GMW 14668 Minimum Performance Requirements for Decorative Chrome-Plated Plastic Parts

The “GMW 14668 Minimum Performance Requirements for Decorative Chrome-Plated Plastic Parts” standard, developed by General Motors Worldwide (GMW), describes the minimum performance requirements for decorative chrome-plated plastic parts and components used in automotive applications.

This specification is particularly important for automotive components made from plastics with multi-layer metallic coatings (typically containing copper, nickel, and chromium), such as ABS and ABS+PC. It defines requirements aimed at ensuring the decorative coating maintains its appearance, adhesion, corrosion resistance, and durability throughout the vehicle's lifespan.

The primary goal of the GMW 14668 standard is to ensure that decorative chrome-plated plastic components have adequate functional and cosmetic durability. Chrome-plated plastic parts are commonly used for items such as:

• Interior decorative frames
• Door handle cover
• Gear lever and transmission control cover
• Dashboard decorations
• Switch and control frames
• Exterior decorative cladding
• Grills and wall sconces
• Decorative components related to the rim.

This standard defines the finish primarily as decorative and allows for different looks, including gloss, satin, brushed, dark, and grained finishes, depending on vehicle styling and applicable appearance requirements.

A typical coated plastic component consists of a non-conductive plastic substrate followed by several metallic layers. The copper layer helps provide leveling and corrosion protection, nickel provides most of the corrosion resistance and structural thickness, and chromium provides the characteristic appearance and surface properties.

The GMW 14668 standard covers copper-nickel-chromium electroplated plastic parts, including assemblies. This standard also distinguishes plating approaches and thickness requirements according to the intended application.

A key feature of the GMW 14668 standard is that the requirements vary depending on the component's location and service intensity. For example, the current specification defines indoor applications as follows:

• Type 1: Standard indoor use
• Type 2: Applications requiring higher coating thicknesses or where Type 1 performance is insufficient.

Examples of components that might require a higher-performance Type 2 approach include interior door handles, manual gear shift lever decorative rings, and frames. Exterior components are generally exposed to more severe environmental conditions and may therefore require additional corrosion and mechanical damage resistance.

The GMW 14668 standard also recognizes different chrome plating processes:

• Code C / Code D: Processes using hexavalent chromium etching and trivalent chromium plating.
• Code E / Code F: Processes using etching and trivalent chromium plating that do not contain hexavalent chromium.

Codes are related to internal and external applications.

This distinction is important because automotive manufacturers are increasingly controlling the chemical and environmental properties of coating processes while maintaining the required performance.

GMW 14668 standard is not just an appearance specification. It includes a series of tests designed to evaluate a complete coated plastic system. Key evaluations include:

• Adhesion: The coating must remain firmly adhered to the plastic substrate and between the separate metallic layers. The saw grate adhesion test is one of the methods associated with GMW 14668. Poor adhesion can result in:
  • Peeling
  • blistering
  • Exfoliation
  • Local coating separation
• Coating thickness: Coating thickness is a critical parameter because insufficient thickness can reduce corrosion resistance and durability. This standard includes coating thickness assessment, and the ISO 1463 standard is specified for thickness measurement. For multilayer coatings, controlling the thickness of individual layers is particularly important because the performance of the finished system depends on the interaction between the layers.
• Temperature storage: Coated components are exposed to high or low temperatures to determine if the substrate and coating system remain stable. Temperature exposure can reveal the following:
  • blistering
  • Cracking
  • Layer separation
  • Dimensional effects
  • Loss of adhesion
• Thermal cycling: Thermal cycling testing evaluates the ability of coated plastic to withstand repeated temperature changes. This is important because plastic and metal have different thermal expansion coefficients. Repeated temperature changes can therefore cause stresses at the plastic/coating interface. This standard is also listed for rapid thermal cycling testing by accredited laboratories.
• Corrosion resistance: Corrosion testing is used to determine whether the coating system protects the substrate and maintains an acceptable appearance after exposure to corrosive environments. Tests related to this standard include combinations of CASS (copper-accelerated acetic acid salt spraying) and thermal cycling. Historical versions also distinguish different test combinations depending on the application and coating configuration.
• Stone impact resistance: Outdoor coated plastic parts may be exposed to stones and road debris. Where necessary, the coating system is subjected to stone impact testing followed by appropriate environmental/corrosion assessments. The GMW 14668 standard test tables specifically specify combinations of stone impact, CASS, and thermal cycling tests. This is particularly important for outdoor coatings.
• Substrate surface tension test: This standard includes specific substrate surface tension assessments for different plastics. For example,
  • Substrate surface tension test for ABS.
  • Surface tension test for ABS+PC substrate.

These tests are important because stresses in the plastic substrate can contribute to cracking, blistering, or deterioration of the coating surface.

One of the most important aspects of the GMW 14668 standard is the control of coating thickness. A coating that is too thin may indicate:

• Poor corrosion resistance
• Early visual deterioration
• Reduced durability
• Increased risk of substrate exposure.

On the other hand, excessive coating can increase the following:

• Cost
• Production time
• Material consumption
• Dimensional variation

Therefore, the goal is not simply to maximize coating thickness, but to achieve the specified thickness suitable for the component and service conditions. For complex plastic components, local geometry can make achieving uniform coating thickness challenging. Industry sources indicate that coating simulations/analyses are used for challenging geometries where the minimum thickness may be uncertain.

The GMW 14668 standard is important because decorative chrome-plated plastic parts are subjected to a combination of mechanical, thermal, chemical, and environmental stresses. A component may look perfect immediately after manufacturing, but may subsequently develop the following:

• Cracks
• Peeling
• bubbles
• Corrosion
• color change
• Loss of brightness
• Stone impact damage
• Layer separation

Therefore, this standard provides a common performance framework for material suppliers, coating suppliers, component manufacturers, laboratories, and General Motors engineering organizations.

In conclusion, the GMW 14668 standard goes beyond simply specifying the appearance of chrome, controlling properties such as adhesion, coating thickness, thermal stability, corrosion resistance, substrate behavior, and mechanical/environmental durability.

For many years, our organization, with its competent and experienced staff, has been understanding the needs of its clients, providing the advanced testing services they require, and assisting them in the establishment, implementation, and improvement of management systems. Within this framework, we also provide testing services in accordance with the GMW 14668 standard.