Diamond Systems Tritan Guía para resolver problemas descargar pdf (Pagina 26)

Good temperature control of the nozzle is important. If a

thermocouple at the threaded end of a long nozzle is controlling

a heater band or bands at the other end, temperatures at the

heater band end can be more than 55°C (100°F) higher than the

thermo couple is able to sense. This can be checked by inserting

a needle pyrometer to different depths in the nozzle opening.

The remedy is to either reduce the setpoint of the controller

or, preferably, to use a nozzle ﬁtted with a thermocouple in the

center of its length with heater bands located uniformly on both

sides, as shown in Figure 30.

Long nozzles may require more than one thermo couple/controller/

heater band along their length for uniform heating. Nozzles with

gas-charged heat pipes have been used successfully to heat the

full length of long nozzles more evenly. Temperature control

problems in the nozzle show up as appearance problems at or

near the gate. Nozzles with a removable tip require special

attention to verify that the tip bottoms out on the shoulder

below to prevent a dead space where polymer can degrade; if

this happens, black specks can form and reenter the melt stream.

Periodic inspection

The screw, check valve, and nozzle assembly should be taken

apart, cleaned, and inspected periodically to measure wear and to

look for cracks or any other spots where material can collect and

degrade. Small cracks or unseated threads can be big enough to

cause streaking or degradation.

Drying

Drying is an absolute necessity to prepare polymers for molding.

All polymers readily absorb moisture. Desiccant dryers must be

used to dry the pellets prior to processing in the injection

molding machine. A typical desiccant dryer is shown in Figure 31.

If pellets are not dried, the moisture will react with the molten

polymer at processing temperatures, resulting in a loss of

molecular weight. This loss leads to lowered physical properties

such as reduced tensile and impact strengths.

Molded parts may not show any noticeable defects such as

splay but may still exhibit lower physical properties

Equipment

Multibed desiccant dryers. These dryers have two or more

desiccant beds and are recommended to properly dry the

polymer. Dryers having three or four beds typically have shorter

start-up times because of quicker bed regeneration. Desiccant

dryers are available from many suppliers. Work with your

desiccant dryer vendor to select the optimum dryer for the

molding job. Locating the drying hopper on the feed throat of

the molding machine is preferred. Planning should include

consideration for throughput rate, ease of maintenance, reliability,

and low variability of the four elements necessary for proper

drying (drying temperature, drying time, dryness of air, and

airﬂow, which are discussed in the section, “Elements necessary

for proper drying” on page 27).

Heater bands

Thermocouple

well

Check nozzle tip

seat for correct fit.

Check nozzle seat

for correct fit.

Nylon configuration suggested for copolyester

Figure 30 Injection nozzle

1 2 ... 21 22 23 24 25 26 27 28 29 30 31 ... 43 44

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