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April 22, 1999

7 Min Read
Primer for molding flame-retardant plastics

Editor's note: Many plastics applications, particularly in the electrical and electronics industries, require a material classified as flame retardant. FR compounds contain additives designed to improve their flammability characteristics. Jim Johnson, senior technical service engineer for compounder Lati USA (Mt. Pleasant, SC), tells IMM the additives also influence the molding process. To consistently produce high-quality FR parts, Johnson offers the following guidelines, which include a basic understanding of the additives and how they affect molding.

Flame retardants, as most molders know, are additives that help a plastic compound resist burning. In addition, other compounds known as synergists are often used. The synergists work by improving the efficiency of the FR additives.

Flame-retardant additives are typically classified as either halogenated or nonhalogenated. Halogenated flame retardants are compounds containing either chlorine or bromine, and nonhalogenated additives are based on red phosphorus, nitrogen, or metal salts. Each of these additive systems has advantages and disadvantages, but they both will have an influence on the processability of the plastic compound.

While the lower thermal stability of flame-retardant plastic compounds should be something to note, it shouldn't scare you away. It only means you need to be aware of a few important points, which, if followed, will help guarantee success. Generally speaking, the chlorinated flame-retardant systems tend to have the poorest thermal stability and the brominated systems the best. Red phosphorus and metal salt flame-retardant systems also have very good thermal stability. The better the thermal stability, the less likelihood they will be degraded during processing.


Drying.Drying is more critical for flame-retardant compounds because of the reactions that can occur during processing. These compounds should always be dried according to the supplier's recommendations. Flame-retardant additives in plastics can react with moisture in the barrel of the injection molding machine to cause out-gassing, which can result in problems such as excessive mold deposits or plugged vents. Excess moisture can also react with the additives to form acids that can be corrosive to process equipment and tooling. Vented barrel injection molding machines are not a substitute for proper drying of materials and are not recommended for processing flame-retardant compounds.


Temperatures and Settings.Because many FR additives have low thermal stability, melt temperature control is critical. It is preferable to set the barrel temperatures on the low side of the recommended temperature range to minimize the possibility of thermal degradation. While this may reduce flowability of the compound, increasing mold temperature can compensate for it.

One should also pay close attention to other variables that could increase melt temperature, such as backpressure, screw speed, injection speed, and shot size. Backpressure should be set as low as possible, typically between 25 and 50 psi. Screw speed should be adjusted so screw recovery is complete within 2 to 3 seconds of mold opening. Screw speeds of 60 to 100 rpm are typical. Fast injection speed can also cause excessive shear heating, which could lead to thermal degradation of the material or higher-than-normal mold deposits.

Proper sizing of the injection molding machine is also very important. The general rule of thumb when molding flame-retardant compounds is there should be no more than 2 to 3 shots in the barrel. If the barrel capacity is more than this, barrel temperature setting should be reduced to minimize the possibility of overheating of the material.

Another area often overlooked is the cushion size. The cushion should be set to approximately 1/8 to 1/4 inch. Anything larger than this allows the material to sit and absorb additional heat, which could lead to thermal degradation.


Hot Runner Systems.Hot runner systems have gained in popularity because they allow for the elimination of waste sprues and runners, so they eliminate the problems associated with generating and using regrind. However, many flame-retardant compounds are not recommended for use with hot runners. Again, the additional residence time and higher temperatures, which are often necessary with hot runners, can quickly lead to thermal degradation. This is especially true with heat sensitive materials such as PET, PBT, and polycarbonate, which can also suffer from hydrolytic degradation if processed with high moisture content.

There are FR compounds available that offer improved thermal stability for use with hot runner systems. Make sure to inform your material supplier if you plan to use hot runners for processing flame-retardant compounds.

If you must use hot runners, ensure that they are streamlined and contain no dead spots where material could hang up and degrade. It is also preferable to use externally, rather than internally, heated systems. Externally heated systems provide more uniform heating and are less likely to cause degradation to the material due to hot spots. If a hot runner system has multiple drops, it is also an advantage for each drop to have its own individual temperature control. In this way, the temperature of each drop can be more precisely controlled and adjusted.


Molding Machines.While flame-retardant compounds can be processed using standard molding machines, there are a few things to note that could help minimize any unexpected problems or machine downtime.

Because of the corrosive nature of some FR additives, you may want to take a little extra time in selecting the proper screw and barrel material, particularly if the machine will be dedicated to running only flame-retardant compounds. Screws and barrels produced from nickel or cobalt-based materials claim to offer excellent resistance to corrosion; however, they tend to be very expensive. In many cases, stainless steel works well at a much lower cost. Chrome or nickel plating is sometimes used, but is not as effective in combating corrosion, especially if the corrosion is caused by the formation of acids caused by degradation of the plastic compound.

It is also a good habit to purge the injection molding machine if it will be shut down for any length of time. This will ensure that the compound will not degrade in the barrel, where it could cause additional corrosion.


Tooling.The issue of corrosion also pertains to the molds used to produce parts. Depending on the quantity of parts to be produced over the life of the tool, it may be advantageous to build the tool from a more corrosion-resistant material. Stainless steel is normally the material of choice, especially for long production runs. However, because of its lower thermal conductivity, cycle times with stainless steel molds may be longer than with an alloy steel mold. Chrome plating and titanium nitride coatings have also been used successfully to reduce corrosion and prolong tool life.

Molds used for processing flame-retardant compounds should not contain components produced with beryllium copper. Beryllium copper may react with certain flame-retardant additives, which could cause discoloration of the molded parts or lead to excessive corrosion in longer production runs.

Good venting in the tool not only helps minimize mold deposits but can help reduce corrosion by allowing corrosive gases to escape from the tool. Placing vents in slug wells at the intersection of runners also helps to vent gases before they reach the mold cavity. Ensuring that runners and gates are adequately sized can help minimize outgassing or degradation due to shear, which, in turn, reduces mold deposits and corrosion.


Safety.Processing compounds that are flame retardant pose no significant additional dangers compared to nonflame-retardant compounds. Utilize good safety practices, which should include allowing for adequate ventilation, especially during purging. Purging the melt into a bucket of water also helps reduce the amount of vapors released into the air. Gloves and long-sleeved shirts should be worn at all times to avoid the possibility of being burned by hot plastic. Degraded material should be purged from the barrel as soon as possible to avoid a possible buildup of pressure in the barrel of the injection molding machine.

Hot tips for FR compounds

  • Ensure that the compound is dry before processing.

  • Keep the melt temperature and residence time to a minimum.

  • Consider using corrosion-resistant materials or coatings for processing equipment (screws and barrels) and tooling.

  • Size gates and runners to minimize shear degradation.

  • In general, avoid hot runners and beryllium copper alloys.

  • Always use proper ventilation when purging or processing.

Contact Information
Lati USA Inc.
Mt. Pleasant, SC
Jim Johnson
Phone: (843) 856-8219
Fax: (843) 856-8277

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