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Understanding

Design of Cooling Channels for Moulds and

Cooling Lines with Cooling Tower for Injection moulding shop.

Prabodh C. Bolur


Introduction

Injection moulding process is cyclic in characteristic. Cooling time is about 50 to 75% of the total cycle time. Therefore, optimising cooling time for best performance is very important from quality and productivity point of view.

Cooling time is proportional to square of wall thickness. Therefore part design should ensure more or less uniform wall thickness through out the part.

Part design should also ensure that the melt flow is uniform in all direction from the gate and melt should reach the boundary of the part more or less at the same time.

Cooling channel design - location and size and type - should ensure that melt freezes uniformly inside the mould. Cooling channel design can be perfected with the help of MOLDFLOW analysis.

It is necessary to understand Heat Exchange and Cooling Channel design in the mould.

Heat Exchange in mould

During every injection moulding cycle following heat transfers take place:

  • from the hot melt to mould steel (heat input to the mould) and
  • from mould steel to coolant flowing through cooling channel of the mould. (heat removal from the mould)

If heat input is more than heat removal, then the mould temperature would keep on increasing from cycle to cycle. Therefore moulding quality would not be constant from cycle to cycle. The moulding quality would be erratic- i.e. varying from cycle to cycle. Therefore, there is a need to balance between the heat input and heat removal in the mould after the desired mould surface temperature is reached. In other words, removal of heat by circulating coolant through the mould cooling channel would arrest the rise of mould temperature above the desired value. In practice, it may not be possible maintain constant mould temperature with respect to time. However, the mould temperature would fluctuate between two values around the desired value.

During injection moulding cycle heat flow takes place from polymer melt to mould steel by

  • effective thermal difusivity of polymer melt and
  • conduction.

This heat is to be removed by circulating cooling fluid through the cooling channels in core as well as cavity during cooling period in order to maintain the desired temperature. Uneven temperature of the mould surface results (uneven shrinkage) in parts with moulded-in stresses, warped sections, sink marks, poor surface appearance and varying part dimensions from cycle to cycle and even cavity to cavity.

Cooling Channel Design for Mould- Design tips

Moulds are usually built with cooling channels. These channels are usually connected in series with one inlet and one outlet for water flow. The water flow rate may not be enough for turbulent flow because the water pump capacity itself may not be adequate. This obviously leads to random temperature variation on the mould surface. With the result, uncontrolled temperature drift, varying part dimensions and irregular warped surface appears on mouldings.

The mould designer should take care of following points:

  • Thermal conductivity of mould steel influences the rate of heat transfer though mould steel to cooling channel.
  • Pure Ethylene glycol can be used as Primary fluid transfer medium in closed loop cooling system. Ethylene glycol does not produce rust and mineral deposits in cooling channels. Mixture of water and Ethylene glycol can also be used for circulation through the cooling channel.
  • Cooling channel diameter should be more for thicker wall thickness:
  • For wall thickness upto 2mm, channel diameter should be 8 - 10 mm.,
  • For wall thickness upto 4 mm, channel diameter should be 10 - 12 mm.,
  • For wall thickness upto 6 mm, channel diameter should be 10 - 16 mm.
  • Cooling channels should be as close as possible to the mould cavity / core surfaces. The distance of cooling channel from mould surface should be permissible by the strength of mould steel against possible failure under clamp and injection forces. It could be 1.2 to 2 times diameter of cooling channel.
  • Cooling system of the mould should have adequate number of cooling channels of suitable size at equal distance from each other and from cavity walls. The center distance between adjacent channel can be 1.7 to 2 times diameter of the channel. This is also governed by the strength of mould steel.
  • The difference between the inlet and outlet water temperature should be less than 2 to 5 degrees C. However, for precision moulding, it should be 1 degree C or even 0.5 degree C.
  • Cooling circuits should be positioned symmetrically around the cavity. There can be sufficient number of independent circuits to ensure uniform temperature along the mould surface.
  • The coolant flow rate should be sufficient to provide turbulent flow in the channel.
  • There should be no dead ends in the cooling channels. It could provide opportunity for air trap.
  • Many a times it is difficult to accommodate cooling channels in the smaller cores or cores with difficult geometry. In such case the core should be made of Beryllium copper which has high thermal conductivity. These core inserts should be located near the cooling channel.
  • The seals of coolant system should not leak inspite of application of frequent clamping force and mould expansion / contraction due to thermal cycle during moulding. The O-ring should be positioned so that there is no chance of them being damaged or improperly seated during mould assembly. Seal and O-ring grove should be machined to closely match the contour of the seal. It should ensure that seal is slightly compressed when the mould is assembled.
  • Mould temperature above 90 degree C normally requires oil as the heating medium. Heat transfer coefficient of oil is lower than that of water.
  • There is enough scope for confusion while giving water connection to mould when there are more number of cooling circuits particularly on bigger moulds. A sketch indicating cooling circuits should be available during mould set up.
  • Hot runner mould should be provided with compression resistant insulating plate between back plate and machine platen. This is to prevent the heat flow from mould to machine platen, which can create an unbalanced heat flow in the mould. With out insulating plate machine platen will act like a big heat sink, there by destabilising the possible balance between heat given to the mould by the hot melt, and heat taken away by circulating water through mould.
  • The cooling channel layout is suitable when the isothermal i.e. the equi-potential lines, are at a constant distance from surface of the mouldings. This ensures that heat flow density is same everywhere.
  • Provision for thermocouple fixing should be available at specific one or two places in core as well as cavity to monitor the temperature of mould.
  • Use efficient sealing methods and materials to eliminate cooling leaks.
  • Poor mould surface temperature control can cause following quality problems: Axial eccentricity, Radial eccentricity, Angular deviation, Warpage, Surface defects, Flow lines,

