A common misconception in plastics injection molding is that coolant temperature is the one true path to achieve productivity and profitability. The reality, however, is that turbulent flow is the primary force behind efficient cooling and a key driver in the ability to achieve operational efficiencies, increase profits and consistently produce high quality products.
Here’s what to know about turbulent flow in plastics molding, as well insight into the most recent advances in technology that allow processors to monitor and control flow, in addition to temperature.
Colder is NOT Always Better
Although precise temperature control is always critical, don’t assume necessarily that lowering the chiller temperature will help to reduce cycle times. Lowering a chiller operating temperature can have many negative effects.
Most chillers are rated (in tons) for 50˚F supply to process, which is just an arbitrary standard that the North America domestic plastics industry has accepted over the years. An important “rule of thumb” is that the capacity of a chiller is reduced by 2% per ˚F below 50˚F. So, if the temperature is reduced by 5˚F to 45˚F, the chiller capacity is lowered by 10%. This can cause the need to increase chiller capacity in order to properly cool the mold.
On the other hand, raising the temperature above 50˚F will, likewise, increase capacity. For example, increasing the temperature to 65˚F (the typical maximum) will result in an additional 30% capacity. This means more capacity for cooling additional molds using the same chiller.
Another negative effect involves the necessity to use a glycol antifreeze solution, which is typically required for operation at or below ~47˚F. This has huge, multiple effects on heat transfer.
As an example, for a chiller to operate at 40˚F, the refrigerant temperature in the evaporator will typically be 30˚F – obviously below the water freezing point. In this case, a 25% ethylene glycol solution will be required (freezing point @ 11˚F), protecting the chiller down to ~20˚F below the evaporating temperature. The results are ominous for the heat transfer process and can result in a vicious circle.
- Chiller capacity is reduced by 7%.
- Mold cooling capacity is reduced by the same amount.
- Pressure drop (ΔP) is increased by 21%. This means more pumping capacity to achieve the same result. Also, the additional pump motor heat will reduce chiller capacity, over and above the reduction due to the lower operating temperature.
- The subsequent result of the above is increased energy cost.
Turbulent Flow IS Always Better
In addition to accurate temperature control, optimizing flow through a mold is critical and, quite often, results in decreased cycle times and increased product quality and repeatability, even after increasing chilled water temperature. There are three stages of flow – laminar, transient and turbulent. This depends on a dimensionless factor call Reynold’s Number, which, for straight pipe, is a function of fluid velocity, internal diameter, fluid density and fluid viscosity (“resistance” to flow). The “flipping” point to turbulent in straight, clean a pipe is defined as 4000, but much higher R values are usually sought for injection molds, considering factors like bubblers, channel geometry, circuiting (i.e. series v. parallel), etc.
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