Design for Manufacturing Injection Molding

There are many design elements involved when manufacturing plastic parts for injection molding - design for cost, design for quality, design for assembly, design for manufacturability. And navigating these environments can be challenging at times. At DAYIN, we provide automated design analysis of your CAD model, highlighting features in your part design that can be adjusted for moldability. This is a great design resource at your fingertips. To keep these moldability recommendations to a minimum and optimize your part design, we created this helpful kit of different injection molding resources.

Material Selection

Thermosets
Thermosets undergo a chemical reaction when processed. This reaction forms bonds in the polymer chains. The chemical reaction is irreversible and can only occur once, so it is not possible to use recycled materials. However, the trade-off is strength and high temperature resistance. Common types of thermosets are epoxies, silicones, polyurethanes, and phenolics.
Thermoplastics
Thermoplastics soften when heated and are easily injection molded. They do not undergo chemical changes like thermosets. Therefore, they can be reprocessed after the initial manufacturing process. There are two different types of thermoplastics - semi-crystalline and amorphous.
Amorphous
Amorphous plastics have a random, tangled orientation of their polymer chains. They are stronger and better suited for structural applications. While they are strong, they are susceptible to stress fractures. They also do not work as well as semi-crystalline plastics on bearing surfaces. Here are some of the most common types:
ABS
Acrylic
Polycarbonate
PVC
Semi-crystalline
Semi-crystalline plastics have a random, crystalline portion of their structure. In other words, they are a mixture of an amorphous structure and a fully crystalline structure. They make excellent bearing surfaces, living hinges, and offer good chemical resistance. The downside is that they tend to shrink and warp more easily than amorphous plastics. Here are some of the most common types:

Polyoxymethylene
Nylon
Polybutylene terephthalate
High-density polyethylene
Low-density polyethylene
PET
Polypropylene
Semi-crystalline
The above materials are often altered using additives and fillers. In the simplest application, the molder manufacturer can change the color of the material using color additives. The color can be changed on the molding machine or at the material manufacturer. Material manufacturers can make more advanced materials or "engineered materials." For example, a manufacturer can add varying levels of glass fiber to a material such as polycarbonate. The material will still be delivered to the molder in pellet form, but the strength will be significantly increased with the addition of glass fiber. Other fillers such as talc and carbon fiber are also common.

Fillers and additives can change other material properties. These can include UV protectants, antioxidants, antistatic agents, antimicrobial agents, lubricants, and more. Remember that they are application specific. Rather than trying to learn everything about a material, start with what your part needs and focus on that.

Key Principles of Design for Manufacturing in Injection Molding:

Simplified Part Geometry: The complexity of a part’s geometry can significantly impact both the cost and time required for manufacturing. DFM principles suggest simplifying designs by avoiding complex features that would require intricate tooling or additional processing. Features such as deep undercuts, thin walls, or excessive fillets should be avoided or minimized. Instead, parts should have simple, uniform shapes that can be easily produced with minimal mold modifications.
Minimize Undercuts: Undercuts are areas on the part that are difficult to access during the ejection phase, such as features that cannot be molded using standard tools. Undercuts often require more complex molds, such as side-action mechanisms or multi-slide molds, which can increase cost and production time. Designers should strive to avoid undercuts wherever possible by designing parts with straight pulls or adding features like draft angles to allow for easy ejection.
Uniform Wall Thickness: Parts with consistent wall thickness throughout are easier to mold and require less cooling time, reducing cycle times and material waste. Variations in wall thickness can lead to issues like warping, sink marks, or uneven cooling. A good rule of thumb is to keep wall thickness consistent, generally between 1.5 mm to 4 mm for most thermoplastics, while also avoiding excessive thickness that could lead to part distortion.
Add Draft Angles: Draft angles are slight slopes incorporated into the design of the part to help it release easily from the mold. A part without a draft angle may get stuck in the mold, making it difficult to eject and leading to damage or defects. Typically, draft angles of 1 to 3 degrees are added to vertical surfaces to ensure the part comes out smoothly without excessive friction. Larger draft angles may be required for deeper or larger parts.
Gate and Ejection Design: Gates are the points where molten plastic enters the mold cavity. Proper gate design ensures the smooth filling of the mold and avoids issues like cold spots, short shots, or uneven filling. The number, location, and size of gates must be strategically chosen to ensure uniform flow of material. Additionally, ejection systems such as ejector pins, sleeves, or air blasts must be considered to ensure the part can be properly removed from the mold without damage.
Incorporate Radii and Fillets: Sharp corners and edges should be avoided in injection-molded parts as they can cause stress concentration, which may lead to cracks or breakages. Incorporating radii or fillets (rounded edges) helps to distribute stress more evenly across the part and improves both durability and aesthetics. It also ensures that the mold can be easily produced without the need for additional modifications.
Optimize Parting Line Placement: The parting line is the seam where the two halves of the mold meet. The placement of the parting line should be considered during the design process to ensure it doesn't interfere with functional or aesthetic features. Ideally, the parting line should be placed in areas that will not affect the part’s appearance or functionality, such as along flat surfaces or hidden sections.
Consider Material Selection: Material selection is an essential component of DFM for injection molding. Different materials have varying melting temperatures, flow characteristics, and cooling rates, all of which influence the design process. Choosing the appropriate material for the specific application ensures the part’s functionality, strength, and performance. Additionally, designers must consider the material’s cost, availability, and recyclability.
Draft for Tooling Accessibility: Designing parts with considerations for tooling accessibility can drastically reduce production complexity and costs. If parts have difficult-to-reach features or require additional tooling such as inserts, side cores, or sliders, the cost of the mold will increase significantly. Incorporating features that can be molded using standard tools and processes will reduce tooling time and costs.
Design for Part Interchangeability: In many manufacturing processes, especially in high-volume production, parts are often designed to be interchangeable or modular. This allows for greater flexibility, reduces the need for multiple molds, and allows for simpler assembly. A good DFM approach in injection molding will consider how parts will fit together in the final product and ensure that they can be easily assembled and disassembled if needed.

