Lost foam casting

10/10/2018

 

In 1958, H.F. Shroyer invented and patented the technology for making metal castings from expandable foam models. The original model was made from polystyrene (EPS) sheets and was molded with a binder-containing molding sand. Germany's Grünzweig and Harrtmann purchased the patent and developed it. The technique of producing castings using binderless dry sand was later patented by T.R. Smith in 1964. The use of a binderless dry sand process prior to 1980 must be approved by the Full Mold Process, Inc., after which the patent is ineffective.

The most common and practical method is to put the refractory coated model into the sand box, the model is filled with dry sand and compacted, and the liquid metal is poured instead of the foam plastic mold. This casting process is called: lost foam casting ( EPC), gasification die casting and solid casting. The Lost Foam Casting Committee of the American Foundry Association adopted "foam casting" as the name of the process. Precision casting

Lost Foam Casting is an innovative casting process that can be used to produce parts for both colored and ferrous metal power systems, including: cylinder blocks, cylinder heads, crankshafts, gearboxes, intake manifolds, exhaust pipes and brake hubs. The process flow of lost foam casting is as follows:

1) Pre-expansion

Model production is the first process of the lost foam casting process. Complex castings, such as cylinder heads, need to be made separately from several foam models, and then glued into a whole model. Each block model requires a set of molds for production. In addition, a set of tires may be needed in the gluing operation to maintain the accurate positioning of each block. The molding process of the model is divided into two steps. The first step is to Polystyrene beads are pre-expanded to a suitable density, typically by rapid steam heating, which is known as pre-expansion.

2) Model molding

The pre-expanded beads are first stabilized, then sent to the hopper of the molding machine, and fed through the feeding hole. After the mold cavity is filled with the pre-formed beads, steam is introduced to soften the beads. Expanding, squeezing all the voids and bonding them together completes the manufacturing process of the foam model, which is called autoclaving.

After molding, the model is cooled by a large flow of water in the water-cooling cavity of the mold, and then the mold is opened to take out the model. At this time, the model temperature is increased and the strength is low, so care must be taken during demolding and storage to prevent deformation and damage.

3) Model cluster combination

Before the model is used, it must be stored for a suitable time to make it mature. The typical model storage period is up to 30 days. For the model formed by the uniquely designed mold, it only needs to be stored for 2 hours. After the model is matured, it can be divided. The block model is glued together.

The block model is glued using a hot melt adhesive on an automatic glue machine. Glued joint seams should be tightly sealed to reduce the possibility of casting defects

4) Model cluster dip coating

In order to produce more castings per box casting, sometimes many models are glued into clusters, the model clusters are immersed in refractory coatings, and then dried in air circulating ovens of about 30-60 C (86-140 F) for 2 to 3 After the hour, after drying, put the model cluster into the sand box, and fill the dry sand to make the vibration tight. The inner cavity of all the model clusters and the dry sand of the periphery must be compacted and supported.

5) Pouring

After the model cluster is filled in the sand box by dry sand vibration, the mold can be poured, and the molten metal is poured into the mold (casting temperature is about 760C/1400F, cast iron is about 1425C/2600F), and the model gasification is The metal is replaced by a casting. Figure 1 is a schematic view of a flask and casting of a lost foam process.

In the lost foam casting process, the casting speed is more critical than conventional hollow casting. If the casting process is interrupted, the sand pattern may collapse and cause waste. Therefore, in order to reduce the difference between each casting, it is best to use an automatic pouring machine.

6) Falling sand cleaning

After casting, the casting solidifies and cools in the flask and then falls. Casting sand is quite simple, and the tilting sandbox castings fall out of the loose dry sand. The castings are then automatically separated, cleaned, inspected and placed in a casting box for transport.

Dry sand can be reused after cooling, and other additional processes are rarely used, and metal scrap can be remelted in production.

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