Almost any reasonable design looks good on paper (or even as a prototype)—but that doesn’t mean it’s a sure thing when it comes to manufacturing it. In some ways, designing the product is easy compared to making sure it can be manufactured in a streamlined, efficient, validated, and repeatable way.
Manufacturability impacts profits. By improving the part design, material selection, and/or the process design (enhancing manufacturability) to make sure a high-quality product is manufactured in a fast and accurate way to meet or exceed customer objectives, a huge amount of money can be saved over the lifecycle of the product.
Quite simply, “part design with manufacturing in mind” will determine the success or failure of a project.
The first 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. Feasibility reviews early in development can avoid costly tooling. Production manufacturing should always be the end target when designing parts and tooling. Decisions made during these design meetings typically account for about three-quarters of the total final cost of the part or product—so a lot of money is on the line.
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. Mold flow evaluation is also undertaken to determine the best type of gate and gate locations, in addition to determining proper vent locations.
Taking these steps up front is the best way to eliminate wasted effort and rework, which adds significant cost to the tooling budget.
Manufacturability review includes confirmation of standard plastic design practices and incorporation of tooling details to create the most robust design possible. Tooling specifications and tooling sources are finalized and purchased component sources qualified. A comprehensive process failure mode effects analysis (PFMEA) is also completed. Below are some key goals when designing a product with manufacturability in mind:
Review material supplier recommendations for selected resins
Design parts with nominal wall thickness whenever possible
Where possible, eliminate or combine steps to increase manufacturability
Gate into the thickest wall section of the part, with a gate designed for minimal to no trimming
Design parts with adequate draft for proper release from the mold without the use of mold release agent
Design parts to minimize secondary operations, such as assembly or finishing
Design parts with generous radii at part geometry intersections to enhance melt flow and reduce stress
Consider aspects of the tool design when designing the part (such as keeping knit lines and end-of-fill locations in mind when choosing gate locations)
Once a reasonable design has been made, complete a mold flow analysis, including melt flow, warp cooling pressures, knits, etc.
For good reason, customers are concerned about cost. The best way to reduce cost, and maximize overall profitability, is by working closely with the injection molder to consider all the design details that will, in the end, result in the highest manufacturability of the product. It will be time well spent.
Designing 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.
More manufacturing companies—especially automotive—are becoming aware of converting existing metal products or parts to plastic. Plastic parts have the same tight tolerances and are just as tough as metal parts. Plastics can be engineered to have specific characteristics for particular applications that are better than metal. Plastic parts are typically up to 50 percent lighter in weight than metal parts and converting from metal to plastic can significantly reduce total manufacturing costs.
Manufacturers in the automotive industry are more familiar with metal-to-plastic conversion because they are using this technology to reduce vehicle weight and meet tougher federal emission standards. Engineered plastics that are chemical-resistant and heat-resistant are especially good for fuel systems, fluid handling systems, and under-the-hood applications.
OEMs in other industries that currently use metal parts can also benefit from conversion to highly engineered plastics parts, but don’t know a lot about the technology or the following advantages it provides:
Tensile strength comparable to metal
Reduced part weight
Highly repeatable process (less scrap)
Lower manufacturing costs
Enhanced regulatory compliance
Greater design flexibility
Increased market stability for material cost
Lower packaging and shipping costs
One of the greatest advantages of plastics is the availability of more than 25,000 engineered materials for manufacturing applications. New blends and hybrids can also be custom-designed to meet very specific performance requirements. Plastics also dominate over metal when it comes to aesthetics—plastics are available in a variety of colors, surface finishes, and textures that are much more eye-catching than metal.
A key cost-reduction aspect of making a plastic part is the ability to combine multiple components into a single mold design, compared to making these individual components out of metal and welding them together. This reduces the number of secondary operations and assembly steps required, greatly reducing cost. Joints that would otherwise have to be welded in a metal part can be seamless in an injection-molded counterpart, often without a parting line—ideal for cooling systems and other fluid handling components where leakage is a big concern. Scientific injection molding processes can produce plastic parts right out of the mold with tight tolerances that require no secondary machining.
