fullpacdigi

CYCLE TIME

Any manufacturing activity would like to have optimised productivity and quality. In injection moulding of plastics, if quality is taken care of by part design, mould design and mould precision, then productivity is also ensured on account of zero defect moulding with out rejection and optimised cycle time.

Cycle-time optimisation starts at design stage. Cooling time takes up over 50% of cycle time. Therefore understanding of cooling in the mould becomes very important.

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 2 to 2.5 times diameter of cooling channel.

• 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.