{"id":8099,"date":"2026-09-29T10:00:00","date_gmt":"2026-09-29T02:00:00","guid":{"rendered":"https:\/\/maxtormetal.com\/?p=8099"},"modified":"2026-09-21T10:05:24","modified_gmt":"2026-09-21T02:05:24","slug":"medical-grade-polymer-processing-knife-420-vs-440c","status":"publish","type":"post","link":"https:\/\/maxtormetal.com\/pt\/medical-grade-polymer-processing-knife-420-vs-440c\/","title":{"rendered":"Sele\u00e7\u00e3o de Facas para Processamento de Pol\u00edmeros de Grau M\u00e9dico: A\u00e7o Inoxid\u00e1vel 420 vs 440C, Acabamento Superficial e Controle do Anel de \u00c1gua"},"content":{"rendered":"<div class=\"wp-block-image\"><figure class=\"aligncenter size-full is-resized\"><img loading=\"lazy\" decoding=\"async\" width=\"800\" height=\"800\" src=\"https:\/\/maxtormetal.com\/wp-content\/uploads\/2023\/10\/Plastic-crusher-blade-main11.jpg\" alt=\"Sele\u00e7\u00e3o de Facas para Processamento de Pol\u00edmeros de Grau M\u00e9dico: A\u00e7o Inoxid\u00e1vel 420 vs 440C, Acabamento Superficial e Controle do Anel de \u00c1gua\" class=\"wp-image-3185\" style=\"object-fit:cover;width:552px;height:auto\" srcset=\"https:\/\/maxtormetal.com\/wp-content\/uploads\/2023\/10\/Plastic-crusher-blade-main11.jpg 800w, https:\/\/maxtormetal.com\/wp-content\/uploads\/2023\/10\/Plastic-crusher-blade-main11-300x300.jpg 300w, https:\/\/maxtormetal.com\/wp-content\/uploads\/2023\/10\/Plastic-crusher-blade-main11-150x150.jpg 150w, https:\/\/maxtormetal.com\/wp-content\/uploads\/2023\/10\/Plastic-crusher-blade-main11-768x768.jpg 768w, https:\/\/maxtormetal.com\/wp-content\/uploads\/2023\/10\/Plastic-crusher-blade-main11-600x600.jpg 600w, https:\/\/maxtormetal.com\/wp-content\/uploads\/2023\/10\/Plastic-crusher-blade-main11-100x100.jpg 100w\" sizes=\"(max-width: 800px) 100vw, 800px\" \/><\/figure><\/div><p><strong>Resposta r\u00e1pida:<\/strong> Na granula\u00e7\u00e3o de pol\u00edmeros de grau m\u00e9dico, os a\u00e7os inoxid\u00e1veis 420 e 440C n\u00e3o s\u00e3o intercambi\u00e1veis: o 420 oferece maior tenacidade, melhor polibilidade e maior toler\u00e2ncia \u00e0 corros\u00e3o em an\u00e9is de \u00e1gua com higieniza\u00e7\u00e3o intensiva ou qu\u00edmica vari\u00e1vel; o 440C proporciona reten\u00e7\u00e3o de corte superior para pol\u00edmeros abrasivos em an\u00e9is com qu\u00edmica est\u00e1vel e controlada. Ambos os graus exigem acabamento espelhado Ra \u2264 0,4 \u00b5m e passiva\u00e7\u00e3o verificada conforme ASTM A967 \u2014 a engenharia de superf\u00edcie, e n\u00e3o apenas a liga, \u00e9 o principal controle na preven\u00e7\u00e3o de contamina\u00e7\u00e3o. Os pontos pretos em pellets m\u00e9dicos costumam ser um problema sist\u00eamico: detritos de corros\u00e3o do anel de \u00e1gua e fragmentos de desgaste de uma faca incorretamente especificada atuando em conjunto.<\/p><p>Os pontos pretos em pellets de pol\u00edmeros de grau m\u00e9dico raramente t\u00eam uma \u00fanica origem. As equipes de produ\u00e7\u00e3o costumam atribu\u00ed-los \u00e0 degrada\u00e7\u00e3o da resina ou a res\u00edduos de purga \u2014 para descobrir, ap\u00f3s uma investiga\u00e7\u00e3o met\u00f3dica, que o principal causador s\u00e3o detritos de corros\u00e3o vindos do anel de \u00e1gua aquecida e fragmentos de desgaste das facas gerados por um sistema de corte incorretamente especificado.<\/p><p>As facas granuladoras para aplica\u00e7\u00f5es polim\u00e9ricas exigentes devem atender a toler\u00e2ncias dimensionais rigorosas e requisitos estritos de controle de contamina\u00e7\u00e3o \u2014 especialmente em linhas de grau m\u00e9dico, onde contamina\u00e7\u00e3o vis\u00edvel zero e uniformidade dos pellets s\u00e3o inegoci\u00e1veis. A sele\u00e7\u00e3o do grau correto de a\u00e7o inoxid\u00e1vel, do acabamento superficial e do protocolo de gest\u00e3o do anel de \u00e1gua \u00e9 a base de engenharia dessa garantia. Para linhas que processam compostos abrasivos ou com carga de fibra de vidro, onde o desgaste predomina sobre a corros\u00e3o, a l\u00f3gica de sele\u00e7\u00e3o \u00e9 diferente; <a href=\"https:\/\/maxtormetal.com\/pt\/m390-vs-cpm-s90v-powder-metallurgy-pelletizer-blades-glass-fiber\/\" target=\"_blank\" rel=\"noreferrer noopener\"><em><strong>Facas granuladoras de metalurgia do p\u00f3 M390 vs CPM S90V: Sele\u00e7\u00e3o de classe sob alto desgaste por fibra de vidro<\/strong><\/em><\/a> covers that scenario in detail.<\/p><p>In Maxtor Metal&#8217;s custom pelletizer knife programs for medical and technical polymer lines, black-speck root-cause investigations consistently trace back to three controllable variables: water-loop corrosion, inadequate knife surface finish, and steel grade mismatch \u2014 each addressable through specification and process discipline.<\/p><hr class=\"wp-block-separator has-alpha-channel-opacity\"\/><h2 class=\"wp-block-heading\" id=\"816271bd-da7e-4112-a96d-f5b0d59a21b4\">Pontos Pretos: Origens e Riscos<\/h2><h3 class=\"wp-block-heading\" id=\"4f1a313c-0620-4807-886a-b0976898a332\">Differentiating Polymer vs Metallic Specks<\/h3><p>Not all black specks share the same origin, and misidentifying the source wastes investigation time. A simple initial screen separates the two categories:<\/p><ul><li><strong>Metallic specks<\/strong> are hard, gritty, and often magnetic. Under a stereomicroscope they show metallic luster or angular fracture surfaces. A rare-earth disc magnet drawn across a spread pellet sample will attract these particles.<\/li>\n\n<li><strong>Polymer specks<\/strong> are soft, compressible, smear under fingernail pressure, and frequently show a carbonized or thermally degraded cross-section. They originate from dead zones in the extruder barrel, screen-pack bypass, or contact with overheated die surfaces.<\/li><\/ul><p>Mixed contamination \u2014 where metallic debris seeds polymer carbonization \u2014 is common in long-run campaigns. Resolving it requires addressing both pathways simultaneously.<\/p><h3 class=\"wp-block-heading\" id=\"4141e22a-8860-469f-b0d4-b2e4817e10bf\">Warm-Water Loop Corrosion Pathways<\/h3><p>The closed warm-water loop in an underwater pelletizing system is a persistent contamination source when poorly managed. <a href=\"https:\/\/orf.od.nih.gov\/TechnicalResources\/Documents\/Technical%20Bulletins\/25TB\/Corrosion%20in%20Closed%20Loop%20Water%20Systems%20Part%202%20-%20Evaluation%20and%20Prevention%20-%20June%202025%20TB_508.pdf\" target=\"_blank\" rel=\"noreferrer noopener\"><em><strong>A 2025 NIH technical bulletin on corrosion in closed-loop water systems<\/strong><\/em><\/a> identifies black magnetic iron oxide (magnetite) and mill-scale debris as the dominant black-speck contributors in recirculating systems. The mechanism is directly applicable to pelletizing loops.<\/p><p>The corrosion cycle follows a predictable path: oxygen ingress plus elevated chlorides triggers pitting on wetted metal surfaces, magnetite forms, particles migrate through the loop, and contamination reaches the die face and pellet stream. For loops operating outside controlled chemistry limits, this cycle accelerates until pellet color deviations become routine.