At the heat

07-03-22 Plastic mould heat transfer

5 common challenges at moulding heat transfer

The basics would be like: 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.

Mold cooling can prove to be more complicated in practice. So, plastics processors often encounter the following cooling-related challenges.

1. 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.

2. Inadequate flow
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 (-16°C)

Q= specific heat of material x Btu/shot (hr)

3. Inadequate turbulence

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.

Fluid density at temp x velocity x diameter of the pipe/dynamic viscosity of the fluid at a temp. = Reynolds number.

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 litres per minute (lpm).

If a recalculation 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.

4. 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.

5. 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.