In general, companies can expect to achieve an overall cost savings of 25-50 percent by converting to plastic parts. Multiple parts can be combined into one mold, eliminating the need for fasteners and assembly. Colors can be molded in, eliminating secondary operations for welding, painting, and laser marking. Plastic has a nominal impact on part cost; sheet metal, however, has a much bigger impact thanks to the need to weld, grind, rework, and add dent and scratch resistance and noise dampening.
Perhaps the most exciting advantage of metal-to-plastic conversion is the design freedom it provides engineers. They can think in more creative ways about complex geometry, performance in harsh environments, shielding considerations, weight and structural limits, thermal management, and product differentiation—both in performance and how it looks on the shelf.
As material suppliers continue to develop high-strength thermoplastics that are increasingly impact-resistant, corrosion-resistant, and heat-resistant, more companies across a range of industries are converting from metal components to plastic.
Kaysun Corporation has performed metal-to-plastic conversion for many companies across a wide range of industries, including automotive and defense. Contact us to consult with our engineering team about the feasibility of converting your metal products to plastic. With full in-house testing and analysis capabilities and deep polymer expertise, we can help you comply with regulations, lower total costs and/or reduce product weight while delivering improved design, flexibility, and performance with the use of plastic.
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. For example, a glass- or mineral-filled material can hold tighter tolerances than an unfilled material. Different materials also have different shrink rates—the higher the shrink rate, the less repeatable the tolerance.
Crystalline materials don’t hold tight tolerances as well as other materials because of their higher shrink rates and the potential for continued crystal growth. Polyethylene and acetal are good examples—these materials can continue to grow more crystalline, even at freezing temperatures. As a result, the overall structure gets tighter and more compact, shrinking the size of the part. This can continue to fluctuate over time.
Therefore, to create tight-tolerance parts, the injection molding environment must be designed to ensure that the highest state of crystallinity is achieved quickly. 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.
Moisture absorption also affects tolerance. Hygroscopic materials—especially nylons—change in size as they absorb water and humidity levels fluctuate. Mechanical properties can change as well. A good example is using molded, unfilled nylon for the throttle linkage in boat engines. In these hot and damp conditions, the nylon linkage gets longer as it absorbs more heat and moisture, throwing off the timing of the carburetors. Kaysun Corporation solved this problem by making the linkage from aluminum rods with overmolded polybutylene terephthalate (PBT) snap-fit ends. Not only was the aluminum lighter in weight, it had a lower thermal expansion that also matched the thermal expansion of the engine block.
To find the best material for your injection molded product that maximizes tight tolerance and superior quality, be sure to collaborate with the Kaysun engineering team early in the design process to discuss performance expectations, the end-user environment, and a production process that maximizes manufacturability. This will ensure the highest possible performance and the least amount of shrinkage/variance over the lifecycle of your part or product.
There are plenty of factors that impact tight tolerance of injection-molded parts—all of which need to be controlled with precision to achieve those specifications. The greater the number of factors, the harder it is to achieve tight tolerances consistently. That’s why it is so important to consider tight-tolerance goals during the product design process—it is here where tight-tolerance factors can be minimized through design modifications, without impacting quality or performance. In fact, depending on tolerance needs, working with an experienced injection molder like Kaysun can actually tighten those tolerances even further if needed, by modifying the design, materials, or production process.
Product design 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.
Not many product designers have in-depth injection-molding experience—they know the basics, but that is usually not enough when it comes to factoring injection molding into the design and production process.
Sometimes companies contract a design house for their products—this can sometimes work, but more often than not the design house focuses on eye-appeal of the final product and cost, without enough consideration for ease of manufacturing and meeting specs.
