Tips to simplify designs
Injection molding is a powerful technique that uses relatively high-cost tooling to manufacture highly repeatable net-shape parts in a wide variety of polymer and composite materials. The approach allows designers to exploit a very high degree of design complexity and the integration of multiple functions and even multiple materials into a simple to manufacture finished part.
Advantages of injection molding in manufacture
Applied with good adherence to manufacturing, assembly and component performance issues, injection molding empowers the designer to make high quality and precise finished parts that require little or no post-molding work to perform high stress and high cosmetic tasks. Injection molding offers:
- High production efficiency, as the process allows for rapid production of parts, enabling high-volume manufacturing with short cycle times.
- Consistent quality and precision ensures high repeatability and consistency, producing parts with tight tolerances and highly controllable variability between parts over long runs and across multiple suppliers..
- Complex geometries and intricate shapes that would be difficult or impossible to achieve with other manufacturing methods are essentially straightforward to achieve through injection molding, allowing the designer considerably increased freedom to innovate.
- Material versatility results from a remarkable range of materials, including various types of plastics and composites and even polymer-bonded metal powders, can be used in injection molding, offering extreme flexibility in design and functionality. In particular, polymer additives expand the variety of properties considerably.
- Minimal waste results, as the process generates little to no non-component material, and excess plastic can often be recycled and reused, making it an environmentally friendly option in some regards. In high-volume adapted tooling, material waste can be reduced to effectively zero by use of internal heaters that keep feeder channel contents molten (hot runners).
- Enhanced strength results from fillers/additives such as talc, glass fiber, Carbon fiber etc. being added to the injection molding material, heavily modifying the strength and durability of the finished component.
- Injection molding can be highly automated, reducing labor costs to essentially zero and greatly reducing part costs. In addition, the automation of inspection can remove a layer of skilled labor, for additional cost benefit.
- Surface finish and aesthetics are typically excellent, as parts produced via injection molding typically offer good surface finish, reducing the need for post-molding processes.
- Cost-effectiveness for large runs is easily achieved, despite the initial tooling cost being typically quite high. The per-unit cost decreases dramatically in large production runs, making it cost-effective for mass production.
- Integration of multiple parts/functions can be designed into a molded part, reducing assembly complexity, part count and streamlining the manufacturing process. Moving parts/functions from assembly, back upstream to low labor, high precision and auomatable processes is the central target of Industry 4.0 adaption.
Yoav A
Head of Design
DFM Principles
There are a wide spectrum of potential errors in the design of parts for injection molding, and this makes the DFM process particularly critical in the design of parts for this manufacturing process. With an effective design process that avoids both the simple and the more esoteric failure modes, high quality and highly repeatable parts can be produced at low cost, when the volume justifies the use of this manufacturing method.
Design considerations for process
Design for uniform wall thickness ensures consistency in section-weights that prevents the distortion that otherwise results from non-uniform shrinkage, and the internal stresses that this induces.
Varying thickness can lead to uneven cooling, intricate dimensional or surface finish defects and both subtle and gross variability between moldings, during and between production runs.
Excessive flow restrictions can have serious consequences in the filling of the cavity, resulting in voids, scorching, poor quality weld lines etc.
Restraining the use of undercuts and complex features, by simplifying the design, reduces the need for complex tooling. Tooling complexity will typically increase costs and manufacturing time, while reducing tool durability. Avoiding undercuts reduces the complexity of mold design and allows for easier part ejection.
Where undercuts are necessary, allowing sufficient space for them to be robust in the tool and integrating multiple undercuts into a limited number of common sliders is key to reliability and cost-control.
Adding appropriately generous draft angles facilitates the easy ejection of parts from the mold. This reduces the risk of damaging the parts during ejection, as shallow draft surfaces become scuffed by sliding, textures get smeared by poor disengagement and parts stick and become distorted by ejector forces remote from the stuck areas.
Tooling elements that must slide close to each other to seal the cavity – for example where undercuts without sliders meet – must be heavy drafted to avoid tool wear at the seal contact point. Often referred to as blanking, this is a key component of tools that allows undercuts without sliders, so it’s important to understand their optimization.
Tool design issues in part design
Position gates to ensure even and low turbulence material flow and minimize weld lines. Proper gate location helps achieve uniform filling and reduces the chances of defects such as irregular surfaces from turbulent flow, and weld lines from poor flow planning. Plan for gate position and type in design where possible.
