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I was just asked this question…

To determine the correct size chiller you must first calculate the cooling time, total amount of heat to be removed, the cooling power, the cooling power per line, and the required volumetric flow rate.

A basic way to calculate the cooling time for this purpose is to use a common rule: 2 X [wall thickness (mm)]^2

To calculate the total amount of heat to be removed use the equation:

Q= (combined mass of the parts and feed system) x Cp x (Tmelt -Teject)

Where: Cp is the specific heat of your plastic, Tmelt is your melt temperature, and Teject is the temperature of your parts at ejection. This answer should be in Joules. 

After calculating the amount of heat to be removed divide it by your cooling time to get an answer in the units of J/s or Watts. This is the power you will need to cool your part. 

The cooling power per line can be calculated by dividing the power you need to cool your part by the number of cooling lines in the part. Example: You had a 1000W estimate for the required power to cool your part and you have 4 cooling lines. The result is 250 W per cooling line.

To calculate the volumetric flow rate you will need to cool your parts you must:

Divide the cooling power per line by: [allowable increase in coolant temp x density of the coolant x specific heat of the coolant]

This calculation should result in a number with the units of m^3/s or ft^3/s. 

You can now use the estimated coolant flow rate to determine which machine will meet your needs. 

-Andy

Closed-Loop Molding Machines: 1st stage injection on these molding machines is based on maintaining a consistent screw velocity. If there is adequate injection pressure available, and the machine is operating properly, the polymer will leave the barrel with a consistent shear rate. If the injection speed is high enough to cause shear thinning, the volume that is injected should be consistent over a long period of time. Although there will be normal variations in material viscosity, a process using 95% fill during 1st stage injection should maintain a short shot over an extended period of time.

Open-Loop Molding Machines: 1st stage injection on these machines is based on maintaining a consistent injection pressure. In this case, the screw velocity will drop as the resistance to flow increases… resulting in a constantly decreasing shear rate. Since this machine is incapable of maintaining a consistent shear rate, viscosity shifts will significantly affect the fill time as well as the volume of material which is injected. For this reason, an open-loop machine cannot maintain a consistent short shot during 1st stage. As a result, the most consistent processes, on open-loop machines, tend to result from completely filling and packing the part during 1st stage injection.

It is completely possible to mold acceptable parts using a complete 1st stage fill with a closed-loop machine, or a 1st stage short shot with an open-loop machine… it’s just that the process will require significantly more adjustment as the viscosity of the material shifts over time. The above comments is strictly focused on what 1st stage injection strategy generally provides the least process variation.

-Andy

In general, the filling behavior of open-loop and closed loop molding machines are such that a different machine-independent process sheet is typically preferred.

-Andy

I was just asked the question…

You can calculate the Intensification Ratio of your machine two ways if you do not know the hydraulic cylinder diameter. The first way to calculate the intensification ratio is to divide the peak plastic injection pressure by the peak hydraulic injection pressure.

The second way to calculate your intensification ratio is to use the machine graph if it has one. The graph usually has lines relating the plastic injection pressure and hydraulic pressure for different screw diameters. Find the line for your screw diameter and divide the peak injection pressure by the hydraulic pressure to calculate the intensification ratio of the machine.

-Andy

I was recently asked…

If your tool is designed for vertical injection in a vertical molding machine you are not limiting yourself to using a vertical machine. A tool that is vertically fed can also be installed in an inline machine and produce parts. If the tool is designed for parting line injection you are limiting yourself to one machine type. If the vertical machine were to break and only an inline machine was available, parts could still be produced on the inline machine assuming machine specs were sufficient for part production.

If parting line injection is better suited for the application and your company can facilitate it, then always use the preferred method for the application.

-Andy