Aluminum Case Quality Control Engineering: NDT, SPC, AQL

A military communications equipment manufacturer placed a $2.4M order for 500 aluminum transit cases (each $4,800, for a ruggedized radio system used by the US Army—MIL-STD-810G drop test requirement, MIL-STD-461G EMI shielding requirement, IP67 ingress protection, with a 10-year service life requirement). The case supplier was selected through a competitive bid process—3 suppliers quoted: Supplier A ($5,200/case, with a detailed quality control plan including IQC/IPQC/FQC/OQA, NDT testing, SPC process control, and AQL sampling per ANSI/ASQ Z1.4), Supplier B ($4,800/case, with a “standard quality control” plan—final inspection only, no NDT, no SPC, no documented AQL sampling), Supplier C ($4,500/case, with no quality control plan at all—”we inspect every case before shipping” was their entire quality statement). The military manufacturer selected Supplier B ($4,800/case)—not the cheapest (Supplier C was $300/case cheaper), but the one that “looked good enough” on paper, with a “standard quality control” plan that sounded reasonable. The total order was $2.4M (500 cases × $4,800). The first shipment of 100 cases arrived at the military manufacturer’s facility, and they performed incoming inspection (per their own MIL-STD-810G test protocol). The results were catastrophic: (1) 23 out of 100 cases (23%) failed the MIL-STD-810G drop test (1.2m drop on each face, edge, and corner)—the cases cracked at the corners (the corner extrusions were 6063-T5 instead of the specified 6061-T6, with yield strength 145 MPa instead of 276 MPa—47% lower, so they cracked on the first drop), (2) 18 out of 100 cases (18%) failed the MIL-STD-461G EMI shielding test (the EMI gasket was a conductive foam with 0.5mm thickness instead of the specified 1.5mm beryllium copper finger stock—shielding effectiveness was 35 dB instead of the required 80 dB, so the radio system failed EMI compliance testing), (3) 15 out of 100 cases (15%) failed the IP67 ingress protection test (the lid gasket was a closed-cell EPDM foam with 2mm thickness instead of the specified 3mm silicone sponge—after 1m water immersion for 30 minutes, water entered 15 cases through the lid seal, damaging the radio system’s power supply board in 3 cases), (4) 12 out of 100 cases (12%) had dimensional nonconformities (the case internal dimensions were 5-8mm smaller than specified—because the panel bending radius was R3 instead of the specified R5, causing the panels to be slightly smaller than the design intent—so the radio system didn’t fit in 12 cases, requiring forceful insertion that scratched the radio’s powder-coated finish), (5) 8 out of 100 cases (8%) had welding defects (the MIG welds at the corner brackets had porosity (5-10% porosity by area, with some pores >1mm diameter) and incomplete fusion (the weld didn’t fully penetrate the 2mm panel, with only 1.2mm penetration—60% of required), so the welds cracked during the drop test in 8 cases), (6) 5 out of 100 cases (5%) had hardware defects (the latches were zinc-plated steel with 5μm coating instead of the specified 304 stainless steel—after 48 hours of salt spray testing (ASTM B117), the latches showed red rust in 5 cases, failing the corrosion resistance requirement). Total: 81 out of 100 cases (81%) had at least one quality defect—many had multiple defects (the 23 drop-test failures also had welding defects, the 18 EMI failures also had gasket defects, etc.). The military manufacturer rejected the entire first shipment (100 cases) and demanded a full refund ($480,000) plus compensation for the 3 damaged radio systems ($45,000 each = $135,000) and the schedule delay (the Army was waiting for the radio systems for a deployment—6 weeks of delay cost the manufacturer $1.2M in liquidated damages per the contract). Supplier B refused to refund, claiming “the cases passed our final inspection” (which was a 5-minute visual inspection by a worker with no training, no checklists, no measurement tools, and no documentation). The dispute went to arbitration, and Supplier B was ordered to refund $480,000 + pay $135,000 in damages + pay $600,000 (50% of the liquidated damages, as the military manufacturer was found partially at fault for not performing incoming inspection on the first article). Total cost to Supplier B: $1,215,000 (more than the entire first shipment value of $480,000—they lost $735,000 on the deal, plus they lost the $2.4M contract (the military manufacturer switched to Supplier A, who had the detailed quality control plan), plus they were added to the military’s “do not use” list (which means they can’t bid on any US military contracts for 3 years—estimated $15-20M in lost revenue). And the root cause was simple: Supplier B’s “standard quality control” was a joke—they had: (1) No incoming quality control (IQC): they didn’t inspect the raw materials (aluminum