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Calibre polycarbonate resins from The Dow Chemical Co. are heat and impact resistant. They also have good clarity and serve in a variety of ITEs such as flat-screen monitors. |
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Calibre polycarbonate resins commonly used to build PDAs and other ITE devices can be modified to improve colorability, ignition resistance, and UV stability. |
Here are the top 10 tips for molding PC according to experts at Dow Chemical Co., Midland, Mich. (www.dow.com/engineeringplastics).
Account for the high-thermal diffusivity. PC melt tends to lose heat faster from the mold, barrel, nozzle, and air than most plastics. This can lead to delamination. The thermal diffusivity of the selected PC is a factor that must be taken into account when working with new resins or transitioning to a new resin supplier. Making molders aware of any changes in thermal diffusivity will make it easier and faster for them to tweak molding parameters if needed and also help minimize the likelihood of parts not meeting spec.
Fine-tuned machines. Due to thermal diffusivity, PC melt temperatures can be difficult to control. Proper temperature control constants can help reduce the time needed to stabilize the process after start-up and prevent melt temperature override. The relatively high viscosity of PC resins requires screw velocity and plastic pressure-control constants that provide a high rate of increase.
Temperature control. Some recommended systems for the optimal temperature control for PC would include high-density, high-response mineral-filled bands and an auto-tune controller. Be aware of the temperature-control zone used for barrel end caps. On older machines, this zone has the highest mass of steel and is controlled by the last thermocouple in the barrel. Even though the barrel is at the set point, it's possible the end cap is 100°F cooler than the intended temperature, which leads to streaking and burning-related defects.
Know your end cap. End-cap designs on older machines often have many transitions in the flow path, which can "shear" PC and degrade it. Such end caps typically don't seal well against the higher pressures used in PC processing. Newer designs have only three consolidated components, including the nozzle tip, and a constant-taper flow path to help streamline PC flow.
Watch screw design. PC products will not mold well on machines with "general-purpose" olefinic screws. These screws tend to degrade materials in the rapid compression (transition) sections. Screws with moderate feed lengths (7 turns) and long, gentle compression sections (8-10 turns) often process more efficiently. The nonreturn valve (NRV) in the screw is also important. For example, the correct valve for polypropylene generates shear heating of PC products and requires "suck-back" to seat. Reputable vendors can provide screw designs to accommodate different materials.
Be aware of moisture. Drying PC can be a challenge because the pellets tend to adsorb moisture rapidly from the air as they are moved to the molding machine or while waiting to be molded. These ambient conditions can affect the molding process and typically occur in May through August, commonly known as "Splay Season."
Dry thoroughly. Molding undried PC not only causes splay, it destroys physical properties like tensile and impact strength. For optimal performance, PC products should be dried to less than 0.02% moisture with a desiccant drier. The temperature, dew point, and time required depend on product, weather, and drying equipment.
Heat soak prevents splay. Adding a heat-soak step helps eliminate splay for screws not optimized for PC. With this approach, heat soaking softens the pellets in the feed section. This helps the material through the rapid heat transition as it moves through the screw.
Use wetted parts that are chromed and smooth. PC products tend to adhere to iron-rich alloys and pitted metals. Therefore, screws should be plated to create smooth surfaces and reduce contact with the screw base metal. Chrome-plated end caps, internal surfaces, and flow paths reduce the chances that PC will stick to machine parts as they degrade with use.
Thoroughly purge machines. Without careful purging, PC materials can "weld" two pieces of steel together. For example, the valve can be welded to the end cap and will break off when the screw is turned. Or, if the screw is not given enough time to warm up before it is turned, the melted PC can become like glue and pull the plating off the screw. To avoid this, the machine should be thoroughly purged after processing PC.