Interview Questions for Plating Rack Design

Designing plating racks for high-volume production necessitates a holistic approach balancing efficiency, cost-effectiveness, and consistent plating quality. The primary considerations include:

• Throughput Maximization: The rack design must allow for the efficient loading and unloading of a large number of parts within a given timeframe. This often involves optimizing rack geometry to maximize part density while maintaining accessibility.
• Material Selection: Choosing durable, corrosion-resistant materials like titanium, stainless steel (various grades), or even plastics (depending on the plating process) is crucial for longevity and preventing contamination of the plating bath. The material’s compatibility with the plating chemistry is paramount.
• Uniform Current Distribution: The rack’s design directly influences current distribution. Uneven current flow leads to non-uniform plating thickness. Strategies to address this are discussed in later answers.
• Automation Compatibility: In high-volume scenarios, automation is key. The rack should be designed to be easily handled by robotic systems for loading, unloading, and transfer between tanks.
• Cost Optimization: While quality is paramount, minimizing material usage and manufacturing costs is essential for profitability. This requires a balance between robust design and efficient use of materials.
• Part Handling and Protection: The design must safeguard parts from damage during racking, processing, and unracking. This might involve specialized fixtures or cushioning materials.

For example, a high-volume automotive parts plating operation might utilize a custom-designed, automated racking system with precisely positioned contacts to ensure uniform current distribution across thousands of identical parts per hour.

The choice of material for plating racks depends heavily on the specific plating process, the chemicals involved, and the desired lifespan. Several materials are commonly employed:

• Titanium: Excellent corrosion resistance and high strength make titanium ideal for aggressive plating solutions. It’s expensive but crucial for long-term use in harsh environments.
• Stainless Steel (various grades): A cost-effective option, with different grades offering varying corrosion resistance. 316 stainless steel is frequently used due to its superior resistance to many plating chemicals.
• Copper: Relatively inexpensive, but prone to corrosion, limiting its use to less aggressive plating processes. Often used as a conductive layer for other materials.
• Polypropylene and other plastics: Suitable for some applications, especially those involving non-corrosive solutions. They are lightweight and offer good chemical resistance but have limited conductive properties and might require conductive coatings.
• Nickel-plated Steel: Provides a balance between cost and corrosion resistance. The nickel layer enhances the base steel’s resistance to various plating solutions.

For instance, in a chrome plating operation involving highly acidic solutions, titanium racks are preferred due to their exceptional corrosion resistance, ensuring longevity and avoiding contamination. For simpler processes with less aggressive solutions, stainless steel might be a suitable and more economical alternative.

Minimizing part distortion during plating is critical, especially with delicate components. Strategies include:

• Proper Part Support: Design the rack to provide adequate and even support for each part, preventing warping or bending due to the weight of the parts or the plating process. This often involves multiple contact points distributed strategically across the part’s surface.
• Controlled Contact Pressure: Excessive contact pressure can cause distortion. Design contact points with appropriate force to ensure good electrical contact without deforming the part. Spring-loaded contacts or carefully designed clamping mechanisms can be beneficial.
• Balanced Rack Design: Ensure even weight distribution across the rack to prevent tilting or uneven loading in the plating tank, reducing the risk of uneven plating and part deformation.
• Material Selection: Select materials that offer sufficient rigidity to withstand the stresses of the plating process while minimizing the risk of distortion.
• Pre-Plating Inspection: Thoroughly inspecting parts before plating for any existing imperfections or stress can prevent further deformation during the process.

Imagine plating thin, intricate circuit boards: using a flexible material and minimizing contact pressure are vital to avoid bending or damaging them. A carefully designed fixture with multiple low-pressure contact points would be ideal here.

Designing racks for complex-shaped parts presents unique challenges. The primary concerns are:

• Achieving Uniform Current Distribution: Complex shapes often lead to uneven current distribution due to varying distances and surface areas. This necessitates careful placement of contact points and potentially the use of auxiliary anodes or shields.
• Part Support and Masking: Developing appropriate fixtures that provide secure support without obstructing access for plating to all surfaces can be complex. Masking might be required to prevent plating in specific areas.
• Increased Rack Complexity: The design becomes more intricate and potentially more expensive to manufacture, impacting production costs.
• Accessibility for Loading and Unloading: Ensuring efficient loading and unloading of complex parts can be a significant design challenge.

