Diamond Systems Tritan Guia de Resolução de Problemas descarregar pdf (Página 18)

Hot runner systems

Design guidelines

Hot runner systems are common in applications using polyester

materials. When properly designed, these systems can eliminate

sprue and runner regrind, mold with lower pressures, reduce

cycle times, and improve processing windows. The selection of

a suitable hot runner system can vary greatly depending on the

size of the part, polyester formulation, and part design. Therefore,

it is critically important that runner design and selection be

discussed jointly by the molder/end user, the tool builder, the

hot runner supplier, and Eastman to arrive at the appropriate

runner-system design to be used.

Good hot runner systems will not have holdup spots in the

manifold or gate areas. They will also be designed to avoid sharp

corners, extremely small gates, and other high shear areas. In

general, polyester materials are more shear- and heat-sensitive

than many “commodity” polymers. The system selected should

be designed with that in mind.

Uniform heating and good heat control

Excellent thermal control and good cooling at the gate is critical

for molding polyester materials. The mold should be designed

so that heat is quickly removed from the gate. This is best

accomplished by the gate oriﬁce being an integral part of the

cavity steel, rather than the hot runner system being an insert

projecting through the cavity into the part. When the gate is

in the cavity, cooling channels (drilled water lines or annular-

shaped passages) can be incorporated to provide the cooling

needed for the cavity in the gate area. Some hot runner suppliers

offer gate-cooling inserts. Drooling, sticking, and stringing may

occur if the gate does not cool properly. Steel that is directly

heated as part of the hot drop should not contact the part

directly; it should be insulated from the cooled portion of the

mold.

We suggest separate cooling loops with individual ﬂow and

temperature control for hot drop gate cooling. The additional

control is very useful in debugging and optimizing gate

appearance and performance.

Eliminate holdup spots

The ﬂow channel for the plastic should be streamlined and

uninterrupted. Any crevice or pocket where material can collect

and degrade will probably cause defective parts.

Minimize shear heating

The diameter of the ﬂow path needs to be large enough to

minimize the shear heating that can be caused by sharp corners

or edges in the ﬂow path at the gate or elsewhere. Mold ﬁlling

analyses can show shear heating and indicate potential problems

during the design stage.

Hot drops

Externally heated

Externally heated hot drops such as the one shown in Figure 25

are suggested for Eastman

™

polymers.

Figure 25 Externally heated hot drop

With this type of hot drop, the polymer is completely enclosed by

a heated tube. All surfaces of the melt channel ID are maintained

in the desired melt temperature. Heat ﬂow from outside to the

center results in a homogeneous melt temperature across the

melt channel diameter. This allows for excellent temperature

control, minimizing the potential for material degradation or

crystallized material due to poor temperature control.

Part

Cold

steel

Cold

steel

Melt

Heated

tube

Land

Insulation

High-

temperature

rated and

nonreactive

insulator

material

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