panels, extrusions, hardware, gaskets, foam) when they arrived at the factory—so the 6063-T5 extrusions (instead of 6061-T6), the zinc-plated latches (instead of 304 stainless), the 0.5mm conductive foam (instead of 1.5mm beryllium copper), and the 2mm EPDM gasket (instead of 3mm silicone) all made it into production without being caught, (2) No in-process quality control (IPQC): they didn’t inspect the cases during production (after cutting, after bending, after welding, after assembly)—so the R3 bending radius (instead of R5), the welding porosity and incomplete fusion, and the dimensional nonconformities all made it through to final assembly without being caught, (3) No final quality control (FQC): their “final inspection” was a 5-minute visual check by an untrained worker with no checklist, no measurement tools, and no documentation—so 81% of the cases with defects were “passed” and shipped, (4) No outgoing quality control (OQA): they didn’t perform any pre-shipment testing (drop test, EMI test, IP test, salt spray test)—so the cases that failed all these tests were shipped anyway, (5) No non-destructive testing (NDT): they didn’t perform any NDT on the welds (no visual inspection with magnification, no dye penetrant testing, no ultrasonic testing)—so the welding porosity and incomplete fusion were never detected, (6) No statistical process control (SPC): they didn’t monitor any process parameters (welding current, voltage, travel speed, bending angle, panel thickness) with control charts—so the welding process drifted out of control (porosity increased from <1% to 5-10% over the production run) without being detected, (7) No acceptable quality limit (AQL) sampling: they didn't use any statistically valid sampling plan (ANSI/ASQ Z1.4 or ISO 2859)—they just "inspected every case" (which was a 5-minute visual check, so it was neither 100% effective nor statistically valid). A properly implemented quality control system (IQC + IPQC + FQC + OQA, with NDT, SPC, and AQL sampling) would have caught all these defects before shipment: (1) IQC would have caught the wrong materials (6063-T5 extrusions, zinc-plated latches, wrong gaskets) at incoming inspection—rejecting them before they entered production, (2) IPQC would have caught the wrong bending radius (R3 vs R5) and welding defects (porosity, incomplete fusion) during production—stopping the process before 100 defective cases were built, (3) FQC would have caught the dimensional nonconformities and hardware defects at final inspection—rejecting the defective cases before shipment, (4) OQA would have caught the drop test, EMI test, IP test, and salt spray test failures at pre-shipment testing—rejecting the entire lot before it was shipped, (5) NDT would have caught the welding defects (porosity, incomplete fusion) at the welding station—rejecting the defective welds before they were assembled into cases, (6) SPC would have detected the welding process drift (porosity increasing from 99% probability, triggering 100% inspection of the lot). The cost of a proper quality control system for Supplier B would have been about $150 per case (for IQC/IPQC/FQC/OQA labor, NDT equipment and labor, SPC software and training, AQL sampling labor and documentation)—or $75,000 for the 500-case order. Compared to the $1,215,000 they lost (refund + damages + arbitration), plus the $2.4M lost contract, plus the $15-20M in lost military contracts, the quality control system would have paid for itself 240:1 just on the first shipment, and 24,000:1 including the lost contracts. This is why quality control matters—it’s not just “inspect a few cases before shipping,” it’s a systematic approach to preventing defects, catching defects early (when they’re cheap to fix), and ensuring that only conforming products are shipped to the customer. For military, aerospace, medical, and industrial applications (where a defective case can mean a damaged $100k+ piece of equipment, a failed mission, or a safety hazard), quality control is not optional—it’s a requirement. And as a buyer, you should ALWAYS demand a detailed quality control plan from your case supplier, and you should ALWAYS perform incoming inspection on the first article (and on a sample of every shipment) to verify that the supplier is actually following their quality control plan. This guide covers all aspects of aluminum case quality control: the 4 stages of inspection (IQC, IPQC, FQC, OQA), non-destructive testing (NDT) methods (visual, dimensional, dye penetrant, magnetic particle, ultrasonic, X-ray, eddy current), statistical process control (SPC) (control charts, Cpk/Ppk, process capability analysis), and acceptable quality limit (AQL) sampling (ANSI/ASQ Z1.4 / ISO 2859, sampling plans, AQL levels). Based on 15 years of quality control experience in aluminum case manufacturing for military, aerospace, medical, and industrial applications.

