Vacuum Bag Molding for Fiberglass: Lightweight Precision with a Controlled Finish

Vacuum bag molding is a reliable fiberglass molding method that offers excellent control over part quality without the cost or complexity of fully closed molding systems. At Arrowhead, vacuum bag molding for fiberglass is one of the many in-house processes we use to produce custom composite parts with a focus on dimensional consistency, laminate integrity, and lightweight strength.

This method is often selected when your part needs structural integrity with a clean finish on one side, along with better material consolidation than open molding alone can provide. It’s a smart middle-ground process that brings repeatability and improved fiber-to-resin ratios to low volume projects that may not need full RTM, SRTM or compression molding.

What is Vacuum Bag Molding?

Vacuum bag molding—also referred to as vacuum bag lay-up—is a semi-closed molding process that starts with a single-sided rigid mold. After dry fiber reinforcement is placed into the mold and wet out with resin, a flexible membrane or bag (often made from silicone, latex, or urethane) is applied over the mold cavity and sealed.

Vacuum is then applied to evacuate the air between the mold and the bag. This pressure draws the membrane tightly against the laminate, compressing the materials and helping remove excess resin and air.  After curing, the bag is disposed of and the part is removed from the tool.  The result is a well-consolidated, high-quality composite part that offers better strength-to-weight ratios and reduced voids compared to traditional open molding.

When to Use Vacuum Bag Molding

Vacuum bag molding is best suited for fiberglass parts that fall somewhere between basic open molding and fully enclosed systems like RTM or compression molding. It’s ideal when:

• A clean, finished surface is needed on one side
• Controlled laminate thickness is important
• You want better consolidation than hand lay-up alone can provide
• You’re working with curved or complex surfaces
• Resin content, part weight, or consistency need tighter control
• The project is lower volume and doesn’t justify full RTM tooling investment
• The part is very large

This process is especially helpful when repeatable results and lighter parts are the goal.

Benefits of Vacuum Bag Molding

Improved Laminate Quality
Vacuum pressure removes trapped air and compresses the laminate, resulting in fewer voids and better resin distribution across the part.

Lighter Part Weight
By removing excess resin, vacuum bag molding creates lighter parts with strong fiber-to-resin ratios—ideal for structural or weight-sensitive applications.

Dimensional Consistency
The process improves uniformity across the laminate, helping parts meet thickness and performance specifications more reliably than open molding alone.

More Controlled Finish
While only one side of the part is formed against the mold, the vacuum bag ensures even pressure across the surface, improving the cosmetic appearance and reducing print-through or surface imperfections.

Lower Tooling Costs
Compared to closed mold methods, vacuum bag molding requires only a single rigid mold, making it a more cost-effective option for mid-volume production.

Typical Applications for Vacuum Bag Molding

Arrowhead uses vacuum bag molding to manufacture fiberglass parts across a wide range of industries. Common applications include:

• Structural panels for trucks, buses, or equipment
• Curved marine components with clean outer finishes
• Lightweight interior or exterior covers
• Reinforced brackets, access doors, and composite trays
• Custom-shaped parts with compound curves or complex geometries
• Parts requiring enhanced fiber alignment or high strength-to-weight ratios

It’s a versatile process that bridges the gap between manual lay-up and more automated methods.