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moldmakingtechnology.com 15 FIGURE 4 This graph depicting plastic pressure versus fill speed shows 0.32 seconds as optimum fill time and peak pressure as 10,954 psi for a PEEK resin part that was direct-gated with a valve-gate hot runner system. Case in Point To help demonstrate the importance of these resin and hot runner develop- ments, consider the processing requirements and issues that were encountered when molding a mobile device housing (back cover). These parts were produced with a PEEK/PAEK-grade resin ideal for molding thin-wall parts with high strength and stiffness requirements. This experimental project had a few challenges. The first was gating. Direct- gating these parts with hot-tip-style gates was not an option, because the PEEK resin would freeze off at the normal processing temperature. Raising the tem- perature of the tip (at the gate) to more than 340ºC could result in resin degra- dation, so torpedo-style hot tips are not recommended for gating PEEK parts. Other gating styles to consider include valve gates and standard thermal sprue gates (see Figure 2). With these gating styles, the gate area is maintained at a higher temperature as the melt heat is brought to the gate via the nozzle/valve stem or the gate tip. This part was direct-gated with a valve-gate hot runner system. The plaque, measuring 4 inches in length and 1.5 inches in width with 1-mm wall thickness, was molded from a single-drop-offset hot runner mold in a 180-ton injection molding machine. The single-drop-offset hot runner consisted of a valve gate nozzle with a through-bore-tip-gate style (see Figure 3), which has no material insulation area and conducts heat into the gate through the nozzle, keeping it warm, as required with semi-crystalline resins such as PEEK. PEEK grades are semi-crystalline thermoplastics with melt temperatures in the range of 350 to 400ºC and a mold temperature in the range of 150 to 200ºC. Since the processing temperatures for PEEK resins are high, the hot runner sys- tem, including the heated manifold and nozzle, must be capable of maintaining high temperatures to keep the resin in molten state. Pneumatic valve-gate actua- tors perform well in these extreme operating conditions.