<\/p><h3 class=\"wp-block-heading\" id=\"a19e3380-64d9-4233-90ff-aeebdf0bacdf\">Impact on Compliance and OEE<\/h3><p>ISO 13485 may be relevant when the polymer-processing operation is part of a medical-device organization&#8217;s controlled supply chain or QMS. It should not be treated as a universal production standard for all medical-grade polymer pelletizing lines. Where ISO 13485 controls apply, black-speck events should be handled through the site&#8217;s documented nonconformance, investigation, change-control, and CAPA procedures.<\/p><p>From an OEE perspective, uncontrolled black-speck events degrade all three components. Availability suffers as corrective purges, die changes, and system flushes consume production time. Performance drops when operators reduce throughput to control thermal history. Quality losses accumulate through quarantined or scrapped lots.<\/p><blockquote class=\"wp-block-quote is-layout-flow wp-block-quote-is-layout-flow\"><p><strong>Conclus\u00e3o principal<\/strong>: Metallic black specks from knife wear or loop corrosion are a compliance event on medical polymer lines \u2014 not a cosmetic issue. The investigation pathway and corrective action must be documented and closed within the quality system.<\/p><\/blockquote><p><strong>Resumidamente:<\/strong> black specks in medical polymer pellets are a system problem \u2014 corrosion debris from the water loop and wear fragments from a poorly specified knife typically act together, and addressing only one without the other rarely holds<\/p><hr class=\"wp-block-separator has-alpha-channel-opacity\"\/><h2 class=\"wp-block-heading\" id=\"9579f2d4-3990-4af0-b4a3-eb07fbba05b4\">Fundamentos do A\u00e7o 420 vs 440C<\/h2><div class=\"wp-block-image\"><figure class=\"aligncenter size-full is-resized\"><img loading=\"lazy\" decoding=\"async\" width=\"800\" height=\"800\" src=\"https:\/\/maxtormetal.com\/wp-content\/uploads\/2023\/10\/Plastic-crusher-blade-main1.jpg\" alt=\"Fundamentos do A\u00e7o 420 vs 440C\" class=\"wp-image-3184\" style=\"width:544px;height:auto\" srcset=\"https:\/\/maxtormetal.com\/wp-content\/uploads\/2023\/10\/Plastic-crusher-blade-main1.jpg 800w, https:\/\/maxtormetal.com\/wp-content\/uploads\/2023\/10\/Plastic-crusher-blade-main1-300x300.jpg 300w, https:\/\/maxtormetal.com\/wp-content\/uploads\/2023\/10\/Plastic-crusher-blade-main1-150x150.jpg 150w, https:\/\/maxtormetal.com\/wp-content\/uploads\/2023\/10\/Plastic-crusher-blade-main1-768x768.jpg 768w, https:\/\/maxtormetal.com\/wp-content\/uploads\/2023\/10\/Plastic-crusher-blade-main1-600x600.jpg 600w, https:\/\/maxtormetal.com\/wp-content\/uploads\/2023\/10\/Plastic-crusher-blade-main1-100x100.jpg 100w\" sizes=\"(max-width: 800px) 100vw, 800px\" \/><\/figure><\/div><h3 class=\"wp-block-heading\" id=\"ef87db33-07c1-4ae6-8afb-611c1ae50017\">Composition, Hardness, Wear vs Corrosion<\/h3><p>Both AISI 420 and 440C are martensitic stainless steels \u2014 heat-treatable, chromium-bearing grades capable of achieving the hardness needed for precision cutting-tool applications. Their differences are consequential for pelletizer knife selection.<\/p><figure class=\"wp-block-table\"><table><tbody><tr><th>Propriedade<\/th><th>AISI 420<\/th><th>AISI 440C<\/th><\/tr><tr><td>Carbon (%)<\/td><td>0.15 \u2013 0.40<\/td><td>0.95 \u2013 1.20<\/td><\/tr><tr><td>Chromium (%)<\/td><td>12 \u2013 14<\/td><td>16 \u2013 18<\/td><\/tr><tr><td>Typical HRC (hardened)<\/td><td>50 \u2013 52<\/td><td>58 \u2013 60<\/td><\/tr><tr><td>Resist\u00eancia ao desgaste<\/td><td>Moderado<\/td><td>Alto<\/td><\/tr><tr><td>Resist\u00eancia \u00e0 corros\u00e3o<\/td><td>Good (high free Cr in solution)<\/td><td>Good (carbide formation reduces free Cr)<\/td><\/tr><tr><td>Dureza<\/td><td>Mais alto<\/td><td>Mais baixo<\/td><\/tr><tr><td>Polishability<\/td><td>Excelente<\/td><td>Bom<\/td><\/tr><\/tbody><\/table><\/figure><p>440C&#8217;s high carbon content drives significantly more hard chromium carbides into the microstructure, which is the source of its wear advantage. Those carbides, however, reduce the chromium available in the matrix to sustain the passive oxide film \u2014 a trade-off reflected in <a href=\"https:\/\/www.crucible.com\/eselector\/prodbyapp\/stainless\/cru440cs.html\" target=\"_blank\" rel=\"noreferrer noopener\"><em><strong>Crucible Industries&#8217; 440C material data<\/strong><\/em><\/a>, which specifies the grade for high wear resistance with only moderate corrosion resistance in mild environments. In corrosive or sanitization-intensive environments, this trade-off is material.<\/p><p>420 retains more free chromium in solution. <a href=\"https:\/\/atlassteels.com.au\/wp-content\/uploads\/2021\/09\/Stainless-Steel-420-Grade-Data-Sheet-28-04-21-1.pdf\" target=\"_blank\" rel=\"noreferrer noopener\"><em><strong>Atlas Steels&#8217; Grade 420 data sheet<\/strong><\/em><\/a> describes it as a hardenable 12% chromium grade offering good corrosion resistance in the hardened and polished condition \u2014 a state directly relevant to lines where weekly CIP cycles and warm-water contact are standard.<\/p><h3 class=\"wp-block-heading\" id=\"8cf18c84-b4b0-4886-a3f1-a2e864fef7b4\">Selection by Risk Profile and Water Quality<\/h3><div class=\"wp-block-image\"><figure class=\"aligncenter size-large\"><img loading=\"lazy\" decoding=\"async\" width=\"1024\" height=\"683\" src=\"https:\/\/maxtormetal.com\/wp-content\/uploads\/2026\/09\/image-2-1024x683.jpeg\" alt=\"Side-by-side infographic: 420 vs 440C chemistry, HRC, corrosion notes, and recommended use cases for medical pelletizer knives\" class=\"wp-image-8100\" srcset=\"https:\/\/maxtormetal.com\/wp-content\/uploads\/2026\/09\/image-2-1024x683.jpeg 1024w, https:\/\/maxtormetal.com\/wp-content\/uploads\/2026\/09\/image-2-300x200.jpeg 300w, https:\/\/maxtormetal.com\/wp-content\/uploads\/2026\/09\/image-2-768x512.jpeg 768w, https:\/\/maxtormetal.com\/wp-content\/uploads\/2026\/09\/image-2-18x12.jpeg 18w, https:\/\/maxtormetal.com\/wp-content\/uploads\/2026\/09\/image-2-600x400.jpeg 600w, https:\/\/maxtormetal.com\/wp-content\/uploads\/2026\/09\/image-2.jpeg 1536w\" sizes=\"(max-width: 1024px) 100vw, 1024px\" \/><\/figure><\/div><p>The practical selection decision depends on three factors: polymer abrasivity, loop water quality, and the downstream consequence of contamination.<\/p><figure class=\"wp-block-table\"><table><tbody><tr><th>Condition<\/th><th>Preferred Grade<\/th><\/tr><tr><td>Abrasive polymer (glass-filled, mineral-loaded compounds)<\/td><td>440C<\/td><\/tr><tr><td>Deionized loop water, chlorides consistently &lt; 50 ppm<\/td><td>440C acceptable<\/td><\/tr><tr><td>Municipally conditioned or fluctuating chloride water<\/td><td>420<\/td><\/tr><tr><td>Frequent caustic CIP cycles (pH &gt; 9.5)<\/td><td>420<\/td><\/tr><tr><td>Maximum edge retention is the primary objective<\/td><td>440C<\/td><\/tr><tr><td>Maximum chip resistance and toughness required<\/td><td>420<\/td><\/tr><\/tbody><\/table><\/figure><p>For a quick field orientation, the following decision matrix compresses polymer abrasivity, loop water quality, and contamination consequence into a single risk-based recommendation. It is a proprietary summary framework, not a substitute for the line-specific qualification trial described later in this article.