Wednesday 1 March 2023

Long Fiber Plastic Processing - How to Not Make Long Fibers Short (Rule of Thumb Series)

Hello and welcome to this blog post which is part of our Rule of Thumb series. Today we discuss the processing of long glass fiber plastic compounds using injection moulding. 

Long glass fiber (LFT) plastics 

LFT compounds have a fiber length between 9 mm and 12 mm and most are produced via pultrusion technique. Aim in processing is to keep the fiber length as long as possible since it is needed to build up a 3D-network consisting out of entanglements. Having an entangled system allows for high impact and strength properties as well as increased thermal performance of the final part. 

How to not make Long Fibers short - processing recommendations

In general we have to avoid excessive fiber breakage during all phases of injection moulding in order to have a long enough fiber length to form an entangled three dimensional network. 

1. Injection moulding setup: the feed hopper must be large with a slope angle of 45°; the hopper flanges do not need to be cooled and it is enough to have the same temperature as the feeding section of the barrel. A general three section screw can be used (screw diameter above 35 mm), however if possible use a longer feeding zone (> 60% of total length; compression and metering section 20 % length). Regarding the compression ratio, 2:1 or 2.5:1 is recommended. Avoid the use of mixing and shear elements along the screw sections. There are long-fiber screws available and they dramatically improve fiber length. Figure 1 compares the fiber length reduction over the different screw sections of a long-fiber screw and a standard 3 section screw. 

Figure 1: Glass fiber length reduction of a long-fiber screw and a standard screw [3]

2. Processing temperatures: in terms of processing temperature it is advantageous to have a temperature above the melting temperature before the compression section, together with the avoidance of shear heating. Avoid shut-off valves since they induce fiber breakage. Reduction of shear can be achieved by using a flat or reverse barrel temperature profile. Aim to ease melting in the compressing section. 

3. Plasticizing unit: keep the screw speed rotation in a low to medium range (< 100 RPM) together with low back pressure (0 to 10 bars). 

4. Injection moulds: reduction of glass fiber breaking is achieved by using large gate sizes (>80% of wall thickness). Proper venting of the tool will support weld line quality of the final part. Hot runner system can be used, however shut-off valves are not recommended.

5. Recycling: using post industrial waste or post consumer waste is possible up to 10 weight %. 

Additionally, most LFT parts I have seen had a fiber length between 2-3.5 mm and it was efficient enough to have 1.5 mm in average length for good strength properties. With 1.5 mm already ~80% of the strength was reached and after that it leveled off into a plateau with increased fiber length.

Key properties of LFT compounds can be found here and more Rule of Thumb posts here.

Thanks for reading and #findoutaboutplastics


Herwig Juster

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[3] K. Kikikawa History of development of injection molding machine technologies and future perspectives


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