Curing rate and flowing properties of silicone rubber at injection molding.

Generally, silicone rubbers are mold-cured after mixing the rubber and peroxide cutting agent with a two-roll mill or a kneader (ref. 1). Typically this is done at pressures of 5 MPa to 10 MPa and at temperatures between 120[degrees] to 200[degrees]C. Compression molding, transfer molding and injection molding are common molding ways for silicone rubbers. Recently, injection molding techniques are developing rapidly that have the advantages of molding automatically with high cycle mechanisms.

Silicone rubber is an ideal material for the injection molding process due to its good flow characteristics and very fast cure at high temperatures. As early as the middle 1950s, two men had developed a practical system for the injection molding of silicone rubber (ref.1). At this time, vinyl containing silicone rubbers were just becoming available.

Injection molding machines are equipped with molds and an injection device to pour various materials into the molds. The rubber is injected into the heated mold through an injection nozzle device at high pressure and cured inside the mold. The cured rubber molding is then taken out of an opened mold.

The combination of the energy absorbing characteristics of silicone rubber, combined with its compressibility, makes it essential that a constant volume of material be in the injection cylinder each cycle. Otherwise, the variation of pressure and fill of the cavity would vary enough to affect the quality of the moldings being produced.

To reduce the molding time and to make a precision part, both the flowing and curing properties of a particular rubber compound will be important. A cure meter (ref. 2) was used for checking the curing properties. The curing characteristics were measured as shear stresses of curing material by curemeter.

With injection molding however, the rubber begins to cure white flowing in the heated mold. So, the curemeter cannot be applied by means of measuring curing properties of injection molding rubber.

As we began to study some cure testers, we found the Rheovulkameter (Goettfert) which can measure injected pressure, injected volume and curing rate of a compound all at the same time (ref. 3).

In this article, correlations between the curing and the flowing properties of silicone rubber are investigated by using the Rheovulkameter device.

Experimental

Apparatus

The Rheovulkameter assumes simulation of a injection molding machine. The test sample in the supply chamber is pressurized constantly with the above piston and pushed into the mold through the injection nozzle. The injected volume of the test sample is checked continuously. The test sample is injected into the center of the mold through an injection nozzle of 2 mm diameter and flows into the chamber at a width and depth of 2 mm each. Total volume of the test chamber is 3,200 [mm.sup.3].

[TABULAR DATA OMITTED]

Samples

The silicone rubber compound utilized was KE-195-U. The curing agents used are shown in table 1. To check the peroxide oxide curing rates a DSC30 (Mettler) and TC10A (Mettler) utilized. Figure 1 shows typical events that might be observed when a silicone rubber compound mixed with curing agent is scanned at 10[degrees]C/min. from 50[degrees]C to 250[degrees]C in inert gas ([N.sub.2]). DSC data are tabulated in table 1. The curing of the silicone rubber compound is exothermic reaction.

Results and discussion

Injection velocity

As seen in figure 2, the injected volume of this silicone rubber without curing agent increased proportionally to the injected time. Temperature dependencies of injection velocities at each injection pressure are shown in figure 3. The injection velocities increase with the injection pressure. And the injection velocities increase because of the decrease of viscosity at higher temperature.

On the other hand, test results between injection time and injected volume of silicone rubber contained curing agents are shown in fugure 4.

The injection volumes increase rapidly with injection times and reach states of equilibrium. Because crosslinking reactions of silicone rubbers processing with injection time cause its viscosity increase, the hardened silicone rubbers are hard to flow and the ratio of increase of injection volumes are down. The states of equilibrium mean the balance of injected pressure and viscosities of silicone rubbers.

Injected volume

Silicone rubbers adjusted above 2.2 were injection-molded with the Rheovulkameter. Discussions on injected volumes of states of equilibrium displayed in figure 4 were made. Injected volumes were measured at each condition shown in table 2.

Pressure dependencies of injected volumes at each mold temperature are shown in figure 5, curing agent, 2,5-dimethyl-2,5-di(t-butyl-peroxy) hexane, figure 6, curing agent, dicumylperoxide, and figure 7, curing agent, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane.

