How Can You Design Parts for Injection Molding and Avoid Costly Mistakes?

Struggling with parts that are hard to mold or have defects? Poor design choices for injection molding often lead to expensive rework and production delays.

Designing for injection molding¹ (DFM) means anticipating manufacturing limitations. Key considerations include uniform wall thickness², draft angles³, avoiding sharp corners, managing undercuts, and selecting appropriate materials to prevent common pitfalls.

I’ve seen countless product designs over my years at CavityMold, and one thing is clear: a little foresight in the design stage saves a lot of headaches and money down the line. Designers like Jacky, who understand the nuances of injection molding, consistently produce better, more cost-effective parts. It’s not about restricting creativity; it’s about channeling it within the practical boundaries of the manufacturing process. Let’s dive into some common pitfalls and how to steer clear of them, ensuring you Master Molding Right from the get-go.

How Can Ignoring Uniform Wall Thickness Wreck Your Molded Parts?

Frustrated by sink marks, warping, or inconsistent cooling in your parts? These are classic symptoms of non-uniform wall thickness, a common design oversight.
Non-uniform wall thickness causes uneven cooling and shrinkage, leading to defects like sink marks, voids, warpage, and internal stresses. Maintaining consistent thickness is crucial for part quality.

Diving Deeper: The Critical Role of Consistent Walls

When Jacky sends over a new part design, one of the very first things I check is the wall thickness. It’s fundamental. Plastic shrinks as it cools. If one section of a part is significantly thicker than an adjacent section, the thicker area will cool and shrink more slowly and by a greater amount.

• ### Understanding the Problem: This differential shrinkage pulls on the surrounding material.
  • Sink Marks: On the surface opposite a thick section (like a rib or boss), the material gets pulled inwards, creating an unsightly depression.
  • Voids: Sometimes, the shrinkage occurs internally, creating a bubble or void within the thick section, compromising structural integrity.
  • Warpage: The uneven stresses can cause the entire part to twist or distort out of its intended shape.
• ### Design Solutions: The ideal is perfectly uniform walls, but that’s not always practical.
  • Core Out Thick Sections: If a thick section is unavoidable for strength, try to core it out from the non-visible side to create a more uniform wall, perhaps adding ribs for support.
  • Gradual Transitions: If wall thickness must change, make the transition as smooth and gradual as possible. A good rule of thumb is to keep thickness variations within 10-25% of the nominal wall.
  • Rib Design: When adding ribs for stiffness, make sure their base thickness is no more than 50-60% of the wall they are attached to. This minimizes the risk of sink on the opposite surface.
    At CavityMold, we often use mold flow analysis to predict these issues and advise on design modifications before tool manufacturing begins. It’s a proactive step that saves a lot of trouble.

    Why Are Sharp Corners a Nightmare for Injection Molding?

    Seeing cracks or weak points in your molded parts, especially near corners? Sharp internal and external corners are often the culprits, creating unseen problems.
    Sharp corners concentrate stress, making parts prone to cracking. They also impede smooth plastic flow during molding and can cause tool wear or sticking issues.
    !

    Diving Deeper: The Case Against Sharp Edges

    I always advise designers like Jacky to be generous with radii. Sharp corners, both internal and external, might look crisp on a CAD model, but they create multiple problems during and after the molding process.

• ### Stress Concentration: This is the biggest issue. Sharp internal corners act as stress risers. When the part is subjected to load or impact, stress concentrates at these points, making them much more likely to crack or fail. Even thermal cycling can induce stress that targets these weak spots.
• ### Impeded Plastic Flow: Molten plastic prefers to flow along smooth, curved paths. Sharp internal corners can disrupt this flow, leading to incomplete fill (short shots), air traps, or weld lines where flow fronts meet turbulently.
• ### Tooling Challenges and Wear: Machining very sharp internal corners in a mold cavity is difficult and expensive. These sharp features in the tool are also more prone to wear or chipping, especially with abrasive materials. For external sharp corners on a part (which mean internal sharp corners in the mold), the plastic can also "stick" more, making ejection harder.
• ### Recommended Radii:
  • Internal Radii: Aim for a minimum internal radius of at least 0.5 times the wall thickness. A radius equal to the wall thickness is even better.
  • External Radii: Add an external radius equal to the internal radius plus the wall thickness to maintain consistent wall thickness around the corner.
    A small radius can make a huge difference in part strength and moldability. It’s a simple DFM fix we always look for.

    What Happens When You Forget About Draft Angles in Your Design?

    Parts sticking in the mold, scuff marks, or slow cycle times plaguing your production? A lack of adequate draft angles is a very common reason.

Without draft angles (tapers on vertical walls), parts will scrape against the mold during ejection. This causes surface defects, part damage, and can even damage the mold itself.

Diving Deeper: The Necessity of Draft

Draft angles are one of those non-negotiable aspects of injection mold design. As plastic cools, it shrinks and tends to grip onto the mold cores. Without a slight taper on the vertical faces of the part, it’s like trying to pull a perfectly straight-sided object out of a tight-fitting container – there’s a lot of friction.

