How Injection Molded Plastics Measure Up In Aerospace Applications
Given well thought out choices in design, materials selection and processes, plastic parts can meet or exceed specifications for tight tolerances.
“Tight tolerance” is a term that can sometimes be loosely defined, but in aerospace manufacturing, precision is an important factor in aircraft performance. Critical-use parts with improperly executed tolerances can underperform or possibly fail, with costly results.
For years as aircraft punched into the frontiers of high-speed travel, metal parts were trusted to deliver the needed tight tolerances for complex yet reliable operation. Recently, advancements in materials and production are proving plastics capable of replacing metal in aircraft operating systems. Doing so eliminates secondary operations like machining, making it easier to procure mating parts, and increases the potential for full metal-to-plastic conversion—all of which help reduce costs.
Traditionally, aerospace engineers sought out and used metals based upon their ability to maintain tight tolerances. This dominance is being usurped by a growing array of plastics that can meet a typical tight tolerance for injection molding of ±.002 inches, though many are capable of meeting a very tight tolerance is of ±.001 inches. To capitalize on emerging opportunities suitable for most aircraft parts, engineers must address four factors—part design and complexity, materials selection, injection molding tools, and process and control.
Start with Part Design and Complexity
The importance of design is underscored by the tight tolerances required in the critical-use parts used in aeronautics. The design phase is the first place to find and address issues that threaten tight tolerances. Making improvements during the design phase helps achieve repeatable tight tolerances, plus improves manufacturability, quality, and customer satisfaction, all while reducing costs.
Part geometry, overall size and wall thickness requirements can also influence tolerance control. Thick walls may have differential shrink rates that, in turn, make it difficult to hold tight tolerances since the shrinking can vary within a section. Part size is integral because the smaller the part, the easier it is to hold tight tolerances. Further, the larger the part dimensions, the larger the overall shrink. This introduces potential challenges to control and maintain tight tolerances.
Part complexity can impact tolerance if shrink and warp are not repeatable. Upfront 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 parts quickly during molding, maintaining proper tooling temperature and managing the cooling process are important for tolerance control. Moldflow analysis software provides a cost- and time-saving computer simulation of how the specified material will fill a mold’s cavities and identifies areas of concern. The software also provides accurate predictions regarding mold heating and cooling, shrinkage, and warpage—all of which affect tolerance.
For aerospace systems, your designers must have a clear understanding of the environment wherein the part or product will be used. Why is this so important? The environment influences the behavior of plastic, which in turn, affects tolerance. To better illustrate, consider that plastics typically have large thermal expansion coefficients. That means parts may have to be measured at a consistent temperature to ensure accuracy in determining the part’s ability to maintain a tight tolerance.
In the sky, aircraft parts will be exposed to temperature extremes during normal operation and they will expand and contract. Knowing how a part will react during temperature fluctuations may signal to designers that exploring alternative options to a tight-tolerance part or product is possible, thereby shortcutting the process and saving time and money.
Material Options Affect Tolerances
Different resins can produce different tolerances for the same part. As such, engineers are sometimes faced with making a tradeoff in tolerance expectations and the physical properties of the resin to achieve their goals.
For injection molding, shrinkage has a big impact on tight tolerances, specifically repeatability, since materials can have different shrink rates. The higher the shrink rate, the less repeatable the tolerance. Therefore, knowing the general properties of plastics is key.
Amorphous materials don’t experience a drastic volume change when melted and they shrink less, so they generally hold tighter tolerances than crystalline materials.
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.
Using crystalline materials makes the process difficult for holding size because the degree of crystallinity can vary. Plus, certain crystalline 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. To maximize crystal growth, add a nucleating agent to the melt 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, although it’s important to note that different types of fillers can also increase or decrease the degree of shrinkage. Even though plastic parts can be held to tight tolerances in a climate-controlled environment, these parts will not necessarily maintain these dimensions as the temperature changes, conditions typically faced by aerospace parts. This aspect 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.
Holding Tolerances Inside the Mold
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. Features such as undercuts or threads, for example, typically require more mold components. Other components can be added to a mold to form complex geometries, such as rotating devices and hydraulics, floating plates and multi-form slides. However, it is essential that these components transfer heat away from the melted plastic quickly, which may necessitate the use of special cooling line placements or mold component materials.
For complex parts or products, the injection molder can strategically place temperature transducers in the 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 backup just in case the cooling lines or unit fail; upper and lower limits can be set on these sensors to monitor the cooling rate.
A production-capable process with in-mold pressure sensors is also important for establishing a benchmark for reference when making changes to tooling material, process, or molding machines. These sensors can monitor and document the process 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 truly run the same. Matching cavity pressure curves assures that the variation in processing conditions on the alternate machine still produces the same part.
Finally, other tooling design considerations that impact tolerance include the grade of steel, location of waterlines to maximize cooling and minimize warping, and gate types and locations to facilitate flow, fill pressure and part stability.
Keeping an Eye on Process Design and Control
Almost any reasonable design looks good on paper (or even as a prototype), but translating the concept on the injection molding plant floor is no guarantee the part will be made to specifications. The key to molding tight tolerance parts and being able to maintain repeatability is setting up a well-thought-out process.
In the initial run, engineers trained in scientific molding play a pivotal role in increasing the potential for successful production. They know the molding process inside and out, and have a fully developed understanding of what happens with the material inside the mold, specifically in terms of viscosity. Their insights are invaluable in designing successful molds and molding processes that deliver parts to specification.
Once production is underway, 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 instead of using machine parameters such as time, temperature and pressure minimize lot-to-lot variation. Conducting injection-molding operations in a climate-controlled facility also reduces process variation.
Part design and tool design come together on the injection molding plant floor. They are dependent on each other, and they 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. They can also evaluate physical and chemical properties of the selected resin so they can align it with the proper mold steel and review mold cooling. Moldflow analysis may also be conducted during the design phase to ensure higher quality parts and optimized cycle times and tooling trials.
In evaluating the part and the process, the team may conclude looser tolerances work for a particular aerospace application. 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 lead to increased production 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.
Though performance is crucial in the aerospace industry, manufacturers want to also contain costs. Injection molded plastic parts, in many cases, can meet tolerances the same way metal can, but at a reduced cost and in a variety of materials that can deliver other project benefits.