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.

Injection moulding is a cyclic operation. The cycle consist of

• Mould close and clamp, (few seconds -depends on machine speeds)
• Injection – Fill (speed) phase, (few seconds) Switchover and Pack (pressure) phase,(few seconds)
• Cooling time, (40 to 60% of cycle time)

You will observe that total injection stroke is divided in to two phase by a switch over point on the scale of injection stroke. First phase has 80-95% of injection stroke and remaining part of stroke is for pressure phase.

The melt gets compressed in plasticising cylinder prior to entry of melt in to the mould. At the end of filling phase inside the mould, melt is relaxed - i.e. it expands resulting in filling up of remaining space in the mould. This causes pressure peak inside the cavity.

Initially there is no resistance to flow of melt. Resistance increases as the cavity is being filled up -as more and more resistance felt-. This is seen as pressure rising before switch-over. Fill pressure is the measure of resistance to flow of melt. Resistance is directly proportional to melt viscosity and maximum length of flow and inversely proportional to thickness of flow. It means Viscosity ∓mp; Flow ratio.

During pressure phase, the melt flows into the mould in order to compensate for the shrinkage due to falling melt temperature.

Fill pressure drops to minimum with increasing fill time in the beginning. This is due to isothermal behaviour of melt before switchover. There is enough space for melt expansion resulting in fill pressure drop during shorter fill time.

Further increase in fill time rises the fill pressure. This is due to heat exchange in mould. Falling melt temperature means increasing melt viscosity. This in-turn responsible for increasing pressure.

At lower melt temperature, fill pressure is higher on account of higher melt viscosity.

With glass reinforcement, melt visosity increases, hence fill pressure increases.

This equation gives minimum cooling time.

Alfa is the thermal diffusivity of the material, h, is the wall thickness, T_w is the mold wall temperature, T_m is the melt temperature, capital T_e is the ejection temperature.

An example calculation has been shown here with typical values for the different variables. In the example, the minimum cooling time for the part centerline to reach the ejection temperature is calculated to be 23 seconds.

It is observed that cooling time is proportional to square of wall thickness.

Cooling time increases in a non-linear fashion with increasing part wall thickness.

The cooling time for a semi-crystalline material like Polybutylene Terephthalate is always higher than that for an amorphous material like a blend of Polycarbonate and ABS.

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.

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