UL 94 Test Standards for Plastics

Innovative Polymers’ product line includes flame-retardant polyurethanes that are specially formulated for use in manufacturing medical devices, electrical/electronic housings and components for items such as appliances, and automotive parts.  

To ensure a plastic’s ability to perform in these types of applications, material samples are typically tested according to the Underwriters' Laboratories (UL) Standard UL 94 Tests for Flammability of Plastic Materials for Parts in Devices and Appliances. UL has assigned 12 different flame classifications to materials based on the results of lab-scale flame tests. The classifications, shown below, are used to distinguish a material's burning characteristics after test specimens have been exposed to a specified test flame under controlled laboratory conditions.

• Six of the classifications relate to materials commonly used in manufacturing enclosures, structural parts and insulators found in consumer electronic products (5VA, 5VB, V-0, V-1, V-2, HB).  Innovative Polymers casting polyurethanes fit in this grouping.  
• Three of the remaining six classifications relate to low-density foam materials commonly used in fabricating speaker grills and sound-deadening material (HF-1, HF-2, HBF).
• The last three classifications are assigned to very thin films, generally not capable of supporting themselves in a horizontal position (VTM-0, VTM-1, VTM-2). These are usually assigned to substrates on flexible printed circuit boards.

 

Horizontal versus vertical positioning

To test a plastic material, molded specimens are oriented in either a horizontal or vertical position and subjected to a defined flame ignition source for a specified time period. In the horizontal burn (HB) tests, the test flame is only applied to the plastic sample once.

Samples during vertical burn test. The three vertical ratings, V-2, V-1 and V-0 indicate that the material was tested in a vertical position and self-extinguished within a specified time after the ignition source was removed. To comply with the 20-mm UL 94 V-O vertical burn test, each of five specimens must extinguish in less than 10 seconds.  A second flame is then applied, the total flaming and glowing time of the sample must be less than 30 seconds.  Total flaming time for all five samples must be less than 50 seconds.  In addition, to attain the UL 94 V-O rating, specimens can not burn through their entire link and cannot drip flaming particles onto a cotton indicator located below the sample.

For UL 94 5VA and 5VB testing, plastic samples are exposed to the flame ignition source as many as five times to determine the polymer’s suitability for use in severe conditions.  These small-scale tests measure the propensity of a material to extinguish or spread flames once it becomes ignited.

Processes

Cartridge Dispensing

Rapid prototyping is neat, fast and cost effective with cartridge dispensed polyurethanes. Low viscosity systems formulated with 1:1 and 2:1 mix ratios are particularly well suited for easy cartridge filling as well as mixing, dispensing and application from disposable static mix nozzles. Read more...

Hand and Mass Casting

Easy to mix Shore A and Shore D hardness polyurethane systems are widely used for hand pouring and mass casting to produce thin walled prototypes as well as parts up to four inches thick. Read more...

Machine Dispensing

Rapid casting polyurethanes that gel in less than 60 seconds at room temperature are formulated for automated meter mix dispensing equipment that quickly injects materials into the mold. Read more...

Pressure Casting

To ensure complete mold filling for thin walled and other difficult to inject parts, mixed polyurethane resin and hardener are de-aerated, poured into a mold and then cured in a pressure pot. Read more...

Rotomolding

Rotational molding or rotocasting is an economical method for producing hollow parts. Thermosetting polyurethanes are ideal for rotomolding rigid shells used for seating, fluid containers and theme park display fascia. Read more...

Vacuum Assisted Casting

Polyurethanes are often injected into a mold under a vacuum to eliminate voids for applications such as medical equipment prototypes, medical training models, clear lenses, furniture and automotive components. Read more...

Vacuum Assisted Casting

Polyurethanes are often vacuum cast to ensure void free parts. The process calls for a mold to be placed in a vacuum chamber and the mixed resin system injected under a vacuum of 20 to 25 inches of mercury until the mold is completely filled. The vacuum is then released and the part is allowed to cure. Vacuum cast polyurethanes are ideal for prototyping medical equipment and models, clear lenses, furniture and automotive components.

Polyurethanes designed for use in vacuum casting have the following properties:

  • Low viscosity
  • Shore A hardness systems exhibit great elongation and tear strength for producing flexible gaskets, seals and fixtures and for overmolding rigid plastics
  • Shore D hardness products include white, clear, translucent and tan systems that are colorable and feature properties such as flame retardance, heat resistance, high flexural modulus and high impact strength

Pressure Casting

To ensure complete mold filling for thin-walled and other difficult to inject parts, mixed polyurethane resin and hardener are de-aerated. Next, the filled mold is placed in a pressure pot at 90-120 psi and 77ºF (25ºC) until fully cured. By curing the resin system under pressure, any bubbles that may form are compressed to a microscopic size that is visually undetectable.

Polyurethanes are well suited for pressure casting with a combination of such properties as:

  • Easy handling with many systems that feature 1:1 or 2:1 resin to hardener mix ratios
  • Gel times as long as 25 minutes at room temperature to accommodate the degassing, injection and installation of the mold in the pressure pot before the system begins to gel
  • Formulations that accommodate pressure casting of thick walled as well as thin walled components

Cartridge Dispensing

Rapid prototyping is neat, fast and cost-effective with cartridge-dispensed polyurethanes. The 400 ml cartridges can be filled with premeasured amounts of resin and hardener in either a 2:1 or 1:1 ratio. Filled cartridges are then ready to be installed in a DS-400 pneumatic dispensing gun along with a static mixing nozzle that ensures thorough system blending. When the gun handle is depressed, resin and hardener are mixed in the disposable nozzle and can be dispensed into the mold.

Polyurethanes formulated with convenient 1:1 and 2:1 mix ratios can be used with cartridges. Low viscosities facilitate filling, mixing, dispensing and application.

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