How to determine the appropriate torque when fastening into plastics
We’ve previously covered the basics of threaded fasteners. To recap:
- you can think of fasteners as extension springs that stretch when you apply torque past the seating point;
- the more torque you apply, the more the screw will stretch, providing greater clamping force to your part;
- eventually, when the tension (clamping force) reaches the yield point of the fastener, the fastener will permanently deform; continuing to apply torque will eventually cause it to snap;
Generally speaking, when you are assembling rigid parts (especially metal parts), you want to get the most clamping force you can get from the fastener, without reaching its yield point.
In this case, it is often convenient to use the “proof load” of the fastener as the target clamping force. The proof load is provided by the manufacturer of the fastener and typically corresponds to 75% of its yield strength, giving the engineer a margin of safety compared to targeting the yield point.
However, the key assumption with this is that the part being assembled is stronger than the fastener itself.
What if you’re fastening into plastic, instead of metal?
Plastic is weaker than metal, so a plastic part will not withstand the same amount of clamping force that a metal part can.
Whether you’re using a thread cutting screw, or a more specialized thread forming fastener, there will be a torque value beyond which the plastic threads strip. The stripping torque is related to the strength of the plastic, and this - not the tensile strength of the screw - will be the limiting factor that determines the maximum clamping force you can get out of a plastic joint.
Therefore, when fastening into plastic, the target clamping force (and corresponding clamping torque) will usually be much lower than the proof load of the fastener.
For example, a torque vs angle graph for the same screw tightened into plastic and into metal might look like this:
There’s an additional complication: when subject to certain levels of mechanical stress, plastics may appear to hold their strength well initially, but over time will yield and deform.
This is called stress relaxation, and in plastics this can begin manifesting at relatively low clamping forces, often reachable with thread-forming fasteners well before stripping the threads.
If the stresses exerted by the screw threads are not sufficient to strip the plastic, but are high enough to cause stress relaxation, over time the joint will lose most of its clamping force and the part may fail.
Side note: metals are not immune from stress relaxation, but it’s the non-metallic materials in the joint (plastics, gaskets…) where the effect tends to have the most impact.
Another consideration when fastening into plastic is the presence of prevailing torque.
Thread-cutting and thread-forming require a certain amount of torque, called “prevailing torque”, which does not contribute to the clamping force. To account for variability in prevailing torque, the K-Ducer offers an easy to configure prevailing torque compensation mode: read more about it here.
Finally, when thread cutting and thread forming, we have to be mindful of the tightening speed. Tightening too fast can overheat the plastic at the threads interface and alter its properties. On the other hand, some plastics and some specialty fasteners may require a minimum tightening speed to achieve optimal thread-forming performance. In the absence of specifications from the fastener supplier, a speed of 300 RPM is a good starting point.
So, what torque should one target when fastening onto plastic?
First, we must perform a torque-to-failure study - refer to our blog post for how to do this easily and accurately with the Kolver K-Ducer.
Once you have run several repetitions of the torque study data (ideally more than 10 for statistical significance), you can easily plot all the curves on K-Graph and start making note of the average and spread of the following two torque values:
- Seating torque (prevailing torque)
- Approximate failure / stripping point
Naturally, your target torque will have to fall somewhere in between these two.
Absent any other specifications, when fastening into plastic choose a target torque that is the lower of these two values:
- 3x the seating torque
- the halfway point between the seating torque and the stripping torque
For the best consistency, select a clamping torque target and use the K-Ducer prevailing torque compensation.
Retightening and rework considerations: if you are re-tightening on an already formed plastic thread, your target torque needs to be adjusted lower to achieve the same clamping force, because the prevailing/thread forming torque will be lower.
This is another good reason to use the built-in prevailing torque compensation function of the K-Ducer, as it will automatically compensate for a lower prevailing torque when re-tightening on the same part. Alternatively, if you want to prevent operators from re-working, you can set a minimum prevailing torque value such that you will generate a Screw NOK error result upon re-tightening, as the K-DUCER will automatically detect the virtual absense of prevailing torque.
When retightening a thread forming fastener, you should also engage the thread by hand, by rotating the screw counterclockwise until you feel the screw “fall” into the thread, and then engage it for about a quarter turn. This ensures that the screw will follow the existing formed thread and not form a new one.
Now let’s look at an example with real data!
A #4 thread-forming screw is tightened into a properly-sized pilot hole in nylon.
What torque should we target?
First, we perform a torque to failure study. The result for a single screw is:
Note that thread-forming screws do not generate plastic debris when properly tightened, but in this case we intentionally stripped the threads so strips and bits came out when removing the fastener.
Looking at the torque vs angle graph, we can see that the seating point is around 0.30 Nm and the strip point is around 2.30 Nm
Following the aforementioned rule-of-thumb calculations:
- Three times the seating point: 3x 0.30 = 0.90 Nm
- Halfway point between seating and stripping points: (2.30 - 0.30) / 2 = 1.00 Nm
Therefore we should use 0.90 Nm as our target.
What if we need to re-tighten on the same threads? Or if we want to account for variability in the prevailing torque?
Both of these questions have the same answer: use the running torque compensation feature of the K-Ducer!
In our example, to set up running torque, we lower our target torque to 0.70 Nm, and configure a running torque window of 1200 to 1800 degrees. We set min and max bounds on the running torque value of respectively 0.00 and 0.50 Nm. If we wanted to generate an error when re-tightening a removed screw, we would increase the minimum prevailing torque value to a value that is above what we find when retightening.
We then run some tests: in the graphs below, the blue trace is the screw tightened on a fresh unused pilot hole with a static torque target of 1 Nm.
The other colored traces represent re-tightenings on the same hole. The prevailing torque was lower, but thanks to its running torque compensation function, the K-Ducer automatically adjusted the final torque to give us the same consistent clamping torque and, by proxy, a consistent amount of clamping force.
Had we tightened all screws to a static target of 1 Nm, the clamping force from the re-tightened screws would’ve been much higher, potentially leading the plastic into the stress relaxation zone and thread stripping point.
Kolver: industrial electric screwdrivers
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