Hello and welcome to a new blog post. Today we discuss the injection molding of polymers with flame retardant (FR). In general, flame retardants are added to lower the flammability of the used polymer. There are three main classes of flame retardants: nitrogen-phosphorus systems, halogenated systems, and metal-hydroxide systems. They have different working mechanisms (chemical reactions; building up a protective layer; release water for cooling) and Table 1 summarizes the advantages and disadvantages of the different flame retardant systems.
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| Table 1: Comparison of the advantages and disadvantages of the different flame retardant systems |
Furthermore, the standard ISO 1043-4 shows detailed classifications of different flame retardants.
High performance polymers which have a higher level of aromatic elements in their backbone need few or no additional flame retardant at all. They reach inherently a UL94 V-0 flammability rating with the exceptions are PSU and semi-aromatic Nylons such as PPA and PARA which need additional flame retardants to reach V-0.
Selecting the optimal flame retardant plastic grade during material selection is one thing, processing them by injection molding is another thing, especially when you use hot runner systems.
It is important to keep the melt processing temperature below the decomposition temperature of the flame retardant. FR-based systems are lower processed than the base polymer used. Following the processing recommendations of the material supplier is important. Also to not increase unnecessarily the melt temperature through too much shearing.
10 Important things to Consider for injection molding of FR-plastics
1. Usage of corrosion protected plasticizing unit, hot-runner and tool: some flame retardants are corrosive since they generate acids while reacting with water. Therefore, Polyamides should have a residual moisture of below 0.2%.
2. Limitations in residence time and melt temperature: FR-plastics have a 30-40% lower residence time compared to non-FR plastics. For example, a Polyamide with 25 weight-% glass fiber (Zytel 70G25) hs a residence time of 15 minutes, where else the FR-Polyamide with 25 weight-% glass fiber (Zytel FR 70G25 V0) has a residence time of 10 minutes. Furthermore, the preferred melt temperature shall be around 15°C lower compared to the corresponding non-FR reinforced grade.
3. Design of tool and hot-runner: usage of naturally balanced hot-runner channels (flow length from injection nozzle to injection point)
4. Design of tool and hot-runner: usage of thermal separation is important (hot-runner with melt temperature level and tool is cooled).
5. Design of tool and hot-runner: heating should be casted in and not only be inlaid. This ensures a better heat transfer.
6. Design of tool and hot-runner: enable a uniform temperature system of the hot runner by symmetric heating
7. Design of tool and hot-runner: prevent dead corners in the hot runner by using separator inserts (redirection of melt).
8. Design of tool and hot-runner: make the hot runner nozzles interchangeable in case wear is damaging the nozzles.
9. Design of tool and hot-runner: consider good thermal separation in the nozzle area. Otherwise local high temperatures lead to a reaction of the flame retardant.
10. Design of tool and hot-runner: usage of temperature sensors with PID capability (photoionization detector) to optimally control temperatures of each runner nozzle.
Conclusions
When using a FR-plastic grade, keep your processing temperature as low as possible. Prevent too much shearing over the complete processing length (plasticizing unit, hot runner and tool) and reduce local overheating of the material. Also keep residence time to minimum and always keep in mind that the flame retardant can start reaction during processing.
What are your experiences with flame retardant plastics - let me know in the comment section.
Thanks for reading and #findoutaboutplastics
Greetings
Herwig
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Literature:
[1] DuPont - Serie Kunststoffpraxis Teil 8: Kunststoffe mit Flammschutz, 2003
[2] Erwin Baur, Dietmar Drummer, Tim A. Osswald, Natalie Rudolph: Saechtling Kunststoff-Handbuch, Hanser Verlag
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