There are many advantages to packaging two component adhesives in convenient cartridges for use with static mixers. Packaging your adhesives in cartridge systems can help you deliver significant value to your customers. Cartridges save time by eliminating the need to handle and weigh the two adhesive components. They can reduce exposure to chemicals that may be hazardous to your customers' health. They can make it easy for your customer to accurately mix and dispense your material, ensuring consistently good results.
The care and accuracy with which cartridges are filled often determines the extent to which these potential advantages are realized. Two component adhesive end users want their cartridges to be clean, free of leaks, and they want them to dispense adhesive that is fit for the intended purpose. This paper will concentrate on the ways filling a cartridge can affect your customers' experiences.
Filling Both Sides of a Cartridge with an Equal Amount of Material:
If both sides of a two component cartridge are not filled with exactly the same amount of material, the potential exists for the beginning of the extruded adhesive bead to contain too much of one component and not enough of the other. It is well known in our industry that one should always make sure material is being extruded out of both sides of a cartridge outlet before installing the static mixer. Unfortunately, it is equally well known that this important instruction is often ignored.
Filling both sides of a cartridge with an equal amount of material is an important factor in determining the satisfaction of a cartridge user. Equipment is readily available that can dispense fluids with +/- ½% repeatability. Using equipment with this kind of accuracy and being careful to ensure that it is set up to dispense the same amount of material into each side of the cartridge will provide good results. TAH can recommend filling equipment suppliers able to meet your needs.
Inserting Pistons Without Damaging Them:
If a piston is damaged while being inserted into an adhesive cartridge the cartridge contents are more likely to leak past the piston while in transit or storage. There are several factors that can contribute to potential damage.
Proper alignment of the piston insertion fixture and the cartridge bore. We have seen several incidents with leaking cartridges caused by machinery that did not accurately align the piston with the cartridge bore. A lip seal on the front face of the piston can be nicked, scuffed, or even folded back on itself if it collides with an edge at the inlet of the cartridge bore. In one extreme case the o-ring was being stripped of an o-ring style piston due to insertion by improperly aligned machinery.
Use of a shim or wire to allow the air to escape from the cartridge is a common method of “bleeding cartridges” and it can be very effective. However, the use of a shim or wire with sharp edges or a burr along its length can damage the seal area and adversely affect a piston's performance.
Cartridge and piston design features play a role in how well pistons resist damage during the piston insertion process. The inlet to the cartridge bores should have a generous chamfer to compress the seal area of the piston gently and guide the piston into the bore. The piston seal should be designed specifically to resist damage by employing compliant members and rounded edges. The material of construction of the piston is also an important factor in determining how well it will resist damage during insertion.
Filling Cartridges Air Free:
Air trapped in the fluid packaged in a cartridge will negatively impact the performance of your adhesive system. It causes variation in the volumetric ratio of material A to material B in the adhesive bead. As the dispenser applies force to the pistons an equal amount of adhesive should flow from each side of the outlet of the cartridge. If air is present in the cartridge on the A side it will be compressed and, consequently, less of material A will flow out of the cartridge at the beginning of the dispense cycle. Later in the dispense cycle as the pressure in the cartridge decreases the compressed air will expand causing more of material A to flow out of the cartridge. This variation in ratio will cause variations in the gel time, cure time, color and other properties of the dispensed adhesive or sealant.
Steps must be taken to prevent problems due to air trapped in the liquid or paste material. In some case, air is trapped in the adhesive material during the formulating or mixing process that must be removed by vacuum degassing prior to packaging the adhesive in cartridges. With paste materials, filling the cartridge from the bottom up and ensuring that the paste does not fold over on itself randomly as the cartridge is filled will help prevent the forming of air pockets during the filling process.
“Bleeding” the Air from Between the Cartridge Contents and the Piston:
Air trapped between the cartridge contents and piston will also have a negative impact on the ability of your cartridge system to control the volumetric ratio of what is dispensed.
There are several commonly employed methods of removing the air from between the piston and cartridge contents.
Using a shim or wire. This is the least costly method of bleeding cartridges. It is commonly employed with cartridges that contain high viscosity fluids that will not leak past minor imperfections in a piston's seal feature. As mentioned above, care must be taken to avoid damaging the piston during insertion into the cartridge. There are a few additional potential problems that will offset the cost advantage of this method in some applications. The bleeder can become fouled with adhesive material if it contacts the adhesive, in some instances reducing or eliminating its effectiveness. Shims and wires are also prone to breaking or curling due to the requirement that they be small in cross section. Bleed shims and wires must be replaced periodically and can be a source of downtime in your process.
Vacuum is sometimes employed for this purpose. There are limitations associated with this approach due to technical challenges and the fact that it takes time to draw a vacuum, which can increase the time required to perform the piston insertion operation. Some of the more sophisticated fillers have found applications that benefit from this technology.
Using a piston that employs a “bleed plug”. These pistons include a hole through which the air escapes. The hole is plugged after the piston reaches the point where all the air has been expelled from between the piston and cartridge contents. Many cartridge systems currently on the market employ a bleed plug which must be handled separately and which require labor following the piston insertion to install the bleed plug. TAH has introduced a new “prestaged” bleed plug that is installed at the factory in a position that allows air to bleed from the cartridge. Once all the air has left the cartridge the bleed plug can be automatically advanced to the closed position by the same machine that inserts the piston in the cartridge, eliminating the labor necessary with some systems to handle and manually insert the plug.
The importance of properly filling two component cartridges cannot be over emphasized. A well filled cartridge will provide your customer consistently good results. Poorly filled cartridges may negatively affect the performance of your product. TAH is staffed with knowledgeable people who will help you maximize the potential of convenient two component cartridge packaging systems. We can help you every step of the way, from choosing the right cartridge and piston combination to recommending skilled toll filling companies or providing equipment to make your internal filling operation more efficient.