dispense (MMD) Machinery
Meter-mix-dispense machinery that is used for two part potting materials uses several methods to ratio the two components, mix them thoroughly together and dispense the mixed material in to the part.
These are machines all have the same idea of positive displacement, taking a volume of resin and a volume of the hardener in the proper ratio, but they do it by different methods. Each method has positives and negatives so the selection of the equipment is dependent on the total potting system potting compound, part size and number of parts per hour.
1. Cylinder-piston method
The most widely use method is the cylinder-piston method. The cylinders for the resin and hardener are sized so that the volume ratio is that of the potting compound so when the pistons are pushing in to the cylinders the correct proportion is push in to the mixing area. Normally, this method empties the cylinder and then needs to be refilled before more of the potting compound is dispensed. This is a single acting flow. It can be modified so that there are 4 cylinders 2 for each the resin and the hardener so that continuous dispensing can happen - double acting flow. One set is filling while the other set is dispensing. There are several designs that are used with this method.
a. Metal cylinders with metal pistons with the resin and hardener fed by gas pressure or pumps. Variations of this design include pistons attach to a arm which moves up and down or to one shaft which move back and forth.
b. Metal cylinders with replaceable plastic pistons with gravity feed of the resin and hardener.
The diagrams below show a single acting flow machine and a double acting flow machine. These were provided by Don Leone of Ashby Cross Co. Inc. from his presentation given in June 2001 at Michigan State University - Management Education Center. Sponsored by the Adhesive and Sealant Council and Adhesive & Sealants Industry MagazineSingle Acting Flow Machine:
Double Acting Flow Machine:
2. Rod Displacement Method
This method uses a solid rod in place of a piston to displace the potting compound. The advantage of this method is long life with minimum maintenance with filled potting compounds since they use fewer number of seals.
The diagrams below of rod metering pumps are provided by Sealant Equipment & Engineering, Inc. from Jim Schultz.
Single acting metering Double acting metering
3. Progressive Cavity Method
This method is best described by picturing two screws that are turned by separate motors and controlled by electronics. In simpler words, the screws are turned in the correct speed so that they dispense the resin and hardener in the proper ratio. The screws are sized to give the approximate ratio and the turning speed is adjusted to give the exact ratio. This method is a continuous dispensing method since the screws can keep turning and never have to be refilled. Since the screws speed are controlled separately by the electronics as the screws wear the speed can be adjusted to compensate for this wear.
This diagram was provided by Rick Dawson of Fluid Research Corporation
4. Diaphragm method
This method uses two diaphragm pumps with one side filled with the resin or hardener and the other side with oil. The oil is connected to cylinders that are sized for the volume ratio of the resin system. So this is similar to the cylinder-piston method, but only the oil is in the cylinders so the wear on them is very little. This method also is not continuous unless the system is doubled.
5. Gear method
This method is like the progressive cavity method but is done with precision gear pumps. Advantages are a very consistent flow rates which are critical on continuous flow applications such as bead dispensing
Disadvantages: Limited to only unfilled no filler systems because the gears would wear and the ratio would be incorrect. The cost of the precision gear pumps is higher than piston pumps.
After the resin and hardener are dispensed in the proper ratio they flow in to the mixing area. There are two methods of mixing, static and dynamic.
1. Static Method:
The most common method used is the static type. These are nozzles that fit on the machine and can be disposable plastic or permanent steel. They are called motionless or static mixers. They are tubes that contain a convoluted rod inside that causes the two parts to mix together. It does this by dividing the flow in half and then that part in half again and continues to divide the flow until it is mixed. Most people use the disposable plastic static mixers because if the machine is not used for a time the potting compound hardens in the static mixer and it is easy to just through it away. When using the steel static mixers you would have to burn out the cured resin so it could be used again.
2. Dynamic Method
Dynamic mixers have a rotating blade inside a chamber that mixes the resin and the hardener together. This gives the most complete mixing and is required in some applications. In order not to have the potting compound cure inside the chamber the machines have an automatic timed purging system which will expel the mixed material after a set time.
These machines are wonderful to make the potting job faster, less messy and easier, but they require attention to work properly. The operator of the machine makes all the difference in its operation and its maintenance. Someone who is mechanically inclined is the best person for the job.
Here are some areas to watch:
1. The material phases the resin or hardener comes out before the other.
To test for the machine phasing run a serpentine bead test. Take a
large piece of cardboard and run a bead of the potting compound back a forth down
2. The ratio is incorrect.
3. Air gets in the system.
4. Filler settles in the holding container or the feed pipes.Regular preventative maintenance is the key to reduce down time and rejected parts. If the operator is the person in charge of the machine they will get to know when it is not working properly before the rejected parts start coming off the line.
D. MMD Equipment Suppliers Questions to select equipment options
1. What type is the Potting Compound?
Epoxy, Urethane, Silicone, Acrylic, Other
2. What type of Application Equipment?
Manual Hand-Held Cartridge Dispensing Equipment
3. Please indicate the primary type of flow required for applications in your
_____ Continuous - operator controlled flow
Less than 1.0 gram
Less than 5.0 grams/second
1 1,000 cps
Less than 5 minutes
Yes ___ No ___9. Are these fillers abrasive and what are they?
Yes ___ No ___10. What is the typical container size ordered of the potting compound?
50, 200, or 400 ml dual cartridge
Yes No12. What is your typical annual production (parts/year)?
Yes No16. Do you want the potting compound dispensing systems stand-alone or integrated into a workstation?
Stand-alone Integrated into workstation17. Here is a list of machine options that are available.
Machine controlled flow, Material degassing
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Last Modified: July 20, 2012