HDI, Inc.
C-69 Granada St. Alhambra
Bayamón, PR 00957-2328
Tel. 787-269-3278 Fax 787-778-0864
Email: hdiinfo@hdiinc.net
Basic Guide Lines for water cooling sizing
For Injection Molding:
First – Hydraulics:
To calculate the your hydraulic cooling needs, you need to:
1) Add up the hydraulic pump-motor-size for all your molding machines (eg.: 50 hp + 35 hp +...).
2) Multiply the results by 0.1 ton/hp factor. This will give you the load of your hydraulics in TONS.
Normally, you will use cooling tower water for the hydraulics, since the oil temperature is kept between
90o F and 130o F. There are many companies utilizing chill water for this task and that is fine as long as
your molding machine has a water regulating valve attached to your heat exchanger which the machine
can control for closing if the oil temperature is too cold and opening if the oil temperature is too hot. Old
machines did not have this feature, but it is something you can retrofit easily. New molding machines
already have this feature for controlling the oil temperature with chill water. For this cases (New
machines using chill water only) you can use the followin g factor 0.075 ton/hp in lieu of 0.1 ton/hp .
Second – Molds:
To calculate the your mold cooling needs, you need know the following:
1) Material type
2) Material consumption (lbs/hr): If you do not know this, follow this simple steps:
a) Weight a complete shot (parts + runner if applicable) from your molding machine, (ounces).
b) Read from the machine control the total cycle time in seconds.
c) You have: material / time => ounces / seconds. Convert to lbs/hr by multiplying by 225.
3) The following formula: Now with the use of the table below and this formula you can compute your
mold cooling needs:
Cooling Load = Material consumption (lbs/hr) / Chiller sizing factor (lbs/hr per ton) (from table)
Material lb/hr per ton
HDPE 30
LDPE 35
PP 35
ACRYLIC 35
PC 40
NYLON 40
PPE 40
PS 50
ACETAL 50
ABS 50
PVC 75
PET 45
If your material type is not listed on the table above, you can use the factor number for the worst scenario
which would be that of HDPE.
Chiller Capacity Loss: Chiller are design for 50o F operation. If your process runs below the design
temperature your chiller will lose capacity at a rate of 20% per each 10o F below 50o F. The inverse also
holds true, you will gain 20% capacity per each 10o F above 50o F.
Example: Your cooling needs as calculated above, is 50 tons. But your process is below design
temperature parameters, lets say 45o F. A normal 50 ton chiller will only provide 45 tons at 45o F,
therefore you would need a 56 ton chiller or larger in order to get 50 tons at 45o F.
Third – Feed Throat:
For screw diameters 3.5” or smaller: use 1 ton of cooling
For screw diameters greater than 3.5” Dia. up to 6” Dia.: use 2 tons of cooling
Fourth – Total Cooling needs:
Now you need to ADD up the numbers calculated in all three sections above.
Total Cooling Load = Hydraulic load + Mold load + Feed Throat load
Please note that to this point we have not taken in consideration the cooling load caused by the water
pumps on your water pump tank system. This load can be calculated by multiplying the 0.1 ton/hp factor
by every pump motor on your pump tank system and add them up to your total cooling load calculated
above.
It will always be good to verify your numbers with a supplier, as conditions may vary and other
equipment may be added to the list of varia bles discussed here. Also you would need to size up your
pump tank system accordingly, but that is for another technical bulleting.