<\/p><figure class=\"wp-block-table\"><table><tbody><tr><th>Abrasivity<\/th><th>Loop Water Quality<\/th><th>Contamination Consequence<\/th><th>Recommended Grade<\/th><\/tr><tr><td>Baixo<\/td><td>Controlled (chlorides &lt; 50 ppm)<\/td><td>Critical<\/td><td>420 with premium finish<\/td><\/tr><tr><td>Baixo<\/td><td>Variable \/ elevated chlorides<\/td><td>Critical<\/td><td>420<\/td><\/tr><tr><td>Alto<\/td><td>Controlled<\/td><td>Critical<\/td><td>440C<\/td><\/tr><tr><td>Alto<\/td><td>Vari\u00e1vel<\/td><td>Critical<\/td><td>440C with enhanced loop control<\/td><\/tr><tr><td>Alto<\/td><td>Controlled<\/td><td>Moderado<\/td><td>440C<\/td><\/tr><tr><td>Baixo<\/td><td>Controlled<\/td><td>Moderado<\/td><td>Either \u2014 finish drives outcome<\/td><\/tr><\/tbody><\/table><\/figure><p>Neither grade is universally superior. The selection should be documented in the knife specification and formally linked to the water-quality SOP for the specific line.<\/p><h3 class=\"wp-block-heading\" id=\"ca1217b6-7686-420a-930a-722fbcfce13b\">Finish and Passivation Dominance<\/h3><p>For black-speck control, surface condition often matters more than alloy choice. A properly polished and passivated 420 knife will outperform a poorly finished 440C knife in contamination-sensitive applications. Grade selection sets the performance ceiling; surface engineering determines where the knife actually operates within it.<\/p><p><strong>Resumidamente:<\/strong> the 420 vs 440C decision comes down to which failure mode governs your line \u2014 choose 440C for edge retention under abrasive load in a stable loop, and 420 for corrosion tolerance and polishability when loop chemistry varies or sanitization intensity is high.<\/p><hr class=\"wp-block-separator has-alpha-channel-opacity\"\/><h2 class=\"wp-block-heading\" id=\"1d502ca4-7b00-4529-b06b-5434e3de5f17\">Engenharia de Superf\u00edcie para Facas Industriais<\/h2><h3 class=\"wp-block-heading\" id=\"49dd046f-3089-46a0-acc4-da2f327250dc\">Surface Roughness and Polish Targets<\/h3><p>O <a href=\"https:\/\/www.assda.asn.au\/blog\/304-pickling-and-passivation-of-stainless-steel\" target=\"_blank\" rel=\"noreferrer noopener\"><strong><em>Australian Stainless Steel Development Association<\/em><\/strong><\/a> identifies Ra \u2264 0.5 \u00b5m as a critical surface roughness threshold for maintaining effective passive-film integrity on abraded stainless steel. For medical-grade pelletizer knives, apply the following zonal targets:<\/p><ul><li><strong>Cutting edge and die-face contact zones<\/strong>: Ra \u2264 0.4 \u00b5m (\u2264 0.2 \u00b5m preferred for highest cleanability)<\/li>\n\n<li><strong>Side flanks and blade body<\/strong>: Ra \u2264 0.8 \u00b5m<\/li>\n\n<li><strong>Bore and mounting surfaces<\/strong>: Ra \u2264 1.6 \u00b5m<\/li><\/ul><p>These targets require CBN or diamond grinding followed by mechanical polishing. Electropolishing is optional but improves passive-film uniformity and eliminates the micro-crevice points that trap biomass or corrosion product on conventionally polished surfaces.<\/p><h3 class=\"wp-block-heading\" id=\"7d6ea242-102e-45ac-b28f-adefb7e40619\">ASTM A967 Passivation and Verification<\/h3><p><a href=\"https:\/\/store.astm.org\/a0967_a0967m-25.html\" target=\"_blank\" rel=\"noreferrer noopener\"><em><strong>ASTM A967\/A967M<\/strong><\/em><\/a> is the governing specification for chemical passivation of stainless steel parts. It covers both nitric acid and citric acid treatment protocols, with defined concentration, temperature, and immersion-time windows for each.<\/p><p>Passivation removes free iron contamination \u2014 grinding swarf, embedded tooling particles, iron-tool marks \u2014 that would otherwise become active corrosion sites in service. It does not alter surface roughness or repair mechanical defects. The specification requires the treated part to show no etching, pitting, or frosting on visual inspection after treatment.<\/p><p>Verification methods specified under ASTM A967 include the water immersion test, high-humidity exposure (48 h at 95\u2013100% RH), salt spray per ASTM B117, copper sulfate test, and potassium ferricyanide\u2013nitric acid test. For medical-grade knife supply, the potassium ferricyanide test or salt spray test should be the minimum acceptance criterion, with results documented on the material certificate.<\/p><h3 class=\"wp-block-heading\" id=\"153c25b5-b693-4bc5-a8ec-ee4c66486388\">Coating Choices and Flake Risk<\/h3><p>Thin hard coatings (TiN, TiAlN, DLC) can extend knife life in abrasive polymers, but they carry a flake-risk burden in medical applications. The three coating families used on pelletizer knives differ sharply in what they deposit, how thick they sit, and what they demand of the substrate.<\/p><figure class=\"wp-block-table\"><table><tbody><tr><th>Revestimento<\/th><th>Typical thickness<\/th><th>Hardness \/ friction<\/th><th>Medical-grade suitability<\/th><\/tr><tr><td>PVD TiN \/ TiAlN<\/td><td>1\u20134 \u00b5m<\/td><td>~2,000\u20133,500 HV; COF 0.4\u20130.6<\/td><td>Permitted with validated adhesion; hard and diffusion-bonded to substrate<\/td><\/tr><tr><td>DLC (a-C:H, ta-C)<\/td><td>1\u20133 \u00b5m<\/td><td>2,000\u20136,000 HV; COF 0.05\u20130.15<\/td><td>Lower friction, but internal stress and flake sensitivity require interlayer control<\/td><\/tr><tr><td>PTFE (dry-film lubricant)<\/td><td>5\u201325 \u00b5m<\/td><td>Soft; applied as a low-friction layer<\/td><td>Generally not preferred where the coating contacts the pellet stream \u2014 soft film sheds<\/td><\/tr><\/tbody><\/table><\/figure><p>Two thickness effects matter for a medical pelletizer knife. First, <strong>PVD hard coatings at 1\u20134 \u00b5m usually need no dimensional compensation<\/strong> \u2014 a 2 \u00b5m film adds 2 \u00b5m per coated face, well inside the \u00b10.005 mm flatness and typical edge tolerances for precision pelletizer knives. Second, <strong>PTFE at 5\u201325 \u00b5m is thick enough to shift geometry<\/strong>: a fully coated diameter can grow 10\u201350 \u00b5m, which measurably alters edge relief, slot width, and knife-to-die contact unless the drawing carries an explicit coating allowance. For a knife specified to \u00b10.005 mm\/100 mm flatness, that is not a rounding error \u2014 it is a design change.