Strategic Mold Cooling: Common Causes of Inadequate Mold Cooling

The highest points of cooling lines can potentially trap air. Having turbulent flow will flush out the air pockets, shown with the arrows. Source (all images): DJC Plastic Consulting LLC

 

There are times when seemingly minor issues can negatively affect mold cooling. Here are some causes of inadequate cooling that I have seen.

Air Lock

Air bubbles within cooling lines can restrict flow through the lines, which, in worst cases, can cause “waterfall flow,” resulting in “air-locked” cooling channels. Any high points in the cooling line can potentially trap a bubble that, in effect, reduces the inside diameter of the cooling line. I know of only two ways to eliminate air locks: Increase coolant flow (turbulent flow) to flush out the bubble or add an air vent at the high points within the line.

Starved Cooling Lines

Cooling lines are used to pull heat from the part, but to cool efficiently, there must be sufficient coolant flow. Liquids flow from high to low pressure, with the greater flow where the pressure difference and channel cross section is greatest. It is important to ensure that all coolant lines have sufficient flow, not just by checking them in isolation but also while all the other cooling lines are connected to the manifold. Ideally, each cooling line should have its own flow meter to monitor flow in real time. At the very least, you should have one flow meter capable of checking circuits one at a time.

Some molds have large-diameter cooling lines looped through the mold plates, which can reduce the pressure inside the inlet manifold and increase the pressure in the outlet manifold, thereby reducing flow through smaller cooling lines. Cooling lines in cavities and cores often have small-diameter, tortuous channels with multiple restrictions, such as bubblers or baffles. Too high of flow through the mold plate cooling lines may then starve the cooling lines in the cavities and cores, which most directly affects part cooling.

I know of two quick ways to help even out the flows of the large-diameter lines with few restrictions and the small-diameter lines with more restrictions. One is to use different manifolds and possibly different controllers for large and small lines. The other is to add restrictors to the outlets of the larger lines.

(a) These images show one easy method of restricting flow through mold plates to increase pressure to the cavity and core details. Drilling small-diameter flow channels will also restrict flow, but may be difficult with deep holes. Instead, add a restrictor to the exit to reduce flow. It is easily adjusted, if needed. (b) Adding a pipe plug with an orifice drilled through it is an easy method to restrict flow. This could be a threadless or threaded plug installed at the coolant line exit. (c,d) One method for creating a water fitting with an internal restrictor is to thread the inside of the water fitting and install a pipe plug with an orifice into its threaded end. The fitting and mold base should then be marked to ensure correct assembly after each cleaning.

Restrictors may be adjustable valves or fixed orifices to throttle down the flow and increase the pressure within the large-diameter lines. This will serve to raise the pressure inside the inlet manifold and decrease the pressure in the outlet manifold, inducing a higher flow rate through the cavity and core cooling channels.

The cavities and cores of a mold need to have adequate cooling flow to obtain the best possible cycle time and consistently acceptable parts. Anything that compromises that flow will compromise cycle time and/or part quality.

Lime/Rust in Cooling Lines

Shortly after I started my injection molding career, I first learned how much lime and rust in cooling lines can impact mold cooling. The molder I worked for and another in Europe alternated running two molds because it cost less to ship the molds back and forth than to ship all the parts. One mold made an injection molded packaging box about 4 inches (10 cm) long, 3 inches (8 cm) wide and 2.5 inches (6 cm) deep, and the other molded its snap-fit cover. We had taken over the U.S. molding from another custom molder, so we had no specifications or history on these molds. They both ran with extremely long cycles because of the long cooling time. The cooling lines seemed to be adequately sized, so we checked the coolant flow. Removing one exit line at a time, we found barely a trickle coming out of each line. We bought a recirculating pump and a solution for removing buildup in waterlines, mixed the solution per the instructions and recirculated it through the mold.

After running this a full day, the flow was still anemic and very little sediment came out, so management decided to purchase muriatic acid, a diluted hydrochloric acid solution and run it through the lines. This did the trick, removing a thick sludge out of each line! This was repeated several times until the solution came out clear. The lines were flushed with baking soda and fresh water to neutralize the acid. When the molds were run again, the cycle times were cut in half! (I wondered what the European molder thought when they received the mold, if they realized they could run a faster cycle.)

In this configuration, the flow through the bubblers is connected in parallel, enabling the coolant to flow into the base of each bubbler. To maintain equal pressure on each bubbler, the inlet and outlet lines must be sized with a large enough diameter.

Bubblers and Baffles

If a bubbler or baffle is too long or screwed in too deep, it can choke off the flow at its tip. If it is too short, the coolant does not flow to the end of the hole, leaving stagnant coolant in the deep end of the hole. This can be caused by the baffle or bubbler being cut to the wrong length or assembled in the wrong location. I have also seen where a bubbler was screwed in too deep, so the threaded end choked off flow from the cooling line.

There are two issues with this view. These bubblers are connected in series, reducing the flow through both bubblers. Also, one bubbler has coolant flowing into its base, but the other is reversed, with coolant flowing through the hole in the core block and into the tip of the bubbler.

Bubbler tubes are designed for the coolant to flow on the inside from the base to the tip, impinging onto the steel at the deep end of the drilled hole and then flowing around the tube to the exit. Coolant flowing in the opposite direction will still provide cooling, but the coolest liquid is no longer impinging onto the end of the hole, which can change the filling, cooling and internal stresses of the molded part.

Bubblers are typically much smaller in diameter than the

In this view, the right bubbler has been screwed in too deep. The base of the bubbler is against the back wall of the cooling channel, restricting or even cutting off flow through the bubbler. Make sure all bubblers are installed to the correct depth.

cooling lines that feed them, so they act as restrictors in the line. If two identical bubblers are run in parallel, they should both have close to identical flow and cooling rates. However, if they are run in series, with the exit of the first feeding the inlet of the second, the pressure drops at the exit of the first bubbler and even more at the exit of the second, slowing the flow through both bubblers.

Potential Changes Impacting Cooling

If cooling was acceptable when the mold was new, but later problems arose, something must have changed. Possible causes of the change are:

• A valve to a cooling line is shut off or not fully open
• Lime/rust in lines — even a thin coating can reduce heat transfer
• Bubbler/baffle in the wrong location or screwed in too deep or shallow, choking off the flow
• A piece of debris broke loose and blocked a bubbler or small cooling line
• A waterline diverter plug had been left out or installed in the wrong
  

  The diverter plug is missing from the left cooling channel, enabling the coolant to flow straight through, stagnating the coolant above it, and probably trapping air at the top.

  location, causing flow to bypass a cooling channel
• Mixed up in/out cooling lines, causing coolant to run in the wrong direction
• Adding jumpers that were not original or installed incorrectly
• Inadequate temperature controller output (was a different controller originally used?)
• Hoses to the manifold or mold have different diameters or lengths than the original setup

The cavities and cores of a mold need to have adequate cooling flow to obtain the best possible cycle time and consistently acceptable parts. Anything that compromises that flow will compromise cycle time and/or part quality.