The taper of a copper mould tube is one of the main things that decides billet quality and mould life. This guide explains what taper does, the three common taper types, and what information you need to choose the right one.
What taper does
As steel cools in the mould, the solid shell shrinks. If the mould walls stay parallel, a gap opens between the shell and the copper. The gap slows heat transfer, so the shell stays thin and weak, and the billet can bulge, rhomboid or break out below the mould. Taper means the inside of the tube gets slightly smaller from top to bottom, so the copper follows the shrinking shell and keeps contact.
Too much taper is also a problem: the shell is squeezed, friction rises, and the bottom of the tube wears fast. The goal is to match the taper to how the shell actually shrinks.
The three taper types
- Single taper — one constant taper over the full length. Simple and low cost; works well for a narrow range of steel grades and casting speeds.
- Double or multi taper — a larger taper in the upper part of the tube, where the shell shrinks fastest, and a smaller taper lower down. Better contact over the full length for mills that cast at higher speeds.
- Parabolic (continuous) taper — the taper changes smoothly along the length to follow the shrinkage curve. Gives the best shell contact and is common on high-speed billet casters.
What changes the right taper
Steel grade. Low carbon, peritectic and high carbon steels shrink differently. Peritectic grades are the most sensitive to too much taper.
Casting speed. At higher speed the shell is thinner when it leaves the mould, so contact in the upper mould matters more — a reason to move to multi or parabolic taper.
Section size. Larger sections shrink more in absolute terms and usually need more total taper.
Mould length and corner radius. Longer tubes and different corner designs change how the shell contacts the copper, especially at the corners.
Signs your taper is wrong
- Rhomboidity, off-corner cracks or longitudinal corner cracks — often too little taper or a worn tube.
- Heavy wear or coating loss at the bottom of the tube, and high mould friction — often too much taper.
- Breakouts at higher speeds when the same tube is fine at lower speeds.
What to send for a quote
Send the tube drawing (or caster maker and section), the external length, width and height, the inside section and corner radius, the current taper, the caster radius, your main steel grades and casting speeds, and the quantity. If you want a change, tell us the problem you see today. We will quote the same tube and, where it helps, an alternative taper.
See our copper mould tubes page for grades and coatings, or send your drawing for a quote within one business day.