A close-tolerance gear pump coupled with a properly designed drive system will result in consistent, repeatable delivery of sand. There are fit tolerances for any gear-style pump that must be maintained in order to allow the parts to move against each other.
Mixing of any material or combination of materials is accomplished by moving the materials together against themselves. The mixer, regardless of design or materials to be blended, exists only to accomplish a uniform distribution of the components. Whether mixing concrete, polymers, liquids, powders, or silica sand with differing chemical system, the purpose of the mixing machine is to move the materials against themselves.
In our industry we are mixing sands with very small amounts of different binding agents, all having different and variable properties. The first sand that exits the chamber is obviously not the same as the sand a few seconds behind it, as there is really nothing for the first sand to move against. Even with today’s technologies (timers, flowmeters, valving), we cannot get away from this basic goal: to make the first sand usable, and exactly the same as the sand right that follows it.
It is a simple process to catch the first 1-2 seconds of sand that exits the mixer and then put it in the molds as soon as the pattern is covered. A properly trained and motivated mixer operator will waste very little sand.
The object of a continuous mixer is to produce sand for quality cores or molds using just enough binder to obtain the desired casting results at the lowest possible cost. Chemicals are a high percentage of the casting cost. Lower resin levels reduce costs, improve the reclamation process, and improve the working environment. In order to keep chemicals levels as low as possible it is important to control the temperature variables.
The preferred temperature for sand and resin is in the 85-95°F range. The set time of PUNB is either doubled or halved for every 18°F change in sand temperature. Unless sand temperature is well controlled, production rates will suffer. Even if the strip time is reduced with additional catalyst (expensive) the evaporation of solvents remains retarded due to the low sand temperature resulting in a higher potential for gas defects.
Heating sand with a traditional fluidized resistance element design can be costly because compressed air is cool and therefore uses a lot of power to raise the sand temperature 20-30° at the flow rates required.
This high kW requirement can add to the monthly electric bill if it is a demand-based system. Fluidized bed heaters with hot water require substantially less energy and are more accurate because the residence time in the comparatively larger chambers is longer, but capital costs are usually double the cost of resistance element designs. While the end temperature is important, the consistency and repeatability of temperature is equally important. The capital and operating costs of correctly sized sand heaters are high, but usually can be justified by lowered resin requirements, greater production levels, fewer scrap molds/cores, and more consistent production.
Resin temperatures should be as close as possible to the correctly heated sand. The normal response to cold resins is to install a drum heater or heat lamps. While these are helpful, they can be problematic; if left on too long or at too high a temperature, the characteristics of the resin can be dramatically changed.
As the solvent evaporates or resin advances, the performance, viscosity, and metering characteristics will change.
These vessel heating techniques usually only heat a couple inches into the drum or tote. The majority of the liquid’s temperature is unchanged – and this difference in temperature within a single drum is another variable that needs to be eliminated.
The correct technique is to have a dedicated recirculation pump that takes the chemical from the bottom of the container, runs it through an in-line heating device, and returns it to the top of the container. This system keeps the chemical throughout the container at the same temperature at all times. Additions to this container should never be more than 25% of the container size to allow the heating system to recover as quickly as possible while still in production.