Higher tool temperature often results in better surface quality for injection molded parts. To enhance surface appearance of the parts, heat/cool molding technology is a preferred approach which involves thermally cycling the mold surface temperature within the injection molding cycle. This requires heating the mold surface above the material�s glass-transition temperature (Tg) prior to injection, and then rapidly cooling the tool to solidify the molded part prior to ejection. Heat/cool molding process significantly improves the surface appearance of injection molded parts. In addition, the heat/cool molding process eliminates the secondary operations including primers and sanding to hide surface defects and also painting or powder coating in some cases; reduces molded-in stress, jetting and visible weld lines and increases resin flow lengths to make thin-wall parts. This process also enables the use of glass-reinforced structural materials for high-gloss finish applications.
Heat/cool technology eliminated the surface defects and need for sanding when SABIC Innovative Plastics first trialed an 11% glass-filled Xenoy 1760 PC/PBT resin in an automotive roof-rack rail support bracket that was converted from metal. Further, materials such as polycarbonate and blends like PC/ABS and PC/PBT can be used successfully to minimize surface-appearance issues in applications such as TV bezels, light-guide plates, car audio components, and notebook PC covers employing heat/cool technology.
At the outset of the molding cycle, steam or superheated water is circulated in the tool to heat the mold surfaces to a temperature 10� to 30� C above the Tg of the resin. Thereafter, the injection machine is given a signal to inject plastic into the cavity. Once the injection phase is complete, cold water is circulated in the tool to quickly solidify the plastic and cool it sufficiently for ejection. A valve station is used to switch from steam or superheated water to cold water (and vice versa). After the part has cooled the mold opens and the part is ejected, and the system switches back to the mold-heating phase. A special auxiliary system is required for rapid heating and cooling of the mold surface. For SABIC�s tests, mold heating and cooling were regulated with an alternating temperature-control system from Germany�s Single Temperiertechnik which switches from super-heated water at upto 400 degree F to cold water. For efficient process control, the mold must be equipped with thermocouples that are close to the molding surface to monitor temperature. The injection mold, the molding machine, and the heat/cool controller must be integrated to achieve a stable process.

An overview of the heat/cool process shows the temperature cycle relative to the injection cycle. Injection begins at the “permission temperature.”
The effect of heat/cool technology on overall cycle time depends on the material being processed and, more importantly, on the design and construction of the tool. Cavities and cores insulated from the retainer plates and inserted (rather than cut) into the mold plates helps to minimize the mass of steel, which in turn, helps to speed up the time required to heat and cool. This time can also be reduced by using highly conductive alloys like beryllium-copper and also by placing water lines in proximity to the molding surfaces. Conformal cooling (either by laser sintering or direct metal deposition) is an approach that is well suited to this process, where the pattern of water lines mirrors the part surface geometry. For a test mold, SABIC worked with Fast4M Tooling to develop Fast-Form, which builds the tool from a stack of thin sheets of steel, individually laser-cut and bonded with copper. This method easily incorporates conformal and �flood� cooling channels, as well as extensive venting, at low cost. Heat/cool technology can significantly enhance the aesthetics of injection molded parts, more so in amorphous resins such as PC and blends like PC/ABS and PC/PBT. When the mold surface temperature exceeds the Tg of an amorphous resin, the material does not form a skin during the injection phase and the polymer is free to move and not �frozen� when it touches the mold surface, unlike conventional molding. This allows for improved surface replication of the tool surface and higher gloss. For filled materials, a thin layer of polymer on the outside surface encapsulates the filler, thereby increasing gloss by 50% to 90% and reducing surface roughness. With glass-filled materials, an improvement of 70% in Rmax�a measure of surface roughness�has been achieved. The improvement was greater than 20% for unfilled materials.
Schematic of a water- temperature controller that can both heat and cool an injection mold within a single cycle. (Source: Single Temperiertechnik)
As opposed to conventional molded parts, heat/cool injection molding has a positive influence on the depth and visibility of weld lines thereby eliminating the painting operations on some applications. Further, parts molded with heat/cool process had lower molded-in stress and applying the solvent (carbon tetrachloride) did not result in cracks, thereby eliminating the need to anneal parts before use and increasing part lifetime. There are many benefits in part performance and appearance that can be achieved with heat/cool process technology. Although there are additional costs associated with the technology, it can be cost-effective from an overall systems standpoint, particularly if it can eliminate expensive secondary operations. (Author: Andy May, SABIC Innovative Plastics)