Advantages of Design for Manufacturing in Injection Molding:

Cost Reduction: By designing parts with manufacturing in mind, companies can reduce material waste, minimize machining, and shorten cycle times, all of which lead to cost savings.
Improved Quality: DFM principles help ensure that parts are designed to be produced with high precision and consistency, resulting in fewer defects and better-quality products.
Faster Time to Market: A design optimized for injection molding can reduce the lead time for mold creation and production. By streamlining the design process, manufacturers can get products to market more quickly.
Optimized Production Efficiency: DFM practices lead to the creation of more efficient molds, which can be produced faster and with fewer issues during the molding process.
Enhanced Sustainability: By reducing material waste and optimizing material use, DFM helps contribute to more sustainable manufacturing practices, reducing the environmental impact.

Designing Complex Features for Molded Parts

Undercuts
Undercuts are features such as clips or through holes that cannot be milled in a standard vertical (Z-axis) milling setup and would otherwise prevent ejection from the die. Learn how to design cams and inserts to accommodate undercuts in parts.

Stampings
A punch is a small undercut on a part that can be formed and safely ejected from the die without the need for side cams. Learn about the types of features and geometries that are suitable for stampings.

Side Actions
Side Actions, also known as side pulls and cams, are mechanical components in the die that are used to form undercut geometry. Side Actions are driven by pins and move in response to the opening and closing of the die. Learn how to use side actions to accommodate undercuts here.

Shutoff Valves
A closure is any two faces in a mold that come together to "close" or divert material flow within the cavity when the mold is closed. Some closures, called slide closures or through cores, are used to form undercut geometries. Learn how to combine closures to form undercuts here.

Cores and cavities
In an injection molding machine, one half of the mold (side A) is connected and held in place, while the other half of the mold (side B) is attached to a moving fixture that opens and closes the mold. Learn how to choose the right core and cavity locations here.

Wall Thickness
Wall thickness is a primary consideration during molding to ensure dimensional stability and consistency as the part cools and hardens. If the wall is too thin, it can weaken the part and prevent filling. If the wall is too thick, it can cause the part to bow or sink. Learn how to achieve consistent wall thickness here.