For example, plating a three-dimensional turbine blade requires precise contact points to reach all surfaces while preventing shadows or masking. It might involve a specialized fixture with multiple strategically located contacts and possibly the use of auxiliary anodes to ensure uniform plating thickness across the complex curvature.

Proper current distribution is crucial for uniform plating thickness and quality. Uneven current flow leads to thicker plating in some areas and thinner plating in others, resulting in defects and potentially functional issues.

The goal is to ensure that every surface area of every part receives a similar amount of current density. Factors influencing current distribution:

• Contact Point Placement: Strategic placement of contacts minimizes the resistance pathways and promotes even current distribution. Ideally, the distance from each contact point to every part surface should be relatively consistent.
• Contact Pressure: Insufficient pressure leads to high resistance, causing uneven current flow. Excessive pressure can cause distortion.
• Conductor Size: Using appropriately sized conductors prevents voltage drop along the rack, which can affect current distribution.
• Rack Geometry: The design should minimize sharp bends and obstacles that can hinder current flow.
• Auxiliary Anodes: In complex geometries, auxiliary anodes can be strategically placed to improve current distribution in hard-to-reach areas.

Think of a plating rack as a circuit. A poorly designed rack will act like a circuit with high resistance in certain parts, causing uneven current flow, leading to inconsistencies in plating.

Achieving uniform plating thickness involves a multi-pronged approach that complements proper current distribution:

• Careful Rack Design (as described above): This is the foundation for consistent plating. Even current distribution is the primary factor.
• Agitation of Plating Solution: Keeping the plating solution in motion helps to reduce concentration gradients and ensures that fresh solution is constantly available to all parts of the rack.
Periodic Inspection and Adjustment: Regularly monitor the plating process to check for any uneven plating. Adjustments to the rack design or plating parameters may be necessary.
• Use of Thimbles or Shielding: For parts with complex shapes or areas that are difficult to plate uniformly, using thimbles (small, hollow cylinders around contacts) or shielding can improve current distribution to ensure proper thickness.
• Plating Solution Chemistry Control: Maintaining consistent and correct chemical concentrations and temperatures is vital for predictable and uniform plating.

Imagine plating a batch of precisely sized cylindrical parts. The design should use minimal and symmetric contact points to ensure similar current density reaches every section of each cylinder, ensuring the same plating thickness.

I have extensive experience using various CAD software packages for plating rack design, including SolidWorks, AutoCAD, and Pro/ENGINEER. My proficiency allows me to:

• Create detailed 3D models: This allows for thorough visualization of the rack and its interaction with parts, ensuring proper support, contact, and current distribution.
• Perform simulations: I utilize simulation tools to analyze current distribution, identify potential hotspots or areas of low coverage, and optimize the rack design accordingly.
• Generate manufacturing drawings: Precise and accurate drawings are essential for manufacturing the rack efficiently and to the required specifications.
• Collaborate with manufacturing teams: I work closely with manufacturing personnel to ensure the design is manufacturable, cost-effective, and adheres to industry standards.
• Maintain a design library: A database of past designs allows me to leverage previous work and readily adapt to new projects.

For example, in a recent project involving complex electronic components, I used SolidWorks to create a parametric model, allowing for quick adjustments based on varying component sizes. Simulations helped identify areas where additional contact points were necessary to improve current distribution and ensure uniform plating across the entire batch of components.

Rack shadowing, where parts obstruct the plating solution’s access to other parts, significantly impacts plating uniformity. It leads to uneven plating thickness, potentially causing defects or weakening in shadowed areas. Addressing this involves strategic part arrangement and rack design.

To minimize shadowing, we employ several techniques. First, we optimize part placement, ensuring sufficient space between components for solution flow. This might involve staggered arrangements or using angled fixtures. Secondly, we design racks with open geometries. Avoid dense, clustered designs. Think of it like watering a garden: you wouldn’t place plants so close together that they block each other from the water. Similarly, open racks allow better solution circulation. Finally, we might incorporate agitation systems within the plating tank to improve solution flow and minimize stagnant areas where shadowing is most pronounced.

For example, in plating small, intricate parts, we might use a multi-tiered rack with angled shelves to maximize exposure to the plating solution. Conversely, for large, flat parts, we’d focus on spacing and potentially using strategically placed deflectors to redirect the solution flow.

