If you’re a product engineer or procurement specialist, you know the pain of a bad injection molding design: parts that crack, molds that get stuck, or projects delayed by rework. The good news? Most issues are avoidable with the right design choices. This guide breaks down 7 essential tips for injection molding design, with data, examples, and actionable advice to help you build parts that work—on time and on budget.
What Is Injection Molding Design? A Quick Primer
Injection molding design is the process of creating part blueprints that work with the injection molding manufacturing method. Unlike 3D printing (which builds parts layer by layer), injection molding forces molten material (like thermoplastics or elastomers) into a custom mold. Once cooled, the material takes the mold’s shape—making it ideal for high-volume production (10,000+ units) thanks to low scrap and consistent results.
But here’s the catch: poor design can ruin even the best mold. For example, a part with uneven walls might develop sink marks (ugly surface dents), or a part without draft angles could get stuck in the mold—costing you time and money to fix. The tips below solve these common problems.
1. Choose the Right Material (It Makes or Breaks Your Design)
The first rule of injection molding design: pick a material that matches your part’s purpose. Different materials have unique properties—like strength, heat resistance, or flexibility—and they behave differently during cooling.
Key Material Categories to Know
- Amorphous thermoplastics (e.g., ABS, Acrylic): Great for dimensional stability (they don’t shrink much) and impact resistance. Perfect for parts like laptop casings or display frames.
- Semi-crystalline thermoplastics (e.g., Nylon, Polyethylene): Offer better chemical resistance and durability but shrink more. Ideal for parts like fuel lines or food containers.
Material Selection Checklist
Ask yourself these questions before choosing:
- Will the part be exposed to high temperatures (e.g., engine parts)? Pick heat-resistant options like PEEK or Liquid Crystal Polymer.
- Does it need to be flexible (e.g., gaskets)? Go with elastomers or silicone rubbers.
- Will it touch chemicals (e.g., cleaning tools)? Choose semi-crystalline plastics like Polypropylene.
Real-World Example: A Medical Device Company’s Mistake
A medical firm once designed a syringe plunger using Acrylic (an amorphous plastic) because it was cheap. But Acrylic isn’t resistant to the alcohol-based cleaners used in hospitals—after a few uses, the plungers cracked. They switched to Polycarbonate (a more chemical-resistant amorphous plastic), and the problem vanished. The fix cost \(5,000 in retooling but saved \)50,000 in recalled parts.
2. Master Part Tolerances (Account for Shrinkage!)
Molds are made with ultra-tight tolerances (often 0.005 mm using CNC machining), but plastic shrinks as it cools—and different materials shrink at different rates. If you ignore this, your part won’t fit as intended.
Common Materials and Their Shrinkage Rates
| Material | Maximum Shrinkage Rate | Tolerance Adjustment Tip |
| PLA | 0.5% | Minimal adjustment needed |
| ABS | 0.8–1.2% | Add 1% to target dimensions |
| PEEK | 1.5% | Add 1.5% to target dimensions |
| Nylon (Polyamide) | 1.2–1.8% | Add 1.5% to target dimensions |
For example: If you need a part that’s 100 mm long after cooling, and you’re using PEEK (1.5% shrinkage), design the mold cavity to be 101.5 mm long. This way, when the plastic shrinks, it hits your target size.
3. Get Wall Thickness Right (Avoid Sink Marks)
Wall thickness is one of the most common design mistakes. Too thick, and you’ll get sink marks (depressions from slow cooling). Too thin, and the molten plastic won’t flow properly—leading to weak spots.
Recommended Wall Thickness by Material
| Material | Minimum Thickness | Maximum Thickness |
| ABS | 1.143 mm | 3.556 mm |
| Acetal | 0.762 mm | 3.048 mm |
| Acrylic | 0.635 mm | 12.7 mm |
| Nylon | 0.762 mm | 2.921 mm |
| Polycarbonate | 1.016 mm | 3.81 mm |
Critical Rule: Keep Walls Even
Uneven walls are a disaster. For example, a part with a 3 mm wall next to a 1 mm wall will cool at different rates—the thicker section will shrink more, pulling the surface inward (sink mark).
- If you must have uneven walls, keep the difference under 15% of the nominal thickness (e.g., a 2 mm wall can go up to 2.3 mm, not 3 mm).
- Use tapered transitions (slopes) between thick and thin sections to slow cooling differences.
