You’ve finalized a brilliant product design, only to be told by your manufacturer that it’s difficult or impossible to mold. The required changes add unexpected costs and delay your launch. You’re tired of the frustrating back-and-forth between design and manufacturing, watching your budget and timeline spiral out of control. There has to be a way to design parts that are both functional and easy to produce from the very start.
To slash costs and boost quality, Design for Manufacturability (DFM) focuses on simplifying your part for the injection molding process. This means designing with uniform wall thickness to prevent defects, adding draft angles for easy ejection, and eliminating complex features like undercuts wherever possible. A proactive DFM analysis before cutting steel is the single most effective way to prevent expensive tool rework, reduce cycle times, and ensure a higher-quality final product. It aligns your design with manufacturing reality.
Over the years, I’ve seen two kinds of projects. The first kind is where the designer throws a complex model "over the wall" to the toolmaker. That project almost always ends up late and over budget. The second kind is where the designer and toolmaker work together from the beginning, applying DFM principles. Those are the projects that run smoothly and succeed. DFM isn’t about limiting your creativity; it’s about channeling it in a way that leads to a better, more affordable product. Let’s walk through the tips that have saved my clients thousands of dollars and countless headaches.
How Can Simplifying Your Design Drastically Cut Costs?
Your part design is packed with features you believe are essential, but your tooling quote comes back shockingly high. Every rib, tight tolerance, and piece of text adds complexity that directly translates into higher mold costs and longer cycle times. You’re stuck between compromising your design and blowing your budget. What if a few simple changes could achieve the same function for a fraction of the cost?
Simplifying your design cuts costs by reducing the complexity of the mold itself. Every feature on a part, no matter how small, must be machined into the tool steel. By removing non-essential features, consolidating multiple components into a single part, and optimizing the overall size, you reduce machining time, material usage, and the potential for complex, expensive mold actions. A simpler part almost always means a simpler, cheaper, and more reliable mold.
I once worked with a startup on a new consumer electronics device. Their initial design for the housing was beautiful, but it was an assembly of four separate plastic parts held together with screws. The total cost for the four molds and the ongoing assembly labor was huge. I sat down with their designer, and we spent a day figuring out how to combine those four parts into one, slightly more complex part. The single mold was more expensive than any one of the original four, but it was far cheaper than all four combined, and it completely eliminated the assembly cost. That one DFM session saved their project.
Eliminate Non-Essential Features
Take a hard look at your design and ask, "Does this feature truly add value, or is it just for looks?" Every extra rib, boss, or decorative element has a cost. For example, raised text on a part requires the letters to be carefully machined into the mold cavity. Engraved (recessed) text is often easier, as it can be created with an electrode during the EDM process. Is that fancy, intricate logo critical, or would a simpler version or even a sticker suffice? Each decision impacts the final tool cost. The goal is to achieve the part’s function with the absolute minimum number of features.
Optimize Part Size and Weight
This seems obvious, but it’s often overlooked. The cost of your part is directly tied to two things: the amount of plastic material used and the time it takes to produce it (cycle time). A larger, heavier part uses more material, which costs more. It also takes longer to cool, which increases the cycle time and the cost of running the molding machine. By making walls thinner (while maintaining uniformity) and reducing the overall footprint of the part, you can achieve significant savings over a long production run.
Consolidate Parts When Possible
As in my story, look for opportunities to combine multiple parts into a single molding. This is one of the great strengths of injection molding. Can two pieces that get screwed together be replaced by one part with integrated snap-fits? Can a bracket and a housing become a single component?
| DFM Simplification Tactic | How It Reduces Cost | Example |
|---|---|---|
| Remove Features | Less complex mold, less machining time. | Changing a complex, raised logo to a simple engraved one. |
| Reduce Size/Weight | Less material cost, shorter cycle time. | Reducing wall thickness from 3mm to a uniform 2mm. |
| Consolidate Parts | Fewer molds to build, no assembly labor. | Combining a housing and a battery cover into one part. |
Why Are Wall Thickness, Draft, and Radii the Holy Trinity of DFM?
Your molded parts are coming out with ugly sink marks, they’re getting scratched during ejection, or they’re cracking under stress. These quality issues are causing high scrap rates and forcing you to troubleshoot endlessly. You’re frustrated because the part looks perfect in CAD, but the physical result is flawed. The root cause often lies in three fundamental design principles you may have overlooked.
Uniform wall thickness, draft angles, and corner radii are the "holy trinity" because they solve the most common molding defects. Uniform walls ensure even cooling, which prevents warping and sink marks. Draft angles allow the part to release cleanly from the mold, preventing drag marks and damage. Radii on corners reduce stress concentrations, making the part stronger, and they also help the molten plastic flow more easily into the cavity.
If a young designer asks me for the most important DFM advice, I always tell them to master these three things. Get these right, and you’ve solved 80% of potential molding problems before they ever happen. I remember a project for a medical device where the part had perfectly sharp, 90-degree internal corners. It looked clean in the design, but the parts kept failing stress tests right at those corners. We went back, added a small radius, and the problem disappeared completely. It’s that simple, but that critical.
