Advantages and Design Characteristics
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NALC produces rubber parts expressly formulated for these applications. The advantages and design characteristics of these high heat resistant NR products include the following:
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Cost Benefit
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These bladders are dip-molded from natural rubber latex. This is much less expensive than methods using silicone rubber, or using injection or compression techniques to mold natural rubber. Prototyping is usually quite inexpensive, especially when compared to other options.
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Tooling
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Tools for dip-molding are male molds, as contrasted to female or cavity molds, used in other molding methods. Male molds are usually much faster to make and less expensive than female molds. Either NALC or the customer can provide the necessary mold or molds. Some standard tooling is available for use at no cost. The mold/molds need only resist ~ 180*F and can be made from aluminum, stainless steel, polypropylene, nylons, and sometimes, even an epoxy/carbon fiber part itself may suffice. Prototype molds made via resin printing have been used as well. The strength of the tool and its mounting must be able to resist the force required to strip the rubber part off of the mold. Strip ratios of 7:1 are possible with some geometries, but ratios of 4:1 or less are usually the design norm. Contact NALC to determine what tooling options and design requirements are best for a given application.
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Geometry
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Bladders can be made to suit almost any required shape.
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Rubber Part/Bladder Characteristics
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Remember! These are pliable, rubber parts ---- they can usually be made to do whatever you want them to do!
- The bladder will hold the shape of the mold it was dipped on, making it easy to lay into a cavity quickly.
- Wall thickness can be varied to attain best compression results.
- Surface finish is usually light talc finish.
- No parting lines or creases.
- Bladders usually need little or no mold release, depending on resin character, to allow extraction from the finished part. Compatibility of the bladder with various mold releases is not normally an issue.
- These bladders have excellent tensile strength and tear resistance, even after several molding cycles. This is necessary for easy, efficient extraction.
- NALC bladders are formulated in a bright color which allows for easy inspection of the interior of the finished article, should this be necessary.
- Aging characteristics of the bladder compound is very good, and very stable, so keeping bladders in inventory for extended lengths of time is not usually a concern.
- There are no plasticizers, oils, or other substances which can migrate out of the bladder and into the molded article. This is a tremendous advantage over silicone rubber bladders in some military and aerospace applications.
- NORMALLY, the bladder’s dimensions are chosen such that it must expand/stretch 5-20% when pressurized to fill the cavity and compress the polymer resin/fiber matrix. NOTE: Thinning around a radius can impact bladder lifespan and resin compaction.
- These high temperature resistant bladders are used under various time/temperature/pressure conditions. Routine molding conditions are 45 -120 min. at 250*F-350*F and 40 – 100 psi. However, longer times, and higher temperature and pressure have been used. Only experience in a specific application will determine how many cycles of use the rubber bladder will remain useable. Results vary with the molding parameters. Often a test article can be supplied to evaluate the rubber under a set of molding conditions prior to prototyping.
- NALC’s heat resistant bladders facilitate excellent resin/fiber integration and compaction, resulting in superior strength and surface finish properties in the molded article.
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