4. Add Draft Angles (Prevent Mold Sticking)
CNC machining can make vertical walls, but injection molding can’t. When plastic cools, it shrinks and clings to the mold—especially around the core. Without draft angles (slight tilts on walls), you’ll have to force the part out, damaging the mold or ejector pins.
Minimum Draft Angles for Common Surfaces
| Surface Type | Minimum Draft Angle | Why It Matters |
| “Near vertical” walls | 0.5° | For parts that need a almost straight look |
| Most common walls | 2° | Balances function and aesthetics |
| Closed faces | 3° | Harder to eject, needs more tilt |
| Shallow textured walls | 3° | Texture increases friction with the mold |
| Medium textured walls | 5° | More texture = more sticking |
Example: A Toy Manufacturer’s Fix
A toy company designed a action figure with vertical legs (no draft angles). When they tried to eject the parts, 20% of the legs cracked because the plastic stuck to the mold. They added a 2° draft angle to the legs, and the defect rate dropped to 0.5%.
5. Use Ribs and Gussets (Strengthen Without Thickening)
Want a stronger part? Don’t just make the walls thicker—add ribs (thin, vertical strips) or gussets (triangular supports). They boost strength without causing sink marks.
Rib Design Rule
The thickness of a rib should be 50–60% of the wall it’s attached to. For example: If your wall is 2 mm thick, the rib should be 1–1.2 mm thick. This prevents the rib from cooling slower than the wall (which causes warping).
Example: A Furniture Company’s Success
A furniture brand made a plastic chair seat with 4 mm thick walls—it was strong but had ugly sink marks. They redesigned it with 2 mm walls and 1 mm ribs. The seat was just as strong, the sink marks disappeared, and they used 30% less plastic (saving $0.20 per chair).
6. Add Radii and Fillets (Eliminate Sharp Corners)
Sharp corners are bad for two reasons:
- They weaken parts—molten plastic can’t flow smoothly into corners, creating air bubbles or thin spots.
- They’re expensive to make—molds with sharp corners need special (costly) CNC tools.
Radius Design Formula
- Inner radius: At least 0.5T (T = wall thickness).
- Outer radius: At least 1.5T.
For a 2 mm wall:
- Inner radius = 1 mm (0.5 x 2)
- Outer radius = 3 mm (1.5 x 2)
This improves material flow, strengthens the part, and makes ejection easier—all while lowering mold costs.
7. Design Snap Fits Smartly (Avoid Expensive Side Cores)
Snap fits (plastic tabs that lock parts together) are great for assembly—but they often need undercuts (features that stick out, making demolding hard). Traditional undercuts require side cores (mold parts that slide in and out), which add \(1,000–\)5,000 to mold costs.
3 Ways to Avoid Side Cores
- Use slots instead of undercuts: A narrow slot can act like a snap fit without needing a side core.
- Move the parting line: Adjust where the mold splits to cover the undercut. This works best for outer undercuts.
- Peel undercuts: If your part is flexible (e.g., elastomers), design small bumps that “peel” out of the mold when the part bends.
Tip: Give Clearance for Snap Fits
For peel undercuts, make sure the bump has a 30–45° angle—this lets it slide out of the mold easily. If the angle is too steep, the part will tear.
Yigu Technology’s Perspective on Injection Molding Design
At Yigu Technology, we believe great injection molding starts with design—not just mold making. For clients, we prioritize material matching and tolerance planning first: too many projects fail because teams pick a cheap material instead of the right one. We also advocate for “design for manufacturability” (DFM) checks early—catching a draft angle mistake or wall thickness issue before mold production saves 50% of rework costs. Injection molding is a team sport: engineers, designers, and manufacturers need to collaborate to build parts that are strong, cheap, and easy to make.
FAQ
1. What’s the biggest mistake in injection molding design?
Uneven wall thickness. It causes sink marks, warping, and weak parts. Always keep walls as even as possible, and limit thickness differences to 15% of the nominal size.
2. Do I need draft angles for all injection molded parts?
Yes—even “near vertical” walls need at least a 0.5° draft angle. Without it, parts will stick to the mold, leading to cracks or mold damage. The only exception is parting faces (where the mold splits).
3. How do I choose between amorphous and semi-crystalline plastics?
Pick amorphous plastics (e.g., ABS, Acrylic) for parts that need dimensional stability (no big shrinkage) or impact resistance. Choose semi-crystalline plastics (e.g., Nylon, Polyethylene) for parts that need chemical resistance or durability (e.g., outdoor gear, food containers).
