What is tilt casting?
Aluminum alloy wheels are made by either casting or forging. Cast wheels are either gravity, low pressure or tilt cast. Gravity casting makes up probably 90% of all aftermarket alloy wheels. Low pressure a smaller percentage and tilt cast perhaps 1-2%. Flow forming gained prominence in the mid 2000’s and is now pretty standard for any performance oriented wheel. Most of the high end performance and race oriented aftermarket wheels offered in North America are gravity cast & flow formed. Our upcoming Coto and 6UL wheels are tilt cast & flow formed.
Gravity, low pressure and tilt casting can all produce a strong wheel. Basic strength being the same, gravity will be the heaviest, low pressure a bit lighter, tilt cast lighter still, then forged. We can use as a rough guide, a composite “strength” index that combines elongation, Youngs modulus and ultimate tensile strength. Assuming the same mass wheel; low pressure offers an increase in overall strength over gravity casting, perhaps 10-15%. Tilt casting another 20-25% over low pressure. Forging another 20% over tilt casting. If you’re thinking that a tilt cast wheel can be 40-50% stronger than the same weight gravity cast wheel, you get the idea. Strength being equalized, tilt cast will be about 10% lighter than gravity cast.
Due to the different flow path of molten alloy within the mold, gravity cast wheels will have slightly stronger hubs and spokes than a low pressure cast wheel of the same dimensions. Low pressure cast wheel thus have slightly stronger barrels than gravity cast wheel of the same dimensions. Manufactures can compensate for gravity wheels slightly weaker barrel with flow forming. So gravity can match the overall strength of the low pressure with only a slight weight penalty.
How are wheels cast?
To get a better picture of the difference in materials properties when used in alloy wheel casting, it’s important to have a grasp of how wheels are cast and the typical challenges faced in that process.
With gravity and low pressure casting, the mold is stationary, wheel face down. Molten alloy is poured into the mold and solidifies as it cools. In low pressure, it’s injected from the bottom near the hub first then flows upward. In tilt casting the flow is from the barrel down to the hub. The tilt cast mold starts on it’s side, locked together with the crucible containing the molten alloy. The crucible and mold are then rotated in unison allowing the molten alloy to flow smoothly into the mold. The common analogy is tilting a beer bottle and glass together slowly to eliminate foam vs just pouring straight into a glass sitting on a table. This is an oversimplification but the concept is the same. This method used in tilt casting both reduces unwanted turbulence and allows for more rapid cooling of the alloy. Reducing turbulence helps eliminate oxides and inclusions caused by “foaming” of the liquid alloy flowing through the mold cavities.
All else being equal, the faster the molten aluminum cools and solidifies within the mold, the tighter the grain structure. Tighter grain is denser and stronger than a less dense material. Tilt casting allows faster relative flow with less turbulence compared to gravity or low pressure.
Tilt casting also reduces what is called Cold Joint Defect. This is where the molten alloy flow from one part of the mold meets another part of the flow on the far side of the inlet. The alloy cools as it flows so the coldest part of the flow is where these two rivers meet at the end of mold filling. Gravity and low pressure require compensations in the mold design which can result in added weight and limitations on final product shape. Tilt castling allows shallower draft angles and increased detail in the mold, which gives me and John a bit more freedom in the design (me) and engineering (John) of the wheel.
Gravity casting tooling costs quite a bit less than low pressure tooling which makes smaller volume production runs more cost effective. Gravity casting is a bit slower process than low pressure casting but both are significantly faster than tilt casting. Tilt casting tooling and the actual production process is more expensive than either gravity or low pressure.
These days, most OEM wheels are still gravity cast. Many higher end optional OEM wheels are now being tilt cast as it offers auto manufacturers a more cost effective option than forged.