Remember "The Graduate"? Young Benjamin Braddock is advised to get into plastics. "There's a great future in plastics." Well, Braddock may not have known quite what to do with that recommendation, but others certainly have. Over the last 30 years, plastic materials have seen widely increased use in all types of areas, from consumer goods to aerospace. While most rubber product manufacturers look at plastics from the standpoint of potential competition, there is a useful application for them within our business also, namely cleaning molds.
Where do plastics fit?
The first application of plastic media for mold cleaning occurred in the early 1980s. One of the principal producers of plastic cleaning materials, Maxi-Blast of South Bend, Indiana, notes that interest has continued to increase since then because of several factors:
* it's environmentally safe (few disposal problems);
* it can be effective, and
* it is not as abrasive or corrosive as other media often used (steel shot).
The plastic materials used for mold cleaning fall in a hardness range of 3.0 to 4.0 on the Mohs scale. This bridges the gap between steel grit/shot (very hard) and agricultural products such as walnut shells (very soft). As such, it is able to provide effective cleaning of rubber molds while reducing the risk of damage to the tools.
Plastic media is used in the same fashion that either steel shot/grit or glass is used. It is propelled by air at high pressure using a hand held hose to remove residues on the mold surface. Since the granules are non-abrasive, the mold surface is reportedly not affected.
The most prevalent blast cleaning media are glass beads, followed by steel grit and shot. Both media will cause wear and pitting on the metal surfaces. This, in turn, reduces the useful life of the tool, increasing operation costs. By using the plastic media, tool life is reported to be significantly longer since there is no damage to the metal surfaces.
Agricultural products such as walnut shells and apricot pits will not cause damage or abrasive wear on mold surfaces. However, cleaning time is greatly increased and both products tend to break down into a fine dust that is difficult to collect and tends to hang on to mold surfaces, requiring further handling and cleaning.
In the case of the agricultural products or glass beads, only one grade is typically available. While this may not be ideally suited to the application, an alternative is just not available.
What are some of their properties?
Plastic media has been found to be very versatile as a mold cleaner. Plastic cleaning media can be bought in a wide variety of hardnesses, densities and sieve sizes to meet a wide range of cleaning applications.
Plastic media can be made from a range of resins to suit particular applications for both cleaning and deflashing. On the softer side, polyesters are available. On the hard side, thermoset melamine is available. Urea is the other material which finds some application in rubber mold cleaning and fits between the other two on hardness. Melamine, produced with a density of 1.5 and a hardness of 4.0 Mohs, is reported to work best for cleaning steel molds. It is also a thermoset that will not melt or ignite. As a result, it can be used on hot molds. For aluminum or other materials sensitive to wear, either urea (density 1.5, hardness 3.5 Mohs) or polyester (density 1.15, hardness 3.0 Mohs) is recommended. As the hardness of the media drops and density falls, air pressure should be increased to achieve the best results.
Polycarbonate is the recommended material for cryogenic deflashing. It has a density and hardness similar to walnut shells, however, use has shown it yo be much more durable than walnut shell. Data available indicate that the polycarbonate will last approximately 17 times longer than the shells; i.e. if a company used 3,000 lbs. of walnut shells per month, it would be expected that usage of polycarbonate in the same period would be only 180 lbs.
According to Tracy Rowe of Maxi-Blast, many mold cleaners using either glass beads or steel shot expect to see a shiny metal surface after cleaning. However, this may not be desirable. Many rubber compounds will impart stains that enter into the pores of the metal. If bright metal is noted in the areas with these stains, it probably means that the surface of the mold has been eroded or altered to remove the stain. The objective of mold cleaning is to remove residues on the metal surface that interfere with production of subsequent parts. As long as these residues are removed, any discoloration on the metal is secondary and should not interfere with molding. Chemical cleaning has found favor in some areas. However, chemical cleaning techniques often require disassembly of the mold to clean blind spots and cavities. Almost always, molds must soak a considerable amount of time in the tanks - anywhere from a few hours to one or more shifts. Also, after soaking, they must often be scraped or buffed to remove the softened residues. Chemical cleaners have a number of secondary handling problems as well. In-plant usage of these cleaners and solvents requires additional training as well as special permits in some areas. In addition, use of chemical cleaners can pose a major disposal problem. Virtually all areas in the country are very sensitive to disposal of contaminated solvents.
What about converting to plastics?
Plastic blasting media are lower density than either steel or glass. As a result it can be used at much higher operating pressures. This reportedly allows for rapid cleaning without abrasion to either the part being cleaned or the equipment. Pressures of up to 100 psi are run by some companies with no damage to parting lines or tear trim areas.
Normally, there is very little cost or problem in changing over normal steel shot or glass bead blasting equipment to plastics. The only requirements are a thorough cleaning of the blast cabinet to remove any of the old media. Operating pressure is then increased to approximately 70 psi or greater and the nozzle is changed to 3/16 inch or 1/4 inch.
One report cited by Maxi-Blast is that of Jimona, Inc., an independent test lab. In tests run there, melamine grade plastic was run continuously on a mold surface for 65 minutes at 60 psi air pressure. Dimensional changes of 0.0001 to 0.0034 inch were found in various locations. The report stated that these changes were noted after one continuous, long term blast cycle. This is far different from typical intermittent cycles common in the 15 year industrial time span that was simulated. They concluded that, when evaluated on a per year basis, changes in critical dimensions would be very minimal.
There appears to be a strong argument for use of plastic media for mold cleaning. It would appear to be particularly useful in working with close tolerance, high cost molds with critical dimensional requirements. Compared to other media, information available indicates satisfactory performance while reducing the potential damage to tooling. In addition, there are no particular problems from an environmental consideration and conversion from either steel shot/grit or glass bead media is reasonably easy. While cost of the media is significantly higher than other media noted, the long term durability of the plastic is significantly better than the others and normally more than offsets the higher unit cost.
Table 1 shows a comparison of various methods of cleaning molds. Table 2 shows a cost comparison between plastic and glass bead. The figures compare annual media costs only. Eliminating the mold wear and abrasion extends tooling life, which can be a sizeable cost savings.
Other long term benefits are realized due to reducing wear to vital parts of the blast cabinet (hoses, gloves, nozzles, etc.) The reduced mold and machinery wear may amount to an average annual savings of more than $25,000 per year. [Tabular Data Omitted]
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|Title Annotation:||plastic media for mold cleaning|
|Date:||Aug 1, 1990|
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