Ease of loading and unloading is paramount for efficiency and safety. Poorly designed racks can lead to delays, operator fatigue, and increased risk of part damage. We achieve this through several design elements.

• Modular Design: Break down large racks into smaller, easily manageable modules. This allows operators to handle lighter weights and work more ergonomically.
• Quick-Release Mechanisms: Incorporate features like spring-loaded clips or easily removable fixtures to speed up loading and unloading. Think of it as a well-designed toolbox – everything has its place and is easily accessible.
• Ergonomic Handles and Grips: Provide comfortable and secure grips for safe handling, reducing strain on operators.
• Clear Part Orientation: Design the rack to clearly indicate the correct orientation for each part, preventing misalignment and ensuring consistent plating.
• Accessibility: Ensure sufficient space around the rack for easy access from all sides. Avoid designs that make it difficult to reach specific areas.

For instance, a rack for delicate electronic components might feature individual, easily removable carriers for each part, while a rack for larger automotive parts might employ a trolley system for easier movement and loading/unloading.

Safety is paramount in plating rack design. We must consider several factors to prevent accidents and protect operators and the environment.

• Insulation: Proper insulation prevents electrical shock, especially crucial for racks carrying high currents. We use electrically insulating materials to isolate conductive parts from the operator and the tank.
• Material Selection: Choose materials resistant to corrosion and the chemicals used in the plating process to prevent structural failure and chemical leaching.
• Mechanical Strength: Ensure the rack is robust enough to withstand the weight of the parts and the handling during loading, plating, and unloading, preventing collapse or breakage.
• Sharp Edges and Protrusions: Avoid sharp corners or edges that could cause injury. Smooth surfaces and rounded edges improve operator safety.
• Chemical Compatibility: All materials used must be compatible with the plating solutions and cleaning agents, preventing chemical reactions or degradation.

For example, we’d use chemically resistant plastics for insulating components and high-strength stainless steel for the main rack structure, ensuring structural integrity and operator safety.

The contact material’s choice is critical; it must be conductive, chemically inert in the plating solution, and resistant to corrosion. Poor choice leads to poor current distribution, leading to uneven plating, contact corrosion, and even contamination of the plating bath.

Factors influencing material selection include:

• Conductivity: Materials like copper, brass, titanium, and various specialized alloys offer excellent conductivity. Copper is commonly used due to its cost-effectiveness but might require protective coatings in aggressive environments.
• Chemical Resistance: In acidic or alkaline solutions, some materials corrode rapidly. Selecting a chemically inert material prevents contamination and ensures the rack’s longevity.
• Cost: Balancing material cost and performance is key. While some materials offer superior properties, their higher cost may not be justified for all applications.
• Plating Process: The plating solution’s composition dictates the choice of contact material; some materials might react negatively with specific chemicals.

For example, in chrome plating, where highly acidic solutions are used, a titanium or lead-tin alloy contact material might be preferred due to its superior corrosion resistance compared to copper.

Determining the optimal number of parts per rack depends on several factors; it’s a balance between maximizing throughput and ensuring uniform plating.

• Rack Size and Geometry: A larger rack can hold more parts but may lead to increased shadowing. Rack geometry (open vs. dense) also plays a role.
• Part Size and Geometry: Larger parts require more space, reducing the number of parts that can fit per rack.
• Current Density: Exceeding the optimal current density leads to burning or poor plating quality. This limits the number of parts that can be plated simultaneously.
• Plating Solution Capacity: The plating tank’s size and the solution’s replenishment rate affect the number of parts that can be plated without depleting the solution’s concentration.

We often use iterative testing and simulation to find the optimal number. It’s not a one-size-fits-all answer. For example, plating a batch of large, flat sheets might allow fewer parts per rack compared to plating many small, intricate components.

Insulation plays a crucial role in minimizing energy loss and preventing electrical shock hazards. In plating, electrical current flows through the rack and into the parts. Without proper insulation, this current can leak, leading to inefficiency and safety issues.

Insulation is achieved by using non-conductive materials in various rack components. This is important for several reasons:

• Energy Efficiency: By reducing current leakage, insulation improves energy efficiency, leading to cost savings.
• Safety: It prevents electrical shock to operators by isolating the conductive elements.
• Improved Plating: By reducing current loss, insulation promotes more uniform current distribution, improving plating quality.

