On-Line Solutions To Injection Molding Problems
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.
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.
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.
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.
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.
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.
Cloudy areas can be defined as an imperfection resembling a localized cloud formation or dull areas. It is most apparent throughout a transparent part but can also be evident on the surface of an opaque part.
Some common causes and solutions are listed below.
NOTE: For more detailed information on the causes and solutions of this defect, you can find it in our BOOK, or ONLINE SEMINAR.
Copyright by IPLAS and Douglas M. Bryce
Worldwide Rights Reserved