Frequently Asked Questions (FAQ) on Design for Manufacturing Injection Molding

What is Design for Manufacturing (DFM) in injection molding? DFM in injection molding refers to the process of designing parts with manufacturing constraints and capabilities in mind. It aims to create designs that are easy to manufacture, cost-effective, and efficient while ensuring high-quality products. The goal is to reduce complexities, minimize costs, and shorten production timelines.
Why is Design for Manufacturing important in injection molding? DFM is crucial because it helps ensure that parts are optimized for the injection molding process. By considering aspects like material choice, wall thickness, part geometry, and mold accessibility during the design phase, DFM reduces production costs, improves product quality, and enhances production efficiency.
How can DFM principles reduce production costs? By simplifying part geometry, using uniform wall thicknesses, and eliminating unnecessary undercuts, DFM minimizes tooling complexity, cycle times, and material waste. This directly translates into lower manufacturing costs, fewer defects, and less time spent on rework.
What are the common challenges in applying DFM in injection molding? Common challenges include balancing design aesthetics with manufacturing practicality, avoiding complex features that may increase tooling costs, and ensuring that designs meet the required functional specifications. Additionally, designers must communicate effectively with engineers and manufacturers to ensure the design is feasible.
What role do gate and ejection designs play in DFM? Gate and ejection designs are critical in ensuring that molten plastic flows uniformly into the mold cavity and the part can be easily ejected after cooling. Proper gate placement minimizes issues like short shots, flow lines, and uneven cooling, while efficient ejection systems prevent part damage and improve cycle times.
How do undercuts impact mold design, and how can they be avoided? Undercuts are features on a part that make it difficult to eject from the mold, often requiring complex molds with additional moving parts. Undercuts can significantly increase tooling costs and cycle times. To avoid them, designers can use simple geometry, incorporate draft angles, or modify the design to eliminate features that create undercuts.
What is the ideal wall thickness for injection-molded parts? The ideal wall thickness for most injection-molded parts typically ranges from 1.5 mm to 4 mm. Maintaining uniform wall thickness ensures consistent filling, cooling, and reduces the risk of defects like sink marks, warping, or uneven cooling.
How does material selection impact the design process in injection molding? Material selection plays a vital role in both the design and production phases. Different materials have different flow characteristics, cooling times, and temperature tolerances. Choosing the right material for a specific application ensures optimal part performance, structural integrity, and production efficiency.
What is the purpose of draft angles in injection molding? Draft angles are slight slopes added to vertical surfaces of parts to allow easy ejection from the mold. Without proper draft, parts may get stuck in the mold, leading to damage or deformation. Typically, draft angles of 1 to 3 degrees are used to facilitate easy removal of parts.
How does part geometry affect injection molding efficiency? Part geometry significantly impacts the flow of molten plastic, cooling times, and mold complexity. Simple and symmetrical geometries tend to fill more easily, cool evenly, and are more affordable to manufacture. Complex geometries with thin sections or deep cavities can lead to uneven material distribution, longer cycle times, and higher tool costs.
What are the benefits of designing for part interchangeability in injection molding? Designing for interchangeability means that parts can be easily swapped with other components without major modifications. This reduces the need for multiple molds and simplifies the assembly process. It also improves production flexibility and reduces costs in high-volume manufacturing.
How does DFM contribute to sustainability in injection molding? DFM encourages the use of materials efficiently, reduces waste during manufacturing, and minimizes the use of complex tooling that can lead to higher resource consumption. By designing parts that are easier to manufacture and use less material, DFM promotes more sustainable production practices.
What are the potential consequences of neglecting DFM in injection molding? Neglecting DFM principles can lead to higher production costs, increased defect rates, longer lead times, and reduced product quality. It may also result in the need for expensive mold modifications or redesigns to accommodate manufacturing issues that could have been avoided at the design stage.
Can DFM principles be applied to all types of injection molding parts? Yes, DFM principles can be applied to nearly all types of injection-molded parts. However, the specific design considerations may vary depending on the material, complexity, and intended application of the part. Each design must be tailored to meet the unique needs of the product and manufacturing process.
What is the role of collaboration between designers, engineers, and manufacturers in DFM? Collaboration is key to successful DFM implementation. Designers must work closely with engineers and manufacturers to ensure that the part design is practical, cost-effective, and feasible for production. Open communication ensures that potential issues are identified early, and the design can be optimized for manufacturability.
How can designers address challenges with parting line placement in DFM? The parting line should be strategically placed to avoid impacting the part's functionality or appearance. Designers can use features such as flat surfaces or hidden areas to position the parting line, ensuring that it does not interfere with critical functional or aesthetic elements of the part.
What is the impact of DFM on time-to-market? By optimizing the design for manufacturability, DFM helps reduce mold development time and improves overall production efficiency. This can result in faster time-to-market, allowing companies to launch products more quickly and gain a competitive edge in the market.