Clean Film Packaging - Newsletter
A Guide to Ultra Clean Film Packaging
Introduction: This guide has been written to assist the engineer, purchasing agent, cleanroom manager, laboratory researcher, quality control manger, packaging specialist, etc., in the current technology of clean film packaging. In the space allowed, I have tried to be as thorough as possible in this overview, but I'm sure after reading the article, many of you may still have questions. Please feel free to contact Liberty Industries by phone, mail, fax, or email. Contact information is listed at the end of this newsletter.
Is there one plastic film that will satisfy all packaging requirements?
Unfortunately, the answer is no. It is generally known that no such film "miracle film" currently exists.
Today, depending on specific requirements, there are many different types of plastic films that can be used. This article discusses the advantages and disadvantages of the most common films used in industry today- ACLAR, Nylon and Polyethylene films.
Can I clean film in my own facility?
The answer is yes, if you have access to a laminar flow (unidirectional) cleanroom that employs critical control of temperature, humidity and aerosol particles. It is also essential to have a well trained staff of technicians and operators.
Machinery used in the cleanroom must be designed to eliminate the addition of metal particles and similar contaminants while processing the plastic film. Fluids used to perform the cleaning operation must be filtered to a level of five (5) to ten (10) times in excess of the ultimate test requirements of the clean film. Electrostatic characteristics of the film also play a major role since attraction of free particles will seriously affect end product cleanliness. The technicians and operators who perform the cleaning must be fastidious about themselves, their clothing and their actions when performing the cleaning operation. Final cleaning is performed under the most scrupulous conditions currently attainable in the contamination control field.
Doing your own cleaning can be costly and unwise with out the properly trained personnel and the proper facility and equipment.
Automation and mechanization provide the optimum means of obtaining a superior clean product in conjunction with a reasonable expenditure of funds. No matter whether you purchase clean film or clean your own, once the film has been effectively cleaned, it must be kept in a totally particle free environment and inspected for compliance to the cleanliness levels stipulated in the cleaning manufacturers facility and/or to the ultimate users specification. It is then packaged in a barrier bag(s) and certified by a trained quality assurance representatives in the facility for compliance with physical and mechanical properties. Testing is an essential part of the cleaning process and must be performed on a regular basis.
Types of film:
UCF-POLY: The most common of all clean plastic films in use is a medium density polyethylene (UCF-POLY). At a minimum, this will meet the requirements of LP378 (latest revision). The film is an all purpose film for parts which require a moderate level of cleanliness in the packaging environment and are not particularly prone to puncture, abrasion or cause sloughing of the barrier bag. Water vapor transmission rates are low, gas barrier is relatively poor as is oil and grease resistance.
NYLON 6: Nylon 6 is another film for clean packaging. It is also known as Nylon C, Clean-Pak, etc. The gas barrier properties of Nylon 6 are superior as is its resistance to oil and grease. It has been accepted by the FDA for food packaging of meats and food packaging. Unfortunately, Nylon is hydroscopic which results in an affinity for moisture. Absorption and transmission occurs under higher than ambient humidity conditions with water vapor always moving toward the less moist environment. To preclude the possibility of moisture transfer through the barrier film, firms will either use a nylon-poly laminate (nylon on the inside face) or provide additional package protection for the cleanroom user with a second barrier of six mil polyethylene. Typically the nylon film used for clean packaging is 1.5 to 2.0 mils thick. Nylon 6 has excellent resistance to abrasion, sloughing and punctures. It has been substantiated that it is the most effective of the heat sealable plastic films for long term packaging and packaging of critical parts.
ACLAR: ACLAR, a fluorocarbon plastic film manufactured by Allied Chemical is a film which has come into recent use. The film has a high density resulting in less yield in square inches per mil per pound. Its bulk price is twelve to fifteen times that of poly and three times that of nylon. ACLAR film has served to satisfy one need which other films as yet have not been able to provide, i.e., compatibility to oxidizers such as liquid oxygen, nitrogen tetroxide, etc.
Unfortunately, ACLAR possesses one major undesirable trait in that it abrades and sloughs off plastic particles when rubbed or contacted by a packaged part.
Two grades of ACLAR are commonly in use, with type 33C being the most frequently used. This material has been selected by the majority of clean film users since it may be cleaned with solvents. It has a resistance to absorption or degradation in the presence of cleaning solvents. A more preferable film in the ACLAR family is type 22A which is more flexible and provides somewhat better handling characteristics during packaging, sealing and storage. ACLAR 22A will soften in the presence of certain solvents and therefore various film cleaning firms have been resistant to accept its use. Because of the poor crease resistance of ACLAR 33C, it is not suitable for center folding or pouch masking from center folded stock. Invariably defects will occur at the puncture of the seal and the centerfold. If the centerfold seam is compressed, this will provide a crease line which will fail more readily than any other portion of the barrier pouch. The more flexible type 22A film is preferable under these conditions.
ACLAR films should be used with great caution. Even though the large volume of particles which they generate to the clean part are compatible to oxygen systems, their quantity, size and physical characteristics should restrict use of this material because of its case of physical deterioration.
- Polyethylene carries a cast characteristic which in the heavier thickness of a mil or more, limits its optical properties.
- Nylon 6 has excellent visual properties and the haze factor is relatively low.
- ACLAR films have optical properties and haze characteristics comparable to nylon films.
Ultra clean film specification guidelines:
We will attempt to generalize and provide the potential user with a set of guidelines, which will assist the user in this task. We will focus on the specifications for packaging developed by North American Aviation. They are the following:
- MB0295-005 - Material Cleanliness Level, Precision Clean Packaging
- MB0295-006 - Film, Transparent, Precision Clean Packaging
- MAOH6 - 017 - Supplier Packaging Requirements
These specifications are by far the most complete and they provide the subcontractor in prime aerospace programs with an excellent set of detail guides in performing critical parts and systems packaging.
