The Bag Molding Process

February 15, 2023

While automation has gained in prevalence in the plastic molding industry, there are still numerous manual processes in use today. For example, bag molding is a method of fabricating thermoset plastic materials into a variety of durable reinforced components, a process that relies mostly on manual control. It involves placing a specialized bag over a plastic workpiece while it is inside a mold cavity, and applying vacuum or compressed air pressure around the bag. Rollers squeeze out air pockets as pressure is directed against the bag, and often heat is introduced to aid in the process. The bag itself is usually an impermeable and flexible cover made of parting film or an elastomeric material.

Bag molding products tend to be compact and strong, with thickness tolerances resembling those offered by injection molding. Bag molding is traditionally employed for manufacturing high performance structural components, such as construction materials, equipment housings, automotive bodies, ship hulls, and a range of military products. It is also often used in conjunction with other molding techniques. This combination of machinery and hand-operated devices allows for quality and attention to detail in fabricating plastic materials.

Mold Preparation
In a standard bag molding process, the elastic bag covers the plastic material within the mold cavity. However, before this can occur, the mold itself must be cleaned, pre-treated, and sealed along the edges. The mold is usually coated with a releasing agent and a gel lubricant, although molds featuring a single curve can sometimes be treated with a plastic film instead. A reinforcing layer of glass fiber can be inserted to provide smoother surface quality, although the key reinforcement is usually a preform composed of a glass thread mat. This preform is shaped to size before being placed into the mold along with any other necessary reinforcing agents or additives.

Bagging and Molding
After the mold has been prepared for bagging and the glass fiber reinforcements have been positioned, the actual fabrication process is ready to begin. In a standard bag molding procedure, the following equipment is usually required to perform the manual stages of the operation:

• Metal rollers in both paddle and disc configurations for flattening trapped air

• Laminating rollers with long hair or short hair bristles for applying bonding resins and gel coatings

• Brush cleaners and brush replacements

• Laminating and coating brushes

A liquid catalyzed resin compound that includes pigments, dyes, and other additives is typically dispensed over the mold reinforcement. The entire mixture is then covered with an impermeable bag, while operators apply serrated hand-rollers to create a uniform distribution of resin under the bag’s surface. Manual rolling also helps ensure that the reinforcement is properly bonded with the resin, without any air pockets that may cause the molded product to degrade. This resin-reinforcement bond is crucial for effective bag molding.

Read more: The Bag Molding Process




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