When designing a product for mass production, engineers must take materials into account—not only in terms of strength or weight requirements, but also from the perspective of assembly. Most manufactured components today are made from plastic resins, metal or both. When all the parts in your assembly are made from the same material, choosing a bonding solution can be relatively straightforward: just find “the best epoxy for plastic” or the best adhesive for metal.
However, when you need to join dissimilar materials—such as plastic and metal—the decision process becomes a bit more nuanced. Saving weight has always been a priority for aerospace manufacturers, but with the increasing trend toward lightweighting in the automotive industry, the demand for a reliable way to bond metal to plastic is all the more pressing.
Other applications that bond metal to plastic include:
- Polypropylene to aluminum for vehicle panels
- Urethane to steel for seals and gaskets
- Polycarbonate to aluminum for battery housings
Multi-Material Bonding Challenges
There are plenty of options for joining like materials—whether plastic-to-plastic or metal-to-metal—but the selections for joining plastic to metal are more limited. Plastic parts are typically joined to metal components using either adhesives or mechanical fasteners. Since the latter option often requires drilling holes—which can be expensive and time-consuming—adhesives may be preferable if your goal is to minimize cycle times or unit costs.
Other advantages to adhesive bonding include spreading loads out over the entire bonded surface rather than concentrating them at fastener points, and acting as an isolator between the bonded surfaces to avoiding the chance of unwanted galvanic interactions between substrates and fasteners. By providing more efficient load management, using an adhesive can also allow for reducing the thickness and weight of the bonded materials.
Although your go-to adhesive for plastic may perform admirably in plastic-to-plastic applications, it might not be the best plastic-to-metal adhesive. Selecting the best glue for plastic-to-metal applications means taking multiple factors into account. More specifically, engineers need to consider the relative surface energy, elastic modulus and thermal expansion of the materials they’re bonding.
Surface Energy
To find the best epoxy for plastic, you need to know whether the plastic you’re using has high surface energy (HSE) or low surface energy (LSE). On HSE plastics, such as ABS or polycarbonate, adhesives flow more easily, wetting out the surface to form a strong bond. In contrast, most adhesives and tapes have a tough time sticking to LSE plastics, such as polyethylene or polypropylene.
If surface energy is your sole concern, HSE plastics are the obvious choice; but, as most engineers know, it’s unwise to select a material on the basis of a single property. Cost is always a factor in manufacturing and, unfortunately, HSE plastics tend to cost more than LSE.
Elastic Modulus
Also known as the modulus of elasticity, an elastic modulus represents a material’s resistance to being deformed non-permanently under stress, defined as the slope of its stress-strain curve in the elastic deformation region. The higher the elastic modulus, the stiffer the material.
If you’re working with an adhesive for plastic or metal only, the material’s elasticity isn’t really an issue, but if you need to glue plastic to metal, then you’ll need to consider the elastic modulus of both materials as well as the target value for your plastic to metal adhesive. Typically, when you need to bond metal to plastic, you’ll want to aim for a bond that has an elastic modulus somewhere in between the modulus of your two materials.
Thermal Expansion
The extent to which your assembly—or parts of it—will change in shape, area or volume in response to temperature is always a concern in product design and manufacturing, but it’s exacerbated in cases of multi-material bonding. Metals and plastics have different coefficients of thermal expansion, so the best glue for plastic to metal bonding will accommodate this difference.
Ideally, the coefficient of thermal expansion for the adhesive itself would be negligible. However, it may play a role in the performance of the assembly as temperatures fluctuate. In addition to considering the CTE of the adhesive, the adhesive’s flexibility and elongation will impact whether the bond will move with the substrates as they expand or contract.
How to Join Dissimilar Materials
(Image courtesy of 3M.)
The key question at the heart of any adhesive application is how much bond strength you need.
A typical non-structural adhesive has a bond strength of less than 1,000 psi, while structural or load-bearing adhesives have bond strengths of 1,000 psi and higher.
For non-structural parts, such as membrane switches and instrument clusters, 3M Ultra-Clean Laminating Adhesive 502FL bonds on contact after wiping down the substrates for dust. 3M’s laminating adhesives can also bond LSE plastics to lightly oiled metals with minimal surface preparation, and they can resist temperatures up to 350° F, making them well-suited to electronics assembly applications.
For structural adhesives, 3M Scotch-Weld Structural Adhesives are 2-part acrylics that bond LSE and HSE plastics with strengths exceeding the bonded materials. This can eliminate the extra steps involved in using screws or rivets and avoid the restrictions of plastic welding.
Another adhesive option is 3M VHB Tapes, which are strong enough to replace rivets and screws for many applications. Able to bond both HSE and LSE plastics, the closed cell acrylic foam carrier can absorb impacts, damp vibrations and flex in response to thermal expansion and contraction, making it a good option as a plastic to metal adhesive.
Choosing the Best Glue for Plastic to Metal
(Image courtesy of 3M.)
If you’re looking to glue plastic to metal, you need to select the right adhesive for the plastic you’re using.
Polycarbonate and ABS are typically the easiest plastics to join to metals, while extremely smooth or LSE plastics like polypropylene and polyethylene present a greater challenge. Primers, such as 3M Scotch-Weld Instant Adhesive Primer AC77, can go a long way to increasing bond strength on materials with low surface energy.
Ultimately, multi-material bonding is about finding a compromise between the properties of unlike materials, such as plastic and metal. To successfully join dissimilar materials, you need to consider their relative surface energy, elastic modulus and coefficient of thermal expansion.
If you’re unsure of which adhesive best compromises between the material properties of your substrates, consider reaching out to an expert who can help you identify the best design solutions.
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This post is sponsored by 3M. All opinions are mine. – Ian Wright
3M, Scotch-Weld and VHB are trademarks of 3M.