Frequently Asked Questions

Why are there so many choices of HYTAC materials?

Each material is optimized for performance with different types of plastic, temperature use, machining conditions, surface finish, durability or even pricing. See the CMT Material Selector Guide for recommendations. While every HYTAC product is suitable for a wide variety of applications, making the “best” choice reduces machining costs, improves material distribution, enhances clarity, reduces plastic residue sticking to the plug and maximizes plug life.

What is the difference between thermoset syntactic foam, thermoplastic syntactic foam and solid polymer?

Syntactic refers to the fact that the plug assist material is comprised of hollow spheres held in suspension by some type of binder system. As related to CMT syntactic foam, a “thermoset” binder is a uniquely formulated epoxy system that takes a hard or “set” shape when cross-linked. The use of differing types or blends of epoxies allows for a wide range of performance characteristics in different HYTAC products. A “thermoplastic” binder is actually a high performance plastic that is more durable in use, generally easier and faster to machine and allows for finer detail in plug design. “Solid polymer” plugs offer a smooth surface for high clarity applications, but lack the beneficial thermal transfer isolation characteristics of syntactic foam.

Why do you specify carbide tooling for machining HYTAC?

Tools must be sharp to obtain a satisfactory surface finish when machining HYTAC products. Dull cutters or incorrect geometries will result in poor surface conditions. High speed steel cutters are quickly dulled. Solid carbide cutters are preferred due to the sharp edge and long life that may be obtained. Diamond coated tools may last longer, but are not required and have not been found to provide additional benefit in machining.

Do more flutes provide a better surface finish quality when CNC machining HYTAC materials?

No. Large flute openings are required to give chip clearance and avoid chip welding by quickly removing cut material from the plug. CMT recommends two (2) flute carbide tools designed to cut plastics for best performance when machining most HYTAC materials. Please see our machining guides for specific tool, feed and speed recommendations.

How do I polish HYTAC materials?

Thermoset Epoxy Syntactic foams (HYTAC-W, WF, WFT, FLX and FLXT Series) may be wet or dry sanded using random motion. When following CMT machining/turning guidelines, most users find 220 grit sandpaper provides the appropriate roughness as a starting point. Finer and finer grits may be used for finish rubs until desired the desired surface finish is achieved.

Thermoplastic Syntactic foams (HYTAC-B1X and XTL) may also be polished, but a good starting surface is critical. A surface that has been damaged in the machining process cannot be fixed by sanding. After following CMT machining/turning guidelines, most users find a 400 or 600 grit sandpaper provides the appropriate starting point. HYTAC-B1X should always be dry sanded.

How smooth does a plug need to be?

There is no single answer for this question. The smoother the surface, the higher the friction between the plug and the plastic. Rough spots or inconsistencies in the plug surface typically leave a mark on molded plastic. Beyond these basics, the general rule of thumb is to focus on highly polished plugs when transparency of the final product is critical.

What sizes of HYTAC are available?

HYTAC materials are available in a wide range of rod, sheet and block sizes. Please see our sizing chart for details.

Can syntactic foam be repaired?

Yes, small voids in HYTAC-W, WF, WFT, FLX or FLXT less than 1/8” in depth or diameter may be filled using the HYTAC repair kit (available in the US only due to transportation requirements). HYTAC materials may be satisfactorily bonded together using adhesives shown on the Recommended Adhesives Chart.

Can I drill a hole through the center of a plug for attachment to a base plate?

This method of attaching plugs to a base is not recommended for most syntactic foams due to the stress placed on the plug, the potential for plastic to build up in the drilled hole, and/or a mismatch of Cte between any filler used and the syntactic foam. HYTAC-B1X and XTL are very durable and will withstand the stress. Just be careful of open holes that may trap plastic in the thermoforming process.

Can I thread an insert directly in to a syntactic foam?

HYTAC-B1X and XTL are suitable for direct threads. For all other HYTAC materials, CMT recommends use of slotted inserts bonded into the base of the plug. See HYTAC insert installation guide for more details.

Is it possible to pour/cast a plug into the shape I need for my plug as an alternative to machining?

In many cases, yes. CMT sells a material that may be poured into a mold/part by a customer to create a plug. Special curing cycles and temperature control in appropriate ovens are required and plug quality is directly related to the quality of the mold.

More commonly, customers provide parts/molds to CMT. This allows for a wider range of HYTAC materials to be used and for curing cycles that will provide a high quality plug. Contact CMT to discuss your application requirements.

How do I eliminate surface scratches in transparent plastics?

A very smooth plug surface is one requirement. HYTAC-FLX, FLXT and C1R are optimized for smooth surface conditions. Temperature, choice of sheet material, clarifiers in the plastic, mold design and many other factors are also involved.

How much of a factor is the Coefficient of Thermal Expansion (CTE) in the selection of a plug material?

HYTAC materials have low CTE values, typically resulting in a negligible change to plug size or performance. This allows the plug to be designed to bring plastic closer to the mold wall than with other materials used as plug assists, improving product quality and lowering operating costs by allowing a reduction in sheet temperature.

Friction plays a key role in the performance of a plug. Why isn’t a Coefficient of Friction (CoF) value listed for each of the HYTAC materials?

The plug/sheet interface performance is all about friction…yet nearly impossible to define with any level of accuracy. Temperature, substrate, surface finish, motion, dust and many other factors affect the value. Different surface conditions as a result of machining or polishing a plug material, or even a few degrees of temperature variation in a mold affect CoF values. The value changes for each specific plastic relative to plug material. There is both a static (objects at rest) value and a kinetic (objects in motion) value for the frictional coefficient. The only suitable method for determining a CoF value is experimentation with the actual plug and sheet to be used.

What is the best design for a plug assist?

Wow… now there is a question for which we wish had a straight answer! Best practice in plug design is a complex issue involving many factors. Plastic sheet choice, mold design, process temperature, type of thermoforming equipment, plug material choice and more all play a variable role. The best practice approach is to begin with a “material safe” plug design – meaning as close to the cavity wall and floor as possible (commonly 1/16″/1.5 mm) with a minimal radius at the nose of the plug. This allows for added radius if too much material is carried to the base of the cavity and additional plug taper to further even out material thickness or to remove potential plug marks.