Mold design—critical factor #2

Thermal control of cavity surfaces

Thermal control is critical for injection molding Eastman Tritan copolyester. Keeping temperatures cooled below the heat deflection temperature (HDT) of Tritan helps ensure successful demolding with no dragging or sticking of parts. Preparing a cooling strategy early in the tool design process can pay big dividends in cycle time and processability. 

One key to keeping it cool—just add water.     
It's not that simple. But providing ample and well-positioned cooling water channels is critical to controlling the temperature of the cavity surface and the resin in the mold. Here's why controlling resin temperature is important: as the resin approaches HDT, it becomes sticky (the coefficient of friction increases) and parts cannot be ejected efficiently.

NOTE: Excess heat over time also reduces molecular weight and degrades a polymer’s properties, as addressed in Mold design—critical factor #1

To produce the best low-stress parts with Tritan, steel surface temperatures should be kept between 130° and 150°F. For optimal temperature control when molding Tritan, cooling water circuits should be positioned near the cavity steel surface, following the contours of the molded part in a process known as “conformal cooling.”
 
To remove heat more efficiently, baffles and bubblers may be used to generate turbulent water flow within the core steel.
 
In areas of the cavity where the part geometry makes it difficult to place cooling lines near the molten plastic, Eastman often recommends an alloy with a higher heat transfer coefficient. An alloy such as steel/copper can extract excess heat more quickly and help improve good part quality and appearance.

Being a good gatekeeper—with special attention to the gate area  
Injection molding gates typically have the highest heat load in an injection mold. Therefore, the area around the gate requires the greatest capacity to extract heat quickly. Some considerations to keep in mind:
•  Run the cooling circuit as close to the gate as possible.
•  A water-jacketed gate often works well.
•  For a hot-gated system, we recommend a valve gate that uses a mechanical shutoff rather than a thermal shutoff.  
• Consider using an independent water supply for the gate area. This lets you control gate water temperature independent from cavity cooling circuit-and can help improve processability and gate aesthetics.

Keep the tool cool beyond the cavity surface.
In addition to the cavity surface, be aware of all high-temperature spots that can lead to sticking-even in small areas of the mold. Designers should make sure there are ample cooling channels to address areas such as:
       •  Core pins
       •  Thin steel areas
       •  Areas near sprue and hot runner channels
       •  Insulation around hot runners

Involve Eastman early—and keep us in the loop. 
The Eastman Design Services group can be a great asset during the tool build. We can review your plans and make sure your mold will give you the best results from Tritan.
 
Questions about temperature control and cooling techniques? The Tritan Experts and the Eastman Design Services group are ready to help you find the answers.