<\/p><p>Flake risk can be quantified rather than assumed. Adhesion is measured by scratch testing per <em><strong><a href=\"https:\/\/www.iso.org\/standard\/35450.html\" target=\"_blank\" rel=\"noreferrer noopener\">ISO 20502<\/a>,<\/strong><\/em> which defines progressive critical loads (Lc): Lc1 marks the first cohesive cracking, Lc2 the onset of adhesive chipping, and Lc3 the first spalling. Published TiN-on-stainless studies report Lc values around <a href=\"https:\/\/eprints.whiterose.ac.uk\/id\/eprint\/185139\/6\/Understanding%20of%20fracture%20conditions%20and%20material%20response%20in%20a%20model%20TiN%20film%20stainless%20steel%20substrate%20system%20A%20cross%20sectional%20scratch%20test%20study.pdf\" target=\"_blank\" rel=\"noreferrer noopener\"><em><strong>20 N, 40 N, and 65 N<\/strong><\/em><\/a> for the successive failure stages, while well-adhered DLC duplex systems with chromium interlayers exceed <a href=\"https:\/\/pdfs.semanticscholar.org\/1464\/84e14d4790f91506be802fcb4e57701cba11.pdf\" target=\"_blank\" rel=\"noreferrer noopener\"><em><strong>49\u201351 N<\/strong><\/em><\/a> at first critical load. A practical acceptance framework for medical tooling follows from those values:<\/p><figure class=\"wp-block-table\"><table><tbody><tr><th>Adhesion criterion<\/th><th>Lc2 (adhesive chipping)<\/th><th>Rockwell HF class<\/th><th>Risk interpretation<\/th><\/tr><tr><td>Minimum acceptable<\/td><td>\u2265 30 N<\/td><td>HF 1\u20133<\/td><td>Coating survives service loading; monitor at tool change<\/td><\/tr><tr><td>Robust industrial<\/td><td>\u2265 50 N<\/td><td>HF 1\u20132<\/td><td>Low flake probability under cyclic thermal and mechanical load<\/td><\/tr><tr><td>Preferred for medical<\/td><td>\u2265 70 N<\/td><td>HF 1<\/td><td>Negligible flake risk with controlled interlayer and roughness<\/td><\/tr><tr><td>Reject<\/td><td>&lt; 20 N<\/td><td>HF 5\u20136<\/td><td>Coating not industrially admissible; HF6 is a known fail class even for DLC<\/td><\/tr><\/tbody><\/table><\/figure><p>A coating without a bonding interlayer regularly lands in the HF5\u2013HF6 range and should not be deployed on a medical line. Substrate preparation is not optional: a chromium or Cr\/CrC interlayer plus controlled pre-treatment is what moves DLC from a few newtons of critical load to the 70 N class. Require the coating supplier to report Lc2 values and the Rockwell HF class on the certificate, measured on the same substrate and heat treatment as the delivered knife, not on a generic test coupon.<\/p><blockquote class=\"wp-block-quote is-layout-flow wp-block-quote-is-layout-flow\"><p><strong>\u26a0\ufe0f Aviso<\/strong>: In ISO 13485-regulated lines, any knife coating must be validated for flake-generation behavior under process conditions before deployment. An unvalidated coating change is a formal process change requiring documented risk assessment and change control.<\/p><\/blockquote><p>For most medical-grade polyolefin, PET, and PEEK pelletizing applications, uncoated and well-passivated 420 or 440C is the lower-risk choice. Where a coating is justified by abrasive loading, specify a PVD TiN or TiAlN system at 2\u20134 \u00b5m with a documented Lc2 \u2265 50 N, and confirm the added thickness does not disturb the knife-to-die contact recorded during installation.<\/p><p><strong>Resumidamente:<\/strong> surface finish and passivation are the primary contamination controls in medical polymer pelletizing \u2014 alloy grade sets the ceiling, but Ra \u2264 0.4 \u00b5m and verified ASTM A967 passivation are what keep metallic specks out of the product stream.<\/p><hr class=\"wp-block-separator has-alpha-channel-opacity\"\/><h2 class=\"wp-block-heading\" id=\"2cf61271-a3ee-4d5f-af28-505403c6e19c\">Controle do Anel de \u00c1gua Aquecida<\/h2><div class=\"wp-block-image\"><figure class=\"aligncenter size-full is-resized\"><img loading=\"lazy\" decoding=\"async\" width=\"800\" height=\"800\" src=\"https:\/\/maxtormetal.com\/wp-content\/uploads\/2023\/10\/Plastic-crusher-blade-main21.jpg\" alt=\"Warm-Water Loop Control pelletizing knives\" class=\"wp-image-3186\" style=\"aspect-ratio:1.3333333333333333;object-fit:cover;width:568px;height:auto\" srcset=\"https:\/\/maxtormetal.com\/wp-content\/uploads\/2023\/10\/Plastic-crusher-blade-main21.jpg 800w, https:\/\/maxtormetal.com\/wp-content\/uploads\/2023\/10\/Plastic-crusher-blade-main21-300x300.jpg 300w, https:\/\/maxtormetal.com\/wp-content\/uploads\/2023\/10\/Plastic-crusher-blade-main21-150x150.jpg 150w, https:\/\/maxtormetal.com\/wp-content\/uploads\/2023\/10\/Plastic-crusher-blade-main21-768x768.jpg 768w, https:\/\/maxtormetal.com\/wp-content\/uploads\/2023\/10\/Plastic-crusher-blade-main21-600x600.jpg 600w, https:\/\/maxtormetal.com\/wp-content\/uploads\/2023\/10\/Plastic-crusher-blade-main21-100x100.jpg 100w\" sizes=\"(max-width: 800px) 100vw, 800px\" \/><\/figure><\/div><h3 class=\"wp-block-heading\" id=\"7b8d796e-cc62-4fcf-b1c9-1bf4f26a5383\">Water Chemistry and Numeric Targets<\/h3><p>Applying NIH closed-loop water chemistry guidance and stainless steel corrosion control principles to a warm-water pelletizer system yields the following parameter framework. The water-loop disciplines here apply equally to TPU and EVA lines \u2014 for the intersection of loop control and knife coating selection in those materials, see <a href=\"https:\/\/maxtormetal.com\/pt\/tpu-underwater-pelletizing-knife-gumming-dlc-vs-ptfe\/\" target=\"_blank\" rel=\"noreferrer noopener\"><em><strong>Ader\u00eancia de facas na peletiza\u00e7\u00e3o subaqu\u00e1tica de TPU: sele\u00e7\u00e3o de revestimentos DLC vs PTFE e janelas operacionais.<\/strong><\/em><\/a>.<\/p><figure class=\"wp-block-table\"><table><tbody><tr><th>Par\u00e2metro<\/th><th>Target Range<\/th><th>Action Limit<\/th><\/tr><tr><td>pH<\/td><td>7.0 \u2013 9.0<\/td><td>&lt; 6.5 or &gt; 10.0: investigate makeup source<\/td><\/tr><tr><td>Chlorides<\/td><td>&lt; 50 ppm<\/td><td>&gt; 100 ppm: drain and recharge loop<\/td><\/tr><tr><td>Total iron<\/td><td>&lt; 0.3 mg\/L<\/td><td>&gt; 1.0 mg\/L: flush and identify corrosion source<\/td><\/tr><tr><td>Condutividade<\/td><td>100 \u2013 400 \u00b5S\/cm<\/td><td>&gt; 800 \u00b5S\/cm: check makeup water and dosing<\/td><\/tr><tr><td>Suspended solids<\/td><td>&lt; 5 mg\/L<\/td><td>&gt; 20 mg\/L: increase filtration frequency<\/td><\/tr><\/tbody><\/table><\/figure><p>The NIH technical bulletin targets pH 9.0\u201310.5 for carbon steel service. For a loop dominated by stainless steel contact surfaces, the lower setpoint range of 7.0\u20139.0 is defensible and reduces scaling risk while maintaining passive-film stability.<\/p><blockquote class=\"wp-block-quote is-layout-flow wp-block-quote-is-layout-flow\"><p><strong>Dica profissional<\/strong>: A rising chloride trend in a closed loop is almost always a signal of external ingress \u2014 makeup water contamination, resin leachables, or cleaning chemical carryover. Trend the data; do not treat chloride as a one-time acceptance check.<\/p><\/blockquote><h3 class=\"wp-block-heading\" id=\"1ac86bc7-ee98-4422-8103-0f88e1d270a6\">Filtration Architecture and Hydraulics<\/h3><p>A two-stage filtration architecture is the practical minimum for medical-grade loops:<\/p><ol><li><strong>Coarse strainer (100\u2013200 \u00b5m mesh)<\/strong>: Installed on the main circulation line upstream of the pump and die head. Captures flakes, scale, and large debris. Schedule cleaning against the differential pressure indicator, not a fixed calendar interval.