MOLD MATERIALS MAKING THE MOST OF HIGH—PERFORMANCE MOLD MATERIALS
Understanding high conductivity alloys and optimizing their u can help you build better molds.
桑葚是热性还是凉性By Douglas Veitch, Director, Brush Wellman
Injection 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 the efficiencies to be realized. Once the characteristics are understood, mold builders can optimize their u of high-performance materials to provide a durable, fast-cycling mold for their customers.
Cooling Time世界杯最佳射手
我们仨读书笔记Mold Alloy Thermal Properties
Some characteristics of mold materials enable us to better understand the thermal process that occurs while molding. Three important properties are:
1. Thermal Conductivity
Higher thermal conductivity equates to the transfer of more thermal energy per unit of time under steady state conditions.
2. Thermal Diffusivity
Higher thermal diffusivity means that thermal equilibrium will be reached faster when the temperature changes. A good thermal diffur 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 (e Chart 1).
失与得
Chart 1 |
Mold Material | Thermal Conductivity, W/(cm-oK) | Thermal Diffusivity, cm2/c | 多媒体pc是指Thermal Effusivity |
Steel - P20 | 0.29 | 0.081 | 1.02 |
Copper Beryllium 瑜伽流派 | 1.25 | 0.415 | 1.94 |
| | | |
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. 电脑知识与技术
∙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 tpoint temperature (equal to water temperature) during mold-open, ejection and mold-clo portions of the cycle.
∙Again, the high diffusivity enables the mold to maintain equilibrium at tpoint during mold open, ejection and mold clo. Since the air is a poor thermal medium, the contact between the water and copper is the overriding factor.
Figure 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 Types二次方程
The two main polymer families—mi-crystalline and amorphous—both benefit from higher conductivity mold materials.
Semi-crystalline polymers have a denly packed, uniform molecular structure and include materials such as polyamide (nylon), polyethylene, polypropylene and polyacetal. The polymers become amorphous when melted during processing and will become mi-crystalline again when cooled.
Amorphous polymers have a loo and random molecular structure, so that in some cas 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 increa the crystallization rate and the HDT so the part can be ejected at a higher temperature without deformation.