With 0.45 phr 2,5-dimethyl-2,5-di(t-butyl-peroxy) hexane curing agent (figure 5) injected volumes converge at 1.5MPa, between 180[degrees]C and 200[degrees]C injected volumes are all approximately 500 [mm.sup.3] at 1.5 MPa. As injection pressure increases above 1.5MPa, injection volumes begin to display some diversity.

Using 0.6 phr dicumyl peroxide (figure 6), injected volumes converged closely at the lower injected pressures, although the previous catalyst shown in fugure 5 does not. However, pressure-volume relationships between each catalyst appear very similar.

1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane (0.6 phr) also plots similarly to figure 5 and figure 6 catalysts, the lower injected volumes converged at a fixed point. As mentioned previously, each peroxide has a different decompositional temperature, therefore, of course, resulting plot points do not entirely match.

Our results reveal how injection volumes increase as injection pressures increase at each mold temperature. With any peroxide, higher injected pressures display larger respective injected volumes and, correspondingly, injected volumes decrease as mold temperatures rise. Injected volumes become especially dependent on mold temperatures at higher injection pressures (2.5MPa

3.0MPa). Injected volumes tend to converge at one point as pressures drop to 1.5MPa. Also, at temperatures higher than those shown, the silicone rubber became too difficult to inject at low pressure. But the phenomenon on relation between injected volumes and injection pressure at each mold temperature (figures 5-7) was found similar, because these data would be obtained by using the same base silicone rubber.

Decision TV

Comparison between figure 2 and figure 4 show that silicone rubbers containing curing agents have no flow in the mold and are cured as crosslinking reactions is taking place. So, a curing rate at injection moldings was decided as TV because the current curemeter is designed to measure curing rates of curing characteristics with no flow.

In figure 4, TV is extrapolated from the tangent lines drawn from the time/volume curves. TV is specified as turning point of the curing rate that dominates the flow of silicone rubber compounds prior to the point and the curing of the ones posterior to the point.

TV

TV was measured at each condition shown in table 2. Relations between TV and injection pressure at each mold temperature are shown in figures 8-10.

As seen in figure 8, lower mold temperatures (170[degrees]C

180[degrees]C) display shorter TV with increased injection pressure. Above these temperatures, though, as injection pressure increases, so does the time it takes to vulcanize. In figures 9 and 10, the same basic characteristics hold true.

Although TV differes with each peroxide, the dependence of TV on mold temperature and injection pressure tends to be the same. In other words, TV becomes shorter with increasing injection pressure at low mold temperatures. But TV is not dependent on injection pressure at the higher temperatures.

The curing rate of each sample is influenced by more than one factor. Injection shear stress increases sample temperature which, in turn, accelerates peroxide decomposition. Therefore, curing rate of the sample is influenced by heat generated from injection shear stress. Of course, higher mold temperatures also speed up the rate at which peroxide catalysts decompose.

With these factors in mind, the curing rate at injection molding is dependent upon injection pressure, mold temperature and the specific type of peroxide catalyst. When mold temperature is so high as to decompose the peroxide fast enough, injected pressure had no influenced on curing rate. In this experiment then, TV is not an acceptable relation to injection pressure at temperatures above 185[degrees]C for 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, above 175[degrees]C for dicumylperoxide, and above 160[degrees]C for 1,1-bis (t-butylperoxy)-3,3,5-trimethylcyclohexane. These temperatures agree well with the DSC exothermic peak temperatures shown in figure 1. Curing rate of silicone rubber by decomposition of peroxide above DSC peak temperature is so fast that the curing rate by heat generated from injection shear stress is almost negligible.

Conclusions

* The Rheovulkameter successfully enabled us to simulate conditions which are important for producing quality silicone moldings.

* Injected volume increased with a higher injection pressure and decreased with a higher mold temperature.

* At low injection pressure and high temperatures, injected volumes converged a fixed quantity.

* At low mold temmperatures, TV (curing rate of injection) become short with increasing the injection pressure.

* But at high mold temperature, TV was not so influenced by the injection pressure.

* Although the injected volumes and TV differ for each peroxide, the dependence of injected volumes and TV on injection pressure and mold temperature tend to be the same.

References

[1] W. Lynch, Handbook of silicone rubber fabrication, VAn Nostrand Reinhold Co. (1978).

[2] ASTM D2084.

[3] K.H. Moos, International polymer processing, III2, 86 (1988).

[4] Goettfert, Rheovulkameter manual.