Diamond Systems Tritan Instrukcja Naprawy Pobierz pdf (Strona 12)

Why corners are critical cooling areas

Figure 7 illustrates that there is approximately 3 times the steel

mass on the outside of a corner than on the inside. Thus, it is

much easier to remove heat from the plastic on the outside

than on the inside because there is more steel in which to place

cooling. This concept also holds true for cores in general: they

are more difﬁcult to cool than cavities, especially as the size of

the cores decreases. To overcome this effect, good core cooling

is critical.

Figure 8 shows how ejector pins are commonly placed in the

corners of box-shaped parts. With the difference in steel mass

between the cavity and core of the mold, as well as the air gaps

at the ejector, it is nearly impossible to cool these corners

properly. The outside of the part cools first and solidifies,

whereas the inside cools slowly, resulting in more shrinkage. The

end result is part warpage and high levels of molded-in stress.

Figure 7 Why corners are critical cooling areas

Figure 8 Poor cooling in corners

Figure 9 shows how to resolve this situation. Place a bubbler

or bafﬂe in the corner to remove heat from that section of the

mold. This will help reduce warpage and lower molded-in stress.

The ejector pins or blades will need to be moved to other

locations, or ejection could be accomplished by specifying the

use of stripper plates in a new mold.

Figure 9 Provide good cooling in corners to

reduce warpage

Sprue design

Proper sprue design is important for good molding and easy

removal of the part from the mold. Sprue design for molds

running Eastman

™

polymers is important because:

• Polyester materials tend to stick to tool steel when hot.

• The sprue is so thick that it is the hottest and one of the most

difﬁcult areas to cool.

As shown in Figure 10, a 6.25-cm/m (0.750-in./ft) taper included

angle (about 3.0°) on the sprue and a maximum sprue length

of 80 mm (3 in.) are suggested. To aid ejection, polish the sprue

in the draw direction. Put a generous radius at the junction of

the sprue and runner system to avoid breakage during ejection.

Place an ejector pin under the sprue puller rather than an air

poppet valve. An air poppet here would cause a hot spot and

impede cooling.

Approximately 3X better

cooling on the outside,

which has more steel mass

to accomplish cooling. Good

core cooling is needed to

overcome this condition.

Ejector pin

Bubbler

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