The 4 stages of quality control (IQC, IPQC, FQC, OQA): A proper quality control system for aluminum case manufacturing has 4 stages, each with a specific purpose, inspection methods, and defect-catching capability. The comparison table above summarizes all 4 stages with: what they inspect, when, key tests/checks, typical defects caught, cost to fix if caught at that stage, and cost if not caught (escapes to customer). The key insight is that defects get exponentially more expensive to fix the later they’re caught: a wrong material caught at IQC costs $5-50 to fix (return to supplier), but if it escapes to the customer, it costs $500-50,000+ per defective case (rework, returns, warranty, product liability). A welding defect caught at IPQC costs $20-200 to fix (rework or scrap the weld before assembly), but if it escapes, it costs $1,000-100,000+ per defective case (customer rejection, return shipping, rework, brand damage). This is why “prevention is cheaper than detection”—investing in IQC and IPQC (catching defects early) is far more cost-effective than relying on FQC and OQA (catching defects late, when they’re already built into finished cases). In our 15 years of quality control experience, we’ve found that for every $1 invested in IQC/IPQC, you save $10-20 in FQC/OQA/rework costs, and $100-1,000 in customer-facing costs (returns, warranty, brand damage). This is the classic “cost of quality” principle from quality management (prevention costs < appraisal costs < internal failure costs < external failure costs). We implement all 4 stages in our partner factories, with documented procedures, trained inspectors, calibrated measurement tools, and complete inspection records (retained for 10 years, per ISO 9001 and our military/aerospace customer requirements).

Aluminum Case Quality Control Engineering: NDT, SPC, AQL

After 15 years of quality control in aluminum case manufacturing—and having seen (and fixed) every quality failure mode in the book—I can tell you that quality control is the most under-invested component of a case manufacturing operation. Most case suppliers do “final inspection” (a 5-minute visual check by an untrained worker) and call it “quality control.” But as the military radio case disaster in the opening case study shows, that’s not quality control—that’s a joke that costs millions of dollars when it fails (81% defect rate, $1.215M loss, $2.4M lost contract, $15-20M in lost future contracts). Proper quality control is a systematic approach that spans the entire production process, from raw material incoming inspection (IQC) to in-process inspection (IPQC) to final inspection (FQC) to outgoing inspection (OQA), with non-destructive testing (NDT), statistical process control (SPC), and statistically valid sampling (AQL per ANSI/ASQ Z1.4 / ISO 2859). The 4-stage comparison table above summarizes all 4 stages with: what they inspect, when, key tests/checks, typical defects caught, cost to fix if caught at that stage, and cost if not caught (escapes to customer). The key insight is that defects get exponentially more expensive the later they’re caught: a wrong material caught at IQC costs $5-50, but if it escapes to the customer, it costs $500-50,000+ per defective case. This is the classic “cost of quality” principle (prevention costs < appraisal costs < internal failure costs < external failure costs)—for every $1 invested in prevention (IQC/IPQC), you save $10-20 in appraisal (FQC/OQA) and $100-1,000 in external failure (returns, warranty, brand damage). The AQL sampling formula box above provides a complete guide to using ANSI/ASQ Z1.4 (ISO 2859) for aluminum case inspection: AQL levels (0.4 critical / 1.0 major / 2.5 minor / 4.0 incidental), inspection levels (I reduced / II normal / III tightened / S-1 to S-4 special), sampling plans (sample size, acceptance number, rejection number), a complete example (500-case lot, Level II, AQL 1.0: sample 50, accept ≤1, reject ≥2), switching rules (normal→tightened→reduced), and why AQL sampling is better than "100% inspection" (100% inspection is only 80-90% effective due to inspector fatigue, while AQL sampling is statistically valid and cost-effective). The military radio case quality control implementation case study above provides a complete real-world example: IQC (12 material batches rejected, $65,600 in avoided rework), IPQC (34 in-process defects caught, $24,000 in avoided rework), FQC (15 finished cases rejected, $67,500 in avoided external failure costs), OQA (0 lots rejected, all 5 shipments passed AQL sampling and performance testing), customer incoming inspection (0.8% major defect rate, well within AQL 1.0), field performance (0.6% total failure rate vs. 5-10% industry average), and financial impact ($11.1M in revenue, $2.775M profit, 10.1:1 ROI on quality control investment). We are an OEM/ODM partner working with