<\/li>\n\n<li><strong>Side-stream bag or cartridge filter (5\u201325 \u00b5m)<\/strong>: Polishes approximately 5\u201310% of total loop flow continuously. Effective at capturing magnetite fines that pass the coarse strainer and would otherwise reach the die face.<\/li><\/ol><h3 class=\"wp-block-heading\" id=\"f7ab002e-06be-4a36-9cfe-d3a290954239\">Sanitation and Degassing Strategy<\/h3><div class=\"wp-block-image\"><figure class=\"aligncenter size-large\"><img loading=\"lazy\" decoding=\"async\" width=\"1024\" height=\"683\" src=\"https:\/\/maxtormetal.com\/wp-content\/uploads\/2026\/09\/image-3-1024x683.jpeg\" alt=\"Schematic of a closed warm-water loop with staged filters, UV sterilization unit, pump, and pelletizer \u2014 showing contamination control points\" class=\"wp-image-8101\" srcset=\"https:\/\/maxtormetal.com\/wp-content\/uploads\/2026\/09\/image-3-1024x683.jpeg 1024w, https:\/\/maxtormetal.com\/wp-content\/uploads\/2026\/09\/image-3-300x200.jpeg 300w, https:\/\/maxtormetal.com\/wp-content\/uploads\/2026\/09\/image-3-768x512.jpeg 768w, https:\/\/maxtormetal.com\/wp-content\/uploads\/2026\/09\/image-3-18x12.jpeg 18w, https:\/\/maxtormetal.com\/wp-content\/uploads\/2026\/09\/image-3-600x400.jpeg 600w, https:\/\/maxtormetal.com\/wp-content\/uploads\/2026\/09\/image-3.jpeg 1536w\" sizes=\"(max-width: 1024px) 100vw, 1024px\" \/><\/figure><\/div><p>UV sterilization at 254 nm (minimum 30 mJ\/cm\u00b2 dose) on the main return line suppresses biofilm accumulation that concentrates corrosion products and increases under-deposit attack on wetted surfaces.<\/p><p>A membrane degasser or closed expansion tank with nitrogen blanket reduces dissolved oxygen to below 0.1 mg\/L on critical lines. Dissolved oxygen is the primary accelerant of pitting corrosion in warm recirculating loops \u2014 reducing it is one of the highest-return engineering controls available.<\/p><p>Following any maintenance event involving pipe or component replacement, flush the system of debris and passivate new stainless surfaces before returning to production, consistent with NIH guidance on system startup after tie-ins.<\/p><p><strong>Resumidamente:<\/strong> the warm-water loop is not a utility \u2014 it is a process variable, and without pH, conductivity, and filtration targets codified in a formal SOP linked to the knife specification, loop-sourced black specks will recur regardless of knife grade.<\/p><hr class=\"wp-block-separator has-alpha-channel-opacity\"\/><h2 class=\"wp-block-heading\" id=\"dd1041c6-066a-44bb-8a37-90bd9f0527e7\">Especifica\u00e7\u00f5es e Procedimentos Operacionais Padr\u00e3o (POP\/SOP)<\/h2><h3 class=\"wp-block-heading\" id=\"b2865e30-2ff2-4d26-bd4d-7d6d0b2618a3\">Controlled A\/B Trial: 420 vs 440C on a Medical-Grade PP Line<\/h3><p>The selection logic above becomes concrete when it is tested under a controlled comparison. The following trial was run by Maxtor Metal&#8217;s engineering team on an anonymized medical polymer compounding line producing medical-grade PP pellets. The goal was not to declare a universally superior grade, but to quantify the difference in particle-quality stability and edge retention under identical process conditions.<\/p><p><strong>Trial setup and controlled variables<\/strong><\/p><figure class=\"wp-block-table\"><table><tbody><tr><th>Par\u00e2metro<\/th><th>Control Requirement<\/th><\/tr><tr><td>Melt temperature<\/td><td>\u00b13 \u00b0C<\/td><\/tr><tr><td>Pelletizer speed<\/td><td>\u00b12%<\/td><\/tr><tr><td>Temperatura da \u00e1gua<\/td><td>\u00b12 \u00b0C<\/td><\/tr><tr><td>Water flow<\/td><td>\u00b13%<\/td><\/tr><tr><td>Taxa de transfer\u00eancia<\/td><td>\u00b12\u20133%<\/td><\/tr><tr><td>Die plate<\/td><td>Same plate for both runs<\/td><\/tr><tr><td>Polymer grade<\/td><td>Same grade (medical-grade PP, unfilled, 0% regrind)<\/td><\/tr><tr><td>Knife pressure<\/td><td>Held identical for A and B<\/td><\/tr><tr><td>Item<\/td><td>Case A \u2014 420<\/td><\/tr><tr><td>Ind\u00fastria<\/td><td>Medical polymer compounding<\/td><\/tr><tr><td>Filler \/ regrind<\/td><td>None \/ 0%<\/td><\/tr><tr><td>Line<\/td><td>Underwater pelletizer, ~2\u20133 t\/h<\/td><\/tr><tr><td>Tooling<\/td><td>AISI 420, approx. 52\u201355 HRC<\/td><\/tr><tr><td>Acabamento da superf\u00edcie<\/td><td>Polished, Ra \u2264 0.4 \u00b5m<\/td><\/tr><tr><td>Run period<\/td><td>Approx. 6\u20138 weeks<\/td><\/tr><tr><td>Cumulative output<\/td><td>Approx. 250\u2013350 t<\/td><\/tr><\/tbody><\/table><\/figure><p><strong>Measurement sequence<\/strong><\/p><ol><li><strong>Incoming blade inspection<\/strong> \u2014 hardness, thickness, edge straightness, edge roughness, and visual inspection under magnification.<\/li>\n\n<li><strong>Cutter installation<\/strong> \u2014 record cutter-head runout, blade protrusion, knife-to-die contact, and tightening torque.<\/li>\n\n<li><strong>Start-up stabilization<\/strong> \u2014 the first 30\u201360 minutes of data are excluded from the A\/B comparison, since start-up pellet quality does not represent steady-state production.<\/li>\n\n<li><strong>Production sampling<\/strong> \u2014 every 4 hours: collect a 500\u20131,000 g pellet sample; sieve for fines; inspect 1,000 pellets for size and shape; count black specks per the specified sampling method.<\/li>\n\n<li><strong>Blade condition<\/strong> \u2014 at each planned stop: microscope inspection of the edge, wear-land measurement, micro-chipping check, and cumulative tonnage logged.<\/li><\/ol><p><strong>Observed outcome (A vs B)<\/strong><\/p><figure class=\"wp-block-table\"><table><tbody><tr><th>KPI<\/th><th>420 (A)<\/th><th>440C (B)<\/th><th>Direction<\/th><\/tr><tr><td>Black specks<\/td><td>approx. 18\u201330 ppm<\/td><td>approx. 8\u201315 ppm<\/td><td>\u2193 approx. 40\u201360%<\/td><\/tr><tr><td>Multas<\/td><td>approx. 0.30\u20130.50 wt%<\/td><td>approx. 0.18\u20130.30 wt%<\/td><td>\u2193 approx. 30\u201340%<\/td><\/tr><tr><td>Off-size \/ irregular pellets<\/td><td>approx. 0.8\u20131.3%<\/td><td>approx. 0.4\u20130.8%<\/td><td>\u2193 approx. 30\u201350%<\/td><\/tr><tr><td>Tool life to regrind<\/td><td>approx. 180\u2013260 h<\/td><td>approx. 300\u2013420 h<\/td><td>\u2191 approx. 1.5\u20131.8\u00d7<\/td><\/tr><tr><td>Unplanned knife changes<\/td><td>approx. 3\u20134 per 6 weeks<\/td><td>approx. 1\u20132 per 6 weeks<\/td><td>\u2193 approx. 40\u201360%<\/td><\/tr><tr><td>Production yield<\/td><td>approx. 98.5\u201399.1%<\/td><td>approx. 99.1\u201399.5%<\/td><td>+ approx. 0.3\u20130.6 pp<\/td><\/tr><\/tbody><\/table><\/figure><p><strong>How the failure mode actually developed<\/strong><\/p><p>The 420 blades were not &#8220;non-conforming&#8221; at the outset. For the first 2\u20133 weeks, pellet dimensions were normal, fines held around 0.3\u20130.4 wt%, and black specks stayed low. As cumulative output increased, a predictable drift appeared: the edge dulled slightly, cutting load rose, the die-face contact shifted toward a more abrasive wiping action, and trace polymer residue began to accumulate near the blade and die plate. Cleaning temporarily cleared the symptom, but it returned after further running.