We often use insulating materials like polymers (e.g., PVC, nylon), ceramics, or epoxy coatings on conductive components to achieve effective insulation. The thickness of the insulation is calculated based on the current carrying capacity of the rack and safety regulations.

Accommodating varied part sizes and geometries within a single run requires a versatile and modular rack design. A ‘one-size-fits-all’ approach is often impractical.

Here’s a multi-faceted approach:

• Modular Design: Create a system of interchangeable modules, each designed for specific part sizes or geometries. This allows customization without redesigning the entire rack.
• Adjustable Fixtures: Use adjustable clamps, hooks, or carriers that can adapt to different part shapes and sizes. Think of it like a highly adaptable workbench with various clamps and vises.
• Customizable Inserts: Develop a system of custom inserts that fit into the main rack structure, allowing easy configuration for different parts.
• Universal Mounting Points: Design the rack with standardized mounting points where different fixtures can be easily attached and secured.

Imagine a rack for an electronics manufacturer handling various circuit boards, connectors, and small components. Using modular design with adjustable fixtures allows them to handle all these parts in a single plating cycle, optimizing production and reducing setup time.

My experience spans various plating processes, including electroless nickel plating, chrome plating, zinc plating, and gold plating. Each process presents unique challenges and influences rack design significantly. For instance, electroless nickel requires racks with consistent surface area exposure to ensure uniform deposition. In contrast, chrome plating, known for its high throwing power (ability to plate recessed areas), allows for more complex part geometries and thus, less intricate rack designs. Zinc plating, often used for corrosion protection, necessitates racks designed to minimize hydrogen embrittlement, a potential issue with this process. Finally, gold plating, often used for its electrical conductivity, demands racks made from materials that won’t contaminate the gold bath, and precise contact points to ensure uniform coverage.

The choice of plating process directly impacts the rack material (e.g., titanium for aggressive chemistries), the rack geometry (e.g., open design for better solution flow in electroless nickel), and the contact points (e.g., high conductivity for gold plating). I consider the chemical compatibility, current distribution requirements, and the potential for part distortion during the process when designing racks for each plating application. A poorly designed rack can lead to uneven plating, defects, and increased production costs.

Sustainability is a core principle in my plating rack designs. I focus on several key areas: material selection, durability, and recyclability. For materials, I prioritize using readily recyclable metals like stainless steel or titanium over less sustainable options. This minimizes waste and environmental impact. Durability is crucial; longer-lasting racks reduce the frequency of replacement, thereby lowering material consumption and energy usage in manufacturing. We also employ advanced coatings to improve corrosion resistance, extending the racks’ lifespan even further. Recyclability is ensured through careful design and material choices, enabling easy disassembly and material separation at the end of the rack’s life. This allows for effective material recovery and reduces landfill burden.

For example, I’ve incorporated modular rack designs where components can be easily replaced instead of scrapping the entire rack. This extends the lifespan significantly, reducing material waste and lowering the overall environmental impact compared to disposable racks. Furthermore, I collaborate with suppliers to source materials from responsible and sustainable sources, verifying their commitment to environmentally conscious practices throughout the supply chain.

Common failure modes of plating racks include corrosion, contact failure, warping, and breakage. Corrosion, particularly in aggressive plating solutions, can lead to rack degradation and contamination of the plating bath. Contact failure, resulting from loose connections or poor contact design, leads to uneven plating or incomplete coverage. Warping, usually caused by thermal stress during the plating process, can affect part alignment and lead to plating inconsistencies. Finally, breakage, often due to mechanical stress or fatigue, necessitates costly repairs or replacements.

Prevention strategies involve meticulous material selection for corrosion resistance (e.g., using titanium or high-grade stainless steel), designing robust contact points using appropriate materials and clamping mechanisms, and employing stress-relieving techniques during the manufacturing process to minimize warping. Regular inspection and maintenance, including checking for corrosion, loose contacts, and any signs of fatigue, are essential to extend rack life and prevent failures. Using finite element analysis (FEA) during the design phase can help predict potential failure points and optimize the design for improved durability.

Evaluating the cost-effectiveness of a plating rack design involves a holistic approach, considering both initial investment and lifecycle costs. Initial costs include the material cost, manufacturing cost, and design engineering time. Lifecycle costs encompass factors like maintenance, repair, replacement frequency, and the impact of downtime caused by rack failures. A well-designed rack, despite potentially higher initial investment, should reduce lifecycle costs due to increased longevity, reduced maintenance, and minimized production disruptions.

