Before a mold is created, best DFM practices for plastic injection molded parts incorporate the following critical elements:
Shrinkage is the contraction of the molded part as it cools after injection. All materials have different shrink rates depending on resin family (amorphous vs. crystalline materials), mold design, and processing conditions. Resin may also shrink differently depending on the direction of flow. As a general rule of thumb, a 10% change in mold temperature can result in a 5% change in original shrinkage. In addition, injection pressure has a direct effect on shrinkage rates. The higher the injection pressure, the lower the shrinkage rate. View typical mold shrink rates here.
How features of a part are formed in a mold determines the type of draft needed. Features formed by blind holes or pockets (such as most bosses, ribs, and posts) should taper thinner as they extend into the mold. Surfaces formed by slides may not need draft if the steel separates from the surface before ejection. Consider incorporating angles or tapers on product features such as walls, ribs, posts, and bosses that lie parallel to the direction of release from the mold which eases part ejection.
Uniform wall thickness throughout a part (if possible) is essential to avoid thick sections. Designing non-uniform walls can lead to warping of the part as the melted material cools down.
If sections of different thickness are required, make the transition as smooth as possible allowing the material to flow more evenly inside the cavity. This ensures the whole mold will be fully filled and will ultimately decrease the chance for defects. Rounding or tapering thickness transitions will minimize molded-in stresses and stress concentration associated with abrupt changes in thickness.
Incorporating the proper wall thickness for your part can have drastic effects on the cost and production speed of manufacturing. The minimum wall thickness that can be used depends on the size and geometry of the part, structural requirements, and flow behavior of the resin. The wall thicknesses of an injection molded part generally range from 2mm – 4mm (0.080″ – 0.160″). Thin wall injection molding can produce walls as thin as 0.5mm (0.020″). Work with an experienced injection molder and design engineer to be sure the proper wall thicknesses are executed for your part’s design and material selection.
In addition to main areas of a part, uniform wall thickness is a crucial design element when it comes to edges and corners. Adding generous radii to rounded corners will provide many advantages to the design of a plastic part including less stress concentration and a greater ability for the material to flow. Parts with ample radii also tend to be more economical and easier to produce, with greater strength and appearance.
Many designers think that by making the walls of a part thicker, the strength of the part will increase. When in reality, making walls too thick can result in warpage, sinking, and other defects. The advantage of using ribs is that they increase the strength of a part without increasing the thickness of its walls. With less material required, ribs can be a cost-effective solution for added strength. For increased stiffness, increase the number of ribs rather than increasing height and space a minimum of two times the nominal wall thickness apart from one another.
How features of a part are formed in a mold determines the type of draft needed. Features formed by blind holes or pockets (such as most bosses, ribs, and posts) should taper thinner as they extend into the mold. Surfaces formed by slides may not need draft if the steel separates from the surface before ejection. Consider incorporating angles or tapers on product features such as walls, ribs, posts, and bosses that lie parallel to the direction of release from the mold which eases part ejection.
Surface finish options for plastic injection molded parts vary depending on part design and the chemical make-up of the material used. Finishing options should be discussed early in the design process as the material chosen may have a significant impact on the type of finish implemented. In the case where a gloss finish is used, material selection may be especially important. When considering additive compounds to achieve a desired surface finish and enhance the quality of a part, working with an injection molder that is aligned with knowledgeable material science professionals is essential.
Consideration of these elements is fundamental for integrating engineering and manufacturing expertise to catch mistakes, see opportunities for efficiencies and cost reduction, and even assess the viability of contract requirements. Typically, your injection molder will conduct a detailed analysis of these elements with your team well before the tooling process is initiated.
DFM is not a “stand alone” guideline or principle when it comes to producing plastic injection molded products or parts. It works with other approaches for design optimization like designing for functionality, assembly and sustainability, each of which is discussed further, below.
CNC machining can be a fast and economical method for producing high-precision plastic parts in small and medium quantities. However, the part’s design plays a significant role in the quality of the part and efficiency of the project. Design for manufacturing (DFM) is the process of designing machined parts, subassemblies, or complete products to optimize manufacturing processes while reducing costs and maintaining quality.
