In medical for example, PEEK-OPTIMA™
carbon fibre-reinforced PEEK (CFR-PEEK) has attracted interest.
The material can offer the strength of metal in a composite polymer,
suitable for trauma plates and nails, while remaining lightweight.
With its favourable modulus of elasticity, it seems to provide the
potential for enhanced stress absorption and load-bearing by
allowing the design of a less stiff construct, which may support the
healing process. It has also been shown to have a 50 times greater
fatigue life than equivalent metal plates.*
Metal-free distal femur fracture fixation plate made with PEEK-OPTIMA Ultra-Reinforced polymer, a carbon fiber composite from Invibio; The trauma plate shown is not available for distribution or implantation - © Invibio
The influence of temperature control during injection moulding
Injection moulded PEEK is typically about 35% crystalline. Proper attention must be paid to the
temperature of the injection mould tool to ensure it is hot enough to produce parts of consistent
crystallinity (temperatures in the range 170 - 200°C are typically used in our laboratories): a
common mistake is to use mould temperatures which are too low and this can result in parts with
darker, amorphous skins.
Here’s the difference in extremis on some test specimens:
On real components which are more complex shapes than test specimens, this sort of
appearance is typical if the mould temperature is too low:
So you can see PEEK’s crystallinity is influenced by how it is processed and especially by how it
is cooled from the melt.
If you cool it really quickly – at more than 700°C/minute – the polymer chains do not get time
order themselves into crystalline domains, so you will make an amorphous PEEK. Victrex does
this deliberately for some of its APTIV™ film products
used in thermoforming applications. Consistent with the table above, the transparent,
amorphous APTIV film softens and is easily formable at temperatures around 180°C, at which
point it spontaneously re-crystallises (as the polymer chains are sufficiently free to move and re-
organise themselves into crystalline domains at this temperature) and it becomes more opaque.
Compare that to crystalline APTIV film, which can only be formed at temperatures above about
Secondary crystallisation of PEEK during annealing – scientific debate ongoing
It is also quite common in some applications – like extruded rods, shapes and plates – to post-
anneal the parts, a process which can result in increased crystallinity. Again, annealing takes
place at higher temperatures, typically around 230°C, and some re-crystallisation occurs, leading
to products with up to 40% crystallinity. Additive manufactured PEEK parts, made by Selective
Laser Sintering, can have around 45% crystallinity or more. Secondary crystallisation of PEEK
during annealing has been debated for many years and even today the detailed mechanism is
still the subject of scientific debate!
* Data on file at Invibio. Mechanical Benchmark of Carbon Fibre PEEK-OPTIMA™ Ultra-Reinforced vs
Ti 6AI-4V Plates undergoing Static Testing per ASTM F382-99 (2008)
© Victrex plc. 2017. All rights reserved.
About the author
John Grasmeder, Chief scientist at Victrex
Dr John Grasmeder has over 25 years’ experience in the polymer industry, in R&D, commercial and business leadership positions at ICI, BASF, Hoechst and Shell joint ventures in both the UK and Germany. He has been with Victrex 11 years, and was Technical Director from 2010. He became Chief Scientist in 2016.