To ensure cost-effectiveness, I conduct a thorough cost analysis which includes estimating the total cost of ownership (TCO) over the entire lifespan of the rack. This involves predicting the number of plating cycles, potential maintenance needs, and the cost of replacing components. This allows for a comparison between different design options, identifying the most economically viable solution. This is frequently performed through comparative life cycle costing to optimize the design considering both initial costs and operating costs over the lifetime of the rack.

Prototyping and testing are integral parts of my design process. I typically start with 3D modeling to create a virtual prototype, allowing for design optimization and refinement before physical production. Following this, I fabricate small-scale prototypes using materials that closely mimic the final production material. These prototypes undergo rigorous testing, simulating the actual plating process with representative parts. This allows me to assess factors such as current distribution, ease of loading and unloading parts, and the overall functionality and durability of the rack under real-world conditions.

For example, I may use a small-scale prototype to test different contact point designs or material combinations under simulated plating conditions, measuring current distribution to identify any hotspots or areas of uneven plating. This iterative process allows for refinements until a satisfactory design is achieved, ensuring the final rack meets all performance criteria before full-scale production.

Addressing corrosion and wear in plating rack design requires a multi-pronged approach. Material selection plays a crucial role; I often use corrosion-resistant materials like titanium, high-grade stainless steel (e.g., 316L), or plastics engineered for chemical resistance. Surface treatments like electropolishing or specialized coatings further enhance corrosion resistance. For wear, strategic design of contact points is essential. I often use robust materials for contact points and incorporate features like spring-loaded contacts or replaceable contact pads to minimize wear and tear. Proper lubrication at contact points can additionally mitigate wear.

For instance, in highly corrosive environments, I might choose titanium racks due to its exceptional corrosion resistance. For contact points prone to heavy wear, I might opt for harder materials like tungsten carbide or use replaceable components. The use of Finite Element Analysis (FEA) can identify areas prone to higher stresses, helping optimize designs to mitigate wear-related failures. This is very important in high volume applications and can save both time and money on maintenance and unplanned downtime.

Incorporating automation features improves efficiency and consistency in the plating process. This can include designing racks with features that enable automated loading and unloading of parts, robotic handling systems, and integrated sensors for real-time monitoring of plating parameters. For example, I might design racks with standardized interfaces that allow for seamless integration with automated handling systems. The design should also account for the specific needs of automated equipment, including clearances and robust construction to handle robotic manipulation without damage.

Automated systems can integrate barcodes or RFID tags for tracking individual parts, improving traceability and inventory management. Designing racks with sensors embedded within the structure to monitor temperature or solution levels within the plating tank adds capabilities for real-time process control and data analysis. This leads to increased efficiency, process optimization, and improved consistency. Consideration for compatibility with existing automation infrastructure is crucial during the design phase to ensure seamless integration.

Designing a plating rack for a new part is a meticulous process that begins with a thorough understanding of the part’s geometry, material, and the desired plating specifications. First, I’d obtain detailed CAD drawings or physical samples of the part. Then, I’d analyze the part’s complexity, identifying areas that require precise plating and those where less precise coverage is acceptable. This informs the rack design, determining the optimal number of contact points, their placement, and the overall rack configuration. For instance, a complex part with intricate details might require a more elaborate jig design with multiple contact points to ensure uniform current distribution. A simple part might only need a few contact points in strategically located areas. Following this analysis, I would create a 3D model of the proposed rack, simulating the plating process to predict current flow and plating uniformity before proceeding to physical prototyping and testing.

Consider, for example, a newly designed automotive part with many recessed areas. I would need to design the rack with carefully positioned contacts to ensure that even these harder-to-reach areas receive adequate plating. If we were to only contact the raised surface areas, those recessed areas would be inadequately plated, leading to corrosion and part failure.

