Friction & assembly
Lubricants Inside a Sealer Film: Why Torque Consistency Depends on Them
A sealing film is not only a barrier. In fastener applications it is often the lubricant that makes the assembly line work. The same film can be tuned for corrosion resistance or for friction, and the two objectives pull in different directions.
The three common lubricant families
| Type | How it works | Trade-offs |
|---|---|---|
| Waxes (polyethylene, carnauba) | Melt or soften under local pressure and shear, forming a low-friction transfer layer on the thread flank | Good cost/performance; can migrate in hot storage; affects appearance at high loadings |
| PTFE | Low surface energy, low shear strength; effective over a wide temperature range | Higher cost; can be difficult to disperse uniformly; over-lubrication reduces clamp load |
| Solid lubricants (MoS₂, graphite) | Platelet structures slide over each other under pressure | Dark colour; not suitable where a bright finish is required; can stain |
What the lubricant changes
- Friction coefficient. The primary reason it is added. Wax systems typically land in the 0.13–0.18 range; heavy lubricant loads can go to 0.08–0.12.
- Torque scatter. Uniform distribution matters more than total amount. A film with 3 % wax evenly spread outperforms one with 6 % that pools at drip points.
- Appearance. High lubricant loads can produce a slightly greasy look or visible bloom. Customers who specify a bright, dry-to-touch finish may reject it.
- Adhesion and overcoatability. Some lubricants migrate to the surface and interfere with paint or adhesive bonding later.
- Corrosion resistance. Lubricants are not barriers; beyond a certain loading, they can interrupt the film's continuity.
Why "more lubricant" is not "better assembly"
Lower friction means lower clamp load at a given torque. If the joint was designed around K = 0.18 and the new sealer delivers K = 0.10, the bolts are under-tightened unless the torque is recalculated. In safety-critical joints this is a design change, not a coating change, and it needs engineering sign-off.
Practical check. When evaluating a new sealer on a torque-controlled part,
run a torque-tension test on at least 10 assemblies and compare the mean and standard deviation
of K against the current process. A mean shift is manageable; a wider spread is not.
Questions to ask
- Does the product contain a lubricant, and of what type?
- What friction coefficient range was measured, on what substrate and by which method?
- Does the lubricant migrate at elevated storage temperature (relevant for parts shipped in containers)?
- Is the finish dry to touch, or does it require a specified cure to avoid transfer?
- Will the film accept paint or adhesive if a downstream customer needs it?
Friction and corrosion are both properties of the same 1–3 µm film. Any
change that improves one usually costs something in the other. The engineering task is to choose
which one your application can afford to trade.