An Introduction to Sand Casting Exhaust Manifolds
Exposed to hot, fast moving and corrosive gasses, exhaust manifolds need an uncommon set of qualities if they are to survive and contribute to efficient combustion. Impro successfully sand casts these parts in a specialized metal and by paying close attention to the details of mold-making. If you need to source exhaust manifolds, or want to learn more about the technical challenges and solutions involved, this blog is for you.
Challenging Conditions and Requirements
Burning gasoline or diesel fuel in an engine releases CO2, water, nitrogen oxides and many other chemicals and particulates, all of which are carried through the exhaust manifold to whatever treatment devices are installed. These gasses are 1,400 °F or hotter and must flow without obstruction to help the engine “breathe” efficiently.
The exhaust manifold is the bridge between combustion chamber/cylinder head and exhaust system. It needs high temperature strength, corrosion resistance, and smooth internal surfaces for optimal gas flow.
Heat is a challenge because the manifold will go through thousands of heating and cooling cycles. The mechanical installation constrains how the manifold can expand and contract, which places the metal under repeated strain.
Metals for Exhaust Manifolds
Metals lose strength and deform readily at higher temperatures in a phenomenon known as creep. To address this, exhaust manifolds need to be made from an alloy with low creep, plus high fatigue and corrosion resistance.
For most applications SiMo ductile iron is preferred. “Ductile” is a form of iron where the excess carbon is induced to form spheres or nodules through inoculation with magnesium at the point of pouring. “SiMo” indicates elevated levels of silicon and molybdenum in the alloy. Together, these improve high temperature strength and corrosion resistance.
For the foundry doing the casting, SiMo ductile has two advantages. Castability is good, meaning it flows well, and shrinkage is low, which results in a low level of defective parts.
Sand Casting in SiMo Ductile Iron
The functional parts of an exhaust manifold are the passageways or ducts that steer hot gasses into the exhaust system. The only practical way of forming these is by casting, and sand casting offers the most attractive economics.
Sand casting entails using a pattern replicating the part being made to create a cavity in packed sand. Typically, this cavity is made in two halves, with additional passages incorporated for metal to flow in through.
Forming hollow regions within the cast part is done by placing cores in the sand cavity. These occupy space that would otherwise be filled with metal. In the case of an exhaust manifold, the cores are what form the functional shapes and surfaces, so they must be made and positioned with the greatest possible care.
Cores for Exhaust Manifold Molds
A key consideration when sand casting exhaust manifolds is core removal after metal solidification. As the passages usually follow complex contours the only material option is sand, similar to that used for forming the external shape, as this is readily broken apart.
Exhaust manifold cores are often made from sand coated in A-E/SO2 resin. “A-E” indicates an acrylic-epoxy base and SO2 refers to using sulfur dioxide to activate the bonding reaction after core forming. The cores are produced by blowing coated sand into a vented pattern. SO2 is then blown through to bind the sand particles into a form rigid enough to survive metal pouring yet still readily broken apart.
A second factor with cores is production of the smoothest possible surfaces with no metal intrusion. Wherever possible cores are produced as single pieces without joins and from sand of small average particle size.
Ready for Every Sand Casting Challenge
Exhaust manifolds pose multiple challenges for the foundry tasked with producing them, but the specialists at Impro have both the metallurgical expertise and the casting know-how required. That’s how we produce consistently high quality exhaust manifolds sand cast in SiMo ductile iron.
If you need a source for quality exhaust manifolds, or have other casting requirements, we can almost certainly help. Contact us and let’s talk.