When dispensing small amounts of two-part adhesives or other materials, two key considerations are whether the material is mixed correctly and the accuracy of the amount dispensed. Here Paul Whitehead at adhesives specialist Intertronics shares his thoughts on the precise metering, mixing and dispensing of small amounts of two-part materials.
There is a growing demand for the precise metering, mixing and dispensing of small amounts of two-part technical materials like adhesives, potting compounds, encapsulants and sealants — including electrically or thermally conductive variants. This is driven by manufacturers in the electronics, medical device and automotive industries requiring increased functionality in seemingly ever smaller formats. In parallel, these engineers are looking for high accuracy material application in a robust and consistent process. They want precise quantities, uniformly mixed in the correct ratio, applied in exact locations.
Two-part formulations can offer superior cured performance but can be difficult to process. A typical method involves weighing out the constituent parts, mixing in a container with a spatula and loading into a dispensing syringe barrel for application. There may be a vacuum degassing or centrifuge step to remove entrapped air. Some suppliers offer respite from this procedure by providing their materials in a pre-mixed, degassed and frozen packaging. However, there are extra costs associated with shipping and storage, which are partly offset by the material being ready to use after thawing.
Once the material is mixed (or thawed), curing starts and the material’s viscosity will start to increase. The amounts dispensed directly from the syringe or through a dispensing valve using air pressure will vary with viscosity change and it is difficult to maintain an accurate deposition. Moreover, the mixed material will have a limited working life, and this can lead to waste as it cures too much to still be usable.
What is the solution to the viscosity problem?
One solution is to use a metering and mixing machine, where measured amounts of material are fed through a mixing nozzle and can then be dispensed directly to the part. Systems operating with gear pumps or piston pumps are readily available but are more suited to larger material quantities, such as potting transformers.
If the requirement is for the application of a few millilitres or even fractions of a millilitre, then another technology is considered — metering and mixing based on progressive cavity pumps. The pump comprises a specially shaped rotor inside an elastomeric stator, forming a series of tapering and overlapping spaces or pockets which progress through the pump as the rotor is turned, producing a pulseless material flow. Flow rate is directly proportional to the rate of rotation. Input pressure and material viscosity have no impact on the output of the pump and the system will achieve true volumetric output based on positive displacement. Dispensing at the point of mixing precludes variability due to viscosity change and reduces wastage. Materials may be purchased in larger package sizes, which can reduce purchasing costs.
One example of such technology is the preeflow eco-DUO, a high precision volumetric metering, mixing and dispensing unit, which integrates two pre-eflow eco-PENs through a manifold and static mixing nozzle. Suitable for a wide range of materials with viscosities from water up to pastes, it enables flow rates from 0.1 to 32 ml/minute. It also offers a mixing ratio of up to 10:1, which can be changed dependent on the project. The variable mix ratio means that unlike many other systems, the user is able to reconfigure it for different materials, rather than sending it back to the system manufacturer for modification.
The operator can control the flow to fine limits and calibrate the controller to the system for a very repeatable output. It is readily integrated into automation to ensure that as well as dispensing volume, positioning is extremely repeatable.
One customer success we have seen is with BPR Medical, which manufactures a valve to extinguish fires in the line between an oxygen concentrator and the user’s mask or nasal cannula ─ a lifesaving technology. The customer was dispensing a two-part epoxy to bond two halves of the valve’s body in a precise repeatable volume, metered and mixed correctly, to achieve a bond integrity to meet conformance for CE marking under the Medical Device Directive. Testing revealed that the pre-flow eco-DUO would provide an appropriate solution. The system has proven itself, with its dispensing accuracy also providing a rapid return on investment through a 16 per cent material savings per part.