Turning Experience into Solutions for Shoe Sole Manufacturing Operations
The various shoe-manufacturing techniques and can be broken down into four major categories. These categories are single density unit soles, dual density unit soles, single density direct attach, and dual density direct attach. Chem-Trend’s years of production floor experience working with shoe sole manufacturers around the globe has allowed us to understand and overcome the molding challenges found in each of these processes.
Single Density Unit Soles
For single density unit soles, the polyurethane foam system is poured directly into the mold cavity† (open pour) and a lid is secured over the cavity. At the completion of the cure cycle†, the lid is removed and the completed part is de-molded. Single density unit soles are then attached to the upper material to complete the shoe manufacturing process. In many cases, single density unit soles are subjected to some type of post-mold operation to obtain the desired finished appearance on the part. Basic mold release agent requirements for this type of process include release performance, low build-up in the mold cavity, and compatibility with the post-mold finish operation and final shoe building process.
Dual Density Unit Soles
The dual density process requires bonding a midsole to an outsole in the same molding process. The outsole polyurethane foam is poured into the mold cavity, and a dummy last is used to close the mold. The dummy last is a type of false lid designed to cover the mold cavity, creating the proper thickness for the outsole while leaving room in the mold cavity for the midsole portion of the part. At the end of the outsole cure cycle, the dummy last is removed, and the midsole foam is poured on the backside of the outsole. A new lid is used to secure the mold, and a second cure cycle is completed. The molded soles may be subjected to post-mold finish operations and, as described above, are attached to the upper to finish the shoe. Release agent requirements for dual density unit soles are similar to those described for single density unit soles with one important difference: the mold release agent must be applied to the dummy last in order to separate it from the outsole. As a result, the release agent can be transferred to the outsole, but it must not interfere with adhesion between the outsole and midsole foam layers.
Direct Attach Processes
As the name suggests, the direct attach process is used to mold the sole directly to the upper material, eliminating the need for gluing or sewing to produce a finished shoe. Direct attach requires that the upper is secured to the top of the mold, and the polyurethane system(s) is injected into the mold cavity. For dual density direct attach, a dummy last is used to form the outsole, and the upper is secured into the mold prior to midsole injection. Scrap produced using a direct attach process is very expensive due to the fact that an entire shoe is wasted. It is also very difficult to complete a post-mold finish process on direct attach unit soles without affecting the upper portion of the shoe. For these reasons, mold release agent selection is critical. Direct attach mold release agents must produce a defect free urethane part, as well as impart a uniform gloss appearance. Gloss requirements are defined by the manufacturer, but can cover a broad range from low (matte) to high (shiny). Other important performance attributes for direct attach release agents include release, low build-up in the mold cavity, and dual density adhesion.
No matter what type of process you utilize in your shoe sole manufacturing operation, you can benefit from Chem-Trend’s extensive expertise in polyurethane molding, and our wide range of products developed to overcome the industry’s greatest challenges. Our products are designed to enhance your efficiency and reduce waste, providing you with higher quality soles and lower costs of operation.
To learn more about our expertise in the area of shoe sole molding operations read our sections on shoe soling polyurethane, release agents for shoe soling or ancillary molding products for shoe soling. Alternatively, contact us to speak with one of our shoe soling experts.