The additives in many sealing compounds are very abrasive and it is more challenging than ever to find dosing pumps that deliver this abrasive compound smoothly. However, there are a number of reliable, non-destructive solutions that use a variety of pump types.
Polyurethane sealing compounds have multiple applications in the electronics industry, such as protecting plastic electrical housings, control units, industrial lighting sources, sensors, etc. from dust, moisture and unwanted interference. To ensure that they work effectively, manufacturers of these systems use a wide range of packings to expand the performance of plastic materials.
However, these additives pose a problem for companies using sealing compounds: how to find a metering pump that can deliver these compounds smoothly.
Figure one
Figure 1. Comparison of three types of pumps. The metered material is a polyhydroxy compound with a density of about 1.6 g / cm3 and a viscosity of about 15,000 mPas.
Electronic components of the sealing process is already quite mature. One-component or two-component sealing compound is passed into the low-pressure mixing metering system for processing and added. Metering pumps are an important part of these systems. Its mission is to pump the sealing compound into the mixing device with a precise flow rate without breaking it. There are various types of pumps that operate continuously and intermittently on the market, but both pumps have advantages and disadvantages. The choice of pump type usually depends on the material's viscosity, abrasiveness and required flow rate (see Table 1).
Figure 2 Figure 2. A long-term comparison of a gear pump and a screw pump. Material: Polyurethane sealing compounds; Filler: no abrasion, 66% addition; Particle size: 16 ?; Viscosity: 20,000 ± 1,000 mPas; Density: 1.64-1.68 g / cm3.
Comparison of metering pumps
The pumps listed below are based on the principle of volumetric metering and allow precise control of the specific discharge of material. The timing pressure measurement, volume and material flow depends on the pressure and viscosity. Pumps used to seal compound metering include:
â—† gear pump (internal gear or external gear)
â—† screw pump
â—† hose pump
â—† piston pump
Gear Pumps - one of the most accurate metering pumps, is ideally suited for the metering of lightly abrasive materials, regardless of whether solid filler is added to the material (from liquid to slurry). Wear-resistant coatings and specially designed gear tooth profiles help to extend pump life. External gear pump pumping flow up to 0.1cm3 / revolution, the measurement pressure up to 100bar. For smaller pumping volumes, internal gear pumps can also be used. This pump is very compact. The internal gear pump is connected to the mixing system and measures up to ± 1% with a metered dose of 20mg (minimum metered dose of 1mg).
External gear pump screw pump and other characteristics
Screw Pump - Ideal for liquids and slurries of various viscosity levels. This type of pump does not easily cause material damage and is not sensitive to packing or abrasive materials. However, screw pumps can not achieve the metering accuracy and flexibility of gear pumps because of their high pressure. Therefore, the screw pump pumping volume can not be less than 1cm3, pressure up to 40bar.
Hose pump - only for low-precision metering, because the hose is very sensitive to pressure. They are an economical solution for the measurement of fillers containing abrasive materials.
Piston pump - suitable for almost any liquid or slurry. Pistons push the material out of the cylinder. After a certain number of measurements, the material will refill the cylinder. Pump models vary depending on the application.
Challenging needs
The demand for plastic chemical properties of electronic components has forced manufacturers to add fillers to epoxy, silicone and urethane sealing compounds to achieve the desired properties of the material. This creates a solid, mixture of different liquid and viscous structures that presents a major challenge for dosing pumps. Special attention must be paid to the following fillers:
â—† chalk
â—† aluminum hydroxide
â—† quartz powder
â—† Desiccant / commercial absorbent (zeolite)
Thixotropic agent
â—† hollow microspheres
These materials can cause damage to the metering pump. However, the pump may also change the characteristics of the material. This is usually a gradual process that is difficult to detect in advance, often as the material passes through the wrong sized components.
When designing a hydraulic circuit, the following factors must be considered:
â—† constant flow rate
â—† ideal for the design of the pipeline flow
â—† lower pressure
â—† minimize the material shear force
These factors are the highest priority when choosing a pump (Table 1).
However, there are other decisive factors, such as:
â—† measurement time
â—† volume flow rate
â—† accuracy
â—† Processing order
â—† measuring temperature
As can be seen from the comparison of three different pump types in Fig. 1, the piston pump has the best dispersibility in the measurement of polyols (density 1.6 g / cm3, viscosity 15,000 mPas) and therefore, from the mechanical properties, The best solution.
Figure 2 shows the long-term comparison of a gear pump and a screw pump. The material delivered was a polyurethane sealing compound where the non-abrasive filler (66%) had a particle size of 16 μm. The compound has a viscosity of 20,000 ± 1,000 mPas and a density of 1.64 to 1.68 g / cm3.
in conclusion
Experience has shown that piston pumps have the longest service life and the highest accuracy, but gear pumps are a good alternative as they are more cost effective than piston and screw pumps and operate continuously. Screw pumps are also suitable for continuous metering of filled materials. Pump in this respect there is no difference between good and bad.
The decisive factor is the system's need. If more than one type of pump is available, personal preferences and logistical factors can have an impact on the decision-making process.
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