The white pattern used in the lost foam casting process is, in essence, the same as the "pattern" in the traditional sense. In traditional hand-built sand molding, this pattern is usually a wooden pattern; on automated sand molding lines, it is usually a metal pattern; and in traditional investment (lost-wax) casting, it is usually a wax pattern.
Because of a distinctive feature of the lost foam casting process — the foam is removed by liquefaction before pouring, which avoids the carbon defects caused by incomplete foam decomposition — the material selection tends toward ordinary EPS (expanded polystyrene, Polystyrene). It offers relatively low cost, good molding performance, the longest history of application, and the most mature process.
1. Grades and Properties of Domestic EPS Beads
Grade | Mesh | Particle Size /mm | Properties |
301A | 13–14 | 1.25–1.60 | Density 1.03 g/cm³ |
301 | 15–16 | 0.90–1.43 | Bulk density 600 kg/m³ |
302A | 17–18 | 0.80–1.00 | Blowing agent 6%–8% (mass fraction) |
302 | 19–20 | 0.71–0.88 | Residual monomer <0.1% (mass fraction) |
401 | 21–22 | 0.60–0.80 | Water content <0.5% (mass fraction) |
402 | 23–24 | 0.25–0.60 | Maximum expansion ratio 40–60 |
501 | 25–26 | 0.20–0.40 | — |
The fine fractions of EPS are the main grades used in the foundry field. When selecting bead size, one must first understand the dimensions, wall thickness, and surface quality requirements of the casting to be produced. Generally, the selection is based on one-tenth of the thinnest wall thickness of the casting. For example, for a casting 7 mm thick, a Grade 401 bead should be chosen. If a higher surface quality is required, the next finer grade may be selected.
2. White Pattern Molding Process
White patterns are produced by mold foam molding, as well as by mechanical machining and hand forming. For large and small patterns produced in quantity and in batches, the mold foam molding method is used; for medium-sized and large patterns produced as single pieces or in small batches, mechanical machining or hand-forming methods are used. The specific molding processes are not elaborated here.
3. White Pattern Defects and Preventive Measures
Defect: The pattern looks normal on the outside but has poor internal fusion (welding).
Causes:(1) Short molding heating time, or low steam pressure / low molding temperature;(2) the pre-expanded beads were over-dried, or the aging/curing temperature was too high or the time too long, resulting in insufficient blowing-agent content.
Measures: (1) Raise the heating steam pressure and molding temperature, and extend the foam molding time; (2) control bead drying, and control the aging/curing temperature and time.
Defect: The internal structure is loose, with poor fusion over most of the pattern.
Causes: (1) Non-uniform bead filling; (2) low molding temperature and short time; (3) too-low bead density or low blowing-agent content.
Measures: (1) Change the filling method and conditions; (2)raise the heating steam pressure and extend the heating time; (3) control the pre-expanded bead density and blowing-agent content.
Defect: The pattern is incomplete and its outline is unclear.
Causes: (1) The cavity is not completely filled with beads; (2) improper distribution of mold vents or incorrect placement of the feed inlet.
Measures: (1) Improve the filling method and adjust the air pressure; (2) adjust the number of vents and the position of the feed inlet.
Defect: The pattern melts.
Causes: (1) Too-high heating steam pressure; (2) excessively long foam molding time; (3) too many vents or excessively large vent diameter.
Measures: (1) Lower the heating steam pressure; (2) shorten the molding heating time; (3) adjust the number of vents or their diameter.
Defect: The pattern surface is rough with depressions at the bead interfaces.
Causes: (1) Too-low bead density or incomplete curing; (2) insufficient foam molding time.
Measures: (1) Increase the pre-expanded bead density and use fully cured beads; (2) extend the molding time.
Defect: The pattern is normal immediately after demolding but later shrinks and deforms.
Causes: (1) Too-low pre-expanded bead density; (2) insufficient aging/curing time of the pre-expanded beads.
Measures: (1) Shorten the pre-expansion time and increase the pre-expanded density; (2) appropriately extend the aging/curing time of the pre-expanded beads.
The above is a brief introduction to the white pattern in the lost foam investment casting process, together with a simple overview of the typical defects encountered in domestic foam molding production and their countermeasures.