My experience encompasses a wide range of plating rack configurations, each suited to different parts and plating processes. Barrel plating racks are ideal for mass production of small parts, efficiently processing large quantities simultaneously within a rotating drum. The parts are tumbled, resulting in relatively uniform plating, although it’s not ideal for delicate items. Conversely, hook-type racks are more suitable for larger, individually handled parts. These allow for precise placement and better control over plating distribution, minimizing the risk of damage, making it suitable for complex parts. Jig racks are customized solutions, meticulously designed for specific parts requiring very precise plating and consistent current distribution. These are often used for complex geometries and require detailed CAD design and precise manufacturing. I’ve also worked with tray racks for flat components and rotating drum racks for specific applications needing a constant motion during plating.

The choice depends heavily on part geometry, production volume, desired plating quality, and the part’s material. A high-precision part, for example, would necessitate a dedicated jig rack, whereas mass production of simple fasteners would benefit from barrel plating.

Easy cleaning and maintenance are paramount in plating rack design. I always aim to minimize crevices and hard-to-reach areas where plating residues and contaminants can accumulate. This is achieved through using smooth, easily cleanable materials and minimizing the overall complexity of the rack design. Components should be easily removable for thorough cleaning and inspection. For example, I might design the rack with modular components that can be easily disassembled and reassembled, allowing for individual cleaning and replacement of damaged parts.

Furthermore, the materials selected should be highly resistant to corrosion and chemical attack from the plating bath. Materials like titanium are extremely durable and easier to clean than materials prone to corrosion and chemical attack. The design also incorporates features like drainage holes and sloped surfaces to allow for easy draining and rinsing.

Compatibility with the plating bath is critical, impacting both the plating process and the longevity of the rack. The chosen materials must be inert to the plating solution’s chemicals and the operating temperature. For instance, a rack material that reacts with the plating bath can contaminate the solution, leading to poor plating quality and even damage to the rack itself. I conduct thorough material selection based on the specific plating bath chemistry, considering factors like chemical resistance, conductivity, and the plating process parameters.

A typical example would be choosing a titanium rack for a highly corrosive cyanide bath, while a less aggressive bath might permit the use of stainless steel. Thorough research and testing are crucial to verify compatibility and ensure the selected materials are suitable for the long term use, avoiding premature corrosion and rack degradation. Always check the bath’s safety data sheet for materials compatibility recommendations.

Quality control measures are implemented throughout the design and manufacturing process. This begins with rigorous design reviews, ensuring the rack design meets all specifications and quality standards. We utilize Finite Element Analysis (FEA) simulations to predict current distribution and identify potential issues with plating uniformity. Prototypes are rigorously tested for durability, corrosion resistance, and ease of cleaning. During manufacturing, we employ stringent quality checks at each stage, ensuring dimensional accuracy and adherence to design specifications.

Regular inspections of the racks in operation are also crucial. This includes visually inspecting for signs of wear, corrosion, or damage. Any signs of damage or degradation would signal a need for maintenance or replacement of components. Consistent checks are key for timely intervention to avoid production issues.

The conductivity of plating rack materials significantly impacts plating efficiency. High conductivity materials, such as copper, allow for efficient current distribution across the part, resulting in uniform plating thickness and reducing energy consumption. Materials with lower conductivity, like stainless steel, lead to increased resistance, resulting in uneven current distribution and potential localized heating. This unevenness manifests as thicker plating in some areas and thinner plating in others.

For example, a copper rack would generally provide superior current distribution compared to a stainless steel rack. However, the selection must also balance conductivity with chemical resistance and overall cost. Titanium, despite having lower conductivity than copper, is often preferred for its exceptional chemical resistance in aggressive plating baths. The final choice involves a tradeoff between conductivity, corrosion resistance, and cost, optimizing the plating process for the specific application.

Troubleshooting uneven plating thickness involves systematic investigation, focusing on several key areas. First, I’d visually inspect the rack for any signs of damage, corrosion, or loose contacts. Uneven contact pressure can severely impact current distribution. Secondly, I’d analyze the current distribution by using measuring instruments to assess current flow at various points on the rack and on the parts themselves. This would reveal whether there are areas with significantly higher or lower current density, pinpointing potential problems in the design or manufacture of the rack. Thirdly, a review of the plating bath parameters – including temperature, agitation, and solution concentration – is necessary as these can also affect plating uniformity.

For example, if a particular area of the part consistently receives thinner plating, we would check for contact issues in that area. A weak connection could be causing the decreased current flow resulting in uneven plating. Similarly, if the current density readings are dramatically different on the rack, this could indicate the need for a redesigned rack with improved current distribution.