Why are cooling systems so important in plastics injection molding operations?
Plastic injection molding relies on a process of heat transfer: A heated screw assembly at the head of a molding machine accepts raw material, usually resin pellets or regrind, and adds heat to the resin until it softens into a consistent mix, enabling injection into mold tooling.
Plastics injection molds are equipped with internal cooling channels, which accept a steady flow of circulating coolant (usually water) at a set temperature, pumped from a temperature control unit (TCU). This outgoing coolant flow serves as a medium for heat transfer and is vital to maintaining a consistent temperature on the internal surfaces of the tooling.
So, when a shot of hot plastic enters the tooling, the water cooling first tempers the mold from overheating as the hot material takes shape. Then, it draws out excess heat, cooling the tool and plastic so it can harden into a finished shape.
The warm “returning” water is then carried to a chilling system or cooling tower, which removes the heat from the water before recirculating it through the TCU and the mold once again.
The theory is pretty simple, right?
10 Challenges in Heat-Exchange and Temperature Control for Injection Molding
Mold cooling can prove to be more complicated in practice. So, plastics processors often encounter the following cooling-related challenges.
Using the wrong mold temperatures and cooling rates
Processing temperatures and cooling rates differ widely among various polymer materials. So, it is important to get your cooling water – and the surface of the mold that’s receiving the hot material – to the right “target” temperature to ensure that resin flows into the tooling properly and your parts cool at the proper rate and harden with the right qualities.
So, for example, if you are working with semi-crystalline materials like PEEK or nylon, which have relatively high processing temperatures (e.g. 500-700°F), you can’t just “quench” them by running cold water (e.g. 60°F) through the mold.
Instead, you must cool them more gradually, using warmer coolant from your TCU to increase the temperature of your tooling (e.g. 250-350°F) so that the hot material cools at a more gradual rate—a slower cooling rate is essential to allow consistent crystal formation throughout the part.
Maintaining the proper mold temperature while processing starts with coolant at the right temperature, but involves other factors as well.
Cooling water must also be flowing at the proper rate (gpm) and with sufficient pressure to ensure the proper degree of turbulence. The rate of coolant flow is based on the amount of heat energy that must be withdrawn, the heat transfer rate of a unit of water, and the size of the coolant channel.
Calculating flow rate for a part is relatively straightforward:
GPM = Q / Delta T x 500 ; Q = Energy Btu/hr ; Delta T (3°F)
Q= specific heat of material x Btu/shot (hr)
Ensuring the proper degree of turbulence in flowing coolant is a bit more challenging.
Basically, a turbulent flow forces more of the cooling water to “touch” the surface of the mold cooling channel, thus maximizing its heat transfer. A coolant flow with too little turbulence can become “laminar,” or layered, meaning the portion of water that’s in contact with the mold channel surface doesn’t change.
Laminar flows insulate the water at the center of the channel, preventing it from making contact with the mold and thus wasting its heat transfer potential.
You can calculate the turbulence of a coolant flow with a Reynolds number (Re) calculation.
The goal is to deliver a coolant flow with a Re between 4,000 – the threshold value for turbulent flow – and 8,000, which is a high degree of turbulence. (The circulating pump in your TCU or cooling system is the key to generating both flow and turbulence. Pump sizes are determined by the flow rates they generate in gallons per minute (gpm).
If a Re calculation suggests that you aren’t getting enough turbulence through the mold channels, the typical solution is to pump at a higher pressure, which often requires a more powerful pump. But note more pressure helps only up to a point: Research shows that increasing turbulence beyond 8,000 wastes pump horsepower and offers little additional cooling value.
Mold design issues
If your calculations show that you’re delivering the proper flow and turbulence for your application, but aren’t getting the cooling results you expect, the tooling design may be deficient.
There are a variety of causes, such as too few channels in the mold or tooling, channels that are too small in diameter, channels that are built too far from the surface in contact with the hot plastic, channels that don’t receive enough coolant due to design or flow problems or channels that have become clogged or narrowed by mineral build up.
In molds where conventional cooling channels can’t reach some features of the molding surface, it is possible to use bubblers or baffles, which are two ways to divert a coolant flow at a 90° angle from a main coolant channel into a more restricted area of the tool. However, both of these alternatives tend to narrow the flow area and increase flow resistance, so they should only be used when necessary.
Imbalanced flow manifolds
The best way to circulate coolant through a tool is to deliver it evenly from a “balanced” manifold on the incoming flow side, through the mold, to an output manifold on the other. Ideally, coolant should make a single pass through the tool, with a balanced manifold ensuring similar flow rates through similarly sized channels that remove similar amounts of heat.