The Golden Rule: Uniform Wall Thickness
This is the most important rule in plastic part design. As molten plastic cools, it shrinks. If one section of your part is much thicker than another, the thick section will cool much slower and shrink more. This differential shrinkage creates internal stress, which pulls on the surrounding material and causes defects like:
- Sink Marks: A depression on the surface of the part opposite a thick section (like a rib or a boss).
- Warping: The part twisting or bending out of shape as it cools.
The goal is to design the entire part with the same, consistent wall thickness. If you need to add a strengthening rib, design it to be about 50-60% of the main wall thickness to prevent sink.
| Material | Recommended Wall Thickness |
|---|---|
| ABS | 1.2mm – 3.5mm |
| Polycarbonate (PC) | 1.0mm – 4.0mm |
| Nylon (PA) | 0.8mm – 3.0mm |
| Polypropylene (PP) | 0.8mm – 3.8mm |
The Ejection Solution: Draft Angles
A draft angle is a small taper applied to the vertical walls of your part. Think of a stack of disposable cups; they don’t have perfectly straight walls, they’re tapered so they don’t get stuck together. The same principle applies to injection molding. As the plastic cools, it shrinks onto the mold core. Without a draft angle, the part’s surface would drag against the mold during ejection, causing scratches or "drag marks." In severe cases, the part can get stuck completely.
- Standard: A minimum of 1-2 degrees of draft is recommended for most parts.
- Textured Surfaces: If your part has a textured finish (like a light grain), you need more draft. A common rule of thumb is to add 1.5 degrees of draft for every 0.025mm (0.001 inches) of texture depth.
The Strength Multiplier: Corner Radii
Sharp internal corners are a major weak point in any molded part. They create high stress concentrations, making the part much more likely to crack or break under load. From a molding perspective, they also make it harder for the molten plastic to flow smoothly, potentially causing incomplete fills. By adding a generous radius (a rounded corner), you distribute the stress over a larger area and improve plastic flow.
- Internal Radii: A good rule of thumb is to make the internal radius at least 0.5 times the wall thickness.
- External Radii: The external radius should be the internal radius plus the wall thickness to maintain a consistent wall section.
How Can You Cleverly Handle Undercuts and Plan for Future Changes?
You’ve designed a part with a necessary snap-fit clip or a side hole, but this creates an "undercut." Your molder tells you this requires a complex and expensive side-action mechanism in the tool. Or, you know a feature might need to be adjusted after initial testing, but you’re afraid of the high cost of modifying a hardened steel mold. These challenges can lock you into a costly path with no easy way back.
Handle undercuts by first trying to eliminate them with a clever redesign, such as creating a "pass-through" core that can be pulled in the main direction of the mold. If an undercut is unavoidable, plan for the simplest mechanism possible. For future changes, design to be "steel-safe," meaning you start with features smaller than you might need. It is always cheaper to remove steel from a mold to make a feature larger than it is to add steel back.
Planning for changes is a sign of an experienced designer. I had a client who was developing a new enclosure. They weren’t sure about the final size of the ventilation slots. Instead of guessing and making them large, we designed the mold with the slots much smaller than they thought they’d need. This was "steel-safe." After testing the first prototypes, they found they needed more airflow. It was a simple and cheap job for us to machine the slots larger in the existing mold. If they had made them too big initially, we would have had to weld the steel and re-machine it, a much more expensive and time-consuming process.
Avoiding and Managing Undercuts
An undercut is any feature that prevents the part from being ejected directly from the mold in a straight line. Think of a latch on the side of a box. The main mold halves pull apart vertically, but the latch would get caught. To mold this, you need a "side-action" or "slider," which is a separate piece of the mold that moves in from the side to form the feature and then pulls out before the part is ejected. These mechanisms add significant cost and complexity to the mold.
- First, try to design it out. Can you replace a side hole with a U-shaped cutout that is open to the top or bottom? Can you design a snap-fit that bends out of the way during ejection? A little creative thinking here can save thousands of dollars.
- If you can’t avoid it, simplify it. Work with your molder to design the simplest, most robust slider mechanism possible.
Designing "Steel-Safe" for Future Modifications
Molds are made of hardened steel. Modifying them is like sculpture. It’s easy to carve more material away, but it’s very difficult to add it back. The "steel-safe" principle uses this to your advantage.
- For internal features (holes, slots, pockets): Design them smaller than you think you might need. Making them bigger is easy.
- For external features (bosses, ribs, pins): Design them larger or taller than you might need. Making them smaller is easy.
This simple strategy gives you a low-cost option for tuning your design after you have real parts in your hands.
Using Inserts for Wear and Tear or Iteration
For features that you know will change or that will experience high wear (like a gate area or a clip that sees a lot of friction), you can design the mold to use a replaceable insert. An insert is a separate block of steel that is fitted into the main mold base to form a specific feature. If that feature needs to be changed or if it wears out, you don’t have to remake the entire mold cavity. You just make a new, small, and relatively inexpensive insert. This is a fantastic DFM strategy for parts that are expected to evolve over their lifetime.
Conclusion
DFM is not a barrier to creativity; it’s a bridge to reality. By simplifying your design, mastering the core principles of wall thickness and draft, and planning for challenges like undercuts, you transform your design from a concept into a successful, profitable product. This collaborative approach between designer and manufacturer is the fastest path to reducing costs, improving quality, and getting your product to market on time.
