Diamond Systems Tritan Guia de Resolução de Problemas descarregar pdf (Página 32)

of the screw at the end of the shot indicates a leaking check

valve. A leaking check valve will prevent a cushion from being

maintained and can cause random short shots and shot-to-shot

variability.

Back pressure

Typical back pressure is 7–10 bar (100–150 psi), though it may be

as low as 3.5 bar (50 psi). To improve melt uniformity, increase

melt temperature, or eliminate air entrapment (air splay),

back pressure can be increased to as much as 28 bar (400 psi).

Excessively high back pressures can aggravate drooling into the

mold since decompression is usually kept to a minimum.

Decompression

In general, minimal decompression is used. Decompression

tends to pull air back into the nozzle, causing splay in the next

shot. Small amounts of decompression can be used to reduce drool.

Trial preparation and operation

Before beginning a trial, be sure that all conditions are optimized.

• Check the dryer for proper:

– Air temperature at the hopper entrance

– Dew point of the drying air

– Airﬂow

– Hopper capacity relative to the size of sample to be dried

• Clean the hopper and material-handling system thoroughly.

• Ensure proper mold temperature control is available.

• Check the chiller and thermolator.

• Be sure the molding machine is clean. (See the section on

start-up and purging.)

• Determine trial objectives. The purpose of the trial dictates

the amount of material needed and the quality and number of

parts required. For example, if the purpose is to obtain 5 good

parts for testing, it is typical to run 200 shots on an untried

mold. If the purpose is to make 50 parts, minimize cycle,

or check part variability, the amount of material and time

required will be much greater.

It is important to keep a complete record of operating conditions,

pretrial preparations, changes, etc. during the trial. Forms for

documenting trial procedures are provided for your convenience.

Completing Form 1 (“Pretrial preparation,” page 41) helps ensure

that all needed preliminary operations have been performed.

Form 2 (“Molding conditions record,” page 42) and Form 3

(“Identifying problems that are causing scrap,” page 43) can

be used during the molding trial. Changes made to processing

variables and effects of those changes during the trial or

start-up can be documented on Form 3.

• Retain 30- to 60-g (1- to 2-oz) samples of the pellets and

parts for follow-up testing of Ih.V. (inherent viscosity) or

molecular weight.

• Document and save all setup conditions, changes to conditions,

and their effects on part quality. Add comments regarding

what worked well and what caused problems. Provide copies

to all trial team members and to your Eastman representative.

When the job goes to production, give copies to all persons

involved.

Start-up

Start with a clean machine. If the machine is not purged,

unmelted particles, gaseous splay, or a combination of problems

will result. Ball checks are typically slow to purge and generally

are not recommended; check rings are preferred.

Removing and cleaning the screw, check valve, nozzle, and barrel

are the only effective means of purging difﬁcult-to-remove

high-temperature plastics.

Purge materials

The material most effective in purging is a polymer similar to

the material to be run. Polyethylene and polypropylene should

be avoided because they can mix with the new material and

cause streaks for extended periods of time. For difﬁcult-to-

remove materials, nozzle and front barrel zone setpoints are

sometimes increased up to 300°C (570°F) to soak and purge,

then cooled back to running temperatures. Use caution and

refer to the manufacturer’s recommendations for the material

used in the previous run.

After any cycle interruption of more than approximately 5

minutes, purging 3–5 shots is good practice.

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