Analysis of Piston Metering Pump Designs Used in Potting Machines

Piston pumps are widely used as metering devices in potting machines. Globally,
 most potting-machine manufacturers use piston-pump metering principles in their
 equipment. However, significant differences in performance can arise from
 variations in the detailed design and structure of these systems.

The following is an analysis of a typical piston metering pump design.

Piston Metering Pump Structure

The image below illustrates the structure of a common piston metering pump.

Operating Principle

• Material filling: The piston retracts, the check valve
     closes, and resin is drawn into the metering cylinder.

• Adhesive dispensing: The piston moves forward, the check
     valve opens, and the resin is forced out of the metering cylinder.

The piston stroke determines the amount of adhesive dispensed. Two sets of
 metering pistons and cylinders with different diameters can be used to
 accommodate different adhesive mixing ratios.

Some machines also incorporate a lever mechanism that adjusts the stroke of
 one of the metering assemblies. This allows the adhesive mixing ratio to be
 adjusted within a certain range without replacing or modifying the hardware.

Common Problems

Overall, potting machines based on this metering principle can perform well.
 However, several common problems should be considered.

Check Valve Problems

In this type of potting machine, the metering pump is usually installed beneath
 the material storage tank. Adhesive sedimentation can interfere with the
 operation of the check valve, which can then affect metering accuracy.

Because the check valve performs the opening and closing function, there is
 also a risk of delayed response.

Incomplete Material Filling

Potting machines based on this principle generally assume that the metering
 cylinder is completely filled with adhesive whenever the piston retracts.
 In practice, however, highly viscous adhesive, check valve failure, or piston
 seal failure can disrupt the material-filling process.

Effects of Pipes and Valves on Metering Accuracy

This type of metering pump requires a shut-off valve at the adhesive dispensing
 outlet. There is usually a considerable distance between the metering assembly
 and the shut-off valve. Therefore, the effects of the connecting pipes and
 fittings on metering accuracy must be considered in actual applications.

Conclusion

In our experience, customers have introduced this type of equipment on more
 than one occasion only to find that it could not meet the expected project
 requirements. In some cases, the equipment ultimately had to be scrapped.
 The main reasons were generally the design and operating issues described
 above.

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