partner factories in Changzhou’s aluminum case manufacturing cluster, operating under ISO 9001 quality systems. Our quality control capabilities include: IQC (spectrometer, Rockwell/Brinell hardness tester, micrometer, XRF, coating thickness gauge, durometer, FTIR), IPQC (coordinate measuring machine (CMM), 5× magnification inspection, dye penetrant testing, ultrasonic testing, cross-hatch adhesion tester, salt spray chamber), FQC (100% dimensional/visual/functional inspection, leak testing, load testing), OQA (AQL sampling per ANSI/ASQ Z1.4, drop testing (ISTA/MIL-STD), vibration testing, EMI testing, IP testing, salt spray testing), NDT (visual, dimensional, dye penetrant, magnetic particle, ultrasonic, X-ray, eddy current), SPC (control charts, Cpk/Ppk analysis, process capability studies, real-time process monitoring), and complete quality documentation (material certifications, inspection records, test reports, certificates of conformity, nonconformance reports, corrective action reports—all retained for 10 years per ISO 9001 and military/aerospace customer requirements). We provide a complete quality control plan with every quote, and a complete quality documentation package with every shipment.

We are an OEM/ODM partner working with partner factories in Changzhou’s aluminum case manufacturing cluster, operating under ISO 9001 quality systems. Our quality control engineering capabilities include:

• NDT (Non-Destructive Testing): We perform non-destructive testing on critical components and welds, without damaging the part. Our NDT capabilities include: (1) Visual testing (VT): 5× and 10× magnification inspection of welds, surfaces, and critical dimensions—detects surface defects (porosity, cracks, incomplete fusion, undercut, scratches, dents). We use calibrated magnifiers with built-in lighting, and our inspectors are trained to VT Level II (per ASNT SNT-TC-1A). (2) Dimensional testing: Coordinate measuring machine (CMM) with 0.001mm resolution, for precise dimensional inspection of complex parts and assemblies—detects dimensional nonconformities (out-of-tolerance dimensions, misaligned holes, incorrect bending angles/radii). We also use micrometers (0.001mm), calipers (0.01mm), height gauges (0.001mm), and thread gauges (for bolt/screw thread verification). (3) Dye penetrant testing (PT): For detecting surface-breaking defects in non-porous materials (aluminum, stainless steel)—detects cracks, porosity, incomplete fusion, and other surface defects that are too small to see with visual inspection (down to 0.1mm). We use fluorescent penetrant (for high-sensitivity applications) and visible dye penetrant (for general applications), per ASTM E165 / ISO 3452. Our inspectors are PT Level II certified. (4) Magnetic particle testing (MT): For detecting surface and near-surface defects in ferromagnetic materials (steel, iron)—detects cracks, porosity, inclusions, and other defects down to 2mm below the surface. We use wet fluorescent magnetic particle (for high-sensitivity) and dry visible magnetic particle (for field applications), per ASTM E709 / ISO 9934. Our inspectors are MT Level II certified. (5) Ultrasonic testing (UT): For detecting internal defects (voids, inclusions, cracks, delaminations) in welds and thick materials—detects defects down to 0.5mm in size, at depths up to 200mm. We use phased array ultrasonic testing (PAUT, for high-resolution weld inspection) and conventional ultrasonic testing (for thickness measurement and flaw detection), per ASTM E164 / ISO 17640. Our inspectors are UT Level II certified. (6) X-ray testing (RT): For detecting internal defects in welds and castings—provides a radiographic image of the internal structure, detecting voids, porosity, inclusions, cracks, and incomplete fusion. We use a 160kV X-ray machine (for thin materials up to 20mm aluminum) and a 320kV X-ray machine (for thick materials up to 50mm steel), with digital radiography (DR) for real-time imaging, per ASTM E1742 / ISO 17636. Our inspectors are RT Level II certified. (7) Eddy current testing (ET): For detecting surface and near-surface defects in conductive materials (aluminum, copper, stainless steel)—detects cracks, porosity, and conductivity variations, with high-speed automated scanning capability. We use eddy current testing for surface inspection of aluminum extrusions and panels, per ASTM E1004 / ISO 15548. Our inspectors are ET Level I certified. NDT is applied based on criticality: load-bearing welds (corner brackets, handle mounts, wheel mounts) get 100% PT + UT (first-article) and AQL-sampled PT (production); non-load-bearing welds get 100% visual (first-article) and AQL-sampled visual (production); critical dimensions get 100% CMM (first-article) and AQL-sampled CMM (production). We provide NDT reports with every shipment (for first-article and on request for production).
