The Society of the Plastics Industry (SPI) established a classification system in 1988 to allow consumers and recyclers to identify different types of plastic. Manufacturers place an SPI code, or number, on each plastic product, usually molded into the bottom. This guide provides a basic outline of the different plastic types associated with each code number.
Thermoplastics Curing Process
Thermoplastics pellets soften when heated and become more fluid as additional heat is applied. The curing process is completely reversible as no chemical bonding takes place. This characteristic allows thermoplastics to be remolded and recycled without negatively affecting the material’s physical properties.
Features & Benefits
There are multiple thermoplastic resins that offer various performance benefits, but most materials commonly offer high strength, shrink-resistance and easy bendability. Depending on the resin, thermoplastics can serve low-stress applications such as plastic bags or high-stress mechanical parts.
Hard crystalline or rubbery surface options
Generally more expensive than thermoset
Can melt if heated
Thermoset Curing Process
Thermoset plastics contain polymers that cross-link together during the curing process to form an irreversible chemical bond. The cross-linking process eliminates the risk of the product remelting when heat is applied, making thermosets ideal for high-heat applications such as electronics and appliances.
Features & Benefits
Thermoset plastics significantly improve the material’s mechanical properties, providing enhances chemical resistance, heat resistance and structural integrity. Thermoset plastics are often used for sealed products due to their resistance to deformation.
More resistant to high temperatures than thermoplastics
Highly flexible design
Thick to thin wall capabilities
Excellent aesthetic appearance
High levels of dimensional stability
Cannot be recycled
More difficult to surface finish
Cannot be remolded or reshaped
SHORE HARDNESS SCALES
Durometer Shore Hardness Scale
There are different Shore Hardness scales for measuring the hardness of different materials. These scales were invented so that people can discuss these materials and have a common point of reference. The Shore A00 Scale measures rubbers and gels that are very soft.
The Shore A Hardness Scale measures the hardness of flexible mold rubbers that range in hardness from very soft and flexible, to medium and somewhat flexible, to hard with almost no flexibility at all. Semi-rigid plastics can also be measured on the high end of the Shore A Scale.
As you can see from the chart, there is overlap on the different scales. For example, a material with a Shore hardness of 95A is also a Shore 45D.
Shore D Hardness Scale measures the hardness of hard rubbers, semi-rigid plastics, and hard plastics.
How Do You Measure A Material's Hardness?
The different Shore Hardness scales measure the resistance of a material to indentation. A ‘Shore Hardness' gauge (looks like a round tire pressure gauge) has a needle on a spring protruding from one end. The needle is placed against the rubber or plastic and pressure is applied.
Once the gauge is pressed firmly against the material and the needle has penetrated as far as it can go, the measurement needle will indicate the corresponding hardness measurement.
Shore Hardness is an important factor when considering which mold rubber you should choose
The consideration is "how easily will you be able to extract your original model (and subsequent castings) from the rubber mold once it is cured".
For example, you would not choose a rubber with a durometer of 70A to make a mold of a plaster standing ballerina with thin, protruding arms bent at opposing angles. A 70A rubber (as hard as a car tire) is stiff and would not offer enough flexibility to extract this model without breaking it.
A better choice would be a Shore 30A rubber or softer that would offer enough flexibility to easily extract the model.
Under what circumstances would you use a Shore 70A rubber? A common application we encounter from the world of concrete casting would be making molds of flat original models (paving stones, for example). This model configuration does not require much flex from the rubber mold to extract the original or subsequent castings. Using a 70A urethane rubber also gives you tremendous abrasion resistance and therefore the longest mold life for this application.