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 tight tolerance is not done correctly, parts and products will underperform or possibly fail, resulting in customer dissatisfaction and a tooling and/or process overhaul. Other tight-tolerance benefits may include the elimination of secondary operations like machining, making it easier to procure mating parts, and allowing the possible conversion of metal parts to plastic—all of which help reduce costs.
In general, a typical tight tolerance for injection molding is +/-.002 inches. A very tight tolerance is +/-.001 inches. Major factors that impact tight tolerance include part design and complexity, material selection, tooling, and process design and control.
Part Design and Complexity
Designing the part is the single biggest factor in controlling tight tolerances. Making improvements during the design phase will not only achieve repeatable tight tolerances, but also improve manufacturability, quality, and customer satisfaction, all while reducing costs.
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 sections, making 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 larger the overall shrink, which makes it more challenging to control and maintain tight tolerance.
Part complexity can impact tolerance if shrink and warp are not repeatable—that’s why up-front discussions between the manufacturing and product design teams 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. Moldflow analysis is also needed for accurate predictions regarding mold heating and cooling, shrinkage, and warpage—all of which affect tolerance.
The application’s environment also influences plastic behavior, which affects tolerance. For example, plastics typically have large thermal expansion coefficients; as a result, tight-tolerance parts may have to be measured at a consistent temperature. Temperature is also something to consider during design—in fact, if the tight-tolerance part will be exposed to high/low temperature extremes in normal operation, where it will expand and contract, tight tolerance may not even be necessary.
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.
Shrinkage has a big impact on tight tolerance. Materials have different shrink rates—the higher the shrink rate, the less repeatable the tolerance. Amorphous materials generally hold tighter tolerances than crystalline materials. This is because crystalline materials go through a phase change from a crystalline solid (densely packed structure) to an amorphous melted fluid (less dense structure), resulting in a volume change. Because amorphous materials stay amorphous when melted and don’t experience a drastic volume change, they shrink less. Different types of fillers can also increase or decrease the degree of shrinkage.
Using crystalline materials makes it more difficult to hold size because the degree of crystallinity can be made to vary. Parts molded in “cold” molds don’t really have the environment to develop their full crystalline potential. Parts with thick walls provide a well-insulated center area for crystal structure to develop, whereas crystal growth is quenched in the “skin” of the part in contact with the mold wall. Materials such as acetal and polyethylene can even continue to grow crystals below room temperatures.
Therefore, to create tight-tolerance crystalline parts, the molding environment must be designed to ensure that the highest state of crystallinity is achieved as quickly as is economically possible. In general, high mold temperatures provide the best environment for crystal growth. Crystal growth can also be maximized by adding a nucleating agent to the melt, which provides sites throughout the melt that stimulate crystal growth.
Holding tight tolerances can be a challenge with most plastics because they have high thermal expansion rates (fillers help reduce this). Even though plastic parts can be held to tight tolerances in a climate-controlled environment, this doesn’t mean they will maintain these dimensions as the temperature changes. This must be considered when plastic parts are combined with other material types such as metals, or when the end use occurs in an environment that has big temperature swings.
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 vary and tight tolerances will be harder 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. It is important to be sure these mold components transfer heat away from the melted plastic quickly; special cooling line placements and mold component materials can be incorporated into the mold design to achieve this.
For complex parts or products, it is recommended that temperature transducers be strategically placed in tools, as well as hot manifold tools, to monitor and control the process in real time. 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.
Establishing a production-capable process with in-mold pressure 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. Creating a pressure curve is a must if a mold needs to be run in a different machine. Two identical machines will never run the same. Matching cavity pressure curves assures that the variation in processing conditions on the alternate machine still produces the same part.
Other tooling design considerations that impact tolerance include the grade of steel, location of waterlines to maximize cooling and minimize warping, and determination of gate types and locations to achieve optimum flow, fill pressure, and cooling time. It is also important to locate gates where they won’t impact part performance or appearance (flow marks, shrinkage, warping). Draft, surface finish, and ejection design can also affect dimensional stability.
Almost any reasonable design looks good on paper (or even as a prototype)—but that doesn’t mean making it is a sure thing. Setting up the ideal process for the part, and being able to repeat it, is the key to molding tight tolerance parts.
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 versus simply using machine parameters such as time, temperature, and pressure help eliminate the lot-to-lot variation that is common in the industry. Conducting injection-molding operations in a climate-controlled facility also reduces process variation.
Production manufacturing should always be the end target when designing parts and tooling. A key step is getting the customer design team and the injection-molding engineering team together to discuss every aspect of the project, including intended end use and lifecycle expectations. Part design and tool design are dependent on each other and thus should be done concurrently—hopefully in the earliest design stages. The cross-functional engineering team (tooling, materials, manufacturing, and quality engineers) must be involved early to provide a realistic manufacturing perspective on product design, tolerances, mold component functionality, mold materials, tool design, material performance, operational constraints, and associated costs.
The team especially looks for any potential problems in part geometry or tolerance that might result in poor steel conditions or require special tooling features such as lifters, slides, and threading/unthreading. The physical and chemical properties of the selected resin are also evaluated so that the proper mold steel can be selected and mold cooling be reviewed.
Conducting moldflow simulation during the design phase ensures higher-quality parts and optimizes cycle times and tooling trials. This involves using a sophisticated computer program to predict the flow of plastic during all phases of the injection-molding process, including flow, pack, and cooling. Other parameters that can be evaluated with moldflow analysis are type of gates, gate locations, vent locations, runner systems, mold temperatures, processing conditions, flow-induced shear stress, weld line location, air trap prediction, sequential valve gating, cavity/part temperature differential, coolant temperature, coolant flow rate, shrinkage, and warpage.
“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 than standard tolerance control because the severity of failure is low.
Many designers automatically set a tolerance in the CAD drafting software and all dimensions are toleranced to that number, when in reality the product may not need such a tight tolerance.
Designers need to understand that tighter tolerances may 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. Achieving tight tolerance may be a more expensive process, so if it not required, loosen up the tolerance a bit to reduce production costs.
Filed Under: Rapid prototyping