It’s your first day on a new job, and you’ve been given an assignment: to choose a set of casters for a new piece of rolling equipment.
Sounds simple, right? But as you browse suppliers’ websites and catalogs, you quickly realize that casters, which seemed so simple before you took on this task, can actually be extremely varied in their details.
Casters have been around for hundreds of years, and have a rich history of technical development. You’ll come across terms like axles, kingpins, and bearings; swivel versus fixed; crowned wheels versus flat wheels – and many more!
After you work your way through all of that, your choice may very well come down to the material from which the wheels themselves are made. Let’s supposed that you’ve compared all of the options, and weighed the tradeoffs of one material versus another, and narrowed it down to two possible choices: polyolefin or semi steel.
In other words, this choice comes down to plastic versus metal. Each has its advantages as we’ll see. In the sections below, we’ll define and describe semi steel first, followed by polyolefin, and then give some guidelines for when you might want to choose one over the other.
Semi Steel: What it Isand Isn’t
In the field of metallurgy, some types of metal have been meticulously defined. Cast iron and steel are two such terms. To achieve their goals, metallurgists can precisely vary the blend of iron and additives to achieve their goals. They can then produce specification sheets describing the final product; consumers can read these specifications, and understand the exact makeup of the metal that they are purchasing.
Semi steel is not so precise. It is a loosely defined term, describing a product made when scrap steel is added to molten cast iron. The characteristics of semi-steel therefore, are something of a blend between those of steel and cast iron. To understand this blend, let’s look at the differences between products made from cast iron and forged steel, and then see where semi-steel fits in.
Cast Iron – On a molecular basis, cast iron is a simpler material than steel. It’s made from iron, with a low percentage of carbon. It has a relatively low melting point; products made from cast iron take advantage of this fact – the molten iron, a liquid, is poured into a mold. When cooled, the iron hardens, and the product is removed from the mold and machined where necessary. Seen under a microscope, the metal appears to be composed of flakes of material, oriented in a random fashion. (These flakes will be important when we consider impact resistance, below.)
Forged Steel – Steel has a higher carbon content than cast iron, and has other elements added to it to form alloys. The different alloys are chosen to give properties that will meet the requirements of the final application. The “forged” part of the name Forged Steel refers to the process of forming parts. A hot ingot of steel is placed in a forge, which exerts tremendous pressure on the steel, pressing it into the final shape of the product.
Cast iron parts are easier to produce, easier to machine, are extremely hard and durable – but they can be somewhat brittle. Because of the structure of the flakes, cast iron can be prone to fracturing upon impact. Forged steel parts are also hard and durable, but the higher carbon and alloy content gives them a toughness and elasticity that makes them almost indestructible.
Semi-steel, containing both cast iron and steel, has characteristics that lie somewhere between these two materials. Ideally, it has the hardness of cast iron, and the toughness of steel – but at a lower cost.
Polyolefin: One Tough Plastic
Polyolefin is a type of plastic. The name is often used as a broad term to describe the product formed when small monomers, such as ethylene or polypropylene, are formed into long chains called polymers. Chemists can control the characteristics of the final product based on the monomer they start with—the additional chemicals they add, and the manufacturing process.
Polyolefin products come in a range of textures, from soft materials like plastic bags and shrink film, to denser materials like the foam used in sport shoes, to extremely rigid materials such as the polyolefin from which caster wheels are made.
The important point to remember is that, while some polyolefin materials might be soft or spongy, caster manufacturers will specify custom compositions that are very rigid, and meet their stringent requirements.
Polyolefin wheels for casters are made from an injection molding process, which makes them very economical. They are lightweight, and are non-marking for good floor protection. Their impact strength is excellent, as is their resistance to grease, chemicals, acids, solvents and most oils. Their load capacity is good (but not as high as semi-steel casters).
Look at the Tradeoffs
If you’ve examined all the options and it’s down to a choice between polyolefin,or semi-steel casters, here are some guidelines to help you make your choice.
- Load Capacity – Semi-steel casters can handle much more weight than polyolefin.
- Impact Resistance – Both have excellent impact resistance, but in some circumstances, cast iron might fracture.
- Chemical Resistance – Polyolefin casters are impervious to almost everything; semi-steel casters might rust or corrode if exposed to chemicals and moisture.
- Floor Protection – Polyolefin casters are non-marking and much gentler on floors than semi-steel.
- Chemical Wash-downs – Because of their chemical resistance, polyolefin casters are the clear winners here.
- Noise – Polyolefin casters will be quieter than those made from semi-steel.
Call Douglas Equipment!
We’ve barely begun to describe the difference between polyolefin and semi-steel casters. If you call Douglas Equipment’s customer service department, our experts can recommend specific caster products to meet your needs. Please call 800-451-0030 to learn more. If you’re in Miami, we are at 305-888-3700. You can also contact us online. We look forward to speaking with you soon!
