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In the manufacturing of fiber-reinforced plastic (FRP) storage tanks, the application of peel ply depends on the specific molding process. It serves two primary functions: acting as a release layer to facilitate demolding, and creating an ideal surface roughness for subsequent bonding. Below are the specific application methods for several typical scenarios in tank manufacturing.
Scenario 1: As a Mold Release Layer (Filament Winding/Hand Lay-up)
When fabricating tank liners or shells via filament winding or hand lay-up on traditional molds, peel ply can be applied to the mold surface—either as a substitute for or in conjunction with a chemical release agent.
•Application: Lay the peel ply smoothly over the mold, which should already be cleaned and treated with a release agent. For large, curved surfaces like those of storage tanks, avoid stretching the fabric; allow it to settle naturally against the surface. This prevents the fabric from cracking or becoming overly saturated with resin due to thermal shrinkage during the curing process. When using an inflatable (bladder) mold, applying a peel ply is also a critical step for successful demolding.
•Edge Preparation: When cutting the peel ply, ensure that the edges extend at least 20 mm beyond the outline of the component to facilitate removal after curing.
•Removal: Once the resin has cured and the component has cooled to below 60°C, begin removing the peel ply from a corner. Pull it away gently at a shallow angle relative to the surface; avoid pulling vertically or abruptly, as this could lift or damage the underlying fibers.
Scenario 2: Surface Preparation for Secondary Bonding (Key Application)
Storage tanks are typically assembled from various components, such as cylindrical shells, end caps (heads), and flanges. Applying a peel ply is standard practice for areas intended for subsequent bonding—such as the joints between the shell and the end caps, or the bonding surfaces for nozzles and flanges.
•Precise Placement: After the structural layers have been laid up, apply the peel ply specifically over the areas designated for bonding. Ensure that the peel ply completely covers the bonding area. This prevents the formation of a smooth, "resin-rich" glossy surface after curing, which would otherwise cause subsequent bonding attempts to fail.
•Creating a textured surface: Once the part has cured, removing the peel ply leaves a uniform, fine texture on the surface (similar to the texture seen on the pipe surface in your image). This roughness significantly increases the contact area and mechanical interlocking for adhesives, providing a more uniform and reliable result than manual sanding.
•Timely processing: Bonding operations should be performed as soon as possible after removing the peel ply. Alternatively, the surface must be properly protected to prevent dust, oil, or other contaminants from fouling the newly exposed, active surface.
Scenario 3: Separation and Flow Control in Vacuum-Assisted Processes
When manufacturing storage tanks using vacuum infusion or vacuum bagging, the peel ply serves as a critical component of the auxiliary material system.
•Separation of auxiliary materials: The peel ply is placed over the top layer of the fiber reinforcement, followed by the flow medium (flow mesh), breather fabric, and the vacuum bag. It acts as a separator, preventing the flow medium or vacuum bagging film from bonding permanently to the tank structure during resin curing, thereby facilitating the removal of these auxiliary materials.
•Assisting resin flow and content control: The peel ply is porous; under vacuum pressure, it functions as a network of micro-channels for resin flow. This helps evacuate air bubbles and allows excess resin to pass through into the breather fabric (absorbent layer) above, which helps regulate the fiber-to-resin ratio in the tank.
Material Selection and Operational Considerations
•Material compatibility: For storage tanks cured at room temperature using unsaturated polyester or epoxy resins, PET (polyester) or nylon peel plies are typically suitable; they are flexible, conform well to curved surfaces, and are cost-effective. If a high-temperature curing process is used, a peel ply with higher heat resistance—such as one made from PTFE or fiberglass—must be selected.
•Preventing contamination: Always wear clean gloves during operation. If the peel ply becomes contaminated with oil, grease, or dust, it may stick to the surface in those specific areas, leading to difficulties during removal or causing surface defects.
•Avoid excessive stretching: This is particularly important for the curved sections of a storage tank. While a tightly stretched layer of peel ply may appear smooth, the material can shrink and crack as the resin cures and generates heat; this leads to localized resin saturation, making the peel ply extremely difficult to remove later.