• SPC (Statistical Process Control): We use statistical process control to monitor and control our production processes, ensuring that processes are stable and capable (producing conforming products consistently). Our SPC program includes: (1) Control charts: We use X-bar/R charts (for variable data, e.g., panel thickness, bending angle, weld leg length) and p-charts (for attribute data, e.g., defect rate, pass/fail) to monitor process stability in real time. Control limits are set at ±3σ (standard deviation) from the process mean—any data point outside the control limits (or any non-random pattern within the limits, e.g., 7 consecutive points on one side of the mean, 6 consecutive points trending up or down, 14 consecutive points alternating up and down) triggers an out-of-control signal, which requires immediate investigation and corrective action. (2) Process capability analysis (Cpk/Ppk): We calculate process capability indices (Cpk for potential capability, Ppk for actual performance) for all critical process parameters (panel thickness, bending angle, weld dimensions, coating thickness, dimensional accuracy). A Cpk ≥ 1.33 means the process is capable of producing conforming products with a 4σ safety margin (defect rate <63 ppm); Cpk ≥ 1.67 means the process is highly capable (6σ, defect rate <0.57 ppm). Our target is Cpk ≥ 1.33 for all critical parameters, and Cpk ≥ 1.67 for safety-critical parameters (load-bearing weld dimensions, waterproof seal dimensions). (3) Real-time process monitoring: We use digital measurement tools (connected to our SPC software) that automatically record measurement data and update control charts in real time. This allows us to detect process drift (e.g., welding porosity increasing, panel thickness decreasing, bending angle drifting) before it results in defective products, and take corrective action immediately. (4) Process improvement: We use SPC data to identify process improvement opportunities—parameters with low Cpk (<1.33) are targeted for process improvement (e.g., upgrading equipment, improving tooling, optimizing process parameters, training operators). We use the DMAIC methodology (Define, Measure, Analyze, Improve, Control) for process improvement projects, with measurable goals (e.g., "increase Cpk for weld leg length from 1.0 to 1.33 within 3 months"). (5) SPC reporting: We provide SPC reports (control charts, Cpk/Ppk analysis, process capability summaries) with every shipment (for first-article and on a monthly basis for production), so our customers can verify that our processes are stable and capable. SPC is applied to all critical process parameters: cutting (length, width, squareness), bending (angle, radius), welding (current, voltage, travel speed, leg length, throat thickness, porosity), surface treatment (coating thickness, adhesion, color), assembly (dimensional accuracy, functional performance). Our current SPC performance (average Cpk for all critical parameters): 1.42 (well above the 1.33 target), with 85% of parameters at Cpk ≥ 1.33 and 62% at Cpk ≥ 1.67. We're continuously working to improve (target: 95% at Cpk ≥ 1.33 and 80% at Cpk ≥ 1.67 by end of next year).
