How To Solve Streak and Black Specks?

To determine if the barrel is the issue, run shots of purge compound though the barrel. Be sure that the barrel is completely full to ensure that any material left behind in a dead spot is also being picked up. Examine the purge pile for specks and uneven color distribution. If there are random specks or streaks in the pile, then your problem likely lies with the screw and barrel.

If the purge from the barrel shows a continuous streak in the same spot close to the gate, then the contamination is likely in the nozzle, check valve, or hot runner. If you have determined that the contamination is not in the barrel, then run purge shots through the hot runner to determine whether the contamination is downstream in the manifold or gates. One or more gates may be more problematic than others due to an uneven flow pattern in the manifold, so examine each purge pile individually.

If the color change does not show up as a streak, but as a burst of color, then the contamination may be coming from your feedthroat.

If the streaking or contamination is in the barrel, try to

1.  Raise the barrel temperature by approximately 50–100° F (28-56° C) above the normal operating temperature.

2. Run several short bursts of agitation at higher screw speed. This loosens the color or carbon. Subsequently slowing the screw speed allows the purge to expand and then push the loosened contamination out.

3. Increase the screw rpm and/or backpressure to improve results by adding force and agitation to the purging process.

4.  If your contamination is at the check ring, run several short, high-velocity shots.

5.  Shut down the machine between a resin or color change to allow for additional contamination to be removed from the barrel and screw.

If the streaking or contamination is in the nozzle or hot runners, try to

1.  Raise the temperature of the nozzle or hot runners 50–100° F (28-56° C) to reduce the viscosity of the resident resin.

2. If your hot runners are valve gated and you are experiencing more contamination with one or more gates, close the other gates and purge through the troublesome gate(s) separately to increase pressure and turbulence.

3. Use short, high-velocity shots to dislodge and remove contamination.

If the problem is in the hopper or feeding system, try to

1.Implement a standard procedure to clean the hopper and magnets thoroughly with an air hose and/or soft cloth, both before introducing the purge and before introducing the next resin. Check all auxiliary feeding equipment to ensure that no pellets are caught in corrugations or on hidden shelves.

2.Avoid clumping or “balling-up” in the feedthroat by keeping it cool. The temperature of the throat coolant should be maintained at 80-120 F (27-49 C). It is best to control the feedthroat temperature as close to 100 F (38 C) as possible. In humid weather, the temperature should be just warmer than the dewpoint to avoid condensation. Also, in some cases, keeping the screw turning at a low speed will help to prevent bridging.