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Understanding Physical Properties of Rubbers and Plastics

Our mold making and casting catalogs are full of technical specifications. These are usually in large tables, like this one:

Graph of the physical properties of the EasyFlo series (liquid plastics)

This information can come in handy for those who don’t have much knowledge of rubbers and plastics. That is why we want to clearly describe each specification.

Mix Ratio

Most Schouten-SynTec products consist of 2 components: component A and component B. The mixing ratio indicates how much you need of both components to obtain this product. When these two parts are mixed in the right proportion, they harden at room temperature. It is important to use the correct mixing ratio so that no mix is left that has not been mixed properly.

Mix ratios vary from product to product. This differs even within the same series. In some cases you will notice that there are two mixing ratios listed: one by weight and one by volume.

Shore hardness

The hardness of our rubbers and plastics is measured on Shore hardness. The device used for this is called a durometer.

Rubbers are measured using the Shore OO Scale and a scale. Very hard rubbers and plastics are measured using the Shore D scale. The higher the number, the harder the material.

Pour time

The pour time is in minutes and is the time it takes, after mixing part A and part B, for the mixture to harden to a point where it is no longer liquid. Also, the product must be mixed and poured into a mold within half of the casting time.

Demold time

The demoulding time is stated per minute or hour. The demoulding time is the number of minutes or hours you have to wait before removing your casting. This does not mean that the casting is ready for use at this point. Demoulding too quickly or using the mold will deform the mold.

Specific weight

Density indicates how much higher or lower the density of the product is compared to water. Water has a density of 1.00 g/cc. Products with a specific gravity greater than 1 are denser than water; products with a density of less than 1 are less dense than water.

Curing Color

Cure colors are the colors to which a product naturally discolours. Some products are transparent, others opaque. It often happens that the curing colors of a product do not always have the same color. For example, the 75 series polyurethane rubber can have a transparent color first and a different color 6 months later. Part B darkens with age resulting in a dark curing color.

Cure colors can be changed using PolyColor paint (for polyurethane products) and Silicolor Silicone pigment (for silicone products).

Mix Viscosity

Viscosity is indicated in CentaPoise (cP) and is the resistance of a flowing substance. Example: water has a lower viscosity than syrup and therefore flows more easily.

In this case, the mix viscosity is the viscosity of Part A and Part B when mixed. The EasyFlo series has one of the lowest viscosity of all Polytek products

Here are some examples of the viscosity of common materials:

  • Water: 1 cP
  • SAE 30 Engine Oil: 500 cP
  • Honey: 10,000 cP
  • Sour Cream: 100,000 cP
  • Peanut butter: 250.00 cP

Specific volume

This specification identifies the number of cubic inches occupied by one pound of material (in³/lb). This number is important to determine how much rubber or plastic is needed to fill the empty spaces.

Maximum exothermic

These specifications are generally only listed for SynTec polyurethane and epoxy resins. It indicates the maximum amount of heat generated during curing. It will also state how much sample to use to determine this number.

Other specifications

Not every product has the same specification list and technical bulletins. If you have any questions about the information on our website, our catalog or our technical & physical bulletins, please feel free to contact us.

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