Plastic materials behave differently from metals. When engineers apply DFM principles to your design, you can be assured your plastic machined parts will be designed to produce the best outcome by optimizing manufacturability, quality, and cost-effectiveness.
The design for the manufacturing process must be done early. For every step that must be redone, cost increases. There is a 1-10-100 rule that suggests for every $1 spent on prevention, you would spend $10 on correction and $100 on a failure. Determining there is a problem with machining the part once it is in production costs you time and money as the part goes back to be redesigned, and parts must be reworked or scrapped. It costs even more if the issue isn’t detected until it’s in assembly and holes are off by just enough to create a problem. If a critical part has a field failure, the cost could easily exceed 100x the prevention cost when you tally up the cost of warranty claims, lawsuits, and lost customers.
Most costs for CNC machined parts are driven by material selection, part setup, tooling, and machining time. DFM looks to streamline processes and remove complexity, when possible, to reduce costs. Below are some guidelines that are followed. However, quality, form, fit, and function are always a priority.
Material selection – Material selection significantly impacts a product’s manufacturability, performance, and cost. Sometimes, substituting materials can significantly reduce costs without compromising function or reliability. It will be important that the engineers understand how the product will be used, the stresses it will encounter, the environments it is exposed to and the length of exposure (intermittent or continuous), and other materials it is in contact with.
Part setup, tooling, and machining time – These three elements are interrelated in many cases. Part setup and the need to change tooling can add to machining time.
You may not have considered part setup when designing your product, but it can add cost to a project. For a complex design with features on several sides, a CNC operator may need to use multiple machine setups to produce the part. In many cases, this means manually maneuvering the part to machine all axes. However, this will depend on the type of CNC machine used (e.g., 3-axis, 5-axis). Keep in mind that manually rotating the part requires the machine to be recalibrated and a new coordinate system defined.
Tolerances can impact cost. General tolerances should be used, and tight tolerances should be used only where necessary.
Irregularly shaped parts may require fixtures to ensure tolerances are met. Designing custom fixtures can increase costs and lead times. Non-standard features may require special tooling, which can add costs.
Tool access is one of the limiting factors when designing a part. There are some design guidelines that engineers will look for on your drawing to ensure that time and costs are not unnecessarily added. A few are discussed here.
Most CNC machining tools are cylindrical with a flat or spherical end, restricting the shape. Inside corners will always have a radius. Smaller tools can reduce the fillet size but introduce additional tooling costs. A general rule is the inner edge radius should be 1.3 times the milling tool radius (e.g., have a 13 mm radius if working with a 10 mm milling tool). If possible, don’t use a floor radius so the same tool can be used.
Holes smaller than 20 mm should also use standard drill bit sizes when possible, as a non-standard diameter requires an end mill tool. Reamers and boring tools can be used to finish holes that have a tight tolerance. Standard tools also have a limited cutting length as well. The depth of a cavity is generally 3-4 times the diameter. Deep cavity milling may require special tooling.
Undercuts can’t be machined with standard tools. If undercuts are a feature on an internal wall, tool clearance must be considered. The space between the machined wall and other internal walls should be at least four times the undercut’s depth.
The minimum wall thickness for plastic machined parts is 1.5 mm. Thinner than this, and accuracy and strength are diminished. The heat during machining may soften the walls, causing warping.
Additional considerations beyond machinability may be considered depending on the project and its needs.
Standardizing – Engineers may look across products to see if parts can be standardized to avoid having multiple customized components. They may also look to use standard hardware in product assembly.
Component reduction – In addition to standardizing components, design engineers may evaluate it to see if multiple components can be combined without significantly causing a negative impact on its quality, function, aesthetic, or cost. This will need to be evaluated to ensure it doesn’t add cost to machining or create issues with use.
Life cycle – Depending on your requirements, the engineer may also evaluate your product with its total life cycle in mind. This may include:
We have the experience, knowledge, and tools to ensure your CNC machined plastic part meets your requirements. Applying DFM principles, our design and engineering team will ensure your part can be manufactured most efficiently and cost-effectively. Contact us to get started!