| 摘要 |
A method used in an injection-moulding process for making thermoplastic articles comprising a hollow profile formed using a pressurised fluid and a moulding tool placed inside the machine and containing a mould region, at least one inlet for molten plastic and at least one duct (2) for injecting and evacuating the fluid. Injecting the molten plastic and pressurised fluid into the mould region forms a cavity (1) in the molten plastic, inside which the fluid exerts pressure on the plastic and urges it against the mould region walls. The plastic is allowed to harden until the desired dimensional stability is obtained, then the fluid is evacuated so that the pressure inside the cavity is reduced to around normal atmospheric pressure, and then coolant (preferably carbon dioxide) is injected at 15-25 deg C from a pressurised coolant vessel (10) into the cavity in pulses lasting 0.1-20 seconds separated by intervals of 10 ~mseconds to 10 seconds, using one or more of the fluid ducts or at least one separate coolant duct (2'). A cooling effect is achieved as the coolant pressure is reduced inside the cavity relative to the pressure of the coolant inside the pressurised vessel. The sudden pressure drop causes the coolant to rapidly cool and temporarily solidify as it enters the cavity and then the solid coolant sublimes as it absorbs heat from the plastic. Sublimed coolant is vented continuously out of the cavity using at least one duct not being used for injecting fluid or coolant, or using separate coolant exit ducts (2''), in order to prevent the pressure inside the cavity significantly exceeding atmospheric levels and also to allow heat from the plastic to leave with the sublimed coolant. This improved cooling of the article reduces moulding cycle time. |
