TECHNICAL RESOURCES
What is Syntactic Foam?
The term "syntactic foam" first appeared in the 1960's. It has come to describe a class of material which has pre-formed hollow spheres as a main constituent. The hollow spheres may be made up of glass, ceramic, polymer, or even metal. The binder is generally a polymer but may also be a metal or ceramic. The "syntactic" portion refers to the "ordered structure" provided by the hollow spheres. The foam term is used simply because of the cellular nature of the material. The original syntactic foams were constructed of hollow glass spheres with either polyester or epoxy as their binder.
Today syntactic foams are used in a wide variety of applications. The material has undergone many changes since the early days providing simple buoyancy. It has also been appeared in a variety of ways in text as both misnomer or trademark such as syntact, syntax, syntac, syntec, syntax, syntac foam, syntetic foam, syntectic, or even synthetic foam.
Syntactic foam has become the most widely used plug assist material today as a result of high performance, light weight, low thermal transfer and surface finish capability. Two types of syntactic foam are available for use as plug assists.
This class of syntactic plug assist is a homogeneous blend of microspheres in a low thermal conductive epoxy. The term thermoset refers to the fact that this class of materials takes a “set” shape/form when cured. The selection of epoxy and microsphere size/type allows the creation of a wide variety of materials for different surface finishes and/or use at different service temperatures.
Thermoplastic syntactic plug assist materials are somewhat softer than thermosets, resulting in a more easily machined surface. This allows for the machining of fine details and eliminates dust during the machining process. Thermoplastic syntactic plugs may typically be machined up to 3X faster than thermosets.
Why use Syntactic Foam Plug Assist Materials?
CGP Europe has developed a wide range of plug materials. The right plug material saves money and improves quality in the thermoforming process by:
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Reducing starting sheet thickness by improving material distribution.
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Providing better part clarity.
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Reducing mark-off and chill marks.
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Eliminating plug cracking and chipping using tough syntactic materials.
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Improving part consistency with excellent detail in plug machining.
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Eliminating dust and tool wear when machining plugs.
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Providing greater durability and thermal stability.
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Reducing sheet sticking with smooth plug surfaces.
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Guidelines for Plug Assist Selection and Design (Click Design Guide for pdf version)
Temperature: Sheet temperature is probably the most critical factor for the thermoforming process. Selection of a plug material should be made to ensure the Service Temperature is suitable for use with the sheet and process temperature. NOTE: It is important to realize the settings on sheet heaters are significantly higher than actual process temperatures. Use the specification sheet for the plastic to determine the appropriate Service Temperature. (Relative to a plug material, processing temperature may also be considered to determine the need for low or high surface stick of the plug. See “Stick” for more information.)
Typical* forming temperatures for common plastics:
* Typical temperature values are reference only and are core (center of sheet) temperatures. Actual molding requirements may vary. See supplier material data sheet for details relative to your specific material.
Transparency/Surface scratch: Plug materials that are easily polished are preferred to minimize/eliminate surface scratching and to optimize material distribution for use in high transparency applications. Modifying the binder and the size/type of spheres in syntactic foam results in plug materials that have been created for ease of polishing. Ra values may be used to compare typical surface finish values achieved on a machine. The lower the Ra value, the smoother the surface finish achievable.
Mark off: A mark on the part may result from contact with a plug that is too cool or from a surface condition on the plug. Syntactic plug materials have very low heat transfer and are preferred as the chance of mark off may be isolated to machining conditions or other easily identified variables.
Stick: High forming temperature requirements or certain material properties (i.e. when forming EVOH, PETG, CPET or RPET) can cause a material to leave residue on the plug. In these cases, a plug material formulated for low stick is preferred. Syntactic plug materials impregnated with PTFE eliminate the issues of coating/recoating the surface of a plug.
Durability: Syntactic foams are durable, long lasting plug materials. Fine detail requirements or general manufacturing practices can result in hostile environments which can chip or break the more brittle thermoset syntactics. Thermoplastic syntactic plug designs are preferred for these applications. Use the Impact Rating on the specification chart to compare durability of various plug choices.
Food or pharmaceutical contact: Plastics used to package food or pharmaceuticals are regulated in regard to the plastic used, the amount of recycled material that may be added and the processing methods/tools used. Always select plug materials suitable for use in FDA controlled applications. Utilizing plug materials with an FDA registered Drug Master File (DMF) offers additional security to the thermoformer for pharmaceutical packaging requirements.
Machining: Syntactic foam plugs are machined using sharp, solid carbide tooling designed for cutting hard, or hard abrasive, plastic. Thermoplastic syntactic plugs may be machined at a much faster rate than thermoset syntactic plugs and result in no dust in the machine. Thermoplastic syntactic plugs are also preferred when it is necessary to machine fine detail into the plug. See the appropriate machining guide for recommendations on any syntactic material.
Polish: Syntactics are formed using tiny microspheres as insulators held in suspension inside a matrix material (binder). The size and type of the microspheres, along with the choice of matrix material make some syntactic foams better than others for surface polish. When an ultra smooth surface polish is needed, thermoset syntactics are generally the best choice. FLX and WFT have been optimized for surface polish requirements.
Plug attachment: Syntactic foam materials for plug assists were developed to optimize forming capability within a mold. As the materials generally have poor tensile and shear strength, direct threading is not recommended. Instead, most thermoformers use some type of insert which is bonded and/or threaded in to the plug. The harder, more brittle thermoset syntactic may result in catastrophic failure when using threaded inserts. A scored/bonded insert can typically be repaired and reseated with fresh epoxy if a failure occurs. With thermoplastic syntactic, threaded inserts increase the pull-out load strength, with pull-out failure limiting itself to damage to the threads. It is generally possible to drill and re-tap the plug, use a slightly larger insert and have the plug back on the machine in a matter of minutes. In all cases, inserts should be installed flush with the surface of the plug. This will prevent over tightening if the insert is below the surface and prevent wobbling/cantilever stress should an insert stick above the surface of the plug.
Plug Assist Overview Chart (click on chart to see larger size):
* Ra values from CNC milled surface using 2 straight flute plastic cutting end mill. Measurements taken with Mahr Federal Pocket Surf III.
** Samples did not break. Toughness value at 5% strain reported.
