There are several scales of durometer, used for materials with different properties. The two most common scales, using slightly different measurement systems, are the ASTM D2240 type A and type D scales.

The A scale is for softer ones, while the D scale is for harder ones.

However, the ASTM D2240-00 testing standard calls for a total of 12 scales, depending on the intended use: types A, B, C, D, DO, E, M, O, OO, OOO, OOO-S, and R. Each scale results in a value between 0 and 100, with higher values indicating a harder material.

Durometer, like many other hardness tests, measures the depth of an indentation in the material created by a given force on a standardized presser foot. This depth is dependent on the hardness of the material, its viscoelastic properties, the shape of the presser foot, and the duration of the test. ASTM D2240 durometers allows for a measurement of the initial hardness, or the indentation hardness after a given period of time. The basic test requires applying the force in a consistent manner, without shock, and measuring the hardness (depth of the indentation). If a timed hardness is desired, force is applied for the required time and then read. The material under test should be a minimum of 6 mm (0.25 inches) thick.

A. Thermosets products begin as liquids and solidify or “polymerize” via the reaction of two or more components. The resultant product is completely different than any of the individual components (A + B = C). Thermosets cannot be melted or liquified back to their original states, which is an advantage in certain applications. Thermosets are either hand-mixed and poured or shot through a static mixer using meter-mix dispensing equipment. They are usually associated with Liquid Molding™, compression molding, or roto-molding.

Thermoplastics begin as solid materials, and can be liquified and molded into particular shapes by heating to elevated temperatures. They solidify when cooled. These materials are associated with processes like injection molding, thermoforming, and vacuum forming. The melted thermoplastic is injected into a mold or formed around a pattern, where it is then cooled down, solidified, then ejected out of the mold. Unlike thermosets, they can be recycled, re-melted, and re-used.

A. Vacuum de-gassing expands the air trapped during mixing or pouring, causing the bubbles to grow, rise to the surface, and in most cases, release. After a period of time the amount of trapped air decreases. The material’s viscosity and surface tension will determine how easily the air will escape. Certain materials appear to bubble indefinitely until the vacuum pump is turned off. In order to maximize the vacuum’s potential for air removal, the pump must be capable of pulling 29.6 inHg. When placed under pressure, any air bubbles entrapped from the mixing and pouring process shrink to the point where they are no longer visible. Pressure ranging from 60-80 psi significantly reduces the chances of visible air bubbles. For pressure to be effective, the liquid thermoset material must remain under pressure until it has reached its gel time, otherwise the bubbles may expand once the pressure is relieved.

Q. Is one method preferred over the other?

A. One method really isn’t preferred over the other. Whether you choose to use vacuum or pressure depends on the application, your capabilites, and budget. In fact, we often recommend vacuum degassing your product after hand mixing, pouring into the mold, and then using pressure to make sure all parts of the mold are filled. When using dispensing equipment, there is no air introduced during mixing. That doesn’t mean air cannot be introduced while shooting into your mold, especially one with many thin walls, sharp corners, or intricate details.

If you find yourself in a situation where you absolutely need a bubble free part (e.g., using a plaster mold or a large quantity of expensive material) it’s best to play it safe and employ both methods. If neither option is in your budget and you need a void free surface, we recommend using low viscosity materials, mixing slowly and thoroughly, and brushing a thin coat onto the mold or pattern’s surface. Using a hair dryer while brushing the thin coat will help to ease surface tension and reduce bubbles.

A. Temperature has a marked effect on thermoset polymers. (Epoxies, Polyurethanes, Polymer Alloy Hybrids)

1. Cold thickens or raises viscosity and slows down the reaction or cross-linking.

2. Hot thins the viscosity and speeds up the reaction.

A general rule of thumb is for every 10°C (18°F) the factor is 2. I.E. start point 25°C and the temperature raises to 35°C the speed is twice as fast and the gel is ½. If we raise the temperature to 45°C the cure rate is 2 x 2 or 4 times as fast and the gel is 2 x 2 or ¼. The inverse is true for cold.

The general rule of thumb is useful but not always exact. Other factors also affect the polymer; humidity, mass, etc.

It is very important to control the storage temperature. 18-27°C is preferable, 22°C is the best. It is also important to control the temperature of the mold, resin, etc. They should be kept at 22-25°C for at least 12 hours before use.