The mould has to be heated or cooled depending on the temperature outside mould surface and that of environment. If heat loss through the mould faces is more than the heat to be removed from moulding, then mould has to be heated to compensate the excess loss of heat. This heating is only a protection for shielding the cooling area against the outside influence. The heat exchange takes place during cooling time. The design of cooling system has to depend on that section of part, which requires longest cooling time to reach demoulding temperature.

Cooling Channel layout depends on :

  • part geometry,
  • number of cavities,
  • ejector and cam systems,
  • part quality,
  • dimensional precision,
  • part surface appearance,
  • polymer etc.

The sizing of cooling channels is dependent on the rate of cooling and temperature control needed for controlling part quality. CAE software like MOLDFLOW or C-Mold can be used to determine the optimised dimension of cooling channel and distance from mould surface, distance between cooling channel, flow rate.

CIRCUIT FOR WATER PUMP, COOLING LINES AND COOLING TOWER

Typical Cooling lines with Cooling Tower

for Injection Moulding Shop.

The figure shows number of pumps (each with bypass lines) connected in parallel supplying water through supply line. Two pumps are for main operation. Where as middle pump can be stand by pump. When ever there is problem on any one operational pump, it can be taken up for repair after the standby pump is put on operation. This ensures uninterrupted water supply for moulding shop.

Water reservoir is partitioned to separate cold and warm water. Water from cold reservoir is pumped to process and returns warm to warm part of reservoir. Warm water is again pumped by a separate pump- of same flow rate but lower head- to cooling tower and returns to cold part of reservoir. The partition will have interconnecting hole at suitable height to avoid overflow on account of any unbalance in water transfer. This is shown in separate figure to avoid over crowding of lines.

Each pump should be connected the pump manifold or main line or supply line through flexible connection. This can save time when pump requires to be removes off line for repairs or maintenance.

Pressure at pump side should be between 5 and 6 bar. Pressure loss across mould is about 2 to 3 bar. This pressure loss represents the productive use of power in cooling the mould and heat exchanger of machine.

The supply line as well as return line have additional pressure equalizing line. Supply line with pressure equalizing line forms main supply ring and similarly return line with equalizer line forms main return line ring. Pressure equalizer lines ensure uniform pressure at each supply terminal (inlet valve) on machine as well as mould. In the absence of equalizer lines on supply as well as return line, the inlet pressure would be different at different machine and mould. Highest at the first machine from pump and lowest at the last machine

Any other pressure loss in the system is waste. Therefore, adequate size of pipe should be used for supply and return lines. Pressure equalizer lines should also have same size as that of supply line. Return line and its equalizer line pipes can be of larger size than supply lines as there should not be any pressure loss on return line and equalizer line.

End of supply line and end of return line is connected through pressure differential valve. This valve automatically ensures pressure loss across mould is 2 to 3 bar. In case this valve is not available then, a gate valve should be used. But this requires adjustment of flow when ever there is a mould change.

Connections to mould as well as machine terminals should be through separate gate valve. Connections to Heat exchanger should be through flexible hose pipe. This saves time during regular preventive clean up of heat exchanger. Select correct pipe for heat exchanger as specified by the machine manufacturer.

A manifold with adequate number of connections for in and out of mould should be connected to the terminal inlet valve for mould. It should be noted that there should be no reduction of water passage area from manifold to cooling channel of mould. Normally hose fittings have smaller cross section area inside thereby throttling the flow. This can prevent turbulent flow. In other words, the water passage for 10 mm channel should have hose fitting with minimum internal diameter of 10 mm. Any thing less will not give turbulence. Turbulence is required for efficient heat exchange resulting in power saving. Therefore ensure that mould should have larger pipe fitting to accommodate this point of view.

Please note that at 3 bar pressure loss across the mould;

  • 6 mm channel requires 6.5 lpm to generate turbulent flow,
  • 10-12 mm channel requires 12-20 lpm.

Smaller the diameter of channels higher the pressure drop across. Higher the channel diameter lower the pressure drop across. Therefore, it is better to have all the channels of same diameter through out the mould. If there are different diameters for channels, then the smaller diameter will have larger pressure drop and hence it will have turbulent flow of water, but larger diameter channels will not have turbulent flow. To achieve turbulence in larger diameter channels the flow rate is required to be increased.

Tips for Design of cooling lines and cooling tower.

  1. Compute total requirement of flow rate and pressure for pump selection.
  2. Decide the pump / pumps from manufacturers catalogues.
  3. Decide reservoir size which should be more than 30 min flow.
  4. Decide pipe diameter recommended for the pump and select supply line and return line diameters.
  5. Measure the lengths of each line and prepare Bill of Materials.
  6. Include valves and pressure gauges, hose pipes etc.



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