Contamination can be defined as an imperfection in a molded part caused by the presence of a foreign object or material that is not part of the original molding compound.
Oil Leaks and Grease Drips
Explanation: It is common for most molders to allow small oil leaks to become big ones before they consider repairing them. This oil can find its way into unusual locations, such as a container of raw plastic being prepared to go into the machine hopper. Also, when equipment is greased, it is usually overdone and grease drips can fall into material containers. These contaminants are not compatible with the base resin so they are very evident in the molded part.
Solution: Fix oil leaks as quickly as possible. Clean up grease drips as they occur and do not use the same cleanup rags to wipe out the hopper between material changes.
Explanation: When the molding machine is first built, it is supplied with a cover for the hopper. It is there to keep contaminants from getting to the molding compound. If that cover is not present during production runs, the material can be contaminated with such things as dust from the ceiling, drops of condensation from overhead waterlines, and even bird droppings in large facilities.
Solution: Use the hopper lid. Do not improvise with flattened cardboard boxes, as the paper particles will cause contamination. If the original lid is lost buy a new one. They are designed to do a specific job well.
Improperly Cleaned Hopper
Explanation: When a material change is required, the hopper must be cleaned out. It is not good enough to simply remove the material present. The sides of the hopper must be wiped to remove any material dust or ``fines'' that stick to the sides due to static charges. If not removed, these fines will get picked up by the new material and, because of incompatibility, will appear as contamination in the molded parts.
Solution: Clean the hopper thoroughly between material changeovers. This may require wiping the inside with a cloth slightly dampened with denatured alcohol to remove all traces of fines.
Explanation: Molds with cams, slides, lifters, and other mechanical actions need periodic lubrication. Sometimes there is a tendency to overdo this and the lubricant may find its way into the cavity of the mold. This is especially true for ejector pin lubricants. The oil-based lubricant is not compatible with the base resin and is evident as contamination on the molded parts. In addition, excessive mold release acts as a contaminant and appears on the molded part as blotches, dark spots, smears, and streaks.
Solution: Optimize the use of lubricants and minimize the use of mold release sprays. Clean up any excess lubricants and use only the amount needed for a specific application. A little lubricant goes a very long way. Investigate the reason for using mold release. Usually it is a temporary approach to a more severe problem, and the problem should be solved to eliminate the need for the release agent.
Contaminated Raw Material
Explanation: The most common cause of contamination in molded parts is molding compound contamination. Such contamination is usually the result of dirty regrind, improperly cleaned hoppers or granulators, open or uncovered material containers, and poor quality virgin material supplied by the manufacturer.
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.
Explanation: Regrind can be defined as any virgin material that has seen at least one heat excursion through the molding machine. It can be in the form of molded parts or runner systems.
Solution: Grinding the plastic in a granulator that is specially designed for the purpose creates the regrind. The material is ground until it falls through a screen with specifically sized openings. The larger the openings, the larger the regrind pellet. If the granulator is not cleaned out between changes (including the screen), pellets of incompatible materials may be mixed with the next material and this causes contamination. Granulators must be completely taken apart to perform a thorough cleaning between uses. Do not assume that the granulator will always be used only for a certain material. Plans have a way of changing. As soon as a run is completed, the granulator should be removed from service and cleaned. Use of compressed air, combined with wiping with clean rags will suffice, as long as every component is cleaned, including rotor, blades, container, screen, sidewalls, and feed throat.
Explanation: Excessive moisture should be considered a contaminant. It doesn't belong in the molding compound. Moisture turns to steam when heated in the injection unit, and this steam interferes with molecular bonding. This causes splay, which is a visual defect, but also creates a weak part due to brittleness.
Solution: Although it is commonly understood that non-hygroscopic material does 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 suppliers 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.
Explanation: The machine operator may be the source of contamination in a variety of ways. First, if the operator is allowed to have food or drink at the work station, these may accidentally get spilled into containers holding fresh material ready to go into the hopper. Second, the operator may have been instructed to keep the area clean and sweeping dust into the air may result in contaminating raw material or freshly molded parts. Third, a lack of concern or outright sabotage could be the incentive to purposely add contaminants to the molding compound.
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 contaminant-free areas.
HOME DEFECTS RESOURCES BOOKSTORE SHOWCASE SEMINARS CONTACT US
Copyright Texas Plastic Technologies
Worldwide rights reserved