机械设计英文参考文献.docx

MOLD MATERIALS MAKING THE MOST OF HIGH—PERFORMANCEMOLD MATERIALSUnderstanding high conductivity alloys and optimizing their use can help you build better molds.By Douglas Veitch, Director, Brush WellmanInjection molders and blow molders can benefit from high conductivity alloys by achieving faster cycle times and better part quality. There are certain properties of the mold material and polymer that enable these efficiencies to be realized. Once these characteristics are understood, mold builders can optimize their use of high-performance materials to provide a durable, fast-cycling mold for their customers.Cooling TimeMold Alloy Thermal PropertiesSome characteristics of mold materials enable us to better understand the thermal process that occurs while molding. Three important properties are:1. Thermal ConductivityHigher thermal conductivity equates to the transfer of more thermal energy per unit of time under steady state conditions.2. Thermal DiffusivityHigher thermal diffusivity means that thermal equilibrium will be reached faster when the temperature changes. A good thermal diffuser will react more quickly to environmental temperature changes.3. Thermal Effusivity (conductivity divided by the square root of the diffusivity)Higher thermal effusivity is a measure of the material's efficiency at instantly removing heat from an object at a higher temperature with which it suddenly makes contact(see Chart 1).The following explains what all of this means when molding plastics.1. Heat mold up to operating temperature (via water channels).• The higher diffusivity allows the copper mold alloy to reach equilibrium faster, so the molding operation can begin sooner.2. Inject hot plastic melt into the mold and cool.• Higher effusivity means the mold will begin to instantly and efficiently remove heat from the plastic.• Then the high diffusivity translates to reaching steady state, uniform temperature quickly.• Finally, once at equilibrium the conductivity determines how fast the thermal energy will be removed from the plastic until the part reaches the desired ejection temperature.3. Maintain setpoint temperature (equal to water temperature) during mold-open, ejection and mold-close portions of the cycle.• Again, the high diffusivity enables the mold to maintain equilibrium at setpoint during mold open, ejection and mold close. Since the air is a poor thermal medium, the contact between the water and copper is the overriding factor.uLI QiaarP-20turo- divtribtitioni in a mo id： wkth turbuMot pooling <20 *C wnwr]- rrarrjjMyafMrtf 山・弋dL£；l. HENIJHF ■ »TC■(W «IT Oto WS力 g SW. r4M>liKl£i ClpCJCg-3 厂 mow .surfico MM iUW-砒切和卫 即c【0Figure 1: IR temp distribution. Images courtesy of Brush Wellman Inc.Figure 1 shows pictures from a thermal FEA illustrating the uniform temperature of a copper beryllium mold compared to that of a mold made of P-20 steel.Polymer TypesThe two main polymer families—semi-crystalline and amorphous—both benefit from higher conductivity mold materials.Semi-crystalline polymers have a densely packed, uniform molecular structure and include materials such as polyamide (nylon), polyethylene, polypropylene and polyacetal. These polymers become amorphous when melted during processing and will become semi-crystalline again when cooled.Amorphous polymers have a loose and random molecular structure, so that in some cases amorphous materials are transparent. Both types of polymers can benefit from improved heat transfer and reduced cooling time.The following are some differences that need to be realized to provide a better understanding of the application.• Crystalline materials have a sharp melting point, and thus a latent heat energy that must be added when melting, and removed when cooling. The plastic needs to be solidified and cooled below the heat deflection temperature before ejection from the mold. The heat deflection temperature (HDT) is available on most resin datasheets. Just getting below the melting point is not enough. The part has to be cooled to the point where it is stiff enough to eject. Glass and mineral fillers increase the crystallization rate and the HDT so the part can be ejected at a higher temperature without deformation.• Amorphous polymers do not have a melting point, but as the heat input is increased above the glass transition temperature (Tg), the viscosity of the polymer decreases until it begins to flow. Heat is added until the plastic can flow adequately to fill the mold. Then the heat has to be removed until the polymer is below the Tg—in many cases before the part will be stiff enough to be ejected.In general, crystalline polymers contain more heat energy due to the latent heat. For example polycarbonate—which is amorphous—has a heat capacity of 1.2 J/(g oK) while polypropylene—which is semi-crystalline—has a heat capacity of 1.9 J/(g oK) or 58 percent higher. Molders will experience cycl。