What’s Controlling Your Cycle Time? Part 2

Molds are designed with a set number of cooling circuits. My definition of a cooling circuit is a little broader than most. I consider a circuit a combination of the individual components between the inlet and outlet manifolds on the temperature controller (thermolator), cooling tower, or chiller. Therefore, a circuit would include the fittings on the manifolds, the fittings on the hoses, the hoses themselves, the fittings on the mold, the cooling channel inside the mold, as well as any diversions within the cooling channel, such as a baffle, bubbler, thermal pin—even a 90° bend. Like a chain, the circuit is often only as good as its weakest link. To minimize cycle time, it is vital to ensure that the entire cooling circuit and the coolant flowing through it are used to their maximum efficiency.

Always check that the mold was set up correctly.

The first thing I recommend doing is to check that the mold was set up correctly. Look for anything that could restrict the flow or increase the pressure of coolant going to and from the mold. Are the ball valves in the manifolds the proper size? Do the hoses have a sufficient inside diameter? Are any of the hoses excessively long? Do any of the hoses have a sharp bend in the line? (Hoses have a minimum bend-radius rating.) Do any of the fittings on the hoses have internal automatic shutoff valves? Most suppliers say there is a “minimal” loss of pressure and flow rate with valved fittings. Figure 1 suggests that is not the case.

FIG 1 Pressure loss and flow rates for water-line fittings. (Source: Parker Hannifin Corp.)

The amount of water flow through any component, such as a hose, fitting or water channel, depends on the inside diameter, the overall length, and the pressure applied to the water. Therefore, to have an efficient circuit, you want the largest diameters and the shortest lengths possible, so that you can minimize the required pressure.

Cooling-channel buildup will extend the cycle time.

If it’s a mold you inherited and it looks like it has some miles on it, it would be wise to remove the water fittings and inspect the cooling channels for a buildup of scale, rust, calcium oxide, lime or biofilms, as shown in Fig. 2. Just a few thousandths of an inch of any of these deposits will insulate the walls of the channel and dramatically reduce the thermal conductivity, which will absolutely extend the cycle time. In fact, a buildup of a mere 0.015 in., or the thickness of four sheets of paper, can increase the temperature of the steel by as much as 50° F. This is another reason to have a preventive mold-maintenance plan in place—especially for molds that contain small flow passages, such as baffles and bubblers.

Read more: What’s Controlling Your Cycle Time? Part 2