Diamond Systems Tritan Guia de Resolução de Problemas descarregar pdf (Página 31)

Injection molding

Proper conditions and machine operations for molding Eastman

™

polymers are discussed in this section. It includes sections on

start-up, purging, use of regrind, and shutdown. The recommended

processing conditions are listed on the technical data sheet for

speciﬁc grades of Eastman

™

polymer. These data sheets can be

found online at www.eastman.com.

Molding conditions

Barrel and melt temperatures

The ﬁrst consideration in setting barrel temperatures is how

much shot capacity will be used. Typically, if about half the

machine’s shot capacity is used in each shot, barrel temperatures

are set almost the same from back to front or slightly cooler at

the feed end. If the shot is small relative to machine capacity,

then temperatures are set signiﬁcantly cooler at the feed end

to minimize degradation due to long residence times at high

temperatures. If the shot size is most of the machine’s capacity,

then ﬂat or higher temperatures at the feed end are typically

used. These polymers often require a descending proﬁle with

higher rear-zone setpoints to achieve proper screw recovery.

Another important factor is expected cycle time. For example,

if the expected cycle time is long because of limited mold cooling,

barrel temperatures should be lower. Different screws add

different amounts of shear heat, but it is common to see melt

temperatures 10°–20°C (20°–40°F) above the barrel settings.

Actual melt temperature should be checked with a needle

pyrometer. Melt temperature is best taken when the cycle is

established, and an on-cycle shot is caught in an insulated

container.

(CAUTION: Care must be exercised when taking samples of

HOT molten material.)

Melt temperature is the biggest factor in ease of ﬁlling the

mold. Typically, melt temperatures 5°–20°C (10°–30°F) above

the minimum temperature required to ﬁll a part will give a good

processing window. Melt temperatures on the high end tend to

cause degradation and related problems.

Mold temperatures

Mold temperatures affect overall cycle, shrinkage, warpage, and

other characteristics of the molded part.

Copolyesters require colder molds than some other plastics;

therefore, antici pating cooling needs ahead of time (i.e., via tool

design) pays dividends in reduced cycle time and processability.

High mold temperatures can cause sticking. Even localized hot

spots where sticking might occur can extend the cycle.

Injection speed

To minimize gate blush, splay, or both, the ﬁll speed used for

copolyesters is slower than for some other plastics. Machines

with ﬁll speed programming capability are recommended.

Start the ﬁll at a very slow speed, such as 10%–20% of available

capacity for the first 3%–5% of the shot, then increase to

40%–60% to complete the shot. An average ﬁll rate of 50–250 g/s

(1.76–8.8 oz/s) is typical.

Screw speed

The screw should be run at the minimum rpm that will allow it

to recover 2–5 seconds before the mold opens. This minimizes

viscous heat generation, tends to make the melt more uniform,

and minimizes dead time.

Pack and hold

A common problem with direct sprue-gated parts is a shrinkage

void at the base of the sprue. Long hold times of 8–12 seconds

and lower hold pressures of 275–550 bar (4,000–8,000 psi)

(nozzle plastic pressure) will feed material to the sprue at a rate

that will eliminate voids but not overpack the sprue. Overall

cycle time does not have to be extended if the cooling timer is

decreased by the amount the hold timer is raised. A shrinkage

void can also form with a conventional runner at the junction

of the runner and sucker pin; this can be eliminated by using the

above methodology.

Cushion size

Cushion size should only be large enough to ensure the screw

does not hit bottom and the pack-and-hold pressures are

transmitted to the part. The cushion left at the end of the

pack-and-hold phase of the cycle is typically 3–13 mm

(0.125–0.5 in.), depending on machine size and injection

speed. Larger cushions can increase holdup time in the barrel

and contribute to degradation. Continued forward movement

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