Customers count on their injection molders for expert advice. “Tight tolerance” is a term that is often tossed around loosely in the industry—however, if it’s not done right, parts and products will underperform or possibly fail, resulting in a tooling and/or process overhaul. Therefore tight tolerance is serious business, especially for complex, mission-critical parts.
In general, a typical tight tolerance is +/-.002 inches and a very tight tolerance is +/-.001 inches. Key factors that impact tight tolerance include part design and complexity, material selection, tooling, and process design and control.
Part geometry, overall size, and wall thickness requirements can all have an influence on tolerance control. Thick walls may have differential shrink rates within the thick section, which make it difficult to hold tight tolerances since the variable shrink can “move” within the section. Part size has an impact if the dimension with the tight tolerance is large (it is easier to hold tight tolerances in smaller areas). Also, the larger the dimensions, the higher the shrink rate, which becomes more of a challenge to control and maintain tight tolerance.
Part complexity can impact tolerance if shrink and warp is not repeatable—that’s why up-front discussions with the design team are absolutely essential for working out the best possible part design to minimize shrink and warp. Part complexity also impacts tooling design and material flow, because filling the parts quickly, maintaining proper tooling temperature, and managing the cooling process are important for tolerance control. Advanced moldflow analysis is needed for accurate predictions regarding mold heating and cooling, shrinkage, and warpage—all of which affect tolerance.
Material selection is another decision that must be made early in the design process. Different resins can produce different tolerances for the same part, so sometimes a tradeoff must be made between tolerance expectations and the physical properties of the resin. A glass- or mineral-filled material can hold tighter tolerances than an unfilled material and a crystalline material will typically hold tighter tolerances than an amorphous material. Materials also have different shrink rates—the higher the shrink rate, the less repeatable the tolerance.
Tool design, tool material, and cavitation all impact tolerance. The need to heat and cool tools, and the number of cavities in the mold, can make holding tight tolerances more of a challenge. 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 will assist with part repeatability and tolerance control.
Many parameters and variables must be carefully controlled during injection molding to achieve tight tolerances. Proper process control and process development ensure the part does not experience unnecessary pressure or stress during the molding process. Matching pressure curves verses simply using machine parameters such as time, temperature, and pressure help eliminate the lot-to-lot variation that is common in the industry.
Setting up the ideal process for the part, and being able to repeat it, is the key to molding tight tolerance parts. Process control is critical for achieving the same shrink rate shot to shot—any variation will result is variation in the shrink rates and thus dimensional results will be inconsistent. Utilizing scientific molding is the best approach for designing a repeatable process that maintains consistent shrink rates and repeatable dimensions. Performing injection molding in an environmentally controlled facility eliminates wide variation in temperature and humidity, providing even more control.
“Loose” Sometimes Works, Too
Keep in mind that, depending on the end use of the product, tight tolerances may not be required. Many consumer-type products do not require anything more that standard tolerance control because the severity of failure is low. Designers need to understand that tighter tolerances equate to increased production costs and development costs—therefore tolerances should be as generous as possible in the design phase to keep costs down, unless they are required for proper fit or function of the part/assembly.