How Should Threads, Bores and Sealing Lands Be Masked for Blasting?

Threads, precision bores and sealing lands should be protected with plugs, caps, abrasion-resistant tape or fixture shields selected for the blast energy and geometry. The drawing should define protected depth, boundary and transition band. After mask removal, the supplier should inspect for leakage, adhesive, trapped grit, edge attack and functional gauge acceptance.

Masking material must resist particle impact without shredding or transferring residue. A mask that works for powder coating heat may not withstand direct abrasive impact, and a blast shield may create a shadow larger than the allowable no-blast zone.

How to Select Plugs, Caps, Tape and Fixture Shields

How to Dimension the Mask Boundary

Reference stable datums or feature edges. Define whether chamfers, counterbores, first thread turns and gasket-edge radii remain protected. Allow a realistic transition band where the blast texture feathers. A note such as “do not blast sealing face” should also address the adjacent edge because abrasive crossing the boundary can round it.

Approve a production-intent sample and retain the map. The post-machining route should identify pre-blast dimensions and post-blast gauges for every protected interface.

How Mask Wear and Leakage Should Be Detected

Reusable plugs and shields erode. Tape can lift under repeated particle impact. Inspect masks before loading and replace them by condition or validated life. In production, sample parts from positions with the strongest direct exposure because those masks may fail first.

After removal, use suitable lighting and clean the boundary before gauging. A few abrasive particles in a thread can cause high torque while the visual surface appears protected. Sealing lands should be checked for roughened spots and flatness or roughness where specified.

How Mask Installation and Removal Should Be Standardized

Work instructions should show plug orientation, insertion depth, cap seating, tape overlap, shield location and the sequence relative to degreasing and blasting. A plug installed before oily soil is removed can trap contamination; a plug installed too late can allow media into the first thread turns.

Removal needs controlled tools and direction. Prying against a sealing land can create a dent, while pulling tape across a textured edge can deposit adhesive. Count and reconcile reusable plugs so none remains in a cavity or shipment.

Train with acceptable and failed samples. Periodic layered audits should verify the operator's method and the final boundary, because an installed mask is not proof that the protected feature remained unchanged.

Store clean masks so abrasive and oil cannot transfer into protected features. Separate masks used on contaminated stripping work from those used on clean production. For reusable plugs, establish cleaning, visual inspection and dimensional replacement criteria. A worn plug can remain apparently seated while allowing fine media to reach the thread root.

Mask inventory should be identified by part and revision. Similar-looking plugs with different depth or hardness should not be mixed at the workstation. A simple error-proofing fixture or color code can reduce selection mistakes, but the final functional inspection remains required.

Hypothetical Masked-Flange Release

Consider a hypothetical housing with a circular sealing land, central bore and six tapped holes. A fixture shield protects the land; a plug protects the bore; individual thread plugs protect engagement. The first article measures the no-blast boundary, checks the land and bore, gauges all threads and examines residual media.

One shield location creates a visible texture shadow outside the allowed transition. The fixture is revised before production rather than accepted because the functional land passed. This scenario is not a reported product result.

The sandblasting service specification should tie shield and plug revisions to repeat-order change control.

Effective masking is an engineered interface between blasted and protected surfaces. It is released by dimension, cleanliness and function, not by the fact that a plug was installed.