To compete successfully in today's global market, molders continue to seek the most innovative, state-of-the-art knowledge about their products and operations. And as molders, you know that the art of molding even a well-known material such as polycarbonate, is continually evolving. This is especially true as polycarbonate materials are increasingly used in more next-generation technologies and applications.
Based on experience in molding labs and on-site work with customers, the Injection Molding Core Technology Team of Dow Plastics has identified the following Top Ten tips for processing with polycarbonate products. Both molders and plastics distributors will benefit from including these tips into their efforts to increase productivity and contribute to the successful use of polycarbonates:
Account for the High Thermal Diffusivity. Polycarbonate tends to lose heat from the melt to the mold, barrel, nozzle and air faster than most plastics, which can lead to delamination when processing polycarbonate. Understanding and working with the thermal diffusivity of your polycarbonate product will make a positive impact on the final molding process. Specifically Tune Machines. Due to thermal diffusivity, polycarbonate temperatures can be difficult to control. Proper temperature control constants can help reduce the time needed to stabilize the process after start-up and help melt temperature override. The relatively high viscosity of polycarbonate resins requires screw velocity and plastic pressure control constants that initially provide a high rate of increase, but are more dampened than may be necessary for material like polypropylene.
Control Temperature with Care. In our experience, the optimal temperature control system for polycarbonate products features high density, high response mineral-filled bands and an autotune controller. Be aware of the temperature control zone used for the endcap of the barrel. 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 setpoint, its possible for the endcap to be 100F cooler than the intended temperature, which can lead to streaking and burning-related defects.
Use Newer Endcap Designs. Endcap designs on older machines often have many transitions in the flow path, which can shear the polycarbonate and cause degradation. Such endcaps typically dont seal well against the higher pressures of a polycarbonate process. Newer designs have only three consolidated components, including the nozzle tip, and a constant taper flow path for a more a streamlined delivery.
Watch Screw Design. Polycarbonate products will not mold well on a machine that has the general purpose olefinic screw. These screws tend to develop material degradation in the rapid compression (transition) sections. Screws with moderate feed lengths (7 turns) and long, gentle compression sections (8-10 turns) may process more efficiently.
Also, the non-return valve (NRV) portion of the screw is not universal. For example, the correct valve for polypropylene can cause shear heating of polycarbonate products and require suck-back to seat. A reputable vendor can provide a screw design and NRV that will accommodate the differences in materials.
Try Heat Soak for Splay. If the screw design is not optimal for polycarbonate, heat soak can be added to help eliminate splay. With this approach, the pellets in the feed section are softened by the added heat, which helps the material survive the rapid transition of the screw.
Use Wetted Parts That Are Chromed and Smooth. Polycarbonate products tend to adhere to high iron alloys and in pitted metals. Therefore, the screw should be plated to create a smooth surface and reduce contact with the screw base metal. Chrome-plated endcaps, internal surfaces and nozzle internal flow paths reduce the chance that polycarbonate products will stick to the machine parts as they degrade.
Thoroughly Purge Machines. Without careful purging, polycarbonate materials can weld together two pieces of steel. The valve can break off when the screw is turned because it has become welded to the endcap. Or, if the screw is not allowed time to warm up before it is turned, the melted material can become glue-like and pull the plating off the screw. To avoid these problems, the machine should be thoroughly purged after manufacturing polycarbonate products.
Be Aware of Moisture. Drying polycarbonate can be a challenge since the pellets tend to adsorb moisture rapidly from the plant air during transport to the machine and while waiting to be molded. These ambient conditions are typical from May through August, known as Splay Season.
Dry Thoroughly. Molding undried polycarbonate not only causes splay, it destroys some physical properties like tensile and impact strength. For optimal performance, polycarbonate products should be dried to less than 0.02% moisture with a desiccant drier. The temperature, dew point and time requirements are dependent on the product, weather and drying equipment.
Please contact the Dow Engineering Plastics Customer Information Group for more information at (800) 441-4369 or visit www.dowengineeringplastics.com.