• ### How Draft Works: Draft is a slight angle, typically between 0.5 to 3 degrees (or more for textured surfaces), applied to faces of the part parallel to the direction of mold opening. This means the part is slightly wider at the mold opening than it is deeper in the mold.
• ### Consequences of Insufficient Draft:
  • Drag Marks/Scuffing: As the part is ejected, its surfaces rub against the mold wall, causing unsightly streaks or scuffs.
  • Sticking and Ejection Issues: Parts can get stuck on cores or in cavities, requiring excessive ejector force, potentially damaging the part or ejector pins.
  • Increased Cycle Times: Difficulty in ejection slows down the overall cycle.
  • Mold Wear: The repeated friction can cause premature wear on the polished mold surfaces.

• ### General Draft Angle Guidelines:	Surface Finish	Minimum Draft (per side)
  Polished (SPI A1-A3)	0.5 – 1 degree
  Lightly Polished/EDM	1 – 2 degrees
  Textured Surfaces	1 degree per 0.001" depth of texture (e.g., 3-5 degrees for light textures)

  I often remind Jacky that more draft is generally better, as long as it doesn’t compromise the part’s function or assembly. It’s a simple feature that makes the molding process smoother and more reliable.

  Are Undercuts Secretly Inflating Your Mold Costs and Complexity?

  Wondering why your mold quote is so high for a seemingly simple part? Undercuts, those features that prevent straight ejection, often require complex, expensive mold mechanisms.

Undercuts are part features that obstruct direct removal from the mold. They necessitate side-actions (slides) or lifters in the mold, significantly increasing tool cost, complexity, and maintenance.

Diving Deeper: The Hidden Costs of Undercuts

An undercut is any protrusion or recess on a part that is not in line with the mold’s direction of pull, preventing the part from being ejected directly. Examples include side holes, snaps, or grooves. While sometimes necessary, I always push designers like Jacky to question if an undercut can be avoided or simplified.

• ### How Molds Handle Undercuts:
  • Slides (Side-Actions/Cams): These are moving parts within the mold that insert to form the undercut feature and then retract before ejection. They require precise mechanisms and add to the mold’s footprint and cost.
  • Lifters: These are angled components on the ejector side that move with the ejector pins but also shift sideways to disengage from the undercut feature as the part is pushed out.
  • Collapsible Cores: For complex internal undercuts, intricate (and very expensive) collapsible cores might be needed.
• ### Impact of Undercuts:
  • Increased Mold Cost: The design, machining, and fitting of these mechanisms are labor-intensive.
  • Increased Mold Size: Slides often make the mold base larger.
  • Longer Cycle Times: The movement of slides or lifters adds seconds to each cycle.
  • Higher Maintenance: More moving parts mean more things that can wear out or require adjustment.
  • Potential for Flash: Seams where moving mold components meet (parting lines for slides) can be prone to flash if not perfectly fitted and maintained.
• ### Alternatives to Consider: Can the undercut be redesigned out? Could a "snap-by" feature work, where a flexible material deflects slightly? Can the part be reoriented in the mold? Sometimes a two-part assembly is cheaper than a single part with complex undercuts.
  At CavityMold, we work with clients to find the most cost-effective way to achieve their design intent, sometimes even suggesting clever ways to avoid undercuts altogether.

  How Does Material Selection Impact Your Design Limitations in Molding?

  Did your chosen plastic fail to fill thin sections or show excessive shrinkage? Material properties directly influence what design features are feasible and how the part will behave.

Material properties like shrink rate, flowability (MFI), and mechanical strength dictate design limits such as minimum wall thickness, feature detail, and structural integrity of the molded part.

Diving Deeper: Material Properties as Design Enablers (or Constraints)

The plastic resin you choose isn’t just about the final part’s color or feel; it fundamentally affects the design possibilities. When Jacky is specifying a material, we discuss its implications on the mold design and part features.

• ### Shrink Rate: All plastics shrink as they cool and solidify. This shrinkage varies significantly between materials (e.g., Polypropylene shrinks more than ABS or Polycarbonate). The mold cavity must be machined larger than the final part dimensions to compensate for this.
  • Impact: Higher, less uniform shrinkage can lead to warpage and dimensional control issues, especially in large or complex parts. Different shrink rates for different materials mean a mold designed for ABS might not produce accurate parts in PP.
• ### Melt Flow Index (MFI) / Flowability: This indicates how easily a molten plastic flows.
  • Impact: High MFI (easy flow) materials are better for filling thin walls or intricate details. Low MFI (stiff flow) materials might struggle with long flow paths or delicate features, potentially requiring higher injection pressures or more gates.
• ### Mechanical Strength and Stiffness:
  • Impact: If a part needs to be strong but also thin, a high-strength engineering plastic (like Nylon or PC) might be necessary. A weaker commodity plastic might require thicker walls to achieve the same rigidity, impacting material cost and cycle time.
• ### Other Considerations:
  • Abrasiveness: Glass-filled materials are abrasive and can wear down molds faster, influencing tool steel choice.
  • Corrosiveness: Some materials (like PVC) release corrosive gasses, requiring special mold materials (e.g., stainless steel).
  • Temperature Resistance: If the part sees high service temperatures, a high-temp plastic is needed. This also affects mold cooling design.
    We always recommend discussing material selection early in the design phase, as it has such a profound impact on both the part design and the mold construction. It’s a key part of our "Master Molding Right" philosophy.

    Conclusion

    Avoiding these common injection molding pitfalls through thoughtful design and material choice leads to better parts, lower costs, and faster production. CavityMold helps you navigate these design challenges.