How to Tune the Resistance of Anti-Static PEEK During Injection Molding
A practical guide for molders · fiber length, fiber distribution, and stable surface resistance
Please read this first. This guide is written for carbon fiber reinforced anti-static PEEK. Resistance control for anti-static PEEK filled with carbon black or carbon nanotubes is different, because those fillers have different requirements for dispersion. Please confirm which filler system your material uses before you follow the steps below.
1. Why Anti-Static PEEK Needs Conductive Filler
Pure PEEK is highly insulation,Its volume resistivity can reach 1016 Ω·cm. To give the material anti-static properties (ESD-SFAE), conductive filler has to be added to the resin. The anti-static PEEK from PRES uses carbon fiber as s filler,it can enhance strength and ESD performance
To control the resistance of anti-static PEEK, you need to work with two main settings: melt temperature and injection speed. Both of them decide how long the carbon fibers stay inside the melt, and how the fibers are distributed in the finished part. Once these two factors are under control, you can reach a stable surface resistance.
2. Why Does the Resistance Vary?
The surface resistance value of ESD PEEK pellets that produce by PRES is 108 Ω/sq, after injection it can still keep at 106-109 Ω/sq,but sometimes the value may not meet our expectations.The following reson may help you fix it.
2.1 Shear Breaks the Carbon Fibers
The conductivity of ESD PEEK depends on the conductive filler carrying electrons through the part. Carbon fiber is the most common "conductive backbone" in anti-static PEEK. To build a percolation network, the aspect ratio (L/D) of the fiber is the key variable. Both simulations and real tests show the same rule: the higher the aspect ratio, the lower the percolation threshold.
The aspect ratio of raw carbon fiber is usually above 20:1. If the fiber is cut too short during molding, down to below 8:1, the effective percolation threshold rises a lot. Even a 30% fiber loading may no longer form a complete network. On the shop floor the result is simple: resistance goes up and the part turns into an insulator. Shorter fibers also lower the mechanical properties of the part.
Fiber breakage happens mainly in three places:
1) The gate. The gate is where the shear rate reaches its peak. In a small gate, melt speed can reach tens of meters per second, and the shear rate can jump above 105 s-1 in an instant. Fiber that passes through such a gate will almost always be cut.
Solution: use a hot runner valve gate with a diameter of ≥3.5 mm. Do not use pinpoint gates or tip gates, because they create extreme elongational shear.For the carbon-fiber ESD PEEK formulation evaluated by PRES, a valve gate of approximately 3.5 mm or larger was used in our tested mold to reduce excessive fiber breakage. The appropriate gate size should be determined according to part thickness, flow length, fiber loading, and mold design.
2) The screw. A study on screw plasticizing simulation (Polym. Eng. Sci. 2018) found that fiber breakage increases from the compression section to the metering section, and is most severe where the compression force changes suddenly.
Solution: increase the flight depth of the feed section and the metering section.
3) Melt shear inside the cavity. Shear is concentrated in the skin layer and the subsurface layer. Raising the melt temperature makes the frozen layer thinner, which reduces shear. An injection speed that is too fast also shortens the fibers. Injection speed for ESD PEEK is discussed in detail in section 3.1.
2.2 Preparation Before Molding: Drying
Like all other PEEK grades, anti-static PEEK must be dried thoroughly: 150-160 °C for 4-6 hours, with a tray layer thickness below 30 mm. It is best not to go above 160 °C, because the material oxidizes easily at higher temperature and then flows worse.
If the moisture level is too high, PEEK will hydrolyze at high temperature. This causes bubbles and silver streaks, and it weakens the bond between the carbon fiber and the matrix. Tensile strength may drop by 25~30%.
3. Resistance Control
| Resistance condition | First adjustment | Secondary adjustment | Check |
|---|---|---|---|
| Resistance too high(>109 or higher than your expect ) | Increase injection speed | Increase melt temperature | Check gate size and fiber breakage |
| Resistance too low(<106 or lower than your expect ) | Reduce injection speed | Reduce melt temperature | Check conductive filler distribution |
| Resistance increases after increasing injection speed | Stop increasing speed | Inspect gate design | Check carbon-fiber breakage |
| Resistance varies significantly | Stabilize molding conditions | Standardize drying and testing | Check MD/TD measurement direction |
3.1 Injection Speed
ESD PEEK needs a faster fill than other PEEK grades.
Injection speed is one of the main process variables that can affect the surface resistance of carbon-fiber ESD PEEK.
In the PRES formulation evaluated in our trials, a relatively high filling speed helped achieve the target resistance range by influencing carbon-fiber distribution near the molded surface.
However, injection speed cannot be considered independently from gate design. If the gate is too small, increasing injection speed can significantly increase local shear and fiber breakage. In this situation, further increasing injection speed may cause resistance to increase rather than decrease.
Therefore, gate design should be checked before using injection speed as the primary resistance adjustment variable.
3.2 Melt Temperature
A higher melt temperature raises the packing density of the carbon fibers in the PEEK melt and compacts them, so ESD PEEK conducts better. PEEK is a semi-crystalline plastic, and the mold temperature has a strong effect on the thickness of the skin layer and on the fiber relaxation time. Together with a high mold temperature (for example 160~180 °C), fast solidification of the surface is delayed, which gives the surface fibers more time to compact and rearrange.
3.3 Screw Speed
A relatively low screw speed can help reduce unnecessary shear and preserve carbon-fiber length. For the PRES ESD PEEK grade evaluated in our trials, a screw speed of approximately 50–60 rpm was used as a starting range.
When the resistance was too low, the screw speed could be reduced to approximately 40–50 rpm as part of the adjustment process.
However, screw speed should not normally be the first parameter adjusted. Excessively low screw speed can increase plasticizing time and extend the molding cycle. In practice, injection speed and melt temperature were the primary adjustment variables in our trials.
Summary of the adjustment rules
If the resistance is too low, lower the injection speed and the melt temperature. If the resistance is too high, raise the injection speed and the melt temperature. But first check two things: that the gate design is reasonable, and that the anti-static material really uses carbon fiber filler. With other filler systems, the result may be the opposite.
| Problem | First action | Next step |
|---|---|---|
| Resistance too high (insulating) | Raise the injection speed | Raise the melt temperature; check the gate size (≥ 3.5 mm) |
| Resistance too low (over-conductive) | Lower the injection speed | Lower the melt temperature; drop the screw speed to 40~50 rpm |
| Resistance rises after a speed increase | Stop raising the speed | The gate is breaking the fibers ，fix the gate first |
4. Resistance Testing
One point needs special attention: anti-static PEEK shows clear conductive anisotropy. The surface resistance along the flow direction (MD) and the surface resistance perpendicular to it (TD) can differ by 1–2 orders of magnitude.
For this reason, we recommend that QC always uses a high resistance meter with a weighted concentric ring electrode. The measurement procedure should clearly state and then fix the measuring direction (MD or TD). Only in this way are the data comparable between batches and between suppliers, and only then is quality control really reliable.
