Troubleshooting problems with mold releases.
A sacrificial release agent is a topical coating applied to the surface of the mold. It facilitates part/mold separation "sacrificially" by allowing a portion of the coating to transfer to the surface of the molded part. The chief drawback of sacrificial release agents is that of transfer. By their very nature, these releases leave a residue on the molded part, which can interfere with secondary operations like painting or bonding. Sacrificial releases are relatively inexpensive, and easily applied to the mold. Regarding the issue of transfer, however, their low abrasion resistance, and the overall "cost of use" limit their suitability for many modem and high tech applications.
The most common sacrificial release agents used in rubber molding are silicone and teflon based products. Silicone releases can range from a very simple silicone oil to a complex resin system which incorporates a more durable silicone material. These are relatively inexpensive, and are easily applied to the mold by spraying, brushing or wiping
Semi-permanent mold release agents were first introduced to the industry 30 years ago. A semi-permanent release is designed to form a chemical bond with the surface of the mold, creating a durable non-transferring film which allows the part to be easily removed, while remaining on the mold. Semi-permanent releases are either proprietary chemical formulations, or a combination of technologically advanced silicone resins and oils that have been highly crosslinked to the point where transfer is minimal.
Semi-permanent releases are rapidly gaining preference in rubber molding by eliminating the problem of transfer, and providing superior performance with modern rubber compounds. They require careful mold preparation and application in order to provide optimum release capabilities.
The issue of transfer critical to mold release selection
Transfer of the release agent to the molded part is one of the most critical issues in the molding of technical rubber goods. In some cases, residue left on a molded part will not affect end-product quality. But where post-molding operations such as bonding are involved, transfer can be a costly, time-consuming problem, resulting in knit line or bonding failure, or an additional process to remove the transfer.
Transfer is the primary drawback to sacrificial release agents, which by their very nature leave a residue on the molded part. Silicone-based products leave a highly contaminating residue. Silicone is very difficult to remove, requiring the use of solvents or other chemicals, such as methylene chloride or other chlorinated or ozone-depleting degreasing solvents. These chemicals are expensive and difficult to work with, and increasingly regulated. There is also the issue of hazardous waste disposal. Transfer from teflon-based products is easier to remove, and in some cases may even be noncontaminating.
Semi-permanent releases, when used correctly, will not transfer at all. Improper application, however, or poor mold preparation, will interfere with the chemical bond formation and cause the release agent to transfer, appearing as white patches on the molded part. When accompanied by poor release, this indicates that the mold surface was insufficiently cleaned prior to application. A dirty or contaminated mold will prevent the release agent from bonding to the mold surface, interfering with release and causing transfer to the surface of the part.
Transfer will also occur if a semi-permanent release is overapplied. Excessive application is indicated when the part pulls cleanly and easily, but exhibits white blemishes. While the adage "if a little is good, a lot is better" works with sacrificial releases, the same does not hold true for semi-permanents, which are designed to be applied in thin uniform coats for maximum effectiveness. In each of these cases, the mold must be thoroughly cleaned and the release agent properly re-applied, taking care to allow it to cure adequately in several thin layers.
Newer compounds demand more from a mold release
Compounds being formulated today, such as EPDM, may be compounded with curing agents which react against the surface of the mold during the molding process. Many of the fillers used in rubber compounds tend to be highly abrasive. High abrasion and the chemical reactions that occur while molding will break down the release agent, and make smooth release extremely difficult.
Sacrificial releases are less effective in these cases than with traditional compounds, as they are quickly sheared off the mold surface by the high abrasion. Semipermanent products are much more durable due to the chemical bond they form with the mold, that resists abrasion and shearing.
When working with an aggressive stock, the quality and ease of release can be improved through frequent application of the release agent, and touch up application on trouble spots. Poor release with a sacrificial agent indicates that the compound is too aggressive for the silicone or teflon-based product, and a semi-permanent agent should be considered as an alternative.
Eliminating knit line and bonding failure
Knit line and rubber-to-metal or rubber-to-rubber bonding failures result when contamination from the mold surface interferes with the bonding process. This is a common occurrence when using a sacrificial release agent, which is easily moved off of the mold surface. When molding a part such as an o-ring or gasket, the release agent is pushed ahead of the rubber and creates a gap in the molded part. Similarly, rubber to metal bonding failures occur when a sacrificial release, or some other contamination, transfers to the bond point.
Both of these situations can be eliminated by switching to a semi-permanent release, which, because of its chemical bond with the mold surface, will not be moved, pushed or transferred. Therefore, semi-permanents are the release agent of choice for most molders involved in processes like this. Should knit line or rubber to metal bonding failure occur with a semi-permanent release, the cause is likely to be mold contamination, such as dirt or traces of another release agent, or incomplete bonding of the release agent to the tool. Thorough cleansing of the mold, and correct application of the release agent, will solve this problem.
