The Short Answer
Voids come from uneven compaction or a wrong sintering cycle, and they are invisible to dimensional inspection. A ball with a substantial internal cavity measures perfectly on a micrometer. The screen that finds them is density.
Where Voids Come From
Compaction
PTFE cannot be injection molded — its melt viscosity is far too high. Instead, powder is compacted cold in a die to form a green preform. If pressure distribution across that preform is uneven, some regions consolidate more than others, and the low-density regions become cavities during sintering.
This gets progressively harder as diameter increases, because pressure has to transmit uniformly through a much larger powder mass. It is one of the main reasons most producers stop well short of 300 mm.
Sintering
Sintering heats the preform above PTFE's crystalline melting point so particles coalesce. Two things go wrong here:
- Heating too fast. The outside sinters and seals before the core reaches temperature, trapping air inside with no escape path.
- Cooling too fast. The outside contracts and solidifies while the core is still shrinking, pulling internal cavities open. This is why a large PTFE ball needs a genuinely long cooling ramp — hours, not minutes.
Both are throughput decisions. A producer under pressure to increase output shortens the cycle, and voids are what they trade away for it. Nothing about the finished ball reveals that decision.
Why Voids Matter
- Sudden failure under pressure. A cavity is a stress concentrator. The ball can collapse or split without warning.
- Fluid entrapment. If a void reaches the surface through a wear path, process fluid enters and cannot be cleaned out — unacceptable in food or pharmaceutical service.
- Accelerated cold flow. Reduced effective cross-section means the ball deforms faster under the same seat load.
- Inconsistent buoyancy. In float applications, a void changes effective density and therefore the float line.
How to Detect Voids
Density testing — the practical method
Weigh the ball in air. Determine its volume by displacement in water. Calculate density and compare against 2.14–2.20 g/cm³ for virgin PTFE.
A result measurably below that range means internal porosity. This is straightforward, needs no special equipment beyond an accurate balance, and is the standard incoming check for a new supplier or a large-diameter consignment.
Note that filled grades have different expected densities depending on filler and loading — ask your supplier for the target figure rather than comparing against the virgin range.
Sectioning — the definitive method
Cut a sample ball in half and look. Destructive, so it is a batch-qualification tool rather than a routine check, but it is the only method that shows you void size, shape and distribution directly. Worth doing once when qualifying a new supplier, particularly for large diameters.
Visual and tap tests
Surface-breaking voids are sometimes visible as pits. A large internal cavity occasionally changes the sound when the ball is tapped. Neither is reliable — treat them as free extra observations rather than a method.
What to Ask a Supplier
- What density do you achieve, and do you verify it per batch?
- What is the sintering cycle time for this diameter? (A suspiciously short answer for a large ball is informative.)
- Can you supply a sectioned sample for qualification?
- Is the resin virgin or reprocessed? Reprocessed material carries considerably higher void risk.
A manufacturer can answer all four immediately. A trader generally cannot answer any of them, because the decisions were made by someone else.