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Examining Lost Foam's 'White Side'.

With the foam pattern (or 'white') side representing up to 90% of process control variability, GM shares the important tooling considerations for a tightly controlled lost foam process.

The lost foam (LF) casting process has become more widely accepted since its first patent by HF. Shroyer in 1958. Many jobbing shops and high-volume foundries have incorporated this process as a viable means to meeting a customer's requirements for a part low in cost, high in quality and complex in geometry. Although gaining acceptance, the LF casting process is still perceived as an infant relative to other more mature casting processes.

The LF casting process steps are broken down into three general areas: white side, casting and final part processing. The LF foundryman has coined the term "white side" in reference to the white expanded polystyrene (EPS) patterns, and has learned that 80-90% of the final casting quality is determined during the white side steps of the LF casting process. The white side process steps are bead expansion, pattern molding, pattern aging, pattern and cluster assembly, pattern coating and pattern drying. While other parts of the total process are equally critical (in fact, casting a LF mold has zero similarities to an empty cavity mold), this article concentrates on the white side of the LF process (particularly pattern molding) and its role in making a quality, reproducible casting.

To adequately explain the pattern molding portion of the LF casting process, a discussion on how LF tools are made is necessary.

Tooling Design

Simultaneous engineering efforts among product designers, LF foundrymen and machining personnel are imperative to ensure that the part can meet the product's functionality while still being castable and machinable. The true value of the LF casting process is that part design can be more complicated than the typical cored designs used in sand and permanent mold processes. Machining steps can be eliminated and parts can be combined to eliminate gaskets and/or fasteners. LF castings would be more widely utilized if the product and machining customer truly understood the value a LF component can bring to the product design arena.

The use of 3-D computer workstations, where the product and tooling designers can work and grow the design simultaneously is imperative. The tooling designer can incorporate the draft, relief, blending and scaling factors to the product design. While the product designer refines the drawing, the LF tooling designer can begin to place the split lines, parting lines and core pull-backs necessary to make the foam tooling. The importance of tooling mold wall thickness with the proper use of mold tool inserts and correct placement of vents and fill guns are attributes the casting tooling designer will incorporate as the product designer continues to refine the part design. The use of a computer "wall thickness" checker detects thin-wall problems not only in the part design, but also in the tooling design. Figures 1-2 show many of the attributes of a typical LF tool that have been discussed here.

A manufacturing design center designing LF tools incorporates the use of a "common tool library" of LF tool designs. The "3-D Electronic CAD Library" stores all the common hardware used in the LF tools. The LF casting tool designer utilizes the designs stored in the electronic 3-D library and applies it to the specific parts being designed. The "common tool library" contains project-specific components, purchased commodities (fasteners, locators, cylinders, actuators, electrical hardware, etc.), and also past tool designs with their fill guns, fill gun mounting plates, steam chest designs with their insulation, water-cooling harnesses, core pullbacks, limit switches, etc. The library also contains all the critical mating dimensions and hardware parameters for the molding and gluing assembly machines incorporated by the foam molding shop. Common geometric dimensioning and tolerancing schemes are also stored in the library. If desired, quick-change tools can be designed with disconnects for all the utilities, including steam, water, electrical, air, vacuum, etc.

If true simultaneous engineering efforts are incorporated among the product designer, tool designer, pattern molding operators, and foundry casting the part, this "common tool library" becomes truly valuable. Lessons learned from past jobs can be incorporated and archived in this electronic CAD library so newer part designs are built upon these lessons learned. As an example, while debugging a tool, a mold machine operator discovered that regardless of the machine parameters he adjusts, he could not fill a particular area of a LF tool. The LF tool designer is consulted and a decision is made to add a fill gun and/or vents in the problem area and the fill problem disappears. As the LF tool designer studies the geometry of the problem area, he develops rules of thumb about placement of fill guns and vents to use in future tool designs. A standard bill of design, bill of material, and bill of process can be developed from this library that leads to future robust parts being produced from the LF casting process.

