Variability Reduction in Functional, Multi-Layer Materials and Laminates
Honeycomb-core structures for wall panels and doors, rubber-coated metal for sealing joints, decorative film-on-metal for household appliances, multi-layer architectural coatings for protecting building exteriors, and metal-to-metal laminates for vibration damping in automotive applications: All of these are common place examples of multi-layer materials and laminates developed for increased strength and decreased weight, decorative appearance, specific functional requirements, or reduced costs.
For purposes of discussing variability reduction, a specific, functional laminate is selected: material for vehicle brake noise damping. Such materials are widely used to fabricate parts known as brake shims, brake noise dampers or insulators, or damping shims. Shims are attached to the brake pad component in disc brake assemblies. There are many choices of commercially available materials, depending on the complexity of the noise problem and the customer’s expectations for eliminating it. In any case, customers expect consistency in performance, uninterrupted processing into finished parts, dimensions within tolerance, and agreed upon appearance. Unplanned variations in these may be due to numerous root causes.
The figure illustrates a cross-sectional view of the typical elements found in a brake noise damper: (a) a release liner applied to protect the adhesive prior to attachment; (b) the bonding adhesive; (c) surface pre-treatment for the metal substrate; (d) one of two metal substrates in the laminate; (e) viscoelastic core; (f) primer applied to the substrate; and (g) an outer coating that is often rubber-based.
In-service performance of noise dampers depends on the specified dimensions and the properties of each layer which are functions of two factors: (1) the composition, quality, and durability of each layer; and (2) the composition, quality, and durability of each material-to-material interface.
Consider the most common method for manufacturing these high-volume materials: continuous coil coating. The process begins as a bare coil of metal is unwound into the entry end of the processing line. As the metal strip is pulled through, it passes through a number of separate processes that may include metal cleaning, application of corrosion resistant pre-treatment, coating with a primer, then coated with an adhesive or specialty coating, metal-to-metal lamination, application of a release liner over the adhesive. Whenever a liquid coating is applied in a process, the coating is dried and cured in zoned ovens. At the exit end of processing line, the product is rewound. Clearly, there are many “opportunities” to affect the quality of each layer and interface for better or worse.
In developing an understanding of the root causes of performance variations in such materials, the key is a detailed review of basic aspects of the product: its design and its manufacture. The product design phase must consider manufacturing processes capable of converting the raw materials into finished products with consistency. Designing and Processing for Variability Reduction begins with completion of a detailed Failure Modes and Effects Analysis (FMEA); one for the product design required by the intended application and one for manufacturing material and parts with consistent performance. Anyone who has worked on a FMEA knows of its difficulty and time consumption. A consultant experienced with designing multi-layer laminates, with the processes required to manufacture them, and with root cause analyses of failures may be a key player on the team.
For illustration, following are two potential failure modes and potential causes that are starting points for preparation of a FMEA for a brake noise shim.
1. What are potential causes of inadequate or varying damping performance for noise shims?
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Specifications for raw materials have not been adequately defined.
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Parameters related to performance may not have been identified completely.
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Coating composition varies due to lack of control during mixing or processing.
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Does it matter if 101 pph are used in the mix instead of 100 pph?
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Integrity of coating-to-substrate interface varies.
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Uncontrolled variations in cleaning the metal substrate.
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Uncontrolled variations in applying the surface pre-treatment.
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Chromate-based and chromate-free pre-treatments may have different properties.
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Time-temperature profile for processing the coating may not be appropriate.
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Solvent mixture may not be compatible with smooth drying.
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Residual solvent may be present.
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Degree of cure may be incomplete.
2. What are possible causes of poor attachments of shims to brake pads?
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Tolerance for dimensions or shapes of clips for attachment may be too great.
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Heat bonding parameters for an adhesive may not be accurate.
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Heat bonding equipment may not be capable of the required bonding parameters.
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Shelf life of a thermosetting bonding adhesive has been exceeded.
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Cure agent is too reactive resulting in rapid thermal aging.
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Bonding adhesive and coating on brake pad are not compatible.
This article was written by Lead Consultant, Richard Dickinson, Ph.D., a highly skilled chemist with extensive experience in materials and process engineering. Richard can be reached at 317-536-7030, or via email at
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