Pressed Metals vs Cast
Pumps are available in a wide variety of materials and finishes but one of the most distinct differences in pump construction is that some parts and casings are manufactured from cast metal and others are made from pressed metals. There are big differences between cast and pressed metals but what are these differences and how could they affect the decision on which pump is best for the process, as well as having the lowest lifetime cost?
What are pressed metals?
Pressed metal pumps are manufactured from sheet metal through a forming process. This process changes the shape of the sheet metal into that of the required object. The sheet metal is then cut to size from the required piece, and is then pressed into shape using either hot or cold force. Any fittings which are required such as flanges and screwed connections are welded to the casing. A pump casing can be a combination of between 2-5 separate pieces which are welded to the casing, with the welds being a few mm thick.
Pumps manufactured from sheet metal are usually laser welded using a filler metal, which typically degrades quicker than the casing, and often starts to corrode when it comes into contact with saltwater, or chemical cleaners if in a domestic environment. Pressed metal casings cannot be reworked; the casing wears and as they are only a few mm thick, their lifespan is not the same as a cast pump (which is typically several times thicker). Pressed metal casings aren’t usually designed for high pressure applications where a cast pump would commonly be used.
What are cast metals?
Cast metals are often manufactured from ferrous and non-ferrous metals through sand casting, lost foam/wax, or investment casting. Additives can be incorporated during casting to create variations of metals; such as magnesium to cast iron – to create ductile iron (spheroidal graphite), or aluminium to copper – to create aluminium bronze.
Cast metals: Available in a wide range of materials, ensuring a more flexible approach as to what may be better suited for application, rather than having only a small number of materials to choose from which are potentially not suitable for the application – as is the case with pressed metals.
Pressed metals: Not available in the same number of alloys, as some metals are unsuitable for pressing due to their micro structure not allowing them to be easily worked mechanically. There is also not always the demand for pressed metals to warrant the combinations available in sheet metal.
A cast metals shape is extremely durable and consistent when formed in a mould, as opposed to a pressed part which can be easily misshapen if knocked. Some castings such as those made from ductile iron can withstand small impacts, and stainless steel castings can be dropped without damage. Cast casings have a good rigidity to pipe loads reducing coupling and bearing misalignment.
The assurance with cast pumps is the pump is a single piece of metal without welded parts and have very few limitations with regards to size and thickness and so complex parts which are designed to withstand high pressures and radial loads are easily manufactured. Not only is the body cast but also internal parts such as impellers and wear rings cast, or machined from solid bar.
Which production method generates the most material waste?
Cast metals are easily recyclable and can be dropped back into a cupola (casting pot) at the end of their life, or if they fail quality control.
Whereas pressed metals generate large amounts of scrap. After the initial shape has been cut from the sheet, there are leftover pieces which are too small to make parts from, and pieces are unable to be reworked in the factory; these pieces can be recycled but this is done elsewhere after transportation.
As castings wear there is also the opportunity for them to be reworked involving the application of coatings and this is not possible with pressed parts. Cast pumps have an allowance for corrosion within the casing and the use of wear rings in pumps ensures longevity of the casing by almost eliminating casing wear.
Some specifications of pumps are simply not possible with pressed metals due to their design and limitations such as pumps for higher pressures, or where pumps require an overhung bearing design or designed for a long lifespan in critical plant. Cast pumps are brilliant in solid handling applications due to their robustness and thickness of parts including casing and impellers, ensuring they remain unaffected by solids transported through the pump.
Cast pumps represent the best value for money due to their durability of design, thickness, and variety of materials available in the majority of industrial and marine applications. Cast pumps are a superior choice to pressed pumps and should be selected accordingly where possible.