Semi-permanent releasants for IM.

The process of rubber injection molding requires the use of a mold releasant to prevent adhesive bonding of the rubber to the mold. In addition to providing release, the mold releasant is expected to prevent mold fouling and to ensure a high quality part surface. Two types of external mold releasants, conventional and semi-permanent, are available commercially. These are briefly described below.

Conventional mold releasants

The most common conventional mold releasants on the market today are nonfunctional polydimethylsiloxanes (silicones) which are available in both solvent based and water based forms. These releasants can be applied to molds by spray, wipe or brush methods. Although silicones are often given a few minutes "cure" time to allow the evaporation of solvent, no crosslinking occurs and there is little or no bonding to the mold. Silicone mold releasants function by transferring a portion of the release film to the molded part. In other words, silicones achieve release by cohesive failure of the release film. The nature of this release mechanism makes frequent reapplication of the mold releasant necessary. In the case of rubber injection molding, conventional silicone mold releasants may require reapplication each cycle due to the abrasive nature of the process. The frequency of reapplication would also be dependent on the type of rubber stock being molded. The use of silicone releasants also means that any molded parts which are to undergo painting or bonding processes require extensive cleaning, since a significant amount of silicone is transferred to the part.

Semi-permanent mold releasants

Semi-permanent mold releasants are crosslinkable polymers which are available in solvent based and water based forms. These releasants are usually provided as low viscosity solutions (or emulsions) which can be applied to molds by spray, wipe or brush methods. Semi-permanent mold releasants require a short cure time (a ten minute cure at the rubber molding temperature is usually sufficient), during which the carrier evaporates and the release polymers bond to the mold and crosslink to form a thin, uniform release film. The bonding that occurs between the release polymers and the mold is directly responsible for die durability and abrasion resistance (and hence, multiple releases) provided by semi-permanent mold releasants. The continuous crosslinked release films formed by semi-permanent releasants contribute to low transfer to the part, low buildup on the mold, and a low defect rate.

It is necessary to discuss the relationship between transfer to the part and durability because release film durability is the key to multiple release. Semi-permanent releasants generally exhibit very low transfer to finished parts relative to conventional releasants, but a slight amount of transfer may occur (otherwise, these would be considered permanent releasants). Initially, transfer to the part is due to the removal of the least tightly bound segments of the release film, which are often the most lubricious segments. Once these segments have been removed there is a slow, continuous wear on the release film until it is eventually abraded from the mold. There is a direct relationship between transfer and durability; i.e., you cannot have transfer to die part without also having some amount of wear on the release film. The severity of the wear/transfer is determined by the releasant used, the rubber stock being molded and the molding conditions. Careful matching of the releasant to die rubber stock can minimize the wear/transfer of the releasant and optimize the molding process.

Environmental and safety issues

Historically, semi-permanent releasants have been solvent based formulations. Recent proposals in environmental and safety legislation plan to severely restrict and/or phase out the use of solvents such as CFCs, 1,1,1-trichloroethane, methylene chloride and volatile organic solvents due to risks associated with ozone depletion potential, global warning potential, and health hazards to workers. The majority of new product development in semi-permanent releasants currently concentrates on water based technology. The goal of releasant suppliers and consumers is to match or exceed the performance of solvent based systems with water based products.

This article presents data comparing the performance of water based semi-permanent releasants with solvent based semi-permanent releasants and a conventional releasant for injection molding of various rubbers. The performance parameters evaluated include release ease, transfer and release film durability. The data demonstrate that water based semi-permanent releasants are effective with various rubbers. In addition, the data show that emulsifying the same components of a successful solvent based releasant does not necessarily produce a good water based releasant. Often, changes in chemistry are required. Lastly, the results of lab release testing correlated well with the results on injection molding machines.

Experimental

Laboratory release testing

Matching the releasant to the rubber stock is achieved by laboratory screening of the semi-permanent releasant product line. Parameters considered when selecting a releasant for a particular rubber stock include releasant chemistry, releasant film or coating physical properties, the chemical and physical properties of the rubber stock, and the processing parameters. The following lists the releasants used in this article:

* WA - water based semi-permanent, chemistry A

* WB - water based semi-permanent, chemistry B

* WC - water based semi-permanent, chemistry C

* SA - solvent based semi-permanent, chemistry A

* SB - solvent based semi-permanent, chemistry B

* SC - solvent based semi-permanent, chemistry C

* CMR-W - conventional releasant, water based silicone

Four different rubber stocks were used as obtained from rubber parts manufacturers. These include natural rubber, EPDM, epichlorohydrin and polyacrylate.