The principles applied to proper parts packaging are as follows:
- Size of the bag in relation to the part. Here it is essential to have adequate room within the primary barrier to allow the part to be easily encapsulated.
- Strength of the bag in relation to the part contained. Many bags used have three sides sealed, as contrasted to end-sealed tubing. When a seal test is performed per MILB-116D on a section across the seal versus the folded seam of a bag made from tubing, it will be found that the resulting strength will be comparable or greater for the section fabricated from tubing. The three side seal pouch has a form of rigidity when all four sides are sealed, but this does not necessarily imply additional strength.
- Where small or large parts have sharp edges and projections, it is our suggestion that the part be first wrapped in two to four laps of sheet film of tubing. The layers of film can be secured with PVC tape. After the part is wrapped, it is enclosed in the first barrier bag and sealed. To restrict the motion of the part in relation to the bag, it is suggested that the bag be thoroughly evacuated by cutting a corner of the sealed bag and inserting this slit into the opening of a vacuum tool with an opening of approximately 1.8 x 0.5 inches. After all the air has been evacuated, seal the opening, over bag and repeat the evacuation procedure. In the event of a pin hole in the bag, the pin hole will act as a filter and the size of particles ingested will not be serious. In the event that a major break in the primary barrier takes place this will be well noted by visual examination. The part should be then subjected to reinspection, recleaning and repackaging.
- Where parts have many openings, it maybe necessary to seal each one so that during assembly, they will not be exposed to the environment more than one at a time. Methods for sealing these holes are as follows:
- Cut squares of clean film and then tape them in place on non-critical surfaces.
- Use Mylar tape with low adhesive and residue properties. Fold over a corner for easy removal. The better quality Mylar tapes have shown evidence of leaving no residues when a black light test is performed. (Hydrocarbon free surfaces.) We do not advise the use of this method for critical oxidizer systems.
- For male fitting and tube ends, wrap the protrusions with clean film and draw the film over the critical surfaces and tape on either non-critical portion of the part or over the film itself by stretching the tape. The tape used is a PVC type (PPP-T-60). Connector nuts mated with tubing should be restrained to prevent free motion. After packaging, use PVC tape, tightly securing it over the clean film.
Handling precautions for ultra clean film:
To maintain cleanliness, certain precautions must be taken in the handling of clean film and packaging during the encapsulation of the parts.
Cleanroom garments are the greatest source of contamination within a well controlled cleanroom. Cotton or woolen garments worn beneath the cleanroom uniform have a tendency to aid in transmitting lint and dust particles through the protective uniform into the clean environment. Normal operator motion causes ballooning of the uniform. Air permeating the garment carries with it particles 10 or more microns in diameter. If film is brought anywhere in the vicinity of the uniform, these fibers and particles will be attracted and rigidly adhere to it. Antistat agents used in the film are generally of no avail under these circumstances. Packaging should take place in an environmental area of the same, or greater level of cleanliness to be used for final cleaning and inspection of critical parts.
When the clean packaging film is to be cut, it is suggested that stainless steel scissors be used. It is suggested that the scissor cut be started and carefully be pushed through the film to be separated. A razor blade should be avoided. Also, a hacking action would be detrimental in as much as particles would be generated.
To perform effective sealing on plastic films the all purpose impulse sealer should be used when possible. Films such as nylon, ACLAR and laminates will be most effectively handled with this type of sealer.
Standard testing procedures for testing ultra clean film:
The testing procedure is performed in a cleanroom environment using all standard procedures required to insure that the results reflect the precise cleanliness of the inside of the test sample as follows:
- Seal both ends of the sample tubing or the open end of the sample bag 1/8" from openings
- Follow your company's documented protocol for testing.
- De-contaminate the tools being used with ASC Reagent grade isopropyl alcohol, giving special attention to the following items that will touch the sample or test solution:
- Spray the outside of the test sample with the same quantity of alcohol to insure that the outside contamination is not introduced into the test sample or sample solution. Cut off a corner of the sample at a 45° angle, no more than ¾" wide, so the sample solution can be introduced into the sample. Always cut using a slitting action and not a chopping scissors action. If a chopping cut action is used, material shavings can and will be introduced into the sample.
- Measure 80 - 85 ml of alcohol into a beaker and then pour alcohol into the 45° angle opening trying not to splash solution on the outside of the sample. DO NOT touch the area just cut with your hands. Use the tweezers to help open the bag.
- After the alcohol is in the bag, make a 1/8" fold at the opening. Do not touch the opening of the material with gloves because this could contaminate the test sample.
- Shake the bag from side to side for 15 seconds; unfold the opening (again without touching the opening). Use tweezers to re-open the bag and pour the alcohol back into the test beaker.
- Allow the alcohol to "rest" for 30 minutes before placing in the sampler. "Resting" will allow any bubbles to dissipate, as these bubbles will register as particles in the testing process.
- Place beaker in sampler for particle counting. If sample particle counts do not meet the particulate count criteria set down on the protocol documentation, another sample must be processed per the above procedure. If the material fails to meet the standards on the second test, this is cause for the raw material to be rejected.
How to Save Thousands on Your New Cleanroom - Volume 5, No. 1 in a series of publications from Liberty that makes understanding cleanroom design clear and simple. Utilizing a question and answer format like our previous Cleanroom Primer, Air Shower Primer and Maintenance Primer, the design primer simplifies a complicated subject. 56 pages. Don't buy a cleanroom until you've read this publication. It will save you thousands - I promise.
Published by John J. Nappi, Jr.