Rapid Temperature Cycling - RTC?
This is a new generation of moulding technology resulting in considerable improvements, gloss finishes; elimination of visible weld lines, flow lines and splay marks; at lower pressures; longer flow paths and with reduced moulded-in-stress. The process was developed by Gas Injection Worldwide Ltd (GIWW) in collaboration with Oxford Moulding Technology (OMT), which provides complete solutions with heat transfer control equipment and innovative mould construction technology. The process involves initially heating the mould cavity surface near to plastic melt temperatures following which the plastic is injected to fill the cavity. Then, it involves cooling the mould to solidify the plastic and finally, ejection of the moulded component. RTC? normally uses the medium of steam flowing through conformal channels adjacent to the mould cavity surface to heat the mould, followed by chilled water to cool the mould and plastic. In addition to this, the rapid temperature cycling can also be carried out through other alternative ways: -
--Heating the mould cavity with the injection of steam into conformal channels within the mould and located near to the mould cavity surface.
-- Heating the mould cavity surface when the mould is open by a ?work coil', through which a high frequency electric alternating current is passed creating a magnetic field which rapidly heats the steel surface. After injection of plastic to fill a mould cavity the mould plate is cooled with chilled water.
-- Heating the mould cavity plate hot water at up to 160°C under pressure at up to 10bar to prevent evaporation to steam. After injecting the plastic to fill the mould cavity hot water is replaced with flow of chilled water to cool the mould rapidly. -- Heating the mould cavity plate with hot oil at temperatures up to 350°C after injecting plastic to fill the mould cavity hot oil is replaced with cold oil normally at 40°C to cool the mould and solidify the plastic rapidly.
Steam is an effective medium for heating the mould due to the release of latent heat energy when it condenses to water. The higher the steam temperature the more heat is released. It is well known that if a mould cavity surface temperature is near to the plastic melt temperature the visible quality of the moulding surface can be much improved. In RTC? by steam assisted moulding the mould is heated by flowing steam through channels located near to the cavity surface, and to cool the mould, steam is replaced with chilled water in the same mould channels. In this process, molten plastic is injected into the mould cavity which is set at a temperature near to the plastic melt temperature. This allows the cavity to be filled with the plastic before it starts to solidify. Immediately, or just before, the mould is filled with plastic, the remaining steam and condensed water are expelled with compressed air, followed by water, preferably chilled, to cool the mould and plastic. As soon as the plastic has solidified sufficiently to be self supporting the product is ejected.
The SWC Controller lies at the heart of the RTC? process designed and configured by long established world renowned steam specialists with academic contributions from UK 's Oxford and Swansea Universities . It controls the flow of steam, water and compressed air into the mould. A simple to operate control panel allows timings to be set and monitored in real time. SWC Controller enables controlled steam delivery temperature by regulating the steam pressure from one or more temperature sensing thermocouples positioned adjacent to the mould cavity surface with a pneumatically positioned steam pressure regulating valve. This also provides water delivery control assisted by an installed water pump at up to 200 litres per minute. Further, SWC Controller controls the compressed air to expel steam and condensate after the heating cycle, and to expel water after the cooling cycle before the next moulding. If needed, it includes a vacuum air pump to evacuate the mould cavity to avoid air entrapment during faster than normal plastic injection. In SWC, each stage of the steam / water cycle is PLC controlled and timed. Settings and monitoring are through a user friendly interface colour touch screen. Moreover, features a hand held controller with on-screen user friendly graphics to illustrate real time mould and steam temperatures and pressures. Optionally, the SWC Controller can also house a second independently controlled steam circuit for a large complex mould or for heating the second half of a mould if different timing and temperatures are needed. Other discretionary features include t hree alternative steam output capacities ( to match different mould sizes and configurations), mould cavity evacuation systems, and supply of steam supply systems. The benefits of Rapid Temperature Cycling include reduction of molded-in stress, elimination of the weld lines at the places where the melt flow meets avoiding the need for paint finishing. Additionally, the process is also known to increase the gloss of the matt surfaces and increase the glossiness of the already glossy materials. Also, the company claims the RTC process leads to reduced plastic injection pressures with lengthened flow paths thereby reducing press lock forces and operating costs. The process curbs flow marks including damp material marks and enhances part transparency without internal imperfections in clear moulded components. |
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