Mold build up impairs ease of release and part quality
Cosmetic defects on the molded part, such as loss of exactness or detail, often accompanied by poor release, occur as a result of build up on the mold surface. Build up, either chemical deposits from the molded material or of the release agent itself, requires that the mold be stripped, cleaned, and the release agent reapplied on a regular basis.
While rubber and other thermoset compounds can often be molded without the aid of a release agent, carbon black and other chemicals are deposited on the mold surface with each cycle, and rapidly build up to the point where the part loses its definition or will no longer release. Release agents, both sacrificial and semi-permanent, slow this build up, thereby increasing ease of separation.
The mold release agent itself will also build up on the mold surface, particularly with sacrificial silicones and teflons. The low number of releases per application with these products means that a high concentration of solids is frequently being applied to the mold surface, requiring more frequent stripping and cleaning. Semi-permanents are applied in thin coats and offer a higher number of releases per application, minimizing build up on the mold.
Achieving multiple releases per application
While the actual number of releases per application depends upon specific processing conditions, such as molding temperature and the material being molded, it is generally possible to achieve more than one release. Sacrificial agents can average anywhere between two and ten releases; semi-permanents can last as long as an entire shift.
If a semi-permanent fails to provide multiple releases, there are several possible causes. An unconditioned mold surface (i.e. a new mold, or a freshly cleaned mold) may require frequent application of the release agent for the initial cycles until it is properly conditioned. After the first shift, the release agent should be re-applied on an as-needed basis, when the first signs of difficult or poor release become apparent. A particularly aggressive rubber stock might require a more concentrated release agent formulation. Age of the release agent may also be a factor in the performance of a semi-permanent.
When the mold just won't let go
Complete failure to release occurs infrequently in rubber molding. If this should occur when using a semi-permanent, however, likely causes are an improperly cleaned mold surface, which inhibits the release agent from forming a chemical bond, or an improperly cured release agent. With this exception, failure to release will only come after poor and increasingly difficult releases, due to abrasion of the release interface off of the mold, or excessive build up of residue on the mold surface. In either case, the mold must be taken out of production and the part completely stripped out, followed by cleaning and proper application of the mold release agent.
Microporosity affects semi-permanent releases
Because of their reliance on the formation of a chemical bond, semi-permanents are susceptible to microporosities in the mold surface. Where sacrificial releases are thickly applied, semi-permanents must be sprayed or wiped on in several thin, uniform coats. Any porosity in the mold surface will create gaps or voids in the chemical bond, interfering with proper release, and causing small particles of the molded material to be left on the mold surface. Sealing the mold or applying additional coats of release agent will generally eliminate this problem.
Injection molding demands higher performance
Of the three basic techniques for rubber molding - compression, transfer and injection molding - injection molding is by far the most demanding on a release agent due to the speed of the operation. The material is injected into the mold at high pressure, causing high abrasion on the mold surface. Also, cure cycles are generally much quicker, creating intense chemical reactions on the mold surface that put additional demands on the release agent. Compression and transfer molding involve less force and chemical reactivity, as the material is formed and cured at a slower rate.
Increasingly, processors today are turning to injection molding for its faster cycle times, investing in state of the art equipment to increase productivity and remain competitive. A typical sacrificial release agent is not able to tolerate such high stress conditions, and these are consistently being replaced by the more sophisticated semipermanents.
Trends in mold release technology
Increasingly, rubber molders are turning to semi-permanent releases as a means of maximizing the productivity of their molding operations. While sacrificial agents are still widely used in applications where transfer and bonding are not critical, more processors are realizing the benefits of reduced mold maintenance and increased performance that semi-permanents offer.
Advances in rubber compounds and the growing trend toward injection molding are also causing many processors to consider semi-permanent release agents. The quicker cycling times and accelerated chemical reactions in the mold, as well as more aggressive materials, are placing increasing demand on the mold release agent, and traditional sacrificial releases do not have the ability to perform well in these situations. As governmental restrictions on harmful and ozone depleting chemicals become increasingly complex, mold release manufacturers are developing products that are in compliance with these regulations. Semi-permanent technology is concentrating on the development of water-based and CFC-free formulations. Waterbased silicone emulsions, which have been available for many years, offer one alternative, but these products have the same shortcomings as other silicone products - transfer and mold build up. Water-based semi-permanents, first introduced several years ago, have proven to offer comparable performance to solvent-based products. Most companies offer a line of CFC-free products, but these may still contain other solvents that are potentially harmful to the user and contain high concentrations of VOCs.
The newest technology, introduced recently by leading mold release manufacturers, is an aerosol spray which employs compressed air to replace traditional ozone depleting chemical propellants. Many CFC-free aerosols are available, but the majority of these still rely on flammable gas propellants such as propane and butane, which present storage, usage and disposal hazards. New aerosol products offer water-based releases with compressed air propellants, for a system that is in compliance with existing regulations, and anticipates the stricter laws to come.
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|Author:||Hillman, Stephen J.|
|Date:||Aug 1, 1993|
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