An experienced injection molder like Kaysun (we have over 60 years of tight tolerance injection molding experience) truly understands the tolerance limits that are achievable and how to support product design to make these tolerances a reality. Knowing just the basics from a plastics-manufacturing standpoint often results in flawed designs—for example, die lock conditions, poor areas of fill with abrupt changes in wall thickness, sharp corners, and impossible tolerances. This doesn’t happen with an experienced injection molder.
Is tight tolerance even necessary? Is it required for performance, longevity, or appearance? 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. Achieving tight tolerance is a more expensive process, so if it not required, loosen it up to reduce production costs.
Physical part size affects tight tolerance. Large parts are difficult to hold to tight sizes consistently. Local areas with the large part, however, are easier to manage and can be dimensioned accordingly from a local datum. Plastic material manufacturers also have charts to assist with choosing appropriate tolerance based on part size and wall thickness.
Material type and environment influence 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—if the tight-tolerance part will be exposed to high/low temperature extremes in normal operation, where it will expand and contract, is tight tolerance even necessary?
Crystalline plastics tend to shrink more (move away from tolerance) as time goes on because they become more and more crystalline. This is especially true with acetal-type plastics. This shrinkage also occurs when plastics are exposed to an environment that promotes crystal growth, such as high temperatures. Also, if these plastics are molded in cold molds that don’t allow much initial crystal growth, the tendency for this growth is stronger than if a plastic was molded in a hot mold. These points all need to be considered when selecting materials and mold processing.
Other key design factors that affect tolerance are differences in part wall thickness (can create uneven crystallinity) and moisture (for example, some plastic parts—especially those made from nylon—lose some of their tight tolerance when they leave a tightly controlled atmosphere and are used in environments with a higher moisture content).
Be sure to select an experienced injection-molding partner to work with your design team early in the process and give it a key role in designing the product for tolerance, performance, and manufacturability. This will ultimately reduce cost, time to market, and yield a higher quality product.
Scientific molding is the best way to deliver complex, high-performance parts. It is a highly precise, data-driven process that eliminates any guesswork and maximizes quality and manufacturability. Scientific molding is especially valuable when it comes to decisions about process optimization, molding and tooling design validation, and product quality control. This approach is superior to standard molding procedures because of the high level of scientific control utilized through upfront design of experiments, flow analysis, process monitoring, and quality control that can correct any process variations with-in seconds.
One of the most important steps in developing a robust injection-molding process is utilizing a design of experiments (DOE) on the mold.
The DOE process is a critical aspect of the scientific molding process. After considerable preparation and engineering regarding product design and process parameter selection, the DOE process is established—a big step toward optimizing the process.
However, to ensure consistent and repeatable production of flawless molded parts, the process extremes must be completely investigated and the injection mold evaluated before it’s called into action. This is how tooling weaknesses are identified and corrected. This is one of the most important jobs of the process engineer.
Both the tool engineer and process engineer thoroughly examine every aspect of the mold’s mechanical functionality to make sure everything works as designed, using the material settings provided by the supplier of the material to be molded. They then conduct short-shot testing to assess the dynamic pressure loss and, in a multiple-cavity mold, to check for any imbalance among the cavities. This step is also the stage for a crucial objective: establishing the rheology curve (or viscosity curve) to indicate the best fill rate and pattern.
After this, gate seal studies are performed from both the pressure curve and the weight of the sample parts to see if the gates seal fully, and at what point, on the mold cavity (or multiple cavities). Engineers examine the test parts from the processing extremes for any defects and record their findings along with recommendations for any adjustments in the process or the tool in order to correct the defects. They also record data on the melt temperature, fill time, mold temperature, coolant flow, cycle time, and pressure curves.
The parts then go to quality control for examination of their measurements, shot-to-shot consistencies, and overall quality. That information is used for any necessary adjustments to the tool, before new samples are made. The new samples then undergo the same quality testing, with necessary adjustments made again as needed. Once all the process parameters meet their performance ranges, the mold is ready for action and production process has been optimized.
