CLEAR SPOTS

 

 Clear spots can be defined as spotty, translucent imperfections on the molded
part surface and usually are caused by unplasticized particles of resin. They
might also be caused by improperly dispersed additives, such as colorant.

Machine

Low Barrel Temperature

Explanation: Low injection barrel temperature results in an improper melting and blending of the plastic resin. Some of the unplasticized pellets enter the  melt stream and appear in the molded part as small chunks of clear material. The clearer the base resin, the more obvious the particles are, but even dark opaque materials can display these unmelted particles. And, improperly blended additives can have the same general appearance.

Solution: Increasing the barrel temperature will reduce the amount of unplasticized material. It will also help to mix additives with the base resin. Increase the barrel temperature by adding 10 degrees F to each zone until the clear spots disappear. Wait 10 cycles between adjustments for the barrel heats to stabilize. And, watch for signs of degradation as the material temperature rises.

Low Back Pressure

Explanation: While the heater bands and screw friction usually provide all the required plasticizing, it is usually necessary to use a small amount of back pressure to improve the mix. The shearing action of the back pressure setting adds heat but also helps blend additives into the base resin. Low back pressure will result in improper blending of additives and cold spots of material.

Solution: Use a minimum amount of back pressure but make sure it is enough to do the job. Start at 50 psi and increase in increments of 10 psi until proper blending occurs. Do not exceed 300 psi, as that will degrade the base resin.

Slow Screw Rotation

Explanation: Screw rotation helps to impart shear heat into a material and aids in blending the resin and additives. A slow speed will keep the material from achieving proper heat to fully plasticize and some of the unmelted particles will enter the melt stream.

Solution: The screw rotation speed should be set at the material supplier's recommendation. As a rule-of-thumb you can start at 100 rpm and adjust upwards or downwards in 10-rpm increments until the proper speed is achieved. However, excessive screw speed will degrade the material.

Uncontrolled Heater Bands or Thermocouples

Explanation: Improperly sized or damaged heater bands or thermocouples can cause low barrel temperatures. This will keep some of the pellets from melting all the way and these will show up in the molded part as clear spots.

Solution: Check each heat zone to ensure that all heater bands are working properly, are suitably controlled, accurately sized, and are tight against the barrel. A conductive sealant should be used to ensure full contact with the barrel. Be sure to replace bands with the proper size, voltage, and wattage requirements as stated in the machine manual.

Improper Screw Compression Ratio

Explanation: Compression ratio is a value that indicates how much a screw will compress a material while it is being processed in the barrel of the machine. It is calculated by dividing the depth of the feed section (rear) of the screw by the depth of the metering section (front) and is usually less than 2 to 1. If the compression ratio is too low for the specific material being molded, the resin will not melt properly and unmelted particles will be present. These enter the melt stream and appear as clear spots in the molded part.

Solution: The material supplier can recommend the proper compression ratio for a specific resin. A general-purpose screw can usually be used to provide adequate compression but specific conditions may require a screw that is specially designed for a given material.

MOLD

Cracked Mold (Water Leak)

Explanation: A mold that is cracked due to weak waterlines may cause water droplets to enter the cavity. These will be trapped by incoming molten resin and will show up as clear spots in the molded part.

Solution: Waterlines that are leaking may be welded if the cracking condition is not severe. Tubing can be threaded through the cracked waterline and the water can flow through the tubing much like air within a tire inner tube. Or, a reverse water system can be utilized that sucks water through the mold instead of pressurizing it through the mold.

Contamination from Mold Release

Explanation: Use of mold release may cause a residue pocket that gets picked up and trapped in the melt stream as the material flows through the mold. These trapped pockets will not allow material to bond together and those areas will appear as clear spots in the part.

Solution: Eliminate the reason for the use of mold release. If release is needed, use it sparingly and clean the mold cavity surfaces frequently to remove residue.

MATERIAL

Contaminated Raw Material

Explanation: A common cause of clear spots is contaminated raw material. Often, the raw material containers are left uncovered and pellets from other incompatible resins may get deposited in the container. Because they melt at different temperatures the incompatible pellets may never get hot enough to melt and they will be dragged into the cavity appearing as clear spots.

Solution: This type of contamination can be minimized by dealing with high quality, reputable suppliers and by using good housekeeping practices. Properly trained material handlers will also help reduce contamination.

Improper Regrind Use

Explanation: Although regrind melts at a lower temperature than virgin material, if the regrind pellets are too big or coarse, they may not absorb heat as fast as the virgin pellets and may not fully melt in the barrel. These unmelted particles will form clear spots in the part under the right conditions.

Solution: Use a smaller screen in the granulator. This will create smaller, more uniform regrind pellets that will melt fast enough in the barrel. Also, limit the amount of regrind being used to 15%. At that level, or lower, the regrind tends to get dissipated enough to keep the clear spots from showing.

OPERATOR

Inconsistent Process Cycle

Explanation: It is possible that the machine operator is the cause of delayed or inconsistent cycles. This will result in erratic heating of the material in the injection barrel. If such a condition exists, some pockets of material may not melt all the way and unmelted particles will cause the clear spots. 

Solution: If possible, run the machine on automatic cycle, using the operator only to interrupt the cycle if an emergency occurs. Use a robot if an ``operator'' is really necessary. And, instruct all employees on the importance of maintaining consistent cycles. 

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