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MATERIALS

For structural reasons, an AED unit has a rigid clamshell design in which both halves and the battery door are overmolded with a soft-touch resin.

The gasket areas between the upper and lower housings and between the upper housing and battery door require a water­tight seal. To achieve the seal, three clear windows are molded in as inserts during the first molding step. Gas assist is used to core out thick sections and ensure a surface that is free of sink marks for the subsequent overmolding process.

Return to Article: Material Selection: The Right Resin for Your Design

Requirements for the overmolding resin are high flow length, a good chemical bond, good compression characteristics under temperature, as well as low abrasion and a dry soft-touch feel. For the substrate resin itself, the material must be able to withstand high impact for a drop test, resist chemicals, and bond well to the overmolding material. In terms of regulatory compliance, the material must meet UL 94V-0 flammability rating and environmental requirements from the EU Eco-labeling Board.

Table I. (click to enlarge) A comparison of the material properties of generic resins, with color coding for the best material per desired characteristic for the AED unit in Example 1.

The process for narrowing down material selection for the substrate is illustrated in Table I. First, the list of required properties was generated. Then data were collected from different resin suppliers. It is important to ensure comparability of the data using the same specifications for testing.

To highlight which resin was most promising based on overall performance, the various properties in Table I are color-coded to determine good, better, and best. As the chart points out, all three resin options will bond to the overmolding material and, from a chemical resistance standpoint, are strong enough to meet cleaning requirements. They all have a UL 94V-0 rating at the given wall thickness, as well as the required impact properties to successfully pass the drop test. In this particular case, availability and price are important factors for a final resin selection.

For the overmolding resin, a number of requirements are important. Chemical resistance for cleanability, and achieving an excellent chemical bond with the substrate, are critical. A durometer of 75 Shore A balances low abrasion with sufficient material softness and a dry-touch feel.

The selected material is an ether or ester thermoplastic polyurethane (TPU) with Shore A hardness of 75. Bonding capabilities vary for substrate-to-substrate and substrate-to-overmold combinations. It is important to consider these characteristics, in addition to other requirements.