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Green Sand Metalcasting Foundry News

Preliminary Investigation into the Cause of Pinhole Porosity in Aluminum Metal Castings

Posted by Hill and Griffith Company on Nov 24, 2020 8:00:00 AM

Excerpt from the University of Northern Iowa dissertation by Brodie J. Biersner.

Introduction

Sand Mold Design
There are many molding methods available to produce a metal casting. The majority of metal castings however are produced with sand molds. This method for the production of metal casting has advanced over the years—from the materials utilized to the types of molding equipment and processes. Historically there was a lot of manual labor required for the production of a sand mold. Today, high speed molding machines are used with minimum amounts of human labor required. For sand casting to be successful for foundry applications, certain criteria must be met. The principal feature is good compatibility between the binder system and the sand aggregate chosen. This combination must also be compatible with the metal alloy used to produce the casting.

Core Mold Sand Cast

Functions of a Sand Mold
There are multiple purposes for a sand mold. The main purpose of a mold is to hold liquid metal in a specific shape until the metal solidifies. Sand molding can be broken down into two main categories: permanent and expendable. Permanent molds can be reused after the casting is solidified and removed, while in the expendable process the mold is destroyed. There are two methods that are predominant in expendable mold processes. One process uses an aggregate with a polymer/plastic and mixes the products together and allows the polymer to harden, or react. The other process mixes an aggregate with a mixture of bentonite clay and water. Known as green sand molding, this process produces the greatest amount of castings by weight.
 
Historically, the most widely used aggregate in the foundry industry is silicon dioxide (SiO2), commonly known as silica sand. Silica sand, in the form of quartz particles, is used because it is the most abundant mineral in nature, easily mined, low cost to produce, available in various grain sizes and distributions, along with being an excellent refractory (American Foundrymen's Society, 1962). Other aggregates used are olivine ((Mg,Fe)2SiO4), chromite (FeCr2O4) and zircon (ZrSiO4) sands. This is because of their resistance to react to the liquid metal. These aggregates also have a higher fusion point, and lower thermal expansion, when compared to traditional silica quartz.

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Requirements for an Aggregate to be used for Sand Molding
All sand aggregates used for the foundry process will have to meet certain criteria to produce a satisfactory casting. Every foundry will have its own requirements to produce a casting to the specifications that the end user requests. The majority of these foundries will pay attention to the following characteristics when choosing a base sand for their foundry sand system: acid demand value (ADV), clay content, Grain Fineness Number (GFN), grain shape, loss on ignition, moisture, and pH.
 
Acid Demand Value (ADV)
The ADV was a test initially designed for acid set binder systems. This test is used to measure the reactivity level of any insoluble acid reactive materials found within the foundry sand. Insoluble materials within the sand include carbonate minerals and metal oxides. ADV can measure acid consuming materials that are not water soluble. This phenomenon is a limitation of pH testing. pH (acidity or alkalinity) is only affected by water soluble materials present in the sand. ADV is determined in the foundry industry following the American Foundry Society’s procedure AFS 1114-00-S.
 
Clay Content
Clay content is determined in the foundry industry following the American Foundry Society’s Clay Content procedure AFS 2110-00-S. The test is used to determine the percentage of clay and other particles that settle at a rate of less than one inch per minute in water. Typically, these are materials under 20 microns. Clay testing was originally designed to monitor the total amount of total clay in the green sand molding process. It determines how much clay, dead and active clay, is in a given sand system.
 
Clay content testing has evolved to also be used in chemically bonded sands to monitor clay fines within the aggregate, before it is mixed with a resin polymer. The closer the clay content is to 0.0%, the cleaner the sand. Lower values correlate to the amount of surface area from fine particulate matter that would be required for the resin to coat. The increase in clay content will require more resin to coat each individual particle, because of this increased surface area. The clay content test is helpful for a foundry when sand is shipped to them. Typically a trucking container ships a variety of goods, i.e. cement, sea shells, oil, lime, fertilizer, flocculants, etc. The fine material can easily be picked up from the shipment container and cause issue within the molding and casting processes.

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