Polyester Resins

Polyester Resins

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%.

It is possible to add or reduce strength with the reinforcement chosen. Other parts molded from polyesters have similar properties if the glass % reinforcements are similar. Polyester resin (neat) meaning without filler or reinforcements would be a very brittle resin and would never be used without fillers or reinforcements. "Buttons" are made from Polyester resins but do have fillers and pigments, just no reinforcement as one example of a non-reinforced product.

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 fiberglass parts and can be used with a variety of tooling and processes.

Resource Typesort descending Description
Technical Design Guide for FRP Composite Products and Parts PDF Fiberglass (FRP) composite materials and processes are explained in detail. This design guide outlines various selection criteria with helpful technical data and comparisons to alternative materials.
UL Certified FRP Materials from MFG PDF List of proprietary MFG materials that are UL-approved.
iNVENTA 615UL SMC Material Datasheet PDF iNVENTA 615UL is a UL 94-5V approved non-halogenated reinforced thermoset SMC material. Ideal for applications where non-flammability is crucial.
INVENTA 515UL SMC Material Datasheet PDF INVENTA 515UL is a UL approved reinforced thermoset SMC material. Ideal for HVAC components and other applications where non-flammability is crucial.
iCom 617 SMC Material Datasheet PDF iCom 617 is a non-halogenated fiberglass thermoset sheet molding compound with a UL 723 rating.
iCom 618 SMC Material Datasheet PDF iCom 618 is a UL approved non-halogenated fiberglass reinforced thermoset SMC.
Facts About Styrene 2013 Video Educational video on the effects of styrene. This video is part of ACMA's Risk Communication Program that provides information tools about chemical health risks.