PPT05-notes_probs - ME4 Polymer Processing Technology 5...

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5 Cooling The cooling phase begins when the mould is completely flled and ends when the component is ejected. It occupies 50-75% oF cycle time, so e±ciency is crucial. Although the next shot must be plasticised during this time, it is economically important to make it as short as possible. However, problems can arise iF the moulding is cooled too quickly: poorer material properties and reduced dimensional stability; warping (Notes 12) resulting From uneven or asymmetrical cooling. Good cooling channel design can optimise material proporties and minimise warping while keeping the cooling time down. These notes introduce the basic technology oF cooling, and present a frst-order analysis which allows cooling time to be predicted as a Function oF material properties, moulding parameters and moulding geometry. Learning outcomes AFter completing this section you should be able to… 1 Demonstrate awareness oF the technology oF mould temperature control systems and oF the e²ects on moulded part quality, and on cycle time, oF mould temperature. 2 Demonstrate understanding and make appropriate use oF the Following terminology: ba³e, bubbler, thermal pin, coolant, cooling time, ejection (demould) temperature, thermal di²usivity. 5.1 Mould cooling technology An injection mould is an unsteady-´ow heat changer, with the Following contributions: heat enters as a relatively short pulse by Forced convection From the melt (and conduction From the nozzle); iF there are hot runners, heat enters continuously From them; there is heat transFer between mould and machine by conduction; heat is extracted by continuous ´ow oF a liquid coolant ´owing through channels lying below the cavity surFace. It is impossible to maintain constant cavity surFace temperature: the objective oF the mould cooling system is to maintain a uniform cavity surFace temperature. To provide Feedback, thermocouples can be ftted as mould inserts but usually a single thermocouple in the mould block is the only measurement made — iF any. ME4 Polymer Processing Technology PSL 23 November 2009
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Time (s) Cavity inlet pressure (MPa) 0 0 25 50 5 1 01 52 0 Time (s) Temperature 0 5 10 15 20 T P T t T B T amb Midplane Gate 5.1.1 Coolant circuit A pump maintains fow and recirculates coolant through a Free-standing temperature control unit. A and B mould plates are separately Fed by parallel fow paths. Non-uniForm cooling: UniForm cooling: Good moulding Warped moulding Long cooling time Short cooling time 2 ME4 Polymer Processing Technology 5: Cooling 23 November 2009 PSL
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Coolant For mould temperature <90 °C, water, ethylene glycol or a water/glycol mixture.
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This note was uploaded on 04/07/2010 for the course MECH ENG 207 taught by Professor Levers during the Fall '10 term at Imperial College.

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PPT05-notes_probs - ME4 Polymer Processing Technology 5...

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