I’ve seen it a hundred times. An engineer designs this brilliant part—sleek, functional, elegant. Then they send it to the molding shop and reality hits.
“Yeah, we can’t actually make that.”
Or worse: “We can make it, but it’ll cost three times what you budgeted and take twice as long.”
Design for Manufacturability (DFM) isn’t about dumbing down your design. It’s about creating parts that are brilliant and producible. Parts that come out of the mold consistently, meet spec every time, and don’t drive your manufacturing team insane.
Let’s talk about DFM principles that actually matter when you’re molding parts in Michigan.
Wall Thickness: The Foundation of Everything
If there’s one thing that makes or breaks a molded part, it’s wall thickness.
Keep walls uniform. This is the golden rule. When you have thick sections next to thin sections, they cool at different rates. The thick section is still shrinking while the thin section is already solid, which creates internal stresses, warping, and sink marks.
Ideally, maintain the same wall thickness throughout the entire part. In practice, that’s not always possible. When you need transitions, keep thickness variations within 25%. If your nominal wall is 2.0mm, don’t go thinner than 1.5mm or thicker than 2.5mm anywhere in the part.
Thick sections take forever to cool, which means longer cycle times. They also tend to have sink marks or voids. If you need structural strength, use ribs instead of thickening the whole wall.
How thick should walls be? Depends on the material and part size, but here are starting points:
- Small parts (under 2 inches): 0.040″ to 0.080″ (1.0mm to 2.0mm)
- Medium parts (2 to 6 inches): 0.080″ to 0.150″ (2.0mm to 4.0mm)
- Large parts (over 6 inches): 0.100″ to 0.200″ (2.5mm to 5.0mm)
Thinner walls fill faster, cool faster, and reduce cycle time. But too thin and you risk short shots or insufficient structural strength. Work with your molder to find the sweet spot for your specific application.
Ribs: Adding Strength Without Adding Thickness
When you need structural strength, ribs are your friend. But you’ve got to design them correctly or they cause more problems than they solve.
Rib thickness should be 50-60% of the nominal wall thickness. Any thicker and you’ll get sink marks on the opposite surface. The thick base of the rib cools slower than the surrounding wall, pulling the surface inward as it shrinks.
Height-to-thickness ratio matters. Ribs should be no more than 3 times as tall as they are thick at the base. Taller ribs are hard to fill and prone to warping. If you need more height, use a gusset or a slightly thicker rib with proper draft.
Space ribs properly. Place them at least 2-3 times the wall thickness apart. Too close together and you create a pseudo-thick section that defeats the whole purpose of using ribs.
Add draft to ribs. At least 0.5 degrees per side, preferably 1 degree. Draft helps parts eject cleanly without drag marks or stress.
Here’s something most engineers miss: radius the base where ribs meet walls. A sharp 90-degree corner creates stress concentrations and restricts material flow. A radius of 25-40% of the wall thickness makes the part stronger and easier to fill.
Draft: Let Parts Come Out of the Mold
Draft angles seem like a pain, but they’re essential. Without adequate draft, parts stick in the mold, get damaged during ejection, or require excessive ejection force that causes warping or stress.
Minimum draft is 0.5 degrees per side for smooth surfaces. Textured surfaces need way more—1 to 3 degrees depending on the texture depth. The rougher the texture, the more draft you need.
For deep pockets or tall walls, you need more draft. A core that’s 2 inches deep and has only 0.5 degrees of draft will still stick. Aim for 1-2 degrees minimum on deep features.
And here’s the thing: draft needs to be on both sides of walls and features. If you spec 1 degree of draft on the outside of a boss, make sure there’s draft on the inside too.
I know draft makes parts slightly different sizes at top and bottom, which can complicate tolerancing. But fighting draft with tight specs costs you in tool complexity, ejection problems, and cycle time. Build draft into your design from the beginning and dimension accordingly.
Radii and Fillets: Smooth Transitions Matter
Sharp corners are bad for two reasons: they create stress concentrations that cause parts to crack, and they restrict material flow during molding.
Inside corners need radii of at least 25% of the nominal wall thickness. Better yet, use 40-60%. Larger radii reduce stress and help material flow around corners without hesitation.
Outside corners should have radii too, typically 150% of the inside radius. This maintains uniform wall thickness through the corner, which prevents sink marks and improves strength.