1. Never store materials or molds on the floor. There can be a 5-7°C difference in temperature versus table height.

2. Molds should be at table height or higher.

3. It takes about 12 hours for a Silicone or Hapflex mold to change temperature throughout. They are thermal insulators; therefore, slow to change temperature. Tool-It or Hapcast are thermally conductive; therefore, molds will change temperature in 3 – 6 hours.

4. Certain Part-A and Part-B’s may crystallize when they come in contact with cold. This phenomenon will occur above 32°F. Crystallization may look like a slight milky color, mild crystals to a solid, depending on the degree. When this occurs the crystallization must be reversed with heat. (See product information). If not reversed, improper cure, striations, and uncured areas may occur.

Be aware of the temperature!

A. Post curing is the process of exposing a part or mold to elevated temperatures to speed up the curing process and to maximize some of the material’s physical properties. This is usally done after the material has cured at room temperature for at least 12 hours. In general, thermoset materials will achieve full cure over a period of 7-10 days. After a full cure is achieved at room temperature, post curing will have no effect on the material’s properties.

Post curing will expedite the cross-linking process and properly align the polymer’s molecules. Much like tempering steel, post curing thermosets can increase physical properties (e.g., tensile strength, flexural strength, and heat distortion temperature) above what the material would normally achieve at room temperature. Generally speaking, rigid materials are post cured at 175F for 8-24 hours and flexible materials at 140F for 8-24 hrs. Be aware that post cure temperatures vary for different materials.

An oven is best for applying uniform heating; however, we recommend not using the one in your kitchen. Applying too much heat to some materials may result in dangerous fumes being emitted or a material may melt, ruining your oven. Digital, vented lab ovens are ideal for post curing parts and molds; however, this can also be done in a Molding Chamber such as the 'X-Series' by Hapco Inc. During elevated temperatures, thin-walled parts may bubble or deform. Keeping them in the mold or using a fixture during post cure is recommended. Make sure that anything you place in an oven can take the heat.

A. The simple answer is no, as the FDA has never approved uncured resins. The FDA requires the submission of the final product made up of all its components. It is the customer’s responsibility to have their final product submitted and approved from the FDA for its particular use. Hapco’s Filterbond and Steralloy products have been successfully submitted and approved many times, but the process of submission and approval must be followed by the customer. Hapco’s Steralloy and Filterbond product lines are manufactured with raw materials that, when processed according to Hapco’s guidelines, result in components that are potentially FDA approvable.

A. The Underwriters Laboratory(UL) is an independent product safety certification organization that evaluates products, components, materials and systems for compliance to specific requirements. When a datasheet indicates that an individual product is available in flame retardant(FR), it does not necessarily mean the product is certified by the Underwriters Laboratory. Our suppliers conduct FR testing according to the same procedures developed by UL to categorize a product as flame retardant. In this case the test method is known as 94 V. The letter “V” indicates that the sample is held vertically over the flame during the test. The last digit after the “V” is either a 0, 1, or 2 based on the results of the test.

V-0 Vertical Burn

Burning stops within 10 seconds after two applications of ten seconds each of a flame to a test bar. No flaming drips are allowed.

V-1 Vertical Burn

Burning stops within 60 seconds after two applications of ten seconds each of a flame to a test bar. No flaming drips are allowed.

V-2 Vertical Burn

Burning stops within 60 seconds after two applications of ten seconds each of a flame to a test bar. Flaming drips are allowed.

The following sequence depicts the 94 V test as it is conducted in the lab. With the test sample suspended vertically, the technician moves the flame directly under the sample for ten seconds and then removes it. This is repeated once more when the sample self extinguishes. The times are then recorded and averaged. Depending on the results, the material is given one of three ratings as explained in the chart above.

Although our suppliers have many products that will pass with a V-0 rating, those products have not necessarily been sent to UL for an official rating and cannot be marked with their logo. If you would like more information on our flame retardant materials, or for a list of products that have been certified and hold a yellow card from UL, please contact us.

A. Composites Warehouse Direct does not supply free liquid samples of any products; however, we do supply cured samples of most of our product lines. We are proud to offer them at no cost to the customer. Having the cured samples can give you a better idea of the differences in products and help in choosing a material based on color, stiffness, and machinability. If you need to test a material’s compatibility with your application, quart units are available for purchase.

If you would like to receive cured samples or literature on any of our products, please contact us our technical support staff will help you find the right product for your application.

NOTE: Cured samples have not been post cured and are not for testing purposes.