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Practical Mold Cooling (continued)
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Temperature
Control In some cases you may want slower
flow, and special systems are available for these situations. For example,
you may have a mold that runs only if the cores are a few degrees warmer
than the cavities, or vice versa. To warm up the desired zone, you merely
throttle the cooling flow. A fancy name for this is heat recovery
temperature control. In other words, you recover heat from the molding
process to elevate the mold temperature. You may have noticed that it is
very difficult to settle a mold into a steady-state condition. You’re
always chasing the temperature up and down. If you are ambitious, you
might hook up a mold heater to solve the problem. But there is an easier
way. Heat recovery temperature control systems are available that
electronically monitor cooling circuit temperature and automatically
throttle flow to maintain a set temperature. The temperature sensor is
normally placed in the water return from the mold but can even be placed
in the mold steel. This system will not replace a mold heater in
situations that require adding heat from outside the mold -- when the mold
must be hot before you can start molding, for example. But it can make
life easier when different zones require slightly different temperatures
or when you want to run a mold warmer than tower or chiller water allows
at full flow. These systems allow you to throttle the flow with precise
temperature control. With their modest initial cost and operating energy
savings, heat recovery systems can be an attractive alternative to
traditional mold heating systems. The accompanying box illustrates an
example of this.
Flow
Rates With
the preceding basics in mind, let’s look at some cooling conditions in
actual molding situations and some data taken from lab experiments. Once
you decide you need a certain flow rate, you need to know what it takes to
get there. Table 2 shows cooling data taken from selected operating molds
and from our lab. Design
mold cooling passages as large as practical to reduce resistance to flow.
Next, look at the size of pipe or quick-connect fittings going to the
mold. Note in Table 2 the change to 2.3 gal/min from 1.7 gal/min brought
about just by changing from 200 series to 300 series quick-connect
fittings and changing to a slightly larger drilled passage. The same
effect generally follows increasing hose sizes. For example, our lab has
an available water pressure of 85 psi, but it is impossible for more than
about 2.5 gal/min to flow through 0.25-inch hose. In many plant
situations, water pressure is much lower than 85
psi. Water
manifolds can also make a big difference. It does no good at all to have
12 0.5-inch NPT(National Pipe Taper) fittings coming off a manifold with
only a 0.75-inch NPT inlet to supply all those fittings. A well-designed
manifold should have a supply area roughly equal to the discharge area, or
as large as practical. For example, 16 0.5-inch NPT fittings require about
a 2-inch NPT supply for best performance. Also, the main supply and return
fittings and hoses to the manifolds should not be smaller than the
manifold inlet. Attention to these details can improve the pressure
available at the mold. |