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10 Essential Injection Molding Best Practices for High-Quality Parts
Created on 01.13
Injection molding is a go-to way to make lots of precise plastic parts. But how good the part turns out depends a lot on the early design and tooling choices. Miss those steps, and you might end up with warped parts, surface flaws, or slow production, which all drive up costs. If engineers stick to the usual injection molding best practices, they can plan parts that are simple to make and last a long time.
This guide covers ten basic ways to make parts better and production more reliable.
1. Maintain Consistent Wall Thickness
Maintaining a uniform wall thickness throughout a part is the most effective way to prevent manufacturing defects. When walls vary in thickness, the thicker sections take longer to cool and shrink than the thinner ones. This temperature imbalance creates internal tension. Inconsistent cooling causes sink marks on the surface and warping of the overall part shape.
Recommendation: Keep wall thickness between 1.5 mm and 3.0 mm for most materials. If a design requires different thicknesses, use a gradual transition (a "ramp") rather than an abrupt step to allow the plastic to flow and cool more evenly.
2. Apply Sufficient Draft Angles
A draft angle is a slight taper you add to the vertical walls of a part. If you skip this, the part sticks to the mold walls when you try to remove it. This friction can hurt the plastic or the mold.
- Normal Draft: Aim for at least 1° to 2° of draft for most parts.
- Textured Stuff: If your part has a textured surface, increase the draft. A good rule is to add 1° of draft for every 0.025 mm of texture depth.
The right draft helps the part come out smoothly, without marks or needing too much force.
3. Design Ribs to Prevent Sink Marks
Ribs are there to make a part stronger without bulking up the wall. But where a rib meets the wall, the material gets thicker.
- Rule: Keep ribs 50% to 60% as thick as the main wall. This stops sink marks from showing up on the other side.
- Height and Spacing: Rib height should not exceed three times the wall thickness to prevent filling issues, and multiple ribs should be spaced apart to allow for proper cooling.
4. Use Radii to Reduce Stress
Sharp internal corners lead to stress concentration, which makes the part more likely to crack under load. They also restrict the flow of molten plastic during the injection phase.
- The Solution: Replace sharp corners with rounded edges, known as radii.
- Dimensions: An internal radius should be at least 50% of the wall thickness. For the exterior, the radius should be the sum of the internal radius and the wall thickness. This maintains a constant wall thickness through the curve.
5. Position Gates at Thickest Sections
The gate is where melted plastic goes into the mold. Where you put it matters because it changes how the plastic fills up the mold and where the plastic flows meet.
Always put the gate where the design is thickest. This way, the plastic goes from thick to thin spots, filling everything before it gets hard. Put gates where they won't be seen after you take the part out. This hides any marks from being removed from the gate.
6. Incorporate Proper Mold Venting
As plastic enters the mold, the air inside must be pushed out. If air cannot escape, it becomes compressed and hot, which can burn the plastic or prevent the mold from filling.
- Venting Strategy: Small channels, or vents, should be placed at the end of the flow paths.
- Tolerance: These vents are typically deep enough for air to escape but narrow enough (usually under 0.04 mm) to prevent plastic from leaking out and creating "flash."
7. Manage Undercuts Efficiently
Undercuts are things like holes on the side or tabs that stop a part from popping straight out of the mold. They make things harder and cost more, but sometimes you need them.
Use slides for stuff on the outside and lifters for undercuts inside. If you can, make a design that has a shut-off. This means making a hole in the part so the mold can make the shape without moving extra parts.
8. Use Conformal Cooling for Complex Geometries
Usually, cooling means drilling holes in the mold for water to flow through. But for hard-to-make parts, these holes might not be close enough to some spots, so they don't cool evenly.
Better Way: Use metal mold pieces made with a 3D printer that have cooling lines that follow the part's shape. It keeps the temperature the same all over, which speeds things up and stops big or tricky parts from bending.
9. Select Materials Based on Shrinkage Rates
All plastics shrink when they go from liquid to solid. But some shrink more than others, like polypropylene (shrinks a lot) and polycarbonate (doesn't shrink much).
Mold makers have to make the mold bigger or smaller based on how much the plastic will shrink. You can't just change materials after making a mold. It will mess up the size of the part. Always pick your material before starting the mold.
10. Adjust Design for Surface Finishes
How you want the part to look affects the design. Shiny parts act differently from rough ones when you take them out of the mold.
Shiny parts stick to the mold easier because they're smooth. You need to angle the sides more and use bigger ejector pins. Rough parts need more of an angle on the sides so the texture doesn't get messed up when you take the part out.
Technical Strategies for Large-Scale Production
In high-volume manufacturing, small errors are magnified over thousands of cycles. Experienced molders often use specific techniques to ensure consistency.
Warp Management
In many large, flat parts, some degree of warping is inevitable due to the nature of polymer cooling. Instead of attempting to eliminate it, engineers use "warp management." By controlling the temperature of each mold half independently or changing the gate location, they can influence the direction of the warp. This ensures that the part remains functional or fits correctly into its assembly, even if it isn't perfectly flat.
Post-Molding Fixtures
For parts with extremely tight tolerances, the cooling process within the mold may not be sufficient. In these cases, parts are placed into a "cooling jig" or fixture immediately after being ejected. This tool holds the part in the correct shape while it finishes cooling to room temperature, preventing it from bowing or twisting.
Getting the Most Out of Sourcing and Learning for Engineers
Engineers often check in with industry experts and groups to