<\/p><p>The first corrective attempt was simply to raise knife pressure. Tails dropped briefly, but the black-speck issue persisted and edge wear accelerated. Only then was the grade changed from 420 to 440C while holding knife pressure, pelletizer speed, die plate, and polymer formulation constant.<\/p><blockquote class=\"wp-block-quote is-layout-flow wp-block-quote-is-layout-flow\"><p><strong>Conclus\u00e3o principal<\/strong>: During the controlled trial, black-spec occurrence decreased by approximately 40\u201360% after changing from hardened 420 to 440C blades. This is a trial result under the stated conditions \u2014 not a universal guarantee, since black-spec contamination in medical plastics can also originate from pellet conveying, dust, and material handling rather than the cutting tool alone.<\/p><\/blockquote><p>For context, both grades are established martensitic stainless steels in the medical tooling family, and ASTM F899 lists 420 and 440C within the martensitic stainless system used for surgical instruments. That is why the comparison is a genuine qualification question rather than a manufactured contrast \u2014 and why the honest conclusion is conditional:<\/p><blockquote class=\"wp-block-quote is-layout-flow wp-block-quote-is-layout-flow\"><p><strong>Engineering conclusion<\/strong>: The A\/B trial indicated that 440C provided longer edge retention and lower particle-quality drift under the tested medical-grade PP conditions, while 420 remained a viable option where toughness, machining cost, and moderate wear requirements were prioritized.<\/p><\/blockquote><p>Maxtor Metal&#8217;s own technical documentation lists 420\/440C martensitic stainless with Ra \u2264 0.4 \u00b5m mirror finishing as a material system for medical-grade and food-packaging pelletizer applications. This is a brand-side engineering basis; it should not be presented as independent third-party validation.<\/p><p><strong>Resumidamente:<\/strong> a knife specification that stops at alloy grade and hardness is incomplete \u2014 material traceability, surface finish, passivation verification, and a linked water-loop SOP are what make a pelletizer knife program auditable to ISO 13485 expectations.<\/p><hr class=\"wp-block-separator has-alpha-channel-opacity\"\/><h3 class=\"wp-block-heading\" id=\"675a9c0e-ccb6-4396-b25b-45215dd087e4\">Knife Spec Template (420\/440C)<\/h3><p>A formalized knife specification eliminates ambiguity in procurement and incoming inspection. The following framework reflects the precision-grinding capabilities and documentation practices \u2014 including EN 10204 MTC 3.1 certificates \u2014 that apply to custom pelletizer knife programs for technical and medical polymer lines. Maxtor Metal&#8217;s approach to custom knife manufacturing integrates material traceability, heat-treatment records, and verified surface finish from a single source.<\/p><figure class=\"wp-block-table\"><table><tbody><tr><th>Atributo<\/th><th>AISI 420 Specification<\/th><th>AISI 440C Specification<\/th><\/tr><tr><td>Base material<\/td><td>AISI 420 (UNS S42000)<\/td><td>AISI 440C (UNS S44004)<\/td><\/tr><tr><td>Tratamento t\u00e9rmico<\/td><td>Vacuum hardened + tempered; 50\u201352 HRC \u00b11<\/td><td>Vacuum hardened + tempered; 58\u201360 HRC \u00b11<\/td><\/tr><tr><td>Surface finish \u2014 cutting edge<\/td><td>Ra \u2264 0.4 \u00b5m<\/td><td>Ra \u2264 0.4 \u00b5m<\/td><\/tr><tr><td>Surface finish \u2014 blade body<\/td><td>Ra \u2264 0.8 \u00b5m<\/td><td>Ra \u2264 0.8 \u00b5m<\/td><\/tr><tr><td>Passivation standard<\/td><td>ASTM A967\/A967M, citric or nitric acid method<\/td><td>ASTM A967\/A967M, citric or nitric acid method<\/td><\/tr><tr><td>Passivation verification<\/td><td>Potassium ferricyanide test, pass<\/td><td>Potassium ferricyanide test, pass<\/td><\/tr><tr><td>Material certificate<\/td><td>EN 10204 MTC 3.1<\/td><td>EN 10204 MTC 3.1<\/td><\/tr><tr><td>Dimensional tolerance<\/td><td>Per drawing; flatness \u2264 0.005 mm\/100 mm<\/td><td>Per drawing; flatness \u2264 0.005 mm\/100 mm<\/td><\/tr><tr><td>Revestimento<\/td><td>Sem revestimento<\/td><td>Sem revestimento<\/td><\/tr><\/tbody><\/table><\/figure><p>For a full overview of knife formats and configurations available for underwater and die-face pelletizing systems, see Maxtor Metal&#8217;s <a href=\"https:\/\/maxtormetal.com\/pt\/industrial-blades-for-plastic-processing-guide\/\" target=\"_blank\" rel=\"noreferrer noopener\"><em><strong>industrial blades for plastic processing guide<\/strong><\/em><\/a>, which covers material options, dimensional standards, and application-matched configurations across pelletizer types.<\/p><h3 class=\"wp-block-heading\" id=\"44c1b7d4-ae22-4a76-8977-4516ea02fecc\">Water-Loop Quality Specification<\/h3><p>The water-loop SOP must be formally linked to the knife specification to create a coherent, auditable contamination-prevention program. Key document elements include: makeup water source and treatment with acceptance criteria; chemical inhibitor type, dose, and monitoring interval; filtration change schedule; analytical sampling plan (weekly pH and conductivity; monthly total iron and chlorides during normal operation; daily during startup or post-maintenance); alert and action limits as defined in the chemistry table above; loop flush and passivation procedure for post-maintenance restart; and records retention for a minimum of three years.<\/p><h3 class=\"wp-block-heading\" id=\"d6a89774-b160-4af4-8e9a-3f3c56f320f6\">Maintenance, Inspection, and Records<\/h3><p>For knife assemblies: inspect cutting edges and die-face contact surfaces at each tool change for chipping, pitting, and corrosion discoloration; record regrind cycles per knife set and replace at the minimum height specified in the OEM drawing; retain hardness test and passivation verification records for each incoming lot; log every observed black-speck event with date, lot number, retained pellet sample, and investigative findings.<\/p><hr class=\"wp-block-separator has-alpha-channel-opacity\"\/><h2 class=\"wp-block-heading\" id=\"ee449fd9-74f1-4a6d-9740-4f7e79c9f843\">Diagn\u00f3stico e Recupera\u00e7\u00e3o<\/h2><div class=\"wp-block-image\"><figure class=\"aligncenter size-full is-resized\"><img loading=\"lazy\" decoding=\"async\" width=\"800\" height=\"800\" src=\"https:\/\/maxtormetal.com\/wp-content\/uploads\/2023\/10\/Plastic-pelletizer-knife-main1.jpg\" alt=\"Diagnostics and Recovery pelletizing blades\" class=\"wp-image-3195\" style=\"width:572px;height:auto\" srcset=\"https:\/\/maxtormetal.com\/wp-content\/uploads\/2023\/10\/Plastic-pelletizer-knife-main1.jpg 800w, https:\/\/maxtormetal.com\/wp-content\/uploads\/2023\/10\/Plastic-pelletizer-knife-main1-300x300.jpg 300w, https:\/\/maxtormetal.com\/wp-content\/uploads\/2023\/10\/Plastic-pelletizer-knife-main1-150x150.jpg 150w, https:\/\/maxtormetal.com\/wp-content\/uploads\/2023\/10\/Plastic-pelletizer-knife-main1-768x768.jpg 768w, https:\/\/maxtormetal.com\/wp-content\/uploads\/2023\/10\/Plastic-pelletizer-knife-main1-600x600.jpg 600w, https:\/\/maxtormetal.com\/wp-content\/uploads\/2023\/10\/Plastic-pelletizer-knife-main1-100x100.jpg 100w\" sizes=\"(max-width: 800px) 100vw, 800px\" \/><\/figure><\/div><h3 class=\"wp-block-heading\" id=\"0befb608-57be-4d61-9a4a-eee99c5fdf48\">Microscopy and Magnet Tests<\/h3><p>When a black-speck event occurs, the initial classification requires no laboratory equipment. Spread a 200 g pellet sample on white paper and draw a rare-earth disc magnet across the surface at 5\u201310 mm standoff. Metallic specks migrate toward the magnet; polymer carbonization particles remain stationary.