Tooling Build

Once the LFtool design is complete, it can be built. There now exists a host of competent LF tooling pattern shops that can take the 3-D CAD model and convert it directly to CNC machine tools to cut the LF tools. The new electronic age we now live in with websites and instant transfer of CAD files between all parties leads to shortened lead times to build the tools. Even with all this electronic communication, visits by the tool designer to the pattern shop building the tooling, and the plant running the tool is necessary to ensure a smooth startup. The ideal tool designer has both patternmaking and molding machine experience and understands the real requirements of the LF tool to produce a quality foam pattern.

After the tool is built, all tooling is dimensionally validated, assembled and dry-cycled at the pattern shop before shipment to the foam molding plant. The quality of the tooling plays a crucial role in the production of high quality foam patterns, assemblies and clusters.

'White Side' Importance

Like other processes, thesteps involved in the "white side" of the process must be executed properly to ensure the production of a good casting. In addition, if a previous step is done incorrectly, it is unlikely that a subsequent step can make up for it. If the foam patterns aren't molded or aged properly, problems of pattern misalignment, leaking glue joints and coating penetration are likely. Therefore, great attention must be focused on the tooling and the processes used in these steps. Also, casting characteristics such as surface finish, dimensional accuracy, porosity and retained material can be influenced by the steps.

Foam Pattern Molding

The molding tooling is constructed of aluminum to allow fast heat transfer during the molding cycle. The faster the tool can be heated and cooled, the quicker the molding cycle time. Most molding tool walls are 6-10 mm thick and are contoured on the rear face of the tooling to have a constant wall thickness. Tools for larger castings often require backside ribbing. To maintain wall thickness, multiple inserts (pieces of the cavity shape that are machined separately and bolted onto the mold plate, see Fig. 2) may need to be installed in the molding tooling. In addition, fill gun locations and ribs to add stiffness to the mold plates are also machined into the tooling. Finally, thousands of vents are added to allow air to escape from the cavity during bead fill, and to allow the beads to be exposed to steam, vacuum and air during specific steps in the molding cycle.

While the pattern molding cycle consists of many steps, a basic understanding of the cycle can be achieved by examining a few of the essential steps in detail. The steps that will be covered (as shown in Fig. 3) are preheat, fill, fusion, cooling and pattern removal.

The first major step in the molding cycle is preheat. During the preheat step the molding tooling is heated with steam to remove water from the mold plates. Residual water on the mold plate surface or down in any cavities inhibits the bead flow and causes a pattern defect. During preheat, the tooling reaches a temperature of 200F (93C).

After preheat the tooling closes and the mold cavity is filled with EPS beads via venturi-style fill guns. During this fill step, all drains (and any vent valves) are open to allow the air to escape through the vents in the mold plates and out of the mold cavity. Two fusion steps follow the fill step.

The first step in fusing the pattern is the purge step. During the purge step, the drain valves are fully opened and the steam valves are modulated to achieve a pressure set point. This process yields the initial fusion of the pattern at low pressures. During purge, the pressure inside the steam chest is typically 1.3-1.5 bar (18.9-21.8 psia). After the purge is complete, the next fusion step to occur is the autoclave step. During autoclave, the drains are closed and the steam valves are modulated to achieve a set pressure. The autoclave step reaches higher pressures and temperatures than the purging step and it yields the smooth surface finish desired for the casting. The pressures achieved during autoclave vary depending on the pattern geometry. Typical autoclave pressure setpoints vary from 2.2-2.5 bar (31.9-36.3 psia) and the typical autoclave tooling temperatures range from 245-260F (118-126C).

Two cross-steaming steps can also be used as an alternative to the purge step. These steps involve opening the steam valves on one side of the tooling and the drain valves on the other side. For example, the steam valves on the moving side of the tooling, and the drain valves on the stationary side of the tooling can be opened for a set period of time and pressure. This process allows steam to flow from the stationary side of the tooling through the pattern to the moving side. The next cross-steaming step would be a reversal of the drain and steam valve sequence to allow steam to flow from the moving side of the tooling to the stationary side. The cross-steaming steps are typically followed with the autoclave step.

