How to mold flexible foam products

How do I mold polyurethane flexible foam to make soft products?

Do you want to make soft foam products in molds? Northstar Polymers carries various polyurethane flexible foam resin formulation products to be used to make molded foam products. Molding flexible foam is different from making non-foam solid casting resin products. Instead of the liquid resin filling the mold, foam resin fills the mold by expanding to the inner walls of the mold. It requires a type of molding technique called compression molding.

1. Overview

Compression molding of polyurethane foam involves placing a calculated excess amount of foam resin into a closed mold to generate expansion pressure, ensuring complete cavity filling. Proper mold design, venting, temperature control, and post-molding processing are essential for achieving optimal foam properties.

2. Mold Design and Preparation

· Closed Mold Requirement: The mold must be designed to withstand internal expansion pressure. A simple compression mold consists of an open-top box with a lid that is securely clamped during the foaming/curing process.

· Venting: To prevent voids caused by trapped air, incorporate small vent holes at strategically positioned locations where air is likely to accumulate. Minimal foam leakage through these vents is expected and can be trimmed post-curing.

· Mold Material: Suitable mold materials include metal, plastic, or elastomeric materials. The mold surface should be non-porous to prevent foam adhesion.

· Temperature Control: Metal molds tend to absorb heat from the urethane reaction, which is crucial for proper curing. Metal molds should be preheated to 100–120°F to maintain consistent cure conditions. For plastic or elastomeric molds (e.g., silicone rubber, epoxy, urethane), preheating may not be necessary at room temperature. Test and determine the optimal mold temperature. Higher temperatures may increase shrinkage variations.

3. Compression Rate and Foam Distribution

· Compression Rate Definition: The compression rate refers to the additional amount of resin (in addition to the amount of resin to have the free-risen foam to fill the mold space) introduced into the mold to create extra pressure, ensuring uniform foam distribution.

· Recommended Compression Rate: Typically, a 5–10% compression rate provides sufficient pressure for proper foam filling. For example, when you need 100 grams of free-risen foam to just fill the space, you need to put 105 to 110 grams of the resin for the compression. A higher rate increases foam density and strength but also raises the risk of closed-cell formation to cause shrinkage-related issues.

4. Curing and Demolding

· Curing Time: After 20–30 minutes, the foam should be sufficiently solid for demolding.

· Cell Structure Considerations: Properly cured foam should predominantly have an open-cell structure, though some regions in the mold may develop closed cells. Closed cells can cause deformations due to pressure imbalances as the foam cools post-demolding.

5. Post-Molding Processing

· Addressing Closed-Cell Formation: Certain mold geometries can create localized high compression, leading to closed-cell regions. These areas must be physically crushed to rupture the closed cells, converting them into an open-cell structure.

· Mechanical Crushing: If closed-cell regions are detected (characterized by a balloon-like feel), apply manual pressure using hands, elbows, knees, or designated tools to break the cells. The foam will produce a popping sound as the cells open, reducing shrinkage and restoring the intended shape.

By following these guidelines, polyurethane foam parts produced using compression molding will exhibit consistent density, structural integrity, and minimal shrinkage variations.

Please refer to this page on our website for available flexible foam formulations. https://www.northstarpolymers.com/flexible-foam-systems.html