Mold design and process optimization is vital for creating a highly efficient and low-cost production process by eliminating any problems or quirks before production starts—ensuring top quality and repeatability.
By understanding each phase of product development—especially mold design and process optimization—scientific molding engineers can build the most efficient and robust process possible for your product, saving money on upfront quality and speeding up throughput.
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.
Process Design and 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.
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According to the January 15 issue of US News and World Report, auto manufacturers are moving away from steel and more toward lighter-weight materials to meet fuel economy standards.
“Faced with increasingly stringent benchmarks—the government's new Corporate Average Fuel Economy (CAFÉ) standards require a passenger vehicle fleet that averages 54.5 miles per gallon by 2025—carmakers are taking another approach to achieving better fuel mileage: shaving weight off vehicles by experimenting with lighter materials,” writes reporter Meg Handley.
These include aluminum, plastic, and carbon-fiber composites.
“Right now, only select parts have been revamped with lighter materials including more flexible, shock-absorbing bumpers and roof frames, and manufactures are starting to use more plastic components to lighten heavy engine blocks,” she continues. “But that will likely change over the next several years as auto manufacturers try to take pounds off of heavier vehicles and improve fuel efficiency.”
Kaysun Corporation has helped several automotive clients with this transition. For example, Kaysun worked with Gates Corporation to replace its metal serpentine belt pulley with a durable plastic replacement to reduce weight and improve performance. The plastic pulley—with a tolerance of +/- 0.001 inch—met all required noise, vibration, and harshness specifications and reduced overall production costs for the company.
Plastics manufacturers continue to develop resins that are specially engineered to withstand temperatures in the 400 °F to 500 °F range and resist the corrosive environments in cars and trucks. For example, DuPont’s Zytel® is a strong, stiff, high-performance polyamide that can withstand high temperatures and long-life coolants. It is considered ideal for thermostat housings, water pumps, engine oil systems, brake systems, and transmission components. Scania AB became the first vehicle manufacturer in the world to use a thermoplastic oil pan module/pump made from 35 percent glass fiber-filled Zytel in its commercial truck line.
Another polyamide product is BASF’s Ultramid®, which is designed for continuous use (3,000 or more hours) at 428°F, with temporary temperature peaks as high as 464°F—making it suitable for components near the engine, particularly for the charge air (heat exchange) systems of turbocharged engines. Chrysler is using cam cover injection-molded from Ultramid.
As specifically-engineered resins continue to enter the automotive market and compete favorably with metal in terms of performance, longevity, and cost, automakers will rely more on injection-molded plastic parts in the coming years to improve performance, reduce costs, and save customers money at the pump.
It can occur in just about any injection-molded part or product—yet the experienced injection molder knows how to eliminate warpage from the production cycle and maintain a steady throughput of high-quality product that meets all customer specifications, including dimensional accuracy and tight tolerances.
Warpage occurs when the shape of the final product has deviated from the shape of the mold cavity; sometimes warpage is so slight it is still “within spec” and not a functional or aesthetic problem. In other cases function and fit are compromised and the part or product must be rejected. Warpage can result from one or several variances that may occur during the injection-molding process. However, application of scientific molding principles, combined with a deep understanding of material science and plastic flow behavior, can eliminate this problem.
Temperature differences are the main cause of warpage—whatever variances contribute to these unwanted temperature differences are the production parameters that need to be addressed. These include moisture in the pellets, improperly designed molds and tooling, incorrect melt or mold temperatures, mold contamination, and inaccurate feeding systems.
The cooling system is always critical. A precisely calculated cooling rate must be maintained for uniform cooling and shrinkage; if not, uneven cooling may result in deformation or bending. It is critical to have plenty of cooling channels that are properly located to facilitate even cooling (this can be more challenging in products with complex shapes). The gating system must be properly designed with sufficient gates at the right locations. Wall thickness can also be a problem; even slight variations in wall thickness can result in uneven cooling and warpage.
