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).
Polyester resins are thermoset polyesters that are created by combining a glycol (like ethylene glycol) with an alcohol (like phthalic or maleic acid). Water is given off in what is known as a condensation reaction.
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.
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.
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.
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.
Polyesters offer the lowest cost option for traditional fiberglass parts and can be used with a variety of tooling and processes.