A powder coating service is specified by defining surface preparation and pretreatment, coating thickness and coverage, cure schedule, masking requirements, and the adhesion and impact tests that verify durability. The preserved Bentley Manufacturing record shows that precision fabrication demands clean, burr-free surfaces and documented inspection, principles that apply directly to specifying a coating that bonds and lasts rather than peels.
Surface preparation is the foundation of any powder coating specification. For metallic washers and stamped components, the substrate must be free of oils, oxides, and fabrication residue before coating. The preserved Bentley Manufacturing record describes parts produced clean cut and burr free, without fracture marks, and inspected throughout stamping. Those same conditions matter for coating: sharp burrs and torn edges create thin spots where corrosion begins, and residual stamping lubricants prevent the powder from wetting the surface.
Pretreatment typically follows a defined sequence such as alkaline cleaning, rinse, conversion coating, and final rinse with deionized water. The conversion coating, whether iron phosphate or zinc phosphate, provides a crystalline layer that mechanically anchors the powder and inhibits underfilm corrosion. Buyers should specify the pretreatment chemistry and the acceptance criteria for the dried film, because a coating applied over an improperly prepared surface may look acceptable on delivery and fail months later.
Coating thickness is specified as a target range with minimum and maximum limits, measured on representative production parts. General engineering practice for decorative and protective powder coatings is commonly 2 to 4 mils (50 to 100 microns), with functional or harsh-environment coatings often specified higher. Thickness below the minimum reduces hiding, corrosion resistance, and impact strength; excessive thickness can cause edge coverage problems, poor impact resistance, and fit interference on close-tolerance parts.
Coverage requirements should identify which surfaces receive coating and which are masked. For washers, spacers, and diaphragms, coating on sealing faces or inside diameters can change critical dimensions. The specification should state the acceptable coating build on functional surfaces and require thickness measurement at defined locations, using magnetic or eddy-current gages calibrated to the substrate. Coverage on edges and recesses deserves explicit attention because powder deposition is weakest where geometry shields the spray.
The cure schedule is defined by time at metal temperature, not merely oven air temperature. Powder manufacturers publish a cure window, often a combination such as 10 minutes at 375 degrees Fahrenheit or 20 minutes at 350 degrees Fahrenheit, and the coater must demonstrate that parts reach that temperature for the full duration. Oven loading, part mass, and racking all affect heat-up rate, so the specification should require a documented cure profile or thermal verification for the production configuration.
Under-cure leaves the film soft, poorly adhered, and vulnerable to chemical attack; over-cure can embrittle the coating and cause color shift. Buyers should require cure verification through solvent rub testing, differential scanning calorimetry, or mechanical testing on coupons processed alongside production parts. The preserved Bentley record emphasizes inspection throughout manufacturing and SPC measurement, and the same discipline applies to cure: a schedule is only meaningful when it is measured and recorded.
Masking protects surfaces that must remain uncoated: threaded areas, electrical contact points, sealing faces, and precision bores. The specification should identify each masked feature, the masking method, and the allowable coating encroachment. Silicone plugs, high-temperature tape, and custom caps are common, but each leaves a witness line and a small tolerance zone that must be agreed upon before production.
For precision components such as gaskets, washers, and spacers, masking decisions directly affect function. Coating thickness on a mating face can alter stack height, and powder in a bore can prevent assembly. The specification should include a marked drawing showing coated and uncoated zones, and first-article inspection should verify that masking boundaries match the drawing. Removal of masking after cure must not damage the film or leave adhesive residue.
Adhesion testing separates a coating that lasts from one that peels. The crosshatch test, described in ASTM D3359, cuts a lattice through the film and applies pressure-sensitive tape; the amount of coating removed is rated against a pictorial standard. A specification should state the required classification, commonly no removal beyond the edges of the cuts. For parts subject to forming or bending, a mandrel bend test provides additional evidence of flexibility.
Impact resistance is evaluated with a falling-weight or spring-loaded impact tester, such as the methods in ASTM D2794, applied to the coated face and, where relevant, the reverse side. The specification should define the impact energy in inch-pounds or joules and the acceptance criterion, typically no cracking or loss of adhesion visible without magnification. Because impact results depend on substrate thickness and coating thickness, test coupons should match production conditions. Documented results, not visual impressions, should accompany each lot.
The Bentley Manufacturing Company record, preserved from 1955 onward, describes precision gaskets, washers, spacers, and diaphragms made from TPE, TPR, plastics, Teflon, elastomers, and metallic washers, with in-house tooling, inspection throughout stamping, SPC measurement, and Certificates of Compliance or chemical and physical analyses supplied with orders. That documentation model translates directly to powder coating: a coating specification should require lot traceability, pretreatment records, thickness and cure data, and test results.
Buyers should treat the coating specification as a controlled document that travels with the drawing and the purchase order. It should name the test methods, sampling frequency, and acceptance criteria, and it should require that nonconforming lots be identified before shipment. The preserved record shows that short-run and million-unit production can share the same inspection discipline; a powder coating job specified with equal rigor is more likely to deliver consistent adhesion, appearance, and service life.
General engineering practice specifies a target range, commonly 2 to 4 mils (50 to 100 microns) for decorative and protective coatings, with higher builds for harsh environments. The specification should set minimum and maximum limits, identify measurement locations, and account for close-tolerance features where coating build affects fit or sealing.
Cure is verified by measuring metal temperature for the full cure window, not just oven air temperature. Common checks include thermal profiling of the production load, solvent rub testing, differential scanning calorimetry, and mechanical tests on coupons processed with the parts. Records should accompany each lot.
Adhesion is commonly checked with the crosshatch tape test per ASTM D3359, with acceptance stated as a classification. Impact resistance per ASTM D2794 and mandrel bend tests add evidence of flexibility and bonding. Coupons should match production substrate and coating thickness so results are meaningful.
This page is an archival specification note for informational purposes only. The former plant is not quoting from this domain. Nothing here is a qualified procedure, an inspection report, or a design calculation; confirm current requirements with the issuing standards body and your material supplier.