Plastics that are reinforced with glass fiber or other reinforcement materials have greater potential for warping because of inconsistent orientation of the glass fibers. These types of hybrid materials will not flow exactly as the homogenous plastics do—therefore injection molders must be highly vigilant when monitoring this process, especially temperature, pressure, rate of flow, and fiber orientation. Higher pressure and flow rate will also produce higher shear rates which can also impact molecular orientation, resulting in internal stress, uneven cooling, and warpage.
Molten plastic is a liquid that must be carefully monitored through every phase of the injection-molding process. It “wants” to follow the path of least resistance; any variances in cooling that have created harder sections have also created channels that can affect the flow and cooling of the remaining molten plastic. When cooling rates are inconsistent this remaining material may not even fully harden, creating soft spots.
This is where an injection molder’s experience really comes into play. Warpage must be considered during the earliest design phases so the proper materials, mold design, settings, and process can be designed and tested to make sure warpage is eliminated from the final production process.
Surface finish on plastic composites can vary a great deal, depending on the physical and chemical properties of the polymer blend, as well as the parameters of the injection molding process.
The first objective is working with the client to determine how important the surface finish is for the appearance and/or performance of the final product. For example, does the product need to be eye-catching or simply functional? Depending on the answer, the material selected and the desired finish will determine the settings for the injection molding process, as well as any secondary finishing operations that might be required.
There are also functional tradeoffs to be considered. For example, the coefficient of friction and resistance to wear can be affected by the surface finish; if these characteristics are important to product performance, the design team needs to select a material and process that will create compatible finish. Experienced injection molders have a vast body of scientific data on material chemistry and behavior and processing conditions and can cross-reference this information to accurately determine surface finish according to process parameters.
It is critical to determine surface finish during the design stage because it will impact the types of material, tooling, and processing decisions that can be considered. The texture on the mold steel will limit the part surface finish. Sometimes a rougher texture can be used to hide other surface imperfections, such as sink. Surface finish can also affect the draft required on the part—without appropriate draft, the surface finish could be destroyed during ejection.
Two key surface characteristics—gloss and roughness—can be impacted by the material selected, additives, and different injection molding parameters such as fill rate, pressure, and temperature. Material type is especially important—for example, higher melt temperatures for products made from reinforced crystalline resins increase gloss and reduce roughness, creating a smoother surface. If reinforced amorphous resins are being injection-molded, however, a higher mold temperature will increase gloss and reduce roughness.
Another important consideration for surface finish is additive compounds that are mixed into the resin. This is where knowledge of material science really comes into play—for example, fiberglass content results in a lower gloss. Other additives like carbon black or mica can reduce surface roughness—these are additives to avoid (or find substitutes for) if a certain surface roughness must be maintained for product function or appearance. Adding particulate fillers may increase surface roughness. Engineers can mix and match the right combination of resins and additives to produce specific surface qualities.
Then there’s speed—faster injection speeds, combined with higher melt or mold temperatures, will further enhance the gloss and smoothness of the surface. In fact, a faster injection speed always improves gloss and smoothness, regardless of the other production parameters that have been determined. For any reinforced composite, the fast injection speed combined with high melt and mold temperatures will always provide the glossiest and smoothest part (although a slow injection speed, combined with cooler mold and melt temperatures, may meet other product specifications, they will typically result in a poor surface texture). Quick filling of the mold cavity can help minimize fiber orientation, making the weld line less visible and boosting the aesthetic quality of the product.
These are just some of the methods for controlling surface finish during the injection-molding process. Be sure to consult with your injection molder in the earliest design stages to fully determine surface finish needs—getting this worked out up front will result in the most efficient production process and likely reduce the need for any secondary surface preparations.