Procedure

QD-36 smooth steel panels (available from the Q Panel Co.) are preheated to the rubber molding temperature. Using a Crown sprayer syphon feed aerosol atomizer, three coats of the releasants to be tested are sprayed onto the hot panels using a light, uniform spray. The releasants are cured onto the panels for ten minutes at the rubber molding temperature. All heating and curing takes place on a Model C Carver press, which has platens capable of maintaining temperatures from 65-260 [degrees] C (150-500 [degrees] F) and pressures up to 667 psi (4.6 MPa). After the releasant cure is complete, rubber samples weighing about 0.5 grams are molded between two coated panels using nominal pressure. The release ease is rated on a scale of 1 to 10, where 10 is the easiest release and a rating less than 7 would generally be unacceptable for rubber molding. Coating durability on the Q Panels is rated visually in terms of the observed wear O = none, T = trace, L = light, M = moderate, S = severe). The degree of transfer can be rated visually and by touch using the same scale as the durability rating.

Injection machine testing

Since laboratory testing on the Carver press is more similar to compression molding than injection molding, testing was also conducted on the injection machine at the University of Akron. Molding was carried out on the Boy 30M injection machine using a steel "dogbone" mold.

Procedure

Three light, uniform coats of releasant were applied to the dogbone mold and the flat panel surrounding the injection port. The releasant was cured for 5-10 minutes at the mold temperature (375 [degrees] F/191 [degrees] C). The barrel temperature was set at 210 [degrees] F (99 [degrees] C) and the feed time from the barrel into the mold cavity was estimated to be seven seconds. Release ease from the mold and the panel surrounding the injection port was rated on a scale of 1-10. Durability and transfer were rated by the same methods used in laboratory release testing.

Results and discussion

Tables 1-4 contain the laboratory release tests results for four rubber stocks. Table 5 contains the injection molding results for the same natural rubber evaluated in table 1; however, a higher mold temperature was required to fully cure the rubber in the injection press.

Several observations can be made upon examining the data in tables 1-4. As a first consideration, most semi-permanent releasants outperform the silicone emulsion (CMR-W), particularly with respect to durability. This is not surprising, since the semi-permanents bond to the mold. Second, water based semi-permanents can perform comparable to or better than solvent based semi-permanents, as demonstrated with EPDM rubber and polyacrylate in tables 2 and 4. Here, the water based semi-permanent releasant provided superior release ease and durability compared to solvent based products evaluated. There are instances in which solvent based semi-permanents are still better in performance. Table 3 shows that while the best water based and solvent based semi-permanents release epichlorohydrin rubber equally well, the water based semi-permanent releasant, WB, exhibits slightly more wear and transfer than the solvent based semi-permanent releasant, SC. Also, table I shows the best solvent based semi-permanent releasant releases natural rubber better than the water based products; however, the solvent based semi-permanent does show more wear and transfer after six demoldings in the laboratory test apparatus. While the laboratory data for epichlorohydrin rubber and natural rubber indicate that the performance of the water based send-permanent releasants is slightly worse than that of the solvent based products, the differences are slight and the water based semi-permanents for releasing natural rubber and epichlorohydrin rubber are commercially viable products.

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The third important point to make for the data in tables 1-4 is that correlation between solvent and water based products is not exact; i.e., the chemistry required to make the best solvent based and the best water based products is not the same. Therefore, the successful emulsification of releasant polymers used in a successful solvent based product does not guarantee good performance in a water based semi-permanent releasant. Often changes in releasant chemistry are required when going from solvent based to water based technology. Chemistry C typically produces the best solvent based product whereas chemistry B produces the best water based product.

Fourth, it is important to match the physical/chemical properties of the semi-permanent releasants to the physical/chemical properties of the rubber stock. For each of the rubber stocks evaluated, one or two releasants clearly out-performed the others. Therefore, all releasants will not work on a given stock under a given set of molding conditions. Care must be taken to consider all physical, chemical and process parameters when selecting a releasant.

Finally, a comparison of the results in table 5 (injection press molding) with those in table 1(laboratory testing) is appropriate since the tables contains results on the same natural rubber compound. In general, the laboratory test method seems to be good predictor of injection press results. There are some indications that the laboratory method may be a slightly more critical test for water based semi-permanents. In the laboratory testing, the release ease of the water based semi-permanent (WB) fell off slightly faster than in the actual injection molding trial. Additional injection molding will have to be conducted to confirm this conclusion. Overall, correlation of the lab test results and injection trials was quite good for both the solvent and water based products tested.

Conclusions

Solvent and water based semi-permanent releasants can be formulated to outperform conventional releasants in rubber injection molding processes. When the appropriate semi-permanent releasant is choosen, it provides multiple release, durability, and low transfer properties that far exceed the capabilities of a conventional silicone releasant. Water based semi-permanent formulas can give release performance equal to or better than solvent based formulas, so they are suitable alternatives to solvent based semi-permanent formulas when environmental concerns restrict the use of halogenated solvents and volatile organic compounds.

the results of this study indicated that the chemistry of the best solvent based releasant does not necessarily yield the best water based releasant. Modifications in chemistry are required to make a successful transition from solvent based releasant to water based releasants, but the reasons for this are not fully understood at this time and are under further investigation. A laboratory test method for screening releasants correlates well with the injection molding process based on preliminary test results at the University of Akron and at commercial injection molding facilities.

Additional injection molding trials will be conducted to confirm the correlation with laboratory test results and to support the investigation of new water based chemistries.

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