The function of tooling in evaporative pattern molding.

The Function of Tooling in Evaporative Pattern Molding

The design, construction and function of EPC tooling governs the successful molding of polystyrene beads into accurate evaporative patterns.

Standard tool design for the molding of EPC patterns consists of tool halves backed by steam chambers for each half. The cavity is machined both back and front to maintain consistent wall thicknesses, necessary for equal heat distribution to the inner faces of the cavity. Figure 1 shows a typical tool construction for an evaporative foam pattern.

Pre-expanded evaporative polystyrene bead (EPS) is injected into the cavity via a venturi action fill gun and air is expelled through core vents in the cavity. Often, the tool will not be tightly closed, allowing additional air exhaust through a gap in the parting.

Steam is used in the molding of polystyrene patterns as an economical way of transferring heat. The steam chamber has two basic functions: to heat the mold wall equally, thus supplying heat to the face of the pattern for pattern surface skinning; and for transferring heat into the EPS bead mass to produce expansion and fusion of the material. This is done with corebox vents inserted in the tool wall.

Water is then introduced through directed spray lines to the back surface of the cavity to lower the wall temperature, halt further expansion and to stabilize the molded pattern. Patterns generally will be de-molded from the tool when the tool wall reaches 140F. At this time, the tool can be opened and the part mechanically ejected from the tool face.

The pattern at this stage is soft, with no memory. Deformation by mishandling, warping on ejection and/or improper strip-pin action will be permanent. Therefore, careful consideration of ejecting or robotic picking of the part from the tool face is very important.

Areas in the part configuration resistant to EPS bead flow can be helped by the careful placement of exhaust lands milled into the parting line of the tool, thus widening the parting gap, or by slightly cracking open the parting line on fill, as mentioned above.

Cracking of the parting line, however, must be precise and not more than 10 thousandths in. Otherwise, the soft bead structure will be forced into the parting line gap, creating flash on the pattern. The use of vacuum on or just prior to the fill cycle can also assist greatly.

Crush-Fill

Good cavity fill, as well as the ability to mold walls of thinner section also depends on the diameter of the preexpanded beads employed. However, more successful fill and thin wall molding than the bead size might normally permit can be achieved with the telescoping tool, or "crush-fill" method.

In this method, shown in Fig. 2, an oversized quantity of EPS bead is injected into the cavity. The tool then telescopes down and crushes the medium. It is sometimes superior to molding the pattern in a higher density material, as the only densification occurs at the thin wall areas of the part and the surfaces of heavier geometry.

The method produces higher density material in specific areas, making thin wall sections possible, but with less total material in the overall pattern. The crush-fill method also can be used for producing solid, homogeneous mass in deep pocket areas. Figure 3 shows a casting with 120 thousandths in. walls. The pattern was molded by the crush-fill method.

In all cases, the increase in temperature in the tool necessary for softening, expansion and fusion of the EPS medium will have a measurable effect on the dimension of the tool and therefore, on the pattern at de-molding. This factor must be considered when the tool is cast or cut to pre-determined shrink factors.

Tool Pull-Backs

Complex patterns with internal passages that are geometrically difficult to mold or form are produced in multiple segments and then assembled to produce a final pattern. The standard assembly method is to use glue printers and hot melt adhesives, or robotics and air-sets, or foamed adhesives. It is highly likely with each of these methods that the cost of assembly will be as high as 50% of the cost of the total assembled EPS pattern.

Additionally, a high risk of possible misalignment and failure of the glue line exists in the assembly operation or during sand compaction. The adhesive line will be transferred to the casting and may cause an aesthetic problem, as shown in Fig. 4. Therefore, it is preferable to mold the pattern in one piece, even though the sophistication of the tooling will require a higher initial investment.

Tooling configurations using multiple pulls, collapsible cores and composite actions have been successfully developed for patterns of diverse sizes. Complex pulls in evaporative pattern tooling can be achieved to eliminate assembly and most or all glue lines. Examples of one-piece foam patterns are shown in Fig. 5.

Such tools must be precisely engineered and machined, and should be designed to operate efficiently at the required elevated temperature. This may require considerable apparent misfit of the tool and pulls at the ambient temperature. The tool should be dry-cycled to the optimum temperature before injection of the bead. This is done to eliminate the possibility of excessive flash formation, jamming and tool damage.

Pull-backs in foam pattern tooling must be able to withstand the heating and cooling cycles that occur continuously in the molding of the pattern. Also, some metals may operate smoothly at the ambient temperature but bind at operating temperature. If pulls bind, the injected material may bypass the pull and be introduced into the steam chamber behind, creating considerable cleanup and downtime.

Careful consideration should be given to the heating and cooling of the core pull, as well as the tool surface proper. A solid pull will not heat or cool in the same manner as the wall of the tool, initially creating cold surface conditions, producing poor fusion and surface finish of the pattern. As cycles increase, blistering or shrinkage of the pattern surface away from the hot core will occur. In some cases, the plastic pattern will adhere to the hot core and be forced out of dimension as the core retracts.

Curved pulls must be designed to operate smoothly and repetitively. Meeting cores should be accurately machined to mate perfectly in their expanded position to eliminate flash on the foam pattern.

In all cases, it must be remembered that the major difference between traditional methods of foundry tooling and evaporative pattern tooling is the continual heating and cooling of the tool and the subsequent stresses and geometrical considerations that this condition implies. [Figure 1 to 5 Omitted] Ron Harsley EFP Corp Elkhart, IN