Diamond Systems Tritan Guia de Resolução de Problemas descarregar pdf (Página 25)

Clamping force

Required clamping pressure can be calculated from a mold

ﬁlling analysis where wall thickness, ﬂow length, speciﬁc

material, melt temperature, and mold temperature are taken

into consideration. Clamp tonnage (maximum clamping

pressure available) is typically 40–70 MPa (3–5 ton/sq in.).

Total clamping force needed may also be calculated by

multiplying the part’s projected area on the platen of the

molding machine by 40–70 MPa (3–5 ton/sq in.).

Injection speed

Capability to proﬁle injection speed is another important factor

in choosing a machine. The ability to change the speed smoothly

as the screw moves forward can make molding much easier and

the processing window wider, especially in larger parts.

Screw and barrel design

General-purpose screws (Figure 29) with compression ratios of

approximately 3:1 and L/D ratios of 18:1 to 20:1 have been used

successfully. Screw ﬂight depths are also important. Suggested

ﬂight depths can be obtained by discussing your application

with an Eastman technical service repre sentative. The transitional

zone should be gradual, typically 4 to 7 diameters, so that high-

shear heating of a sudden transition is avoided. These polymers

generally cause little wear on the screw and barrel; therefore,

corrosion of the barrel and screw components is not expected.

While vented barrels have been used with limited success, they

are not a substitute for proper drying. In addition, the vent

should be kept clean when processing clear material. Volatiles

from polymers can accumulate and carbonize in the vent; this

can cause the polymer being processed to be contaminated with

black specks. The middle decompression area on the vented

screw typically causes screw recovery to be sacriﬁced unless

faster screw speeds are used; however, faster screw speeds will

likely result in increased shear heating.

Ring-check (nonreturn) valves are generally preferred to

ball-check valves, although ball-check valves have been used

successfully. Ball-check valves must be carefully designed to

allow free passage of material with an absolute minimum

holdup. The area of ﬂow-through should have approximately

the same cross-sectional area for melt ﬂow as the metering

section of the screw. Check rings need to be replaced periodically,

as they can wear and sometimes even break. Wear could be

indicated when the screw will not hold a cushion and continues

to move forward after the shot and packing are complete. In

extreme cases, frequent short shots will result.

Nozzles

Select nozzles with the minimum length needed to extend

into the mold. General-purpose nozzles of uniform bore or

larger-diameter nozzles that use generous radii to gently reduce

diameters at the exit end are preferred. The inside diameter of

the nozzle should be very close to that of the sprue end but just

slightly smaller so that the sprue can be pulled. Nozzles with

inside diameters of 5–8 mm (3⁄

–5⁄

in.) are typical for smaller

parts; those for larger parts should have a 9.5-mm (3⁄

-in.) or

larger diameter.

Flight length

Feed section Transition section Meter section

Root diameter

Flight depth

(Channel depth)

Outside

diameter

Compression ratio

Flight depth (feed)

Flight depth (meter)

= L/D

Flight length

Outside diameter

Figure 29 Injection screw features and terminology

1 2 ... 20 21 22 23 24 25 26 27 28 29 30 ... 43 44

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