CLOUDY AREAS

  

Cloudy area can be defined as an imperfection resembling
a localized cloud formation or dull area. It is most apparent throughout
a transparent part but can also be evident on the surface of an opaque part.

Machine

Low Barrel Temperature

Explanation: Low injection barrel temperature results in plastic particles that are not fully melted and blended with the main melt. The cloudy appearance forms when groups of these particles are located together.

Solution: Increasing the barrel temperature reduces the likelihood of unmelted particles. Make sure the profile is correct by heating the material higher as it travels from rear to front in the barrel.

Low Back Pressure

Explanation: The use of back pressure improves the mix by adding heat and creating a more thorough blending action to the melt through shearing action. If no back pressure is used, there is a risk that not all plastic pellets will be fully melted and mixed. This may result in a cloudy appearance in areas where these particles have grouped.

Solution: Use a minimum back pressure setting of 50 psi. Then, increase in 10-psi increments until the cloudy appearance disappears. Do not exceed 300 psi, and keep watch on the material as the back pressure increases, to avoid degradation.

Low 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. The cloudy appearance is the result of these unmelted particles.

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.

Excessive Wear Between Barrel and Screw

Explanation: The gap between the inside diameter of the injection barrel and the overall diameter of the screw within the barrel is critical. As a general rule, any gap greater than 0.005'' is excessive and will cause a variety of molding problems. The greatest result of a large gap is unplasticized material. This is caused by an inability to create back pressure, and a slipping of the pellets across the screw flights during travel through the barrel. Of course, if the pellets are not properly melted they can cause a cloudy appearance on the part.

Solution: Screws and barrels should be checked periodically (at least every 6 months) and replaced or repaired when worn or damaged. The greatest wear will be at the front of the barrel or screw because that is where the material is discharged. Barrels can be repaired by fitting with a liner and screws can be replated.

MOLD

Improper Venting

Explanation: Air is trapped in a closed mold and incoming molten plastic will push this air towards the edges of the cavity. If the air cannot get out of the mold, it will be compressed but will interfere with the ability of the plastic to push against the steel of the cavity. The plastic will take on a dull, cloudy appearance in those areas because it cannot replicate the mold finish.

Solution: Vent the mold by grinding thin (0.0005''-0.002'') pathways on the shutoff area of the cavity blocks. Vents should take up approximately 30% of the perimeter of the molded part. Vent the runner, too. Any air that is trapped in the runner will be pushed into the part.

Uneven Packing

Explanation: Improperly gated parts may result in uneven packing of the plastic in localized areas of the cavity. This would be caused, for example, if the part were gated such that the material flows from a thin section to a thick section. The plastic in the thin section will solidify before enough pressure can be applied to the plastic in the thick section. The molecules in the thick section will not be packed against the steel and will appear cloudy.

Solution: Follow proper guidelines for gating, as recommended by the material supplier. Make sure parts are gated to flow the plastic from thick sections to thin sections. And, make sure runners are properly sized. The diameter of the runner should be larger at the sprue than at the cavity if the runner is greater than three inches in length.

Dull Finish on Mold Steel

Explanation: When constructed, the surface of the mold is first machined, then stoned by hand to remove the machining marks, and polished by hand to remove the stoning marks. The final polishing phase can be extensive or brief depending on how much luster is required on the finished product. If polishing is not consistent, there may be areas that are not as highly polished as others. These duller areas will create a dull, cloudy appearance on the molded part.

 Solution:: Inspect the mold for consistent polishing. If dull areas exist, they can be selectively polished to match other areas. If the mold was plated, make sure the plating is uniform. Also, clean any area where staining may be present. This can come from out-gassing of the plastic but can be easily removed.

Poor Mold Temperature Control

Explanation: As a rule-of-thumb, a hot mold produces a shiny part and a cold mold will produce a dull part. If the entire mold is cold, the entire part is dull. But, if there is local reduced temperature due to inconsistent water flow through the mold, that area will produce a localized cloudy finish on the molded product.

Solution: Make sure the waterlines produce turbulent flow. Simply feeling the ``in'' line and the ``out'' line can check this. There should be no more than 10 degrees F difference between the two lines. If there is more than that 10 degree F difference, it means there is inconsistent flow through the mold and hot and cold pockets will be found. The cold pockets will produce a cloudy finish. Turbulence can be created by increasing the flow of water (measured as gallons per minute) into the mold. Finally, make sure the waterlines are hooked up properly. There may not be any water going to certain sections of the mold. Then, the water in the rest of the mold must be set to a colder value to compensate and will cause cloudy parts.

MATERIAL

Excessive Moisture

Explanation: Excessive moisture is a frequent cause of cloudy appearances. Moisture turns to steam when heated in the injection unit, and this steam explodes throughout the plastic, causing voided areas between molecules. The voided areas are not packed together and appear cloudy on the part.

Solution: Although it is commonly understood that non-hygroscopic materials do not require drying, do not take chances. Dry all materials. It may be that fillers used in the material are hygroscopic and they will absorb moisture. Every plastic material requires specific drying conditions, and each material should be dried according to the material supplier's recommendations. The desired moisture content is between 1/10th of 1 percent and 1/20th of 1 percent by weight. This means the dry air being used to take moisture from the material should have a dew point of -20 to -40 degrees F.

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 pellets will not be properly plasticized and will enter the melt stream to create cloudy areas in the molded part.

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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