Diamond Systems Tritan Guia de Resolução de Problemas descarregar pdf (Página 19)

Hot probe

Melt

Cooling line

Land

Part

Excellent thermal control at the tip of the hot drop is critical

to proper operation of this type of system. There is a relatively

small distance between the bottom of the hot drop, maintained

at the desired melt temperature, and the cavity surface, which

must be cooled to the desired mold temperature. Heat transfer

from the heated drop to the surrounding mold steel is minimized

with an insulated gap in the annular space between the hot drop

and the mold steel. Some systems allow the molten polymer

to ﬂow into this gap and serve as the insulating material. This

is not recommended with Eastman

™

polymers, as this material

can degrade and result in black specks or brown streaks in the

molded parts. A more desirable solution is to use a high heat

insulating material such as Vespel

™

to ﬁll this gap. 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.

Many manufacturers offer different thermal tip styles for this

type of hot drop system. In general, full-ﬂow open-tip styles

are suggested for most Eastman

™

polymers. Styles such as a

“spreader tip” design can be problematic with some of the faster

crystallizing Eastman

™

polymers. Consult with Eastman Design

Services for thermal tip suggestions for speciﬁc material grades.

Internally heated probe-type systems such as the one shown in

Figure 26 are not suggested for Eastman

™

polymers.

Figure 26 Internally heated probe hot drop

With this type of hot drop, the polymer ﬂows down the annular

space between the OD of the heater probe and the ID of the

melt channel. Heat generated from the internal probe moves

out from the probe into the melt. A thin layer of polymer

freezes on the colder steel on the melt channel ID. Higher probe

temperature setpoints are often required to keep the melt

channel from freezing completely. The combination of the

frozen layer and higher temperature setpoints can lead to

material degradation and difﬁculty maintaining consistent

processing condition setpoints. Degraded material often results

in black specks or brown streaks in the molded parts with this

type of system.

Valve gates

If possible, a valve system should be used when processing

Eastman

™

polymers (see Figure 27). This has several advantages

when compared with other hot melt delivery systems. With

valve gates, the melt channel is externally heated and the

mechanical shutoff feature allows better gate vestige control.

The gate size is generally larger when compared with other

available systems. The valve pin is retracted during the ﬁlling

process resulting in a less obstructed ﬂow. The end result is less

shear heating and pressure drop.

Figure 27 Valve gate

Part

Cold

steel

Melt

Heated

tube

Insulation

Valve gate

(open position)

Cold

steel

1 2 ... 14 15 16 17 18 19 20 21 22 23 24 ... 43 44

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