Think about it: if you have a sharp inside corner with no radius, the outside corner has to be radiused to maintain wall thickness. That creates a thick section at the corner, which causes cooling problems. Radius both sides and everyone’s happy.
For structural parts or assemblies with snap fits, generous radii can double or triple fatigue life. Stress concentrations at sharp corners are where cracks start.
Undercuts: Expensive and Avoidable
Undercuts are features that prevent a part from ejecting straight out of a two-piece mold. They require side actions, lifters, or collapsing cores—all of which add cost, complexity, and cycle time.
Sometimes undercuts are unavoidable. You need that snap fit or internal feature for functionality. When that’s the case, design them thoughtfully:
Minimize the undercut distance. The less the side action has to move, the simpler and faster it operates.
Avoid deep undercuts that require complex mechanisms. If you need a deep undercut, consider redesigning with a secondary operation or a different attachment method.
Locate undercuts on one side of the part if possible. Having side actions on multiple sides increases tool cost and cycle time.
For simple undercuts on flexible materials, consider using a flexible core or designing the part to flex during ejection. Many snap fits can eject without side actions if designed with the right draft and geometry.
And ask yourself: do you really need that undercut, or is there a better way? Sometimes rotating the parting line or redesigning the feature eliminates the undercut entirely.
Gate Location: Where Plastic Enters the Part
Gate location affects everything: appearance, strength, warp, filling pattern, weld lines. It’s one of the most important design decisions, yet it often gets decided as an afterthought.
Place gates in the thickest section when possible. This ensures the gate freezes last, allowing you to pack the cavity properly.
Avoid gating directly onto cosmetic surfaces unless appearance isn’t critical. Gate vestige always leaves a mark. If appearance matters, gate on a hidden surface or design a feature that conceals the gate.
Consider flow direction. Material flows from the gate in a fountain flow pattern, with molecular orientation parallel to the flow direction. This affects part strength and shrinkage. Gate in a location that orients material properly for your loading conditions.
For parts with complex geometry, multiple gates might be necessary to fill properly. Just be aware that each gate creates a weld line where flow fronts meet. Place gates so weld lines occur in non-critical areas.
Let your molder have input on gate location. They understand material flow, pressure requirements, and how gating affects the process. Sometimes moving a gate an inch makes the difference between an easy-to-mold part and a nightmare.
Tolerances: Specify What You Actually Need
Here’s an uncomfortable truth: overly tight tolerances cost money and time without adding value.
Injection molded parts can’t match machined part tolerances. Physics doesn’t care about your CAD file. Material shrinks, and that shrinkage varies slightly part-to-part based on process variations, material lot differences, and environmental conditions.
Use standard tolerancing based on material and part size. Most molders work to ±0.003″ per inch for dimensions across the parting line and ±0.001″ per inch for dimensions not crossing the parting line.
If you need tighter tolerances, be prepared to pay for it through process development, tighter process controls, and possibly secondary operations. And even then, there are limits.
Tolerance what matters. Critical fits, interfaces, and functional dimensions get tight tolerances. Everything else should use standard or relaxed tolerances. Over-tolerancing the entire part drives up cost for no benefit.
Here’s a Michigan-specific consideration: account for environmental effects on dimensions. Hygroscopic materials like nylon change dimensions with moisture content. Parts that fit perfectly in your climate-controlled facility might be out of spec in a humid summer warehouse or a dry winter one.
Work with your molder to establish achievable tolerances. They can do process capability studies to show what’s actually possible with your design and material.
Bosses and Standoffs: Supporting Fasteners
Most plastic parts have bosses or standoffs for screws, inserts, or assembly features. Design them wrong and they crack, warp, or cause sink marks.
Outer diameter should be 2.5 times the inner diameter for self-tapping screws, 2.0 times for heat-set inserts or ultrasonic inserts. This provides enough material to support the fastener without creating an overly thick section.
Connect bosses to walls with gussets rather than letting them stand free. Gussets provide support and help material flow during molding. Make gussets 50-60% of wall thickness, just like ribs.
Keep bosses short or use hollow bosses for tall features. Solid bosses over 2-3 times their diameter tend to have voids or excessive sink marks.
Add draft to internal boss cores. At least 0.5 degrees, preferably more. Cores without draft stick and cause ejection problems.
