Polyurethane Caster Wheels Material Guide

Polyurethane Caster Wheels – The Ultimate Material

Polyurethane elastomers are a versatile synthetic polymer, invented back in the 1930’s by Dr. Otto Bayer.  Today over 12 Million metric tonnes are used worldwide annually and their application is broad, being used in such items as Polyurethane Caster Wheels, Automotive Bushes, Fork Lift Wheels, Foams, Shoe Soles, Coatings and Adhesives.

All Polyurethanes are manufactured from the same basic elements.  The elements Isocyanate and Polyol (Alcohol) are mixed in a chemical reaction to create the raw polyurethane material.  The chemistry behind the manufacture is relatively complex and there are many different grades available.

There are two types of Polyol which can be used to manufacture Polyurethane Material for Casters, Polyether Polypol and Polyester Polypol.  Polyether Polypol is generally cheaper and has better water resistance.

Polyurethane Caster Wheels hardness can be measured using both the “Shore A” scale and “Shore D” scale.  Shore A is generally used for the softer polyurethanes, whilst Shore D is generally used for the harder materials.  Generally Polyurethane Casters are manufactured towards the harder end of the scale, and hence the Shore D scale is commonly used. 30-50 Shore D is typical for Polyurethane Casters.  To give a comparison, 90sh A is equivalent to 41sh D.

From all the many grades of Polyurethane grades available, they all All fall into the following two categories:

  • Thermoplastic Polyurethane (Abbreviated to “TPU”) –  Thermoplastic Polyurethane generally comes in Pellet Form.  As the Pellets are heated, they become fluid and can be moulded or formed into the required shape.  This process does not change the molecular state of the Polyurethane, and therefore the material will melt if reheated and is therefore suitable for recycling.

 

  • Thermoset Polyurethane (Abbreviated to “PUR”) – Thermoset Polyurethane typically comes in liquid form, which does not need to be heated. The Polyurethane Liquid is mixed with a curative and any additives to give required properties (such as hardness or increased abrasion resistant).  Thermoset Polyurethane is commonly known as a “two part” material, where the two separate liquids are mixed together, which then set (cure).  The curing process causes a chemical reaction where the molecular structure changes to a permanent state.  The Product will cure at room temperature.  Once formed, Thermoset Polyurethane will not melt if heated and cannot be reformed or recycled.

 

Polyurethane Products, Such as Polyurethane Caster Wheels can be manufactured using a range of different techniques, including

Open Moulding –  This is where the two part polyurethane is mixed to give a uniform mixture.  The mixture is then de-gassed (either by using heat or vacuum) and then poured into an open mould.  The mixture sets relatively quickly, and it is therefore important that the process is conducted as quickly as possible and that the material is mixed in small batches to avoid wastage.

Machine Casting – This is almost identical to the Open Moulding technique, however the whole process is conducting in a Casting Machine, saving time and providing a more consistent final product.

Spraying & Coating –  This is a simple process where the polyurethane is sprayed onto a component to provide a coating of Polyurethane

Injection Moulding – Generally only used for Thermoplastic materials, the Polyurethane Pellets are fed into a reciprocating injection moulding machine.  The reciprocating screw is heated which in turn melts the Polyurethane, whilst at the same time feeding the liquid material through the machine and injected into a mould tool.  The material is then left to cool and then removed from the mould.

Machining –  Most Polyurethane Grades can be machined using conventional methods, such as Milling, Turning, Drilling and Grinding, however special techniques are required for softer materials, typically below 50 Shore D.  One Technique for softer materials is to freeze the material before machining to increase its hardness.  It is possible to machine to relatively high tolerances when using hard polyurethane, in the region of +/-0.1mm, which can be further improved by using a grinding process.

 

Polyurethane Caster Wheels       

Polyurethane is one of the most commonly used materials within the Caster Wheel Industry.  It provides many of the characteristics of flexible rubber castors, such as Resilience, High Grip, Shock and Vibration Absorption, whilst also having many of the characteristics of hard Plastic or Nylon Castors such as Extreme Wear Resistance, High Load Capacity and Resistance to many Oils, Solvents and Cleaning Fluids.

Polyurethane Casters can be manufactured from both Thermoplastic and Thermoset Polyurethanes, however most commonly the Thermoset materials are used due to their increased properties.  The molecular changes during curing provide increased strength, wear resistance and material stability under heat.

Polyurethane can be chemically or mechanical bonded to many materials such as Nylon, Polypropylene, Steel and Aluminium.  This allows for a Polyurethane tread to be applied to a vast range of materials.  The thickness of the tread will alter the wheels properties.  A Think 5mm Coating will offer an extremely high load capacity, whereas a 20mm thickness tread will provide a high shock and impact absorption.

Properties of Polyurethane Caster Wheels can include:

  • Non-Marking Wheel protects Floors from Marking & Scratches
  • Resilient Wheel Tread provides Shock, Impact and Shock Absorption
  • Resistant to many Oils, Solvents, Greases and Cleaning Fluids
  • Resistant to Water upto 50 deg C
  • Wear Resistance similar to Nylon and Steel
  • Excellent Electrical Resistivity
  • Low Noise Operation – Excellent for noise sensitive environments such as Offices, Schools and Hospitals
  • High Load Capacity
  • Available in a range of hardness’ – allowing increased load capacity or higher grip and traction
  • Available in almost an Colour providing pleasing aesthetic appearance (Red, Yellow and Orange are most common).
  • Immune to Ozone and Oxygen, meaning the wheels will not crack or degrade if near sources of Ozone, such as near electrical equipment.
  • Some Grades are Mould, Mildew and Fungus resistant, making ideal Caster Wheels for Food and Hospital environments.
  • Weather Resistant, ideal for permanent outdoor use.

 

Bulldog Castors Ltd

Here at Bulldog Castors™ we provide ourselves on providing first class engineering support and solutions.  We employ Mechanical and Polymer engineers to ensure that we offer the best solutions for our customers.  In addition we have a range of Testing & Analysis facilities, at our Leicester based facilities, including the following:

  • Load Testing (up to 2.5 Tonnes In House)
  • Creep and Permanent Set Testing
  • Hysteresis & Damping (Energy Absorption)
  • Metal Hardness Testing (Steel, Brass, Cast Iron)
  • Specific Gravity (Material Density)
  • Material Analysis (Rubber Type)
  • Rubber Hardness Testing (Shore A & Shore D)
  • Ultra Sonic Thickness Gauge (Ferrous and Non Ferrous)

We conduct testing to both ISO & ASTM standards, in addition to our own “Industry Specific” Testing Standard.  Our Casters are inspected in accordance to BS6000 for various mechanical properties to ensure a High Quality, Consistent products is delivered to the customer.

Our Team of Expert Engineers are available to discuss your project, no matter how large or small.  Contact them today on +44 (0)116 2970521 or sales@bulldogcastors.co.uk

To find more information and Buy Castors Online, visit our Polyurethane Caster Wheels section at https://www.bulldogcastors.co.uk/polyurethane-castors

Disclaimer: The above information is provide for guidance only.  Whilst the above information is a excellent guide for characteristics and performance, there are many grades of Polyurethane Caster Wheels available, therefore it is possible that the above comments may not be apply in all instances.  Customers should contact us to discuss the specific application and product to be used.

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