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时间:2017-09-30 15:11来源:毕业论文
Injection molders are warming up to the idea of cycling their tool surface temperature during the molding cycle rather than keeping it constant. This heat/cool process has caught on faster abroad, but more U.S. molders are now also finding t
Injection molders are warming up to the idea of cycling their tool surface temperature during the molding cycle rather than keeping it constant. This “heat/cool” process has caught on faster abroad, but more U.S. molders are now also finding that raising the mold wall temperature above the resin’s glass-transition or crystalline melting temperature during the filling stage, followed by rapid cooling, brings a bevy of benefits in process ability and product performance in applications from automotive to packaging to optics.14007
Processing benefits include longer, more uniform holding pressure, even in areas far from the gate, which can lower injection pressure and clamping requirements; improved flow lengths; reduction of internal part stresses; and reduction or elimination of weld lines, jetting, silver streaks, or sink marks. Other significant benefits are improved replication of minute mold-surface details and improved part surface finish. For example, it reportedly can ensure a smooth, resin-rich surface in glass-reinforced parts or prevent visible bubbles or “splay” in foamed parts. Thermal cycling of the mold can eliminate post-mold downstream operations such as sanding, annealing, priming, and painting to hide surface defects. At least one supplier says it is used to initiate chemical curing reactions in the mold. Heat/cool molding is being increasingly considered for thin-wall applications due to the reduced filling pressure and injection pressure requirements. Parts down to 0.4 mm have already been molded using the process.
Dynamic mold-temperature cycling has been used for flat parts, complex shapes, and large or small parts. Heat/cool or variable-temperature molding is often called “Variotherm” molding, a term coined and trademarked by Werkzeugbau Siegfried Hofmann GmbH in Lichtenfels, Germany. The process has several different variants. It is not for every application, yet because suppliers are making the process more efficient and easier to implement, it is finding an increasing persity of applications from automotive bumpers to television flat-screen frames, cell phone cases, laptop covers, automotive lenses, and niche areas such as lab-on-chip devices with molded micro-capillary structures.

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Early successes in commercializing the various heat/cool technologies are being driven mostly by firms in Germany or France, including Hofmann, Single Temperiertechnik GmbH, Gesellschaft Warmed Kalttechnik mob (GWK), Retool, Whitman Battlefield, and Engel. The different approaches add complexity to the tool, and usually require a temperature sensor in the mold that gives a signal to the injection press or temperature controller. Newer approaches decouple the heating and cooling phases, which allows for mold-temperature increases as rapid as 25° C/sec. Several of the processes enhance cooling by incorporating thermally conductive inserts and/or conformal cooling channels that follow the contours of the part more closely than traditional gun-drilled straight channels.
Molders can raise the temperature of their mold surface using various means:
Convection heating with water or oil through single or multiple heat/cool circuits in the tool. Hot dry air or steam is also used—turned on and off during injection/cooling. (Steam is being used mainly by developers in Japan.)
Conduction (using electric or ceramic heating cartridges in the mold).
Induction by means of an inductor that generates an alternating electromagnetic field. Inductors can be integrated into the mold or built in a “cage” surrounding the mold.
Radiation from infrared lamps.
CONDUCTION SUCCESSES
GWK, a maker of mold-temperature-control and cooling equipment in Kiers, Germany says its Dynamic Mold Temperature Control has been used for many heat/cool applications. Its dynamic Mold Temperature Control process is a combination of high-density ceramic heaters (800W/in.2) mounted just below the tool surface and conformal cavity and core cooling channels. In thin mold sections that cannot accommodate conformal cooling, GWK uses thermally conductive inserts that extend back to the cooling channels. These approaches reportedly can heat at 30° C/sec and cool at 10° C/sec. Temperatures with this process range from 41 F to 750 F says Peter Lip, operations manager for Yellow Box Technologies, LLC, Union Ky. (yellowboxengineering.com), representing GWK in the USA and Canada. 注塑模具加热英文文献和中文翻译:http://www.lwfree.cn/fanyi/20170930/14410.html
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