<\/p><p>For confirmation, mount five to ten particles from each category at 40\u2013100\u00d7 magnification. Metallic particles show angular fractures and bright metallic surfaces, with possible brown-black magnetite smear under oblique illumination. Polymer particles are amorphous, compressible, and show laminar carbonization layers when cross-sectioned with a razor.<\/p><h3 class=\"wp-block-heading\" id=\"d4fba1fd-2baf-4b1d-b79a-ee6a3556cc97\">Correlation with Water Metrics<\/h3><p>Plot total iron and suspended solids values from the loop log against the production record for the same period. A sustained upward trend in loop iron typically correlates with metallic speck events that appear within 24\u201372 hours \u2014 the lag reflecting loop volume and filtration efficiency. When total iron exceeds 1.0 mg\/L without a corresponding maintenance event, treat it as an active corrosion indicator and inspect wetted surfaces before the next production run.<\/p><h3 class=\"wp-block-heading\" id=\"6d64f237-5659-4929-bb23-5dd2585e81fc\">Remediation and Verification Steps<\/h3><ol><li><strong>Drain and flush the warm-water loop<\/strong> with clean deionized water until effluent total iron is below 0.1 mg\/L<\/li>\n\n<li><strong>Inspect the knife set<\/strong> for edge chipping, pitting, and corrosion \u2014 replace if any corrosion pitting is visible on the die-face contact zone<\/li>\n\n<li><strong>Verify passivation<\/strong> on replacement knives before installation (potassium ferricyanide or salt spray per ASTM A967)<\/li>\n\n<li><strong>Restart the loop<\/strong> with a fresh inhibitor charge; verify pH, conductivity, and chloride are within specification before resuming production<\/li>\n\n<li><strong>Run a 15-minute qualification lot<\/strong>: inspect pellets under 10\u00d7 magnification and perform a magnet test before releasing<\/li>\n\n<li><strong>Close the CAPA record<\/strong> with documented root cause, corrective action, and preventive action \u2014 including any spec or SOP revision triggered by the event<\/li><\/ol><p><strong>Resumidamente:<\/strong> when black specks recur after a knife or loop change, the magnet test and microscopy results together tell you whether the source is metallic or polymeric \u2014 treating the wrong source first is the most common recovery delay.<\/p><hr class=\"wp-block-separator has-alpha-channel-opacity\"\/><h2 class=\"wp-block-heading\" id=\"e1714766-abe3-447f-b494-59c545e9e208\">FAQ<\/h2><p><strong>P: Qual \u00e9 a principal diferen\u00e7a entre o a\u00e7o inoxid\u00e1vel 420 e o 440C para facas granuladoras?<\/strong><\/p><p>R: O 420 cont\u00e9m 0,15\u20130,40% de carbono e 12\u201314% de cromo, atingindo 50\u201352 HRC com boa resist\u00eancia \u00e0 corros\u00e3o e maior tenacidade. O 440C cont\u00e9m 0,95\u20131,20% de carbono e 16\u201318% de cromo, alcan\u00e7ando 58\u201360 HRC com resist\u00eancia ao desgaste superior, por\u00e9m menor tenacidade e toler\u00e2ncia \u00e0 corros\u00e3o ligeiramente reduzida devido \u00e0 forma\u00e7\u00e3o de carbonetos. Selecione 440C para pol\u00edmeros abrasivos com \u00e1gua de circuito controlada; escolha 420 para ambientes corrosivos ou com rotinas intensivas de higieniza\u00e7\u00e3o.<\/p><p><strong>P: Por que surgem pontos pretos met\u00e1licos nos gr\u00e2nulos (pellets) de pol\u00edmero de grau m\u00e9dico?<\/strong><\/p><p>R: Os pontos pretos met\u00e1licos geralmente se originam de tr\u00eas fontes: lascamento microestrutural (micro-chipping) do corte da faca por fratura fr\u00e1gil ou carga de impacto; res\u00edduos de corros\u00e3o (magnetita, \u00f3xido de ferro) migrados do circuito de \u00e1gua morna; ou contamina\u00e7\u00e3o por ferro livre em superf\u00edcies de facas inadequadamente passivadas. O teste com \u00edm\u00e3 e a microscopia \u00f3ptica s\u00e3o as ferramentas de diagn\u00f3stico de primeira linha para diferenciar pontos met\u00e1licos de pontos polim\u00e9ricos.<\/p><p><strong>What surface roughness (Ra) should medical-grade pelletizer knives achieve?<\/strong><\/p><p>The cutting edge and die-face contact zones should reach Ra \u2264 0.4 \u00b5m. The Australian Stainless Steel Development Association identifies Ra \u2264 0.5 \u00b5m as the critical threshold for passive-film integrity on abraded stainless steel \u2014 below that value, corrosion resistance improves markedly. Coarser finishes increase both corrosion susceptibility and particulate release into the pellet stream.<\/p><p><strong>What does ASTM A967 passivation accomplish, and is it sufficient to prevent black specks?<\/strong><\/p><p>ASTM A967\/A967M passivation removes free iron, embedded grinding particles, and surface contamination from stainless steel, restoring the passive chromium oxide film. It does not change surface roughness or repair mechanical defects. Passivation is necessary but not sufficient \u2014 the knife must be polished to the target Ra before passivation, and the warm-water loop must be independently managed.<\/p><p><strong>What chloride concentration is safe in a warm-water pelletizer loop running stainless steel knives?<\/strong><\/p><p>Chloride should remain below 50 ppm for 420 or 440C knife service. Concentrations above 100 ppm accelerate pitting corrosion on both grades, and the resulting corrosion debris migrates directly into the pellet stream. The NIH closed-loop corrosion guidance recommends trending chloride as a continuous process indicator rather than treating it as a one-time acceptance criterion.<\/p><p><strong>Should TiN or DLC coatings be used on medical-grade pelletizer knives?<\/strong><\/p><p>Only after full validation of flake-generation behavior under actual process conditions. Thin hard coatings extend knife life in abrasive applications, but coating delamination is a real contamination risk on medical lines. Under ISO 13485, introducing a knife coating is a formal process change requiring documented risk assessment and validation before production use.<\/p><p><strong>How often should the warm-water loop be sampled and treated?<\/strong><\/p><p>Sample pH and conductivity weekly during normal operation; measure total iron and chlorides monthly. After any maintenance event involving pipe or component replacement, perform a full loop flush and recharge before resuming production. If total iron exceeds 1.0 mg\/L without a recent maintenance explanation, treat it as an active corrosion event and flush the loop.<\/p><p><strong>What documentation should accompany a medical-grade pelletizer knife order?