The metal coating improves the moisture stability of SL models by acting as effective moisture barrier as well as reinforcing the model.  RePliForm has measured the moisture stability of several resins and two have been selected to be reported here: Somos 10100 which is a general purpose resin that displays typical moisture sensitivity for this class of SL resins and Somos 7100 which is a high temperature resin that has been found to be one of the least sensitive to moisture that RePliForm has tested.  The moisture sensitivity was tested on 127x12.7x3.2mm samples that were immersed in water at 43°C (110°F) that were periodically removed, dried then were weighed and measured dimensionally.  Uncoated resin samples thermally cycled to 120°C (250°F) were also included in the tests.

The first figure shows the length change of the samples over three weeks of testing.  The uncoated 10100 samples increased in length by nearly 2% while the uncoated 7100 samples increased in length by 1%.  When coated, both resins displayed no dimensional change during 3 weeks of soaking in warm water.

Figure 1.  Length change over time for coated and uncoated SL resin samples soaked in water at 43°C (110°F) for three weeks.

The weight change from the same set of samples is shown in the next figure.  It was found that
the weight change data mirrored the dimensional data in that the weight increase was found to be proportional to the swelling of the samples.  The coated samples had weight gains that were 70 to 90 percent lower than the uncoated resin samples and in generally not consistent from sample to sample.  It is thought to be due to small holes drilled in the parts to hold the samples and make electrical contact on the electroforming racks.

Figure 2.  Weight gain over time for coated and uncoated SL resin samples soaked in 43°C (110°F) water for 3 weeks.

E_p is the stiffness of the model material (1.3-3.3 for unfilled RP resins), E_m is the modulus of the metal coating (125 GPa for copper and 200 GPa for nickel), b is the section width, h is the section height and t is the coating thickness.  This equation predicts a very dramatic increase in the bending moment as the coating thickness increases as shown in Figure 1.

Figure 1.  Predicted increase in moment with application of metal coating in a 3 mm thick RP model section

To verify the effect of the coatings, 127x12.7x3.2mm samples of a variety of SL resins were coated with copper and nickel that ranged from 0 (not coating) to 0.1mm thick.  The samples were the subjected to 3 point bend tests using a span to thickness ratio of 32.  An example of stress-strain curves for coated Somos10100 samples are shown in figure 2.

Figure 2  Stress vs. Strain plots of Somos 10100 resin coated with 0 to 0.1 mm of copper + nickel.

The coating had a profound effect on the apparent stiffness of the samples and had a moderate effect in the strength.  The stiffness properties for three different resins with 0 to 0.1mm of coating applied are shown in Figure 3.  The general trend observed is quite similar to what was predicted in the moment calculations of figure 1.  Nearly order of magnitude increase in bending stiffness is observed with coating thickness that exceed 0.1mm.

Figure 3.  Apparent bending stiffness vs. metal coating thickness for Somos 7100, 9100 and 10100 SL resin.

The rupture strength of the coated samples is shown in Figure 4.  The increase in strength is more modest, only ~2x for a 0.1mm thick coating, but this is achieved at the same failure strain as the uncoated resin.

Figure 4.  Bending Strength vs. coating thickness for Somos 7100, 9100 and 10100 SL resins

Creep performance may be one of the most compelling reasons to look at metal cladding on models.  Several 127x12.7x3.2mm samples of Somos10100 were prepared and half were coated with 0.05mm of copper plus nickel.  A portion of the coated and uncoated samples were then loaded to apply a constant 10 MPa stress on the part in a 3 point bending configuration while a second set was loaded with a constant 20 MPa stress.  The deflection from the load on each the sample was monitored over 500 hour period and is reported as strain over time in the figure below.   

The uncoated samples showed deflection of 0.45% for 10 MPa stress and 1.45% for 20 MPa stress within an hour of being loaded.  The coated models on the other hand showed less than 0.1% deflection at 10 MPa stress and slightly more than 0.2% for 20 MPa stress at 1 hour. At 500 hours, the strain of 10 and 20 MPa loaded uncoated samples is 2.4% and 7% respectively while the coated samples had strains of only 0.11% and 0.35%

In absolute terms, the deflection on the metal-coated samples is less than 5% of the neat resin samples after 500 hours.  In relative terms, the uncoated samples deflection increased by a factor of 6 over the duration of the measurement while the coated samples deflection increased by less than 2 times.  These results further demonstrate the improved stability that can be achieved with the application of a metal coating.