Choosing the best technology for demanding adhesives
Fine tuning ensures the accuracy and quality of the dispensing process
Achieving the perfect result in the adhesive dispensing process can be tricky, requiring a whole series of challenges to be mastered.
Adhesives are becoming more and more complex, and handling in automatic application and dosing systems is becoming more difficult. The choice of the most suitable dosing system depends on the behavior of the adhesive material. Above all, viscosity fluctuations, abrasive solids or a high shear sensitivity occur very frequently.
The basis of each dosing process revolves around the material and the method of application. After successful selection of a material, it is necessary that it is dosed, applied or filled with high precision and repeatability to ensure process reliability. For such applications, various factors must be considered when selecting the dosing technology.
Fig. 1. 2-component adhesive dispensing
Dosing systems can be grouped into two different functional units: Dosing valves and dosing pumps. Dosing valves may include time pressure, volumetric and gravimetric designs, whereas the dosing pumps can be found in rotating and oscillating systems. In an oscillating pump, the material is transported by changing the chamber volume – increasing it and decreasing it. Typical examples include piston or diaphragm pumps.
Dosing components in rotating systems displace a more consistent volume. ViscoTec makes use of the endless piston principle of the progressive cavity pumps in all its dispensing applications. This type of rotary positive displacement pump feeds the material in a self-contained cavity. The combination of a rotor made of hardened stainless steel and a soft elastomer stator can, for instance, gently dose and manage highly filled material. The sealing line between the rotor and the elastomeric stator produces minimal shear on the material and at the same time ensures a seal effect without valve. In this way abrasive media with particles of high grain hardness, for example, can be dosed without increased wear, volumetrically accurate on any one part.
However, there are a number of criteria that must be observed even when selecting the correct pump type. Following are the answers to key questions about the dosing of demanding adhesives.
Common Questions About Dosing Demanding Adhesives
What criteria must be set prior to the sourcing of dosing technology?
Before a dosing technology is selected, it is necessary to contact the manufacturer of the material to be used and find out all relevant material specs. Information, such as specific properties of the material, viscosity, density and filler content are just as relevant as pot life, the type of cross link and the mixing ratio for two-component applications. In this context, the rheological behavior of the material plays an essential role and is subject to various factors, such as type, amount, and duration of the dispense, as well as the processing temperature. Particularly non-Newtonian fluids require high application and processing knowhow, as their viscosity can vary during the dosing process. After the selection of the material, the size of container the material will be delivered in needs to be considered, especially if it is intended that the material will be emptied automatically from its container.
What basic components should be included in the dosing process?
Emphasis should be placed on a coordinated, functional process that is technically specified in close consultation with the customer. Thus the components of the dosing process can be specified in the next step: These can be structured depending on the application in terms of supply of material, material preparation and finally the dispense process itself. The supply of material can be from any kind of container, such as cartridges, cans, buckets or barrels and ensures a reliable removal without air being trapped in the medium. Material preparation is necessary if it already contains air pockets, if it has a tendency to separate or if fillers tend to settle, or if it must only be fed continuously. Depending on these requirements, material processing plants buffer, circulate, homogenize or degas the fluid.
Fig. 2. High quality dispensing devices
The heart of the process is dispensing, which is also the focus of this sourcing guide. Dosing can include any one material but can also consist of a mix of two components which are dosed well before the actual application. In particular, an indication of the required cycle times, the required dosing quantity, the application type and the ambient temperature are of importance for the application. The specifics of the dosing and repeatability of the application have to be determined.
What information is required concerning the setup of the dosing technology?
Along with the knowledge of the material, the container type and the parameters of the application are agreed upon and the details of any open questions are discussed. These include: the required hoses, adapters and connectors of the dosing components. Here it is necessary to pay attention to any possible restrictions in space at the customer`s site and the mechanical mounting of the components. Signal access of the controls and electrical connections represents another important aspect in this detail collecting stage. A semi-automated 'standalone' solution is equally achievable as a fully automated system controlled via PLC, regardless of technologies used such as Profibus, PROFINET or digital I/O. For particularly demanding dosing tasks, testing in the in-house laboratory can be carried out in consultation with the customer. These tests provide an important insight into the process and help to erase any last doubts that the customer may have. In conclusion, based on this information a customized solution will be prepared and an individual offer submitted.
Fig. 3. Endless piston principle
What are the technical challenges to be aware of when dosing abrasive material?
When it comes to the processing of abrasive material, there are a number of details to consider. A multi layered qualification process should be used to specify the correct elastomer for the stator thus to significantly increase its service life despite mechanical load. In addition, it is also important to consider and minimize the friction of the particles in the material onto the dosing components. The wear of both the rotor and the stator can be reduced by adjusting to the lowest possible flow rate. High flow velocities such as abrupt reversals or narrow openings in the dosing pump geometry should also be avoided. These measures can counteract a so-called cavitation in the dispensing pump. Short inlets with large diameters equally prevent the build-up of high pressures and thereby reduce the friction on the inside of the pipe. To prevent a blockage of particles in the dispensing needle, a conical dispensing needle with a diameter of at least the size of the largest grain cross section, should be chosen.