Foam producers aim for improved material control and developments.

Foam Producers Aim for Improved Material Control and Developments

Better understanding of foam pattern shrinkage is the key in achieving net shape castings with the EPC process. This, together with new material developments, helps demonstrate the continued viability of this technology.

Manufacturers of foam for the evaporative pattern casting (EPC) process continue to concentrate their efforts on better understanding and controlling shrinkage of patterns made of expandable polystyrene (EPS), as well as developing new foam materials that may be used as an alternative to EPS, particularly in ferrous casting applications. Work in both areas demonstrates that EPC is a viable metalcasting process and one open to further development.

EPS Pattern Shrinkage

Research conducted by ARCO Chemical Co showed that EPS patterns grow 0.2-0.3% during the first few hours after ejection from the mold and then shrink as much as 0.7-0.8% over the next 30 days. Figure 1 graphically displays EPS shrinkage over time. Shrinkage results from the gas composition in each cell coming into equilibrium with the surrounding air and from molecular relaxation.

According to ARCO's Edward Niemann, gas-air equilibration is the most important cause of shrinkage in the short term. It is a result of diffusion of the blowing agent, typically pentane, out of the cells and its replacement in the cells by air. Freshly molded parts typically hold 2% pentane which decreases to less than 1% in two to three days.

Polystyrene's molecular structure stretches as thin cell walls form during pre-expansion of the EPS beads. This happens rapidly, followed by a cooling which effectively freezes cell walls in stretched positions. Molecular relaxation occurs as these cell walls rebound from the stretched position.

Niemann described research on five EPS processing variables: prepuff density, prepuff age, molding steam pressure, molding method and bead size. Measurement of each variable's effect on pattern shrinkage while holding the other four variables constant was made over a six-month period. Measurements were made 24 hours after molding, then weekly for the first month, biweekly for the second month and monthly thereafter.

Extending prepuff age from four to 24 hours was found to have a slight but measureable effect on shrinkage. Extending prepuff age provides time for more pentane loss and molecular relaxation, resulting in less pattern shrinkage.

Density in the 1.2-1.6 lb/[ft.sup.3] range common for foundry foams did not significantly affect shrinkage, according to Niemann. Shape molding grades where 2.0 lb/[ft.sup.3] density is commonly used showed slightly greater shrinkage difference between the low (1.2 lb/[ft.sup.3]) and high (2.0 lb/[ft.sup.3]) densities.

Niemann also showed that molding at three steaming pressures produced patterns with different degrees of fusion. While low steam pressure produced minimum acceptable fusion, high pressure produced the maximum fusion possible without burning the foam pattern, as shown in Fig. 2.

Results indicate that shrinkage decreases as molding steam pressure is increased, holding other variables constant. Niemann suggested that the high pressure allows greater stress relief during molding, similar to the annealing process.

Molding method was shown to have the greatest effect of all the variables discussed. Vacuum molded parts shrank significantly less than conventionally molded parts, as shown in Fig. 3. Niemann explained that this was probably due to vacuum molding's cooling mechanism, which allows molecular stress relief and faster equilibration of vapor composition within the cells.

He concluded that EPS pattern shrinkage can be reduced by vacuum molding, maximizing molding steam pressures and by increasing prepuff age and prepuff density. Of the variables studied, vacuum molding had by far the greatest effect.

Niemann pointed out that pattern behavior during the first 24 hours and temperature-accelerated shrinkage were not evaluated. Also not studied were density gradient within a part and uniformity of steaming, two other variables that may affect shrinkage. Both density and steaming throughout a pattern should be uniform for consistent, reproducible shrinkage, according to Niemann.

Poor fill, improperly designed vents or partially plugged vents are three factors that could affect part density or steaming. They also should be avoided, he said.

Other Research

In other work conducted on foam for EPC patterns, Dr. Rolland Gellert, BASF, AG, West Germany, identified several process variables that dealt with the nature of the foam. Among these were the composition of the blowing agent and storage temperature.

Gellert said post-shrinkage (i.e., after conditioning the part for 24 hours after mold release) of test samples of 1.25 lb/[ft.sup.3] apparent density was first studied as a function of blowing agent composition, that is, iso- and n-pentane. This was studied because blowing agent concentration is believed to have the most pronounced effect due to its lowering of the glass transition temperature of the polymer.

After four weeks' storage, the n-pentane content of both foam samples had dropped to below 0.1%. At the same time, the iso-pentane content of the second sample was still 0.35% and dropped to 0.1% after 100 days.

"In contrast to these marked differences in pentane release, the post-shrinkages were similar," Gellert said. "Only in the absolute amount was there an insignificant difference discernible." This suggests that factors other than pentane loss contribute to shrinkage.

Further measurements were carried out to determine post-shrinkage during room temperature storage and storage at 70C (158F). Results showed that as temperature increased, so did the proportion of shrinkage attributable to stress relief. In other words, the polymer chain that was extended during the molding process due to thermal expansion recoils faster at 70C, a temperature well below the glass transition temperature of 100C (212F).

Gellert concluded that there is an effect due to pattern density, as well as storage temperature. However, since the shrinkage is small, predictable and constant within the first seven hours, the manufacture of foam patterns can be controlled in a way to optimize processing conditions and assure consistency.

New Foam Materials

Two and a half years ago, Dow Chemical introduced a polymethylmethacrylate (PMMA) resin for use in ferrous EPC applications. PMMA was developed to reduce or eliminate defects that can occur when EPS patterns are used to produce iron or steel castings. In some cases, EPS can produce an irregular surface finish that is often unacceptable for use, according to John Brenner of Dow Chemical.

The major difference between the PMMA monomer and that of EPS is that PMMA contains five carbon atoms compared with eight in EPS. In addition, PMMA contains independent oxygen atoms that link with carbon during thermal decomposition and exit from the pattern as carbon monoxide. PMMA also lacks the carbon-rich benzene rings found in polystyrene.

"This ring is highly stable and is very difficult to thermally decompose," Brenner said. "But at very high temperatures, the benzene ring does decompose to form hydrogen gas and carbon. Since PMMA contains no benzene ring, another source of carbon is eliminated. This is perhaps the most important structural difference between PMMA and EPS."

The two foams also differ in the way they decompose during casting. Polystyrene, Brenner explained, decomposes by a relatively slow, random scission process that initially produces liquids. Due to their higher viscosity, these liquids are exposed to the molten metal for longer periods. This increases the chances for benzene ring decomposition and carbon defect formation.

PMMA, on the other hand, decomposes by a rapid unzipping process. The immediate products of such decomposition are highly volatile gases that rapidly escape from the pattern area. In tests conducted with steel castings, carbon pickup with PMMA patterns was 0.05%, compared to a carbon pickup of 0.1-0.3% using EPS patterns.

Brenner also reported that Dow is working on an improved version of PMMA for introduction in early 1990. It appears that the new material will be similar to the current foam, but with two significant changes: it will contain no chlorofluorocarbon (CFC); and high density of thick-sectioned patterns will require shorter molding cycles. [Figure 1 to 3 Omitted]