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Cooling Parameters

Water pressure, flow and temperature all affect the cooling of the heated plastic inside an injection or blow mold.


In terms of flow, there is laminar and turbulent flow. Both types of flow feature the maximum water velocity in the center of the pipe, however with turbulent flow the difference between the maximum water velocity in the center and the minimum velocity at the inside of the pipe or channel is less. Because we want a high water speed at the outside of the pipe or mold channel. turbulent flow is preferable to laminar flow.


The main component that we can control to get turbulent flow is pressure. (Rough channel surfaces also help). I recommend a pressure of 90 psi or 6 bar at the inlet of the molding machine water supply. Before you do that, make sure all your water fittings and hoses can take that pressure. The goal is to have a delta T between the inlet and outlet temperature of the molding machine of 3º F or 1.5º C. A smaller difference does not improve cooling to a measurable degree, a larger difference is an indication of poor cooling. This temperature is best measured with a digital device such as a voltmeter with a thermocouple socket. I have found that the standard gauges that can be screwed into pipe fittings are not accurate enough. Instead, mount thermocouples with plugs into the pipes and then connect the voltmeter to it when you need a reading. Pressure gauges should also be mounted on the inlet and outlet of each machine. They can help troubleshoot mold problems. 6 bar water pressure is a must for all injection molding of PET preforms. The reason is that the water has to travel through rather small cooling pipes that take it to the front of the injection cores and then back around this pipe. The diameter of these pipes can be as small as 3 mm or 1/8″ and even smaller for neck finishes below 20 mm. During the injection process, the PET will shrink away from the cavity wall in less than a second so the core cooling is the most crucial part of the cooling process. Insufficient water pressure will reduce the flow to a trickle and significantly increase cycle time.


Another possibility is the installation of individual portable chillers for each machine as one visitor to this site has pointed out to me. While this is more costly, maintenance intensive, and space consuming than a centralized chiller, there are clear advantages, especially in the summer time when high humidity causes condensation on the molds. With a centralized system, the temperature for all machines will have to be increased while portable chillers allow each machine to be set to an optimum temperature. Never, never throttle the water flow in these situations. Reduced water flow causes laminar flow and takes away from the cooling properties of the water. There are companies that set their water temperature all year round to 55º F or 13º C to avoid the summer problems. Besides allowing for easier production scheduling it has the added advantage that the addition of anti-freeze can be kept to the minimum of 3% that is needed for the prevention of corrosion. Anti-freeze lowers the cooling properties of the cooling water and if you are running 46º water, you will need 8% to 10% anti-freeze to be sure that no freeze-up close to the compressor will occur. Thus some of the cooling capabilities of the water will have been lost making this an expensive and not very efficient solution.


Every water system should have a filter built in that is easy to clean. For that purpose, a bypass should be installed that allows the cleaning of the filter while water is supplied to the machines. Just imagine you would have to shut down the whole plant because the filter clogged up!


When it comes to choice of mold material, a strong Aluminum alloy with Beryllium-Copper neck, bottom and handle inserts is still the most cost effective solution. For smaller PVC parts, I recommend molds made completely from Beryllium-Copper. The higher cost is justified as these molds will stand up much better to corrosive stress from the hydro-chloric acid that is released during PVC production. I do not see much sense in Stainless Steel PVC blow molds because the heat transfer rate of Stainless Steel is so poor.


Good practice is to have the outlet connections bigger than the inlet connections and never install fittings that act as orifices just because they were handy at the time. Have the proper fittings specified at the time of the mold purchase to make sure they are available when the mold is being installed.


Summary of my recommendations:


  • 90 psi water pressure
  • Monitor pressure and delta T on each machine, look for a delta T of 3º F
  • Raise the water temperature during times of high humidity, do not reduce flow
  • Add no more than 3% anti-freeze to your water system