PTFE Creep: How to Specify Parts Under Compression

PTFE creep is the gradual deformation of polytetrafluoroethylene under a sustained load. A part that meets its drawing immediately after machining may change shape after it is clamped and held in service. For compressed PTFE components, specify the operating load, temperature, confinement and acceptable dimensional change together. A general temperature range or a room-temperature strength value cannot establish whether the assembled part will retain its required fit.

What Changes When PTFE Remains Under Load?

Creep is different from the immediate compression seen while a load is applied. It adds a time dimension to the problem: how much deformation develops during the intended hold period, and what remains after unloading? Engineers also use the term cold flow when discussing this behavior. In its PTFE material-data guide, Parker describes deformation under load and the role of fillers. That explanation is useful background; its compound results are not specifications for another supplier.

For a buyer, the practical question is the remaining function. A spacer might need to preserve separation. A guide may need to retain clearance. A sealing component may need a qualified contact-pressure window. These are different acceptance tasks, even when the starting polymer is the same. Write the task before comparing grades.

Describe the Loaded Geometry, Not Only the Outside Dimensions

Mark the surfaces that actually carry compression. Record whether the part is supported across its full face, trapped in a pocket, loaded through a washer or contacted along an edge. Identify free edges where displacement would interfere with an adjacent feature. An overall drawing size does not explain these boundary conditions.

Keep assembly information with the part specification. Include mating materials, relevant surface requirements, the method used to apply load and whether the load is steady or cyclic. If bolt tightening is involved, ask the assembly designer to establish the load relationship. A metal-fastener torque table is not a validated compression specification for a PTFE component.

Screen Virgin and Filled PTFE by the Complete Duty

RDS lists PTFE products and glass-fiber, graphite and bronze filler options on its PTFE product page. Treat these as candidates to discuss, rather than interchangeable grades. Request the exact offered formulation and evidence relevant to your load and temperature. A filler name alone does not define its proportion, processing history or resulting performance.

Use a separate check for each required function. A formulation considered for dimensional retention must also meet the chemical, sliding or electrical requirements of the application. Do not assume that improving one property preserves all the others. If the supplier proposes a different formulation, record it as a controlled material change and review the affected requirements before approving it.

Specify an Evidence Request That Can Answer the Question

Ask for deformation-under-load information with the test stress, temperature, duration, specimen geometry and measurement sequence stated. Request the meaning of each reported result: deformation while loaded, recovery after unloading, or residual deformation after a defined recovery period. Values with different definitions should not be placed in one ranking column.

Where suitable data are missing, agree on a representative trial instead of inventing an allowable stress. Define the assembled geometry, exposure sequence and measurements before ordering the trial parts. The purpose is to test the required function under agreed conditions, not to demonstrate that every PTFE grade has the same universal creep limit.

Separate Inspection Dimensions from Service Acceptance

Define when and how the machined component will be measured. State the measurement temperature, support method and any agreed conditioning or rest period. For flexible features, also define how the measurement avoids distorting the part. This creates a repeatable incoming-inspection record.

Then define service acceptance separately. Identify the clearance, thickness retention or other functional measure that matters after the agreed load history. Record both the initial measurement and the post-exposure measurement on the same sample. A part can pass incoming dimensions yet fail the assembly trial; that result calls for a design or material review, not an unexplained change to the inspection drawing.

Build a Usable PTFE Compression-Part RFQ

A useful request for quotation includes the drawing revision, PTFE formulation requested or the permission to propose one, stock form, quantities and critical functional dimensions. Attach the loaded-area sketch and describe the temperature and load histories, contacting fluids, assembly method and required operating life. Clearly separate requirements from information supplied only for discussion.

Ask the supplier to list deviations against that request. For example, an alternative stock form, unavailable test condition or proposed tolerance needs an explicit disposition. Link the approved material identity to sample and production lots. This makes a later reorder traceable instead of relying on a description such as white PTFE ring.

An Illustrative Approval Decision

Consider a proposed PTFE spacer that passes the incoming thickness check but has no supporting information for the intended compressed hold. The correct purchasing decision is conditional approval for the agreed trial, not production approval based on the drawing alone. If an alternative filled formulation is offered, preserve the same functional acceptance question and repeat the affected material checks.

The decision record should make three outcomes possible: accept the tested combination within its stated conditions, revise the design or material and retest, or reject the proposal. This keeps an unresolved service requirement visible instead of hiding it inside a tighter machining tolerance.

Questions Buyers Ask About PTFE Creep

• Does a high service-temperature rating establish compression performance? No. Temperature capability and dimensional retention under a specified load are different questions. Review them together for the actual component.
• Is filled PTFE automatically the correct replacement? No. Consider the full duty and qualify the offered formulation; filler identity is only part of that decision.
• Can a short room-temperature check replace a service trial? It may screen dimensions or an obvious problem, but it does not reproduce a different temperature, load duration or assembly boundary condition.