Hot runner mold systems #3

Uniform mold heating and good heat management 
Hot runner valve-gated systems have been used successfully with amorphous copolyesters like Eastman Tritan copolyester for several years. One benefit of the low mold temperature required by Tritan is shorter molding cycle times—but this benefit depends on good management of both heating and cooling.
Best results come from designing a mold that allows good temperature management throughout the drops, mold, sprues, and gates to keep the material in the tool above the glass transition temperature (Tg) until it passes through the gate into the mold cavity—and is ready to fill the cavity on the next shot.
Uniform heating and proper cooling improve success by:
•  Eliminating holdup spots that can degrade the material
•  Avoiding excessive heat that can lead to sticking
• せん断発熱を最小限に抑える
•  Improving processing efficiency
•  Improving part quality 

Early collaboration when designing the hot runner system will pay lasting dividends when molding Tritan in a hot runner system.

Elements of effective temperature management for Tritan
A hot runner system allows you fast cycle times while making parts with good surface appearance. When molding Tritan in a hot runner mold, this means cleanly separating the hot and cold areas of the mold with good insulation systems so that melt temperature is kept uniform within the material's working range of 500°-540°F and the well-cooled mold is maintained at its uniform surface temperature of (100°-150°F)-especially including the area around the gate-to prevent the formation of heat induced sink marks in the molded part.
Planning thermal management early in the design process helps make sure you achieve this combination of mold heating and cooling.  
Mold heating
The melt should be maintained at the same temperature generated at the discharge of the screw all the way through the machine nozzle, mold sprue, hot runner manifold, and hot runner drops and tips.
Hot drops and tips
​Externally heated hot drops are recommended when hot runner molding Tritan. Use a design that improves control by completely enclosing the polymer within the heated drop. This allows for excellent temperature control, minimizing the potential for degradation or crystallized material.
Excellent thermal control at the tip of the hot drop is critical to proper operation of the system.
Heat transfer from the heated drop to the surrounding mold steel can be minimized with an insulated gap in the annular space between the hot drop and the mold steel.
(NOTE: Some systems allow the molten polymer to flow into this gap and serve as the insulating material. This is not recommended with Tritan, as the insulating material can degrade and result in black specks or brown streaks in the molded part. A more desirable solution is to use a high heat insulating material to fill this gap.)
Many manufacturers offer different thermal tip styles for this type of hot drop system. Contact Eastman Design Services for thermal tip suggestions for specific grades of Tritan.
プラスチックの流路は、スムーズで途切れのないものでなければなりません。 Any crevices or pockets that allow material to collect and degrade can cause defective parts.
(Minimize shear heating)
せん断発熱は、ゲートやその他の場所など流路が急に曲がる部分や端で起こります。せん断発熱を最小限に抑えられるよう、直径は十分に大きなものである必要があります。 Mold filling analyses can show potential shear heating areas of concern and indicate potential problems during the design stage. For more information about mold filling simulations, contact Eastman Design Services.
Mold cooling
Eastman Tritan copolyester requires colder molds than some other plastics, so planning cooling design features in advance pays dividends in cycle time and processability. High mold temperatures, even in small areas of the mold, can cause sticking.
Ample mold cooling channels, uniform wall thickness design, good cooling of pins and thin steel areas, good cooling near hot spots such as sprues or hot runners, insulating areas around hot runners, good water supply with few flow restrictions, and thermolators with exact control settings of water temperature all help ensure efficient production of high quality parts.
A cooling circuit or water jacket in close proximity to the gate is also required for heat removal. Plumbing this circuit independent from other cavity cooling channels can be beneficial, as separate water temperature control can be used to optimize molding performance in both the gate area and the mold cavity.
Well-cooled molds require good water flow throughout. Chillers should be considered to cool the water as relying on tower water may prove to be insufficient.
It is important to maintain suggested surface mold temperatures at the interface with the part. An independent cooling circuit in close proximity is always suggested. Another viable solution for temperature control is a water-jacketed insert. These are sometimes custom fabricated but are also available as standard items from some manufacturers. They usually result in a witness line around the gate, which may need to be considered.
In Part 4 of our hot runner series, we’ll focus on gate design—specifically valve gates—when molding Tritan in a hot runner system.