When it comes to composites, you have probably heard this before: “There are so many factors that influence the selection of [fill in the blank].” Some argue that selecting a sandwich core material is even more complex due to the extremely wide variety of products available. Today, there are various tables, matrices and apps available to users that rank different products, all designed to simplify the selection process. The difficulty for core materials, however, is that when these tools assess a product, they only look at the base material. Choosing a sandwich core is not as simple as selecting end-grain balsa or Nomex® honeycomb. Each product has three parts: type, density and configuration. Furthermore, the categories in these evaluation tools often do not align with what is important for each specific application, and the rankings are more often based on assumptions than test data. In addition, one category is almost always missing – cost. A proper evaluation should take all of these elements into consideration with the understanding that density, configuration and thickness can all be tailored to meet the part’s demands. When selecting sandwich core materials, the first thing to do is create a list of critical parameters for your specific application, part geometry and manufacturing process. Ask these two questions:
- What will be built?
- How will it be laminated?
Application consideration has two parts: structural design and environment. Core material properties critical to structural design include shear strength and modulus, compression strength and modulus, and skin-to-core bond strength (tensile/peel). Furthermore, the application may have a particular requirement, such as fatigue or damage tolerance (impact). Damage tolerance is often ranked by measuring a material’s shear elongation. However, elongation is not the only important property with respect to energy absorption under impact; shear and peel strength are also critical. In addition, it’s not as clear-cut as just ranking different materials based on strength, modulus and so on because these properties are dependent on density. Many products, particularly cellular foams, can be manufactured in a very wide range of densities, some of which have greater benefits than others. For example, while low density urethane foams can be weak, higher density urethane-based core materials can be extremely strong. Other behaviors may depend on the configuration or finishing option. For instance, the damage tolerance of styrene acrylonitrile (SAN) foam is excellent if laminated in rigid sheets, but the behavior is vastly different if a flexible (scored) configuration is used. Sometimes the selection process will be straightforward. For instance, the list of core materials with compression strength greater than 700 psi is pretty small. For most other parts, you should first eliminate materials that have insufficient mechanical properties, then evaluate the remaining products that provide the best performance at the lowest density and/or price. In addition to creating your own matrix, there are many simple laminate evaluation tools available that allow the user to compare different materials. Be aware, however, that some do not take the properties of the core into consideration. These will not be very useful if you want to compare sandwich core materials. Aside from mechanical requirements, you should consider the part’s operating environment. Will the part be exposed to water, chemicals or high or low temperatures? Does it need to be fire resistant? Again, product density plays a role. Generally, higher density foams have better corrosion and temperature resistance than lower density materials of the same polymer. The sheet configuration can also affect performance as well as the lamination process. Throughout the selection process, keep in mind that some aspects of a part’s behavior are not exclusively attributed to the core material. For example, changing the resin may vastly improve fire resistance. Another critical consideration is the lamination process. Important features to reflect upon are resin compatibility (styrene resistance), temperature resistance (dimensional stability) and open cell content. For certain foams, resistance to styrene is generally dependent on density. (Lower density is less resistant.) Also keep in mind that foams not designed to operate at high temperature are fully capable of withstanding the heat from resin exotherm, so don’t mistakenly eliminate materials because you fear that the resin will get hot: there is a difference between short-term temperature spikes from exotherm or post curing and long-term exposure with an elevated temperature cure. Once the material is selected, the manufacturing process and geometry of your part will determine the core’s configuration or finish. Here’s where proper core installation comes into play. Often bad performance is blamed on the core material, when poor quality laminate was the cause. Always follow these rules:
- Avoid “never bonds.”
- Fill the kerfs.
- Fillet all core edges.
- Segregate the core from all openings.
This brings us to total cost, which is a combination of the core material, finishing and resin absorption. Let’s take a look at all three:
Material: Look for value. Often the lowest cost product will not produce the least expensive part. It is quite easy to determine which products produce the best “bang for your buck.”
Finishing: Unless you are building flat panels in a press, all core materials (besides honeycomb) should be perforated or otherwise scored to allow air and other gasses to escape from under the core. Sheets must also be cut to conform to curved surfaces. Certain foams may be thermoformed to produce lighter parts. Grooves may be added to promote resin flow.
Resin absorption: Balance conformability with resin uptake. Treatments are also available to reduce the amount of resin absorbed by the core when infused. Overall, the proper selection of a sandwich core material may seem like an arduous task, but many questions answer themselves fairly quickly. If you apply the “right tool for the right job” maxim, then sandwich core materials will help you build better parts with greater quality and improve your bottom line.