<\/strong><\/p><p>Each lot should include an EN 10204 MTC 3.1 material test certificate with heat and chemistry traceability, hardness test results (HRC measured on the supplied parts), surface finish measurement certificate, and passivation verification record (potassium ferricyanide or salt spray per ASTM A967). These records support ISO 13485 supplier qualification and incoming inspection requirements.<\/p><hr class=\"wp-block-separator has-alpha-channel-opacity\"\/><h2 class=\"wp-block-heading\" id=\"76b195b3-396a-4b2e-b87f-c9b444038193\">Conclus\u00e3o<\/h2><p>For medical-grade polymer pelletizing, the 420 vs 440C decision is a risk-matched engineering choice, not a single correct answer. 440C delivers superior edge retention for abrasive polymers in controlled loop-water environments. 420 offers better toughness, polishability, and corrosion tolerance where sanitization intensity or water quality variability is higher.<\/p><p>In both cases, surface finish at Ra \u2264 0.4 \u00b5m on the cutting edge and verified passivation per ASTM A967\/A967M are the primary contamination-prevention controls \u2014 not the alloy grade alone. A codified warm-water loop SOP, with chemistry targets, two-stage filtration, UV sanitation, and degassing, closes the corrosion pathway that generates metallic black specks. Combined with knife specifications tied to EN 10204 MTC 3.1 documentation, these controls create an auditable program fully aligned to ISO 13485 expectations.<\/p><p>The implementation roadmap is straightforward: specify the knife correctly for the polymer and water-chemistry combination, verify surface finish and passivation at incoming inspection, establish the loop chemistry SOP and analytical monitoring plan, and close the loop \u2014 literally and procedurally \u2014 between knife performance and water quality data.<\/p><p>For precision-ground <a href=\"https:\/\/maxtormetal.com\/pt\/produto\/lamina-granuladora-de-plastico\/\" target=\"_blank\" rel=\"noreferrer noopener\"><em><strong>plastic pelletizer blades and pelletizing knives<\/strong><\/em><\/a> manufactured to the tolerances described above, contact the Maxtor Metal engineering team with your machine drawing and polymer program requirements.<\/p><hr class=\"wp-block-separator has-alpha-channel-opacity\"\/><h2 class=\"wp-block-heading\" id=\"153ed552-7293-433c-808e-4fab369d724b\">Normas e Refer\u00eancias<\/h2><p>The specifications and controls in this article are drawn from the following standards and sources. Readers qualifying a specific line should consult the current revision of each document.<\/p><p><strong>Metallurgy and material data<\/strong><\/p><ul><li>AISI 420 (UNS S42000) and AISI 440C (UNS S44004) grade data \u2014 Atlas Steels Grade 420 data sheet; Crucible Industries 440C material data.<\/li>\n\n<li>ASTM F899 \u2014 Standard Specification for Wrought Stainless Steels for Surgical Instruments.<\/li>\n\n<li>ASTM A276 \/ A314 \u2014 Standard specifications for stainless steel bars and billets (440C and 420 forms).<\/li>\n\n<li>ASM Handbook, Vol. 16 \u2014 Machining (tool material selection context).<\/li><\/ul><p><strong>Surface finish and passivation<\/strong><\/p><ul><li>ASSDA (Australian Stainless Steel Development Association) \u2014 pickling and passivation of stainless steel; Ra \u2264 0.5 \u00b5m passive-film threshold.<\/li>\n\n<li>ASTM A967\/A967M \u2014 Standard Specification for Chemical Passivation Treatments for Stainless Steel Parts.<\/li>\n\n<li>ASTM B117 \u2014 Standard Practice for Operating Salt Spray (Fog) Apparatus.<\/li><\/ul><p><strong>Coating adhesion<\/strong><\/p><ul><li>ISO 20502 \u2014 Fine ceramics (advanced ceramics, advanced technical ceramics) \u2014 Determination of adhesion of ceramic coatings by scratch testing.<\/li>\n\n<li>Published TiN-on-stainless and DLC duplex scratch-test critical-load studies (Lc1\/Lc2\/Lc3 data).<\/li><\/ul><p><strong>Water-loop corrosion and control<\/strong><\/p><ul><li>NIH Technical Bulletin (2025) \u2014 Corrosion in Closed Loop Water Systems, Part 2: Evaluation and Prevention.<\/li>\n\n<li>ISO 13485:2016 \u2014 Medical devices \u2014 Quality management systems.<\/li>\n\n<li>FDA 21 CFR 211.67(a) \u2014 Equipment cleaning and maintenance.<\/li><\/ul><p><strong>Brand engineering basis<\/strong><\/p><ul><li>Maxtor Metal technical documentation \u2014 420\/440C martensitic stainless with Ra \u2264 0.4 \u00b5m mirror finishing as a material system for medical-grade and food-packaging pelletizer applications. Note: brand-side engineering basis, not independent third-party validation.<\/li><\/ul><hr class=\"wp-block-separator has-alpha-channel-opacity\"\/><h2 class=\"wp-block-heading\" id=\"8a7cb229-9f2a-48ae-bf42-604fce433ae7\">Sobre o autor<\/h2><p><strong>Nancy Wu<\/strong> is a Senior Manufacturing Engineer in Production Engineering at Maxtor Metal, with 12 years of experience in industrial cutting-blade manufacturing. She specializes in the machining and material behavior of common industrial blade steels \u2014 including SKD11, 420, 440C, D2, M2, H13, powder metallurgy steels, and cemented carbide \u2014 covering their processing characteristics, material properties, and coating behavior, alongside high-precision CNC grinding programming. She holds SME CMfgE, PMP, Six Sigma Black Belt, and ASM International certifications.<\/p>","protected":false},"excerpt":{"rendered":"<p>Quick answer: In medical-grade polymer pelletizing, 420 and 440C stainless are not interchangeable: 420 offers higher toughness, better polishability, and more corrosion tolerance in sanitization-intensive or variable-chemistry water loops; 440C delivers superior edge retention for abrasive polymers in stable, controlled-chemistry loops. Both grades require Ra \u2264 0.4 \u00b5m mirror finish and verified passivation per ASTM [&hellip;]<\/p>\n","protected":false},"author":1,"featured_media":3185,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1,1120],"tags":[1303,1304],"yoast_head":"<!-- This site is optimized with the Yoast SEO Premium plugin v23.6 (Yoast SEO v23.6) - https:\/\/yoast.com\/wordpress\/plugins\/seo\/ -->\n<title>Medical-Grade Polymer Processing: 420 vs 440C Knife Guide<\/title>\n<meta name=\"description\" content=\"Steel selection, surface finish, and water-loop control for pelletizer knives in Medical-Grade Polymer Processing: 420 vs 440C.\" \/>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/maxtormetal.com\/pt\/medical-grade-polymer-processing-knife-420-vs-440c\/\" \/>\n<meta property=\"og:locale\" content=\"pt_PT\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Medical-Grade Polymer Processing Knife Selection: 420 vs 440C Stainless, Surface Finish, and Water-Loop Control\" \/>\n<meta property=\"og:description\" content=\"Steel selection, surface finish, and water-loop control for pelletizer knives in Medical-Grade Polymer Processing: 420 vs 440C.\" \/>\n<meta property=\"og:url\" content=\"https:\/\/maxtormetal.com\/pt\/medical-grade-polymer-processing-knife-420-vs-440c\/\" \/>\n<meta property=\"og:site_name\" content=\"Maxtor Metal | Custom Industrial Blade Manufacturer &amp; 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