The "Art" of Tooling

Posted by Al Elger on May 28, 2013 3:00:00 PM

blank.pngDesigning the mold and its various components (referred to as tooling) for a client’s product is a highly technical and often complex process that requires high precision and scientific know-how to produce top-quality parts with tight, repeatable dimensions. When the right tooling decisions are made, the production process is optimized, costs are reduced, and quality and customer satisfaction are improved.

Tool design, tool material, and cavitation all impact tolerance. In general, the more simple a process is, the greater the chance of achieving and maintaining tight tolerance. More complex parts introduce more variables, such as the number of cavities in the mold, or the need to precisely heat or cool the tools. For example, if tooling is not designed to provide consistent, repeatable cooling, shrink rates will increase and tight tolerances will be harder to achieve. Ensuring the material is at the appropriate and consistent moisture content before melting will also make part repeatability and tolerance control easier to achieve.

In general, more complex injection-molded products require more complex molds. These often must deal with features such as undercuts or threads, which typically require more mold components. There are other components that can be added to a mold to form complex geometry, such as rotating devices (using mechanical racks and gears), rotational hydraulic motors, hydraulic cylinders, floating plates, and multi-form slides.

For complex parts or products, it is recommended that transducers be strategically placed in multi-cavity tools, as well as hot manifold tools, to monitor and control the process in real time. This is a key part of the scientific molding process. Sensors can also be placed on the surface of the tool as a back-up, just in case the cooling lines or unit fail; upper and lower limits can be set on these sensors to monitor the cooling rate, along with the graph template of the cavity pressure graph.

Establishing a production-capable process with in-mold sensors is also important for establishing a benchmark to refer back to when making changes (tooling material, process, or molding machine changes). This way a process can be monitored and documented so that it can be set up and repeated accurately in the future. 

Other tooling design considerations include selecting the proper grade of steel. The correct steel hardness must be determined to maintain the proper balance between wear and toughness, so tooling components that run together don’t wear out prematurely. Another consideration is steel hardness versus steel brittleness. Harder steel is more brittle and therefore not a good choice for mold components that are subjected to side loading or impact, because if it flexes it will crack. Harder steel is also required for molding glass-filled material, which can prematurely wear down tooling, including runner systems and gates. 

Waterlines must be well-placed to maximize cooling and minimize warping. Tooling engineers also need to calculate gate/runner sizing specifications for proper filling and minimal cycle times, as well as determining the best shut-off methods for tooling durability over the life of the program.

Design engineers must also 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).

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.

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.