Because of its rapid cooling characteristics, aluminum is sometimes used for tooling. It can also reduce the time required for building the mold because it is easier to machine than steel, providing faster turnaround and production cycles. However, because it is softer than steel, even hardened aluminum is harder to weld, difficult to maintain, and wears more rapidly—making it most suitable for prototypes and short runs. Depending on the product and mold design, hybrid molds can sometimes be built that are mostly steel but use aluminum in low-wear areas to transfer heat.
Aluminum is not a good choice for complex parts or harder, glass-filled plastics because of premature wear. Copper alloys are sometimes used as an aluminum replacement when rapid heat dissipation is required. Both steel and aluminum molds can be coated with special materials to improve wear resistance and reduce friction, especially when molding fiberglass-reinforced plastics, making tooling last longer. Common coatings are nickel-boron and nickel-teflon the (0.0002 to 0.0004 inches in thickness).
Key Components of Mold Design
Gates are the openings at the end of the runners that direct the flow of molten plastic into the mold cavity. Gates vary in size and shape depending on the part design and resin material. Design engineers must take into account a number of factors to determine gate types and locations to achieve optimum flow, fill pressure, cooling time, and dimensions/tolerance. It is important to locate gates where they won’t impact part performance or appearance (flow marks, shrinkage, warping).
One aspect of mold design that cannot be overlooked is the easy removal of the final product from the mold, with no damage to the surface of the part. This is accomplished by applying a draft angle, or taper, to the walls of the mold. The amount or degree of draft angle depends on several factors, including design of the part, material, depth of the mold cavity, surface finish, texture, and amount of shrinkage. Typically an angle of only a few degrees is applied to the side walls of the mold and creates enough space that the part can be easily removed when the mold is opened. Generally, the deeper the cavity, the more draft is required. Draft angles typically vary from about 1 to 5 degrees.
Finish and texture
Mold cooling and part cooling are critical for determining surface finish. For example, a smooth surface finish on a 50-percent glass-filled resin depends on proper temperature control. The surface must be resin-rich with the fiber glass slightly deeper in the part, which requires a hotter mold—this also means it takes about ten percent longer to cool. Molds can also be designed to apply a texture or pattern to the mold surface—this can actually eliminate assembly steps by creating the company logo in the plastic, for example. Texture can also provide better product function, such as enhanced grip or reduced wear from friction. Types of textures include matte, gloss, graphics, grains, logos, and geometric patterns. Depending on the type, depth, and location of texture, draft may need to be adjusted to facilitate part ejection, which is determined during the mold design process.
Manufacturability and Lifecycle Costs
The main goal of mold design and tooling is to create a product with high manufacturability—a high-quality process that is simple and efficient, long-lasting, easy to operate and maintain, and that meets all customer specifications at the lowest possible cost. Fulfilling these expectations depends on designing the best tooling option for each customer’s needs. To accomplish this, tooling decisions must be made in the earliest design stages. The tool-maker must be involved as early as possible to provide a realistic machining perspective on product design, requested tolerances, tool design, selected materials, and associated costs. Taking this step up front is the best way to eliminate wasted effort and rework, which adds significant cost to the tooling budget. Part design and tool design are dependent on each other and thus should be done concurrently whenever possible. For good reason, customers are always concerned about cost. After all, tool-making is one of the highest expenses in the production process. Properly designing, building, and using tooling for each part requires a highly skilled team of engineers and technicians utilizing the latest in sophisticated design and manufacturing technologies. Labor cost can be optimized, however, by working closely with an experienced, efficient tooling team that makes wise decisions on material selection and design tradeoffs, early in the design process.
In an effort to save costs up front, some companies shop tooling according to price, looking for the lowest bid. There is usually a not-so-good reason behind lowball machining/tooling bids, including poor quality, poor repeatability, inferior tooling, improper materials, low operational skills, and waste/rework. Other companies trying to beat a deadline may select a tool vendor quickly, hoping “things turn out right.” Typically, however, lack of due diligence leads to oversights or cut corners that take much longer to straighten out. Although rushing might get the first shots completed quickly, chances are the final submittal won’t be any faster. The best way to get maximum value for your tooling budget is to consider lifecycle costs, not up-front costs. The ultimate goal is quality and repeatability. This is achieved by working with an experienced injection molder that takes the time to completely understand the customer’s needs, goals for the product, and production expectations and designs the best possible mold/tooling package to meet those needs. Up-front costs for quality tooling may be higher compared to cheaper vendors or offshore suppliers, but the payback come quickly in higher quality, fewer defects, greater throughput, longer-lasting equipment, and over better return on your tooling investment—leading, ultimately, to higher customer satisfaction and loyalty.