Edge gate: Positioned on the edge of the component, feeding at the part line, it’s the most common gate type. Ideal for simple parts, providing even filling and easy removal of the gate vestige.
Pin gate: A small, round gate used in three-plate molds, allowing multiple gates for complex parts and often leaving minimal cosmetic vestiges.
Submarine gate: Also known as a tunnel gate, it’s a hidden gate within the mold parting line. It automatically trims the gate when the part is ejected, useful for aesthetic parts.
Fan gate: A wide (fanlike) gate that spreads the material into the mold, reducing shear stress and ensuring even distribution, suitable for large, flat parts.
Diaphragm gate: Used for cylindrical or round parts, the gate is placed around the circumference, providing uniform filling and reducing the risk of defects.
Tab gate: An extension of the part, it’s designed to minimize stress and warping in thin-walled or delicate parts, requiring trimming post-molding.
Sprue gate: Directly connected to the sprue, it’s used for simple molds with low complexity, often leading to higher residual stresses.
Cashew gate: A curved gate, used in limited space situations, often requiring precise trimming and suited for intricate parts.
Account for material shrinkage during the cooling phase to maintain dimensional accuracy.
Maintain uniform wall thickness, use ribbing and gussets to enhance structural integrity and smooth transitions with radii to reduce risk of warping.
Allows cavity air and molding gasses to escape all around, preventing burn marks and voids. Care must be taken to avoid allowing flash at the part line
Designed venting improves the overall quality of the molded parts. Vents must be deep enough to allow moderate flow and the filled relief channel will require trimming after ejection.
Optimize material selection
Choose material families that suit the intended manufacture method and expected supply chain.
Select materials that match the intended use of the product. Consider mechanical properties, thermal resistance, and regulatory compliance.
Be sure of the material selection, as many aspects of tool design – gate type/size, shrinkage etc. – are hard to alter once tooling is made.
Plan for ejection in design
Uses ejector pins to push the part out of the mold. It’s the most common ejection method, suitable for a wide range of parts. Allow non cosmetic landings for ejector pins.
Utilizes thin blades to eject parts with large flat surfaces or thin walls. It minimizes the risk of part deformation during ejection.
Employ cylindrical sleeves (sleeve ejectors)to eject tubular or cylindrical parts or to assist in ejecting deep features. It ensures uniform ejection and reduces the risk of damage to the part.
Uses an additional tooling plate that moves to push the part off the mold, often around the entire part line. It’s ideal for complex or delicate parts that need uniform force distribution during ejection.
Uses compressed air to blow the part out of the mold. It’s useful for lightweight parts and reduces mechanical stress on the part during ejection.
Utilizes vacuum to assist in part removal, especially for thin-walled or large surface area parts, reducing the risk of damage.
Involves rotating the part during ejection, which can help with parts that have threads or undercuts, ensuring smooth release from the mold.
Design features to exploit the process
Snap fits use sprung hooks that snap together during assembly to remove separate fastener steps/components.
Location features use keys, orientation pins, orientation markers and asymmetry can serve to make parts impossible to fit wrongly or give an obvious indication when this has occurred.
Assembly can be assisted by features that fold/collapse during assembly and spring back into place. Take design care to ensure that over-exercise is not possible and that stress whitening that may result is not cosmetically detrimental.
Consider overmolds for refined functions such as soft touch, seals and branding.
Prototype thoroughly
Early shape-prototypes to test and validate fits and interfaces as the design progresses.
Functional prototypes use close to functional components from more refined processes to validate complex interactions such as snaps, and to evaluate strength issues. These can be proxy materials from 3D printing or vacuum casting or they can be CNC machined in close-to-real materials.
Conclusion
DFM in injection molding component and tool design is a critical process that ensures the creation of efficient, cost-effective, manufacturable and high-quality products. By integrating DFM principles early in the design stage, manufacturers can minimize production challenges, reduce waste, and optimize the overall manufacturing process.
Key considerations are appropriate material selection, simplified part geometry, appropriate wall thickness, and effective gate and ejection system design which all contribute significantly to the success of the injection molding process. All of these aspects can have upstream influence on design aspects, so they must be integrated into an holistic approach.
Well implemented DFM pivotally depends on collaboration between design and manufacturing teams, leading to innovative solutions that enhance product functionality, manufacturability, assembly, and durability. Applying DFM improves manufacturability and reduces costs, while also accelerating time-to-market, delivering competitive advantage when done well.
Daniel V
