Polyester is often referred to as fiberglass – which is not technically accurate. Fiberglass is reinforcement and polyester is the resin.

Polyesters offer ease of handling, low cost, dimensional stability, as well as good mechanical, chemical-resistance and electrical properties. Polyester resins are the least expensive of the resin options, providing the most economical way to incorporate resin, filler and reinforcement. They are the primary resin matrix used in SMC (sheet molding compounds) and BMC (bulk molding compounds).

Thermoset Polyester resins are created by combining an alcohol like ethylene glycol with an organic acid like maleic anhydride.

The low viscosity and raw material cost of polyesters make the additions of filler and reinforcements a matter of practicality. In fact, filler is often called an extender, because it extends the value of the resin – reducing the cost of the final composite on as much as 50%.

Neat polyester, resin without reinforcement, fillers or additives, is very brittle. The low viscosity of polyester resins allow for the addition of low profile agents, pigments, fillers and additives. This allows tremendous flexibility in polyester formulations. Formulations can be tailored to meet class A surface, flame retardancy, physical, mechanical or noise dampening requirements.

Best Processes

Open molding, SMC/BMC, LCM and RTM. The best process for your product is a function of your tooling budget, part specification (such as dimensional tolerance and physical properties) and production volume.


Typically glass

Special Design Considerations

MFG’s dedicated design experts are available to help with your product design. MFG’s Technical Design Guide for FRP Composite Products and Parts provides in-depth information for product designers. You can download it on the bottom of this page.

Technical Details

A wide range of surface qualities are possible to satisfy different application needs, from a cosmetically beautiful Class A automotive/truck body panel to a rugged underground utility box.  Polyester formulations offer tremendous flexibility.

Physical properties are easy to tailor, driven primarily by the amount of glass reinforcement used. Fillers are easy to incorporate in polyesters and commonly used as resin extenders to meet design and cost targets.

Properties can be formulated for your product specification targets. Typical SMC properties with standard 28% glass reinforcement result in 12,000psi tensile strength and 25,000psi flexural strength.  Strength-weight ratio is very good and because of its low density (from 1.70 grams/cc formulated) it often replaces metallic parts that are close to five times heavier.

As an example, the modulus (stiffness) for a typical SMC can exceed 1.5MMpsi. Polyester resins have a normal viscosity of about 700 when cut in styrene (the standard monomer). We can tailor a custom formula to meet your specific design requirements.

Cost Considerations

Polyesters offer the lowest cost option for traditional fiber glass parts and can be used with a variety of tooling and processes.