However, ideal flows are not always possible to achieve, so pressure, heat-transfer, and temperature imbalances can crop up. For example:
- If cooling channels vary in length, then the coolant flows through them will vary, with the shorter/freer flowing channels tending to rob coolant from the longer channels, which have higher backpressure. To remedy this, it is essential to flow-balance the incoming coolant manifold, using valves that direct added flow to the more complex channels so that they can maintain adequate heat transfer.
- If coolant exiting one channel is redirected back through the mold for a second pass instead of to the output manifold, its temperature is going to be increased and its heat-transfer capability reduced. This will create a heat imbalance on the tooling surface and, because even a 10 degree imbalance could affect part cooling and quality, this error should be corrected as soon as possible.
Inability to “hold” steady mold temperatures
Let’s say that you have equipment that has delivered good temperature control in the past, but, for some reason, just isn’t able to hold a steady temperature. You find that tooling temperatures keep creeping up.
It’s very possible that you’ve added some cooling load to your systems—perhaps an additional machine or two.
If so, you’re going to need more cooling capacity to cope.
Another reason for an inability to hold temperatures is a tool that is leaking water internally.
Leaks not only can reduce cooling efficiency, but can also cause part quality problems such as “water marks.” These occur when water leaks through cracks in the steel to the tool surface, where it makes contact with the hot plastic.
There, it flashes over to steam and displaces plastic from the surface of the part, leaving a cosmetic defect, sometimes tinged with rust or mineral deposits from the cooling water.
The ideal solution for a leaky mold is to rework the surface, welding up and refinishing any cracks before use.
However, if that option is impractical, you can also attempt to isolate the leaky circuit, then run it on “negative” pressure. That is, you connect it to a TCU equipped with a negative pressure pump, which pulls, rather than pushes cooling water through. These TCUs (e.g., Conair VacuTrac™ models), are equipped with special valves that allow you to “dial in” just enough suction to solve mold leaks without pulling an excess of air into your cooling system lines.
Because plastics processors have a lot of heavy equipment running on three-phase power, it is not uncommon that a wiring error results in a few pieces of equipment that are running out-of-phase.
Some pieces of equipment run just fine this way, but coolant pumps do not. I can’t tell you the number of times that I’ve investigated reports of malfunctioning equipment or chilling problems, only to find that coolant pumps were wired out of phase and were running in reverse.
If you encounter a sudden cooling problem, be sure to double-check that it isn’t caused by a relatively simple wiring mistake.
Water scaling problems
Water is a cheap and very effective medium for heat transfer. However, ordinary clean water is loaded with minerals such as iron, sulfur, and calcium which happen to be very attracted to warm surfaces like the inside of mold channels, transitional flow areas around bubblers or baffles, or even the heating element of a TCU.
When minerals “stick” on these surfaces, they form deposits called “scale.” And, if scale is allowed to build up, it can act as an insulator that reduces heat transfer, so over time, molds won’t cool as quickly and the efficiency of heat transfer equipment like TCUs or chillers is gradually reduced.
Scaling problems are typically reduced when a coolant system has a “closed” circuit, since these circuits don’t lose/replace much water and therefore, mineral content doesn’t change much over time. However, the risk of scale buildup increases significantly in “open” cooling circuits that rely on evaporative cooling towers.
While cooling towers are a very efficient and inexpensive way to produce cool water, the evaporative process results in a continuous loss of pure water (as vapor) that must be replaced with ordinary, mineral-rich water, so the concentration of minerals in the circulating water increases with each evaporation/replacement cycle. Thus, maintenance requirements are greater.
There are a variety of ways to deal with coolant flow problems that occur from scaling:
- The majority of processors treat descaling as a PM activity. Periodically, they flush chemical descaling agents through cooling systems, auxiliaries, and molds to reduce scale buildup.
- Some organizations run demineralized water in their cooling circuits. While this can dramatically reduce scale-related mold maintenance and cooling problems, this approach is more complex and more costly to manage, particularly in large operations.
- In my experience, the best organizations also test mold coolant flows at regular intervals, connecting molds to pumping/flow measurement tools when molds go into and out of service. By comparing coolant flows before and after mold use, these tests can indicate the need for chemical de-scaling/de-mineralization before future mold use. This approach eliminates guesswork and keeps molds and production running at predictable, optimal levels.
Other water quality problems
Unfortunately, scale isn’t the only water quality problem that processors encounter. Any water system—whether open or closed circuit – can be vulnerable to other water quality problems.
So, periodically check your cooling water system to ensure that it is maintaining proper color/clarity and that it is free of odor, scale buildups, or significant bacterial content.
It is also very wise to perform periodic walk-throughs of your cooling systems, including periodic flow tests to detect any reductions in flow rates that could be caused by scaling, pump or flow control problems, or water supply problems, whether in plant-wide cooling systems or those serving individual machines.