Sponsored By

Tooling Corner: Cooling with baffles or fountains

January 6, 2006

5 Min Read
Tooling Corner: Cooling with baffles or fountains

Make a relatively high, thin standing core in an injection mold and immediately cooling bubblers or fountains come to mind. The designer immediately dives for the parts catalog and looks for components that will fit into the mold. If you believe in the bigger-hammer approach you can make almost any component fit into any mold, but the trick is in getting the mold to work, not the mold component as such.

The Mission

What are we trying to do? Usually the task is to cool a thin core. But how cool is cool enough? And, how thin is the thin core? ?How cool?? is an easy question to answer but a difficult one to understand. We cool the core to get the part to shrink onto it (which it will actually do anyway). However, we also need to cool the core to below the temperature of the cavity in order to overcome the inefficiencies of the ejector system and other phenomena that get in the way of balanced cooling. Our task is more related to heat transfer than to absolute temperatures.

The Method

Our core needs to be cooled but also strong enough to withstand the rigors of molding. Multiple solutions are available:

  • Make the core out of a highly heat-conductive material with an extra-long base. Insert the core in both the mold and the water line. This cools the core and withdraws heat from the plastic.Two variants are available:

    •  Make a steel core, bore out the inside, then press fit an inner core of highly heat-conductive material.

    •  Make the core out of heat-conductive material then plate it to give corrosion/chemical resistance.

    Use gas pins. These are pins with relatively large pockets at both ends, and small holes drilled down the center. They are filled with heat-conductive low-temperature boiling liquids. From the catalogs you can buy what look like straight pins, however in this application, you?d want to have a custom-made pin manufactured for your specific cores geometry.

Going for the conventional baffle or fountain raises some other questions: A baffle is a flat piece of brass or steel jammed into a hole in the core that directs coolant up one side and down the other.

We now have to whip out the calculators to be sure the half diameter portions area is greater than the feed line so that the baffle doesn't impose a restriction. The baffle must also be placed at the top of the core in such a way so that when the flow jumps from one side to the other, this transition doesn?t create a flow restriction (the tool makers/engineers recurring nightmare).It would be nice if the baffle were positioned in the proper orientation for the water to flow properly, yet still be screwed in tightly to avoid leaks.

Bubblers, also known as fountains, take the same approach as baffles. A hole is drilled into the core but this time there is a pipe or tube placed up the center. Normally the water comes up the tube and neatly disperses itself uniformly around the core before it exits. Again there is the concern of a large enough tube to place water in, compared to the hole drilled in the core to provide an exit path for the water yet still be close enough to the tip. Here we can also play metallurgy games, inserting the core or making it entirely with heat-conductive media.

The Hookup

A more fascinating problem with cooling is the concept of daisy-chain or parallel cooling. Let?s say I have an eight-cavity mold with a two-by-four layout.

  • Scenario #1. Most moldmakers put an inlet on one end and an outlet on the other for each row of four, with one cavity feeding into the next. This is called daisy chaining. The problem is that the resistance and heat transfer of the first cavity is passed to the second and so on. This means each cavity will be a different temperature and therefore produce parts of different dimensions.

  • Scenario #2. What if I drilled a line to the left of each cavity going up my row of four from the bottom of the mold and another to the right, going up the same row from the top of the mold? Now I drill lines connecting each cavity to the left and right waterlines. The inlet to the cavity farthest from the molds inlet would have the greatest resistance, yet the outlet from its corresponding cavity would have the least. All cavities would present the same uniform resistance and all would ideally have the same cooling. This is what's called parallel cooling. Parallel cooling is ideal for baffles and fountains. However, it does become tricky positioning the inlets and outlet of the holes.

The Flow

Turbulent flow is the requirement. Without it, there isn't adequate and efficient heat transfer. Without good heat transfer, we tend to cheat by lowering the coolant?s temperature and hoping the problem will go away (usually it doesn?t).

Looking at the discussion boards, there are some lively comments on calculating the proper Reynolds number and therefore gaining the proper cooling. Keep in mind that calculations yield only numbers (it was calculated that there was a 30% chance that the energy produced in the first atomic bomb was sufficient to ignite the oxygen in the atmosphere?obviously the calculations were a either a tad off or 30% was a good number on which to bet the fate of the entire world). Through experience, some elaborate calculations, and a lot of simple common sense, it has been determined that the magic number for getting turbulent flow in each circuit is 1 gal/min.

Flow is the answer. Always go with the flow. If you can?t get it, you can reduce restrictions or increase pressure. I had a client once who said he never had problems with flow but couldn?t understand why he ruptured waterlines consistently. The reason was evident when we checked the plant?s water pressure. It was 120 psi, way too high.

Remember, flow is better than cold, which makes brittle parts that will warp, especially if you paint and bake them. Once you have flow, follow the temperature guidelines from your material supplier.

Sign up for the PlasticsToday NewsFeed newsletter.

You May Also Like