For heat-set or ultrasonic inserts, leave extra material at the top of the boss—at least 0.020″. When you install the insert, displaced material needs somewhere to go. If there’s not enough material, the insert bottoms out before seating properly.
Texture and Surface Finish
Texture hides gate marks, parting lines, and minor surface imperfections. But it comes with trade-offs.
Textured surfaces need more draft—typically 1 degree per 0.001″ of texture depth. Deep textures might need 3-5 degrees to eject cleanly. Factor this into your design from the start.
Texture increases cycle time because textured mold surfaces take longer to fill and parts stick more during ejection.
Light textures are easier than heavy textures. SPI finish standards range from A-1 (diamond polish) to D-3 (dry blast). Finishes B-1 through C-2 are common for commercial parts and still eject reasonably well.
For cosmetic parts, avoid putting texture through text, logos, or fine details. Texture obscures detail. Keep those areas polished or use a different texture.
And remember: texture is expensive if you add it after the mold is built. Spec your desired finish during mold design so it’s built in from the beginning.
Material Selection and Processability
DFM isn’t just geometry—it’s choosing materials that mold easily and meet performance requirements.
Glass-filled materials are stronger and stiffer but harder to mold. They require higher injection pressures, wear tooling faster, and create more visible weld lines and gate vestige. Use them when you need the performance, but understand the trade-offs.
Hygroscopic materials like nylon and polycarbonate need thorough drying before molding. If your production facility doesn’t have good drying equipment, consider non-hygroscopic alternatives like acetal or polypropylene.
Material color affects cosmetic expectations. Dark colors hide sink marks, weld lines, and gate vestige better than light colors. If appearance is critical, choose dark colors when possible.
Commodity resins like PP, PE, and PS are generally easier to mold than engineering resins. They process at lower temperatures, require less pressure, and are more forgiving of design imperfections. If a commodity resin meets your requirements, use it.
Work with material suppliers to get processing recommendations specific to Michigan’s climate. Some materials need different drying conditions or process windows when ambient humidity or temperature varies seasonally.
Designing for Michigan Manufacturing
Michigan has specific manufacturing realities that affect DFM decisions.
Seasonal temperature and humidity changes affect processing. A part that molds perfectly in July might have issues in January if your facility isn’t climate controlled. Design with wider tolerances or specify materials less sensitive to environmental conditions.
Older equipment is common in some Michigan facilities. If you’re designing for a specific molder, ask about their equipment capabilities. Not every shop has the latest servo-electric presses or advanced process controls. Design parts that can run reliably on the equipment your molder actually has.
Lead times for tooling can be long, especially for complex molds. Design parts that can use simpler tooling when possible. Family molds, multi-cavity layouts, and straightforward geometries save time and money.
Labor availability varies. Some operations require secondary work like assembly, insert installation, or trimming. Consider automation-friendly designs if volumes are high enough to justify it.
Working With Your Molder Early
The biggest DFM mistake is designing in isolation. Get your molder involved during design, not after you’ve finalized everything.
Share CAD files early and ask for feedback. Most molders will do informal DFM reviews for free because it’s cheaper than dealing with problems after the tool is built.
Ask about their capabilities. What wall thicknesses do they prefer? What draft angles work best on their equipment? What materials do they run regularly and have good process data for?
Discuss tooling strategy. Should you build a prototype tool first or go straight to production tooling? Single cavity or multi-cavity? Hardened steel or soft aluminum for low-volume runs?
Get process capability data if tolerances are critical. They can tell you what’s achievable before you commit to dimensions you can’t hit.
Good molders want you to succeed. They’ll point out potential issues, suggest alternatives, and help optimize your design for their equipment and process. Use that expertise.
The Bottom Line
DFM is about designing parts that work and are easy to make consistently.
Uniform wall thickness, proper draft, generous radii, smart rib design—these aren’t restrictions that limit creativity. They’re principles that help you create parts that mold reliably, meet spec every time, and don’t require constant process adjustments.
Add in Michigan-specific considerations like environmental effects on materials and regional manufacturing capabilities, and you’ve got a design that won’t just work—it’ll work here.
Start thinking about manufacturability from the first sketch. Work with your molder throughout design. Test and iterate before finalizing tooling.
Because the best design in the world is worthless if it can’t be manufactured consistently at a cost that makes business sense.
And a part designed with DFM principles from the start? That part launches on time, runs smoothly, and lets you move on to the next project instead of fighting fire drills in production.