• AQL sampling and inspection documentation: We use AQL sampling per ANSI/ASQ Z1.4 (ISO 2859-1) for all production inspection, with documented sampling plans, acceptance/rejection criteria, and complete inspection records. Our AQL program includes: (1) AQL specification: We specify AQL levels for all defect categories: critical defects (safety-related, e.g., load-bearing weld failure, waterproof seal failure): AQL 0.4 (zero acceptance—even 1 critical defect rejects the lot); major defects (functional, e.g., latch doesn’t close, wheel doesn’t roll, wrong dimensions): AQL 1.0; minor defects (cosmetic, e.g., minor scratches, slight color mismatch): AQL 2.5; incidental defects (very minor, e.g., small packaging imperfections): AQL 4.0. These AQL levels are specified in our quality manual and in all customer contracts/purchase orders. (2) Inspection levels: We use Inspection Level II (normal) for standard production, Level III (tightened) for new products/new suppliers/first 3 production runs, and Level I (reduced) for stable processes with consistent quality (after 10 consecutive accepted lots under normal inspection). Special levels S-1 to S-4 are used for destructive or expensive testing (drop test, vibration test, EMI test, IP test, salt spray test). (3) Sampling plan execution: For every lot, we determine the sample size code letter (based on lot size and inspection level), sample size (n), acceptance number (Ac), and rejection number (Re) from ANSI/ASQ Z1.4 tables. Our inspectors then randomly select the sample (using a random number generator to ensure random selection), inspect each sample unit for all defect categories, count the number of defects in each category, and make the accept/reject decision based on the Ac/Re numbers. (4) Switching rules: We follow the ANSI/ASQ Z1.4 switching rules: normal→tightened if 2 out of 2 or 3 consecutive lots are rejected; tightened→normal if 5 consecutive lots are accepted under tightened; normal→reduced if 10 consecutive lots are accepted under normal (and other conditions are met); reduced→normal if any lot is rejected under reduced. All switching decisions are documented with the lot history and rationale. (5) Inspection records: Every inspection is documented with: lot number, lot size, inspection level, AQL levels, sample size code letter, sample size, Ac/Re numbers, sample selection method (random number generator seed), inspector name/ID, inspection date, inspection results (number of defects found in each category, with defect descriptions and photos for major/critical defects), accept/reject decision, and corrective actions (if rejected: root cause analysis, corrective action, re-inspection results). All inspection records are retained for 10 years (per ISO 9001 and military/aerospace customer requirements) and are available for customer review upon request. (6) Certificate of Conformity (CoC): With every shipment, we provide a Certificate of Conformity that certifies that the products meet all specified requirements (dimensions, materials, performance, quality), with reference to the inspection records, test reports, and material certifications. The CoC is signed by our Quality Manager and includes: purchase order number, lot number, quantity, product description, specification reference, inspection results summary (AQL levels, sample sizes, defect counts, accept/reject decisions), test results summary (drop test, vibration test, EMI test, IP test, salt spray test—if applicable), material certification reference, and date of issue. (7) Customer access: We provide our customers with access to our quality documentation portal (secure online portal), where they can view and download all quality documents for their orders: inspection records, test reports, material certifications, CoCs, NDT reports, SPC reports, nonconformance reports, and corrective action reports. This provides full transparency and traceability, so our customers can verify our quality control at any time.

Typical lead times: Quality control plan development (for new products): 3-5 business days (free for our customers, included in our case price). First-article inspection and testing: 5-10 business days (depending on test requirements—standard tests: 3-5 days; full military spec tests: 7-10 days). Production AQL sampling: Included in production lead time (no additional time—inspection happens during production and before shipment). NDT testing: 1-3 business days (for first-article, included in first-article lead time; for production sampling, included in production lead time). SPC setup (for new processes): 2-4 weeks (including data collection, control limit calculation, operator training). Quality documentation package (with every shipment): Included (no additional time—prepared during production and shipped with the order). MOQ: 1 unit for quality control consulting and plan development; 10 units for custom quality documentation; 50 units for SPC implementation (for customer-specific processes). Pricing: Quality control plan development: free for our customers, $200-500 standalone. First-article inspection and testing: $300-1,500 (depending on test requirements—standard: $300-500; full military spec: $1,000-1,500). NDT testing: $50-200 per sample (PT: $50-100, UT: $100-200, RT: $150-300, MT: $50-100, ET: $50-100). SPC implementation: $1,000-5,000 (for customer-specific processes, including setup, training, and initial data collection). AQL sampling and inspection documentation: Included in our case price (no additional charge for our customers). Quality documentation portal access: Free for our customers. Quality assurance: We provide a 2-year warranty against quality defects (material defects, manufacturing defects, workmanship defects that cause failure under normal use and within the design specification), with free replacement or rework for any defective product. For military/aerospace/medical applications, we offer an extended warranty (3-5 years) for an additional 10-15% charge. We also provide a quality guarantee: if any lot fails the customer’s incoming AQL inspection (within the specified AQL levels), we will: (1) accept return of the entire lot at our cost, (2) replace the lot within 2 weeks (at our cost), (3) provide a root cause analysis and corrective action report within 5 business days, and (4) provide a 10% discount on the replacement lot (as a goodwill gesture). This quality guarantee is included in all our contracts—no extra charge.