Page 43 - Plastics News April 2024

FEATURE

the check valve prior to material be- Coolant flow rate • Coolant channels have become
ing injected into the mould tool. As stated, maintaining the correct reduced due to scale build-up or

The temperature that the heating mould tool temperature during pro- corrosion.
bands maintain around the barrel is cessing is achieved with the coolant • Coolant does not flow adequate-
set to depend on the material being set at the correct temperature, but ly due to the coolant channel lay-
processed and can be found on the the flow rate of the coolant is also a out
material supplier's specification sheet critical factor. If conventional cooling channels are
and used as a guide. If excessive or The coolant should flow at the rate unable to reach some areas of the
inadequate temperatures are used in required and with sufficient pressure mould tool surface it is possible to
the barrel it can result in quality is- to ensure turbulence in the coolant use bubblers or baffles to divert the
sues in the finished component. If the flow. Baffles can be incorporated into coolant at a 90° angle from the main
barrel temperature is too high it can coolant channels to offset laminar coolant channel to the area of the tool
cause burning and degradation of the flow to create turbulence. Turbulent needing better temperature removal.
material properties, if too low it can flow forces the coolant to the walls of
impact the finished component. the coolant channels whereas laminar The aim is to circulate coolant through
the mould tool to deliver it evenly.
As the material enters the mould tool flow only enables a portion of the wa- This can be achieved by using a mani-
the temperature of the tool is con- ter to be in contact with the walls of fold on the incoming flow side of the
trolled by the coolant which is either the coolant channels. Therefore, lam- tool to an output manifold. Ideally
water or oil. Water cooling is used inar flow insulates the centre of the coolant flow should be designed to
to lower the temperature of the tool coolant flow preventing it from mak- make a single pass through the mould
whereas oil (sometimes backed up ing full contact with the channel walls tool utilising a balanced manifold to
with cartridge heaters) is when the and thereby wasting its heat transfer achieve similar flow rates to remove
mould tool must be held at an opti- potential. The rate of coolant flow is comparable amount of heat.
mum temperature. determined by how much heat ener- It is though not always possible to
The importance of mould tool tem- gy needs to be removed from the tool achieve an ideal coolant flow leading
perature and cooling profile based on the heat transfer of water to temperature, pressure and heat
and the size of the coolant channels
Processing temperatures and cooling in the mould tool. The turbulence of transfer imbalances. Some of the fac-
rates vary for different polymer ma- coolant flow can be calculated using tors that can cause this are:
terials, so it is important that process a Reynolds (Re) number calculation. Cooling channel lengths vary causing
technicians/engineers understand the The aim is to achieve a coolant flow heat and pressure imbalance.
requirements. It is critical that the between 4,000 and 8,000. The key to
coolant gets to the surface of the tool generating good flow and turbulence Coolant that's taken from a cooling
cavities and achieves the correct tem- is central to the pump size in your channel output and redirected back
perature to ensure the resin flows TCU. into another cooling channel for a sec-
into the tool correctly and cools at ond pass through the mould tool may
the required rate to solidify with the Mould tool design causing heat imbalance.
expected quality. If you are struggling to achieve the Another cause of not being able to

However, if you are working with required coolant flow it might be that hold a consistent mould tool temper-
a semi-crystalline material such as the tool design may not be adequate ature, despite in the past achieving it,
PEEK you cannot use water as a cool- with respect to the cooling channels. could be due to adding extra cooling
ing medium as it would reduce the There are a number of areas that can demand to your system maybe with
tool temperature too quickly. Oil is cause problems: the addition of new equipment. This
usually used to allow the material to • Cooling channel diameter is too may mean that additional cooling ca-
cool at a more gradual rate to allow small pacity will be required, an area that is
the consistent material crystallisation • Cooling channels are too distant often overlooked.
of the part. from the tool cavity surface. However, a further cause for not be-

PLASTICS NEWS 45
April 2024