Diamond Systems Tritan Guia de Resolução de Problemas descarregar pdf (Página 11)

NOTE:

• Reynolds number must be calculated for each area of the

mold having different cooling line diameters.

• A water line in parallel should have the actual ﬂow rate

recalculated if the measured ﬂow occurs prior to branching.

• A pressure differential of 0.138 MPa (20 psi) is typically

needed to achieve a good ﬂow rate.

Table 1 Kinematic viscosity

for water

°C (°F)

Viscosity,

centistokes

0 (32) 1.79

4 (40) 1.54

10 (50) 1.31

16 (60) 1.12

21 (70) 0.98

27 (80) 0.86

32 (90) 0.76

38 (100) 0.69

49 (120) 0.56

60 (140) 0.47

71 (160) 0.40

82 (180) 0.35

93 (200) 0.31

100 (212) 0.28

It is common to ﬁnd a pressure drop well below 0.138 MPa

(20 psi) from inlet to outlet supplies in molding shops. This

typically occurs when the number of molding machines has

been increased without upgrading the water supply system.

If there is a large temperature difference from inlet to outlet, it

is NOT an indication of good cooling. Rather, it can be a warning

that greater ﬂow rates are required to remove even more heat.

The optimum condition for heat dissipation and removal is to

have only a few degrees of difference in temperature from inlet

to outlet.

Notes on cooling

• Maintain a clean system. This can be achieved by:

– Glycol additives

– Rust inhibitors

– Stainless steel—no rust, but lower heat transfer

– Demineralized water

– Filtration

– Periodically ﬂushing the coolant channels

• Adding ethylene glycol increases the viscosity of the coolant.

Consequently, the convective heat transfer coefﬁcient and

the rate of heat transferred from the mold are reduced. For

example, doubling the viscosity lowers the heat transfer

coefficient by 30%. A 10-fold increase in viscosity (50%

ethylene glycol compared to water) can reduce the coefﬁcient

by a factor of 3.

• Increasing cooling channel diameters without maintaining

velocity will result in a decrease in the total heat removed

in a given channel. If turbulent ﬂow is maintained, empirical

correlations show that doubling the diameter while keeping

ﬂow volume (gpm) constant results in approximately 40% less

heat transferred in spite of the fact that the area increases.

• Theoretically, for turbulent ﬂow, keeping the same coolant

velocity while increasing cooling channel diameter will provide

a signiﬁcant increase in heat transferred to a given ﬂow

channel. For example, if the diameter is doubled, the heat

transferred should increase approximately 80%.

Note, however, that if one follows the “rule of thumb” on

spacing of cooling channels, fewer larger diameter channels will

ﬁt around the mold cavity and these will be farther away from

the hot plastic. This constraint makes it difﬁcult to show real

gains in heat removal by increasing cooling channel diameter.

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