Copper Casting: Conductivity, Gassing, and the High-Purity Grade Problem

C101 (oxygen-free electronic, OFE, 99.99 percent) and C110 (electrolytic tough pitch, ETP, 99.90 percent) are chosen because they conduct electricity and heat better than almost anything affordable, roughly 100 percent IACS. That purity is precisely what makes them difficult to cast. Pure copper has high thermal conductivity, so it chills the mold fast and freezes before it fills thin sections, and it dissolves large amounts of hydrogen when molten, releasing it as gas porosity on solidification. The oxygen story is worse. ETP copper carries a small amount of oxygen as copper oxide; when remelted in a hydrogen-bearing atmosphere, the hydrogen reduces that oxide internally, generating steam that creates 'steam holes' and the classic hydrogen embrittlement of copper. Foundries casting ETP must control atmosphere tightly, which is one reason oxygen-free C101 or deoxidized coppers are often preferred for casting even though the feedstock costs more. For buyers, the honest position is that casting high-purity copper is a specialized, lower-yield operation. If your part needs maximum conductivity and a complex shape, a copper foundry can do it, but expect porosity controls, possible HIP for critical parts, and a conductivity that may dip slightly below wrought due to cast grain structure and residual gas. Where the part is simple, machining or cold forming from wrought C101/C110 stock almost always beats casting on both conductivity and cost.

Tellurium copper (C145) adds about 0.5 percent tellurium to copper. That small addition forms copper telluride particles that break up chips and give the alloy a machinability rating near 85 to 90 percent, versus pure copper's gummy, smearing 20 percent. Crucially, tellurium does this while retaining 90 to 95 percent IACS conductivity, so you keep nearly all the electrical performance and gain the ability to machine the part economically. In casting, tellurium copper behaves somewhat better than pure copper because the alloy addition slightly widens the freezing range and improves feeding, but it is still fundamentally a high-conductivity copper with the attendant gassing tendencies. The real value of tellurium copper is downstream: cast or wrought, it lets you machine connectors, electrodes, and electrical contacts that pure copper would smear and gall on. The practical pattern in industry is telling. Most high-conductivity copper parts are not cast at all; they are machined or cold-formed from wrought C101, C110, or C145 bar and plate, because wrought copper has better conductivity, no porosity, and predictable properties. Casting copper makes sense mainly for complex shapes, large bus components, motor end rings, certain electrodes, where the geometry justifies the foundry's effort. If a buyer asks to cast a simple copper block or bar, the right advice is usually to machine it from wrought stock instead.

Casting pure copper is justified in a handful of real cases: large or complex high-conductivity parts where machining from solid would waste enormous material or be geometrically impossible (induction coils, large bus connectors, motor squirrel-cage end rings often cast directly onto rotors), and net-shape conductive components at production volume. For these, a specialized copper foundry using oxygen-free or deoxidized copper, tight atmosphere control, and good gating delivers usable parts, sometimes followed by HIP or impregnation to seal porosity for vacuum or pressure-tight conductive applications. The honest alternatives are worth stating plainly. For simple shapes and bars, machine from wrought C101/C110, you get full conductivity and zero porosity. For thin conductive forms, stamp or cold-form from sheet. For high-conductivity parts needing machinability, use tellurium copper (C145) or chromium copper (C18150), the latter being a heat-treatable copper that reaches much higher strength while keeping 80 percent IACS, ideal for resistance-welding electrodes. For parts where some conductivity can be traded for far better castability, the leaded and silicon bronzes and brasses pour beautifully, and many 'copper' parts are actually better served by a copper alloy that casts well. The buyer's decision tree: if conductivity is paramount and the shape is simple, do not cast, machine wrought. If the shape is complex and conductivity is paramount, cast pure copper with a specialist and accept the cost. If you can trade a little conductivity for castability and machinability, move to tellurium copper, chromium copper, or a copper-base bronze.