Once the pattern is fused, it must be cooled to allow removal from the tooling. The first step in the cooling process is the blowdown. During blowdown, the machine's drains (and vent valves) are opened to relieve the pressure built up inside the steam chest during autoclave. Once blowdown is accomplished, a vacuum is pulled inside the steam chest to allow cooling of the foam pattern and the mold plate. Simultaneously, chilled water [at 55F (12C) is sprayed onto the back of the mold plate. The reduced pressure and the heat contained in the mold plate cause the water to undergo a phase change to become steam. This phase transformation is energy-intensive and the energy required to turn the water into steam is supplied by the mold plate. As a result, this process cools the mold plate and the foam pattern. Depending on the size of the foam pattern, the tooling can be cooled down to 80-140F (26-60C). Using only chilled water rather than vacuum and water can also accomplish cooling simultaneously. This process wor ks well for horizontally acting, vertically parted molding machines that allow the water to flow down the drain lines. On vertically acting, horizontally parted machines, however, the water isn't removed from the upper tooling nearly as easily, and the vacuum cooling process is preferred.

Once the pattern is cooled, a vacuum is pulled on the moving side of the tooling to hold the pattern in that tooling half. The machine is then cycled to its open position and an unloader mechanism with vacuum is used in conjunction with machine air to extract the pattern from the tooling. The unloader mechanism is then retracted and the pattern is presented to the operator for inspection and placement on an aging rack.

When removed from the tooling, the foam patterns are undersized and become larger for several hours after molding as the water trapped in the foam pattern evaporates. After several hours, the foam patterns stabilize dimensionally and then begin to shrink with time. With the natural aging, the foam patterns can be used for castings for up to 3 days after molding. A typical aging curve for a foam pattern is shown in Fig. 4. With forced aging (aged in ovens for 3-5 hr), the patterns can be used for weeks.

Molding Defects

Using the proper operating parameters for each step of the molding cycle will consistently produce high quality foam patterns. However, defective patterns can be produced in a variety of ways from mishandling to overfusion. To maintain consistency in the molding operation, the expanded bead properties must be within established specifications in order to properly mold foam patterns. The expanded bead pentane level and density can have a significant influence on pattern quality from a visual, dimensional and casting defect standpoint.

In addition to bead properties, the molding equipment must be operating properly. One of the most common defects is underfill. This defect occurs when an area of the pattern doesn't receive the required amount of beads.

This situation can result from several causes, including:

* water isn't removed from the tooling sufficiently;

* fill guns aren't operating properly;

* bead canister or bead hoses are plugged;

* improper fill gun placement;

* insufficient venting in that area of the tooling.

This defect can also appear to be overfusion simply because the limited mass of beads in that area or feature can't handle the amount of steam available.

Over- and underfusion (Fig. 5) are also common defects and, as their names imply, result from the pattern experiencing too much or too little heat in a specific area. These defects can be fixed by controlling the pressure during the autoclave step of the molding cycle. In addition, insufficient preheat can result in underfusion, and inadequate cooling can result in overfusion. Underfusion makes the pattern weak, and the beads can easily be removed on a broken surface. When patterns are underfused they are more susceptible to handling damage and damage during compaction. Underfusion also results in a rough surface, which can lead to more coating being retained in the gaps between the beads after the casting is cleaned. Overfusion causes the pattern to be extremely flexible and can cause the pattern to have an irregular and often wrinkled surface. As with most other defects, this surface is directly transferred to the casting surface.

Postexpansion can also occur in an area of the pattern that receives inadequate cooling. The inadequate cooling allows the remaining pentane inside the beads to continue expanding while the EPS beads are still soft and pliable. When the tooling is separated, the foam pattern is no longer constrained by the mold plates, and the postexpansion occurs. It can also occur after the pattern is removed from the tooling entirely. Postexpansion is observed when a normally flat surface is raised up and appears "puffy." Postexpansion influences not only the dimensions of the foam patterns but also can substantially impact the application of glue to the patterns during the assembly operation. This defect can typically be fixed by lengthening the cooling time or adding a water nozzle to the tooling in the area of concern.

General Motors: A History In Foam

General Motors is regarded as one of the principal fathers of the production LF process as it is known today. Below are some significant milestones in the automaker's utilization of the process.

1958--A patent for LF casting is granted to MIT art student and metallurgist H.F. Shroyer. General Motors begins early development work at its technical center in Warren, Michigan.

1981--GM starts its first large-scale LF production of aluminum cylinder head at its Massena, New York plant for the 4.3L V6 automotive diesel engine. While short-lived, the experience paved the way for the difficult-to-manufacture 2.0L inline four-cylinder head in 1986. Three years later, Massena's LF cylinder heads completely replace semipermanent mold heads.

1989--With the advent of the Saturn vehicle production in Spring Hill, Tennessee, LF receives a big vote of confidence as a high-volume automotive production process, capable of competing with the older and more established processes, such as permanent molding. Saturn's entire fleet of vehicles and fortunes depend on the LF process for all of its aluminum engine blocks and cylinder heads, as well as all of its iron crankshafts and differential carriers.

Early 1990s--GM facilities alone manufacture more than 50% of the entire world's LF production castings.

1994--Following the completion of an 8-month systems analysis study that entailed more than 6000 formal process manhours of more than 70 experts, GM Powertrain's Manufacturing Manager-Components J. Michael Williams announces LF will be the aluminum engine block and head process of choice.

1996--GM announces expansion of the LF process at its Massena plant to accommodate the cylinder head and cylinder block production for the L850 twin-cam four-cylinder global engine.

1997--A development and validation LF casting line is installed at GM's Advanced Materials Development Center in Saginaw, Michigan to develop and validate new products at the site before transferring them to the regular production site for volume production.

1998--GM announces a $170-million investment at its Saginaw Metal Casting Operations to provide for aluminum LF casting production of. cylinder blocks and heads for a new inline family of six five-and four-cylinder truck engines.

1999--GM announces $110 million investment at its Defiance, Ohio foundry for expanding the L6 and L4 truck engine LF aluminum cylinder block and head. At the Massena plant alone, GM reports that nearly 24 million lb of aluminum castings are shipped for assembly on GM vehicles.

Future challenges Identified for Lost Foam

After describing General Motors successes in lost foam at the Paderborner (Germany) Symposium "Lost Foam 2000" last March, Al Steffe, manufacturing director--castings, GM Powertrain, discussed several of the production hurdles still standing in the way of more widespread acceptance and growth of the LF process.

1. Polystyrene Bead Requirements and Formulations--Since 80-90% of the final casting quality is determined by the quality of the foam pattern, significant attention must be placed on the beads and foam pattern production. On the opposite extreme, major bead manufacturers do not recognize. LF beads as a significant product opportunity due to it representing less than 1% of their total bead market. This results in little R&D effort being placed into an ideal bead for the LF casting process. Much could be gained by a polymer that meets the specific needs of the LF casting process.

2. Coating Formulations and Required Process Controls--Coating plays a significant part in the production of quality castings. It is usually the first element to be modified in attempting to eliminate a casting defect and is also the first one suspected to have changed when a unique casting defect surfaces in production. Better control of coating formulations and manufacturing techniques are definite needs, as are a thorough understanding of which coating variables are important, which need to be monitored vs. controlled and how to consistently monitor and control them to produce repeatable, quality castings.

3. Causes of Casting Defects--Folds and porosity are the major quality issues, and their causes and elimination methodology for various design conditions must be better understood. This is where the previous two elements--beads and coatings--also enter the equation, since the interaction of all elements of the process (beads, coating, sand and metal) all impact the resultant casting quality. A deep understanding of these fundamentals and their interactions is necessary to initially design a product and process that will be free of these defects from the start, and then be able to control them in a robust manner during production.

4. Improved Aluminum Material Properties--Future engine requirements are demanding higher output in smaller and lighter weight packages. Today's LF process and aluminum alloy formulations have peaked the material properties capable of being produced. Another 50% improvement in fatigue and strength properties and significantly reduced property variation over that typically found in 319 aluminum is required:

5. Simulation/Modeling of Fill/Flow for the Full Mild Process--A need exists for gating rules for the LF process. Greater predict ability is required at the time of casting the first product of a new design. Reduced lead times and high initial quality expectations for today's product programs do not allow much casting development time nor condone initial low quality performance at the start of the production ramp-up, as has historically been the experience. Accurate, quick, user-friendly process simulation modeling capability is definite requirement to validate the LF process as an accepted high-volume casting process.
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Article Details
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Author:Biederman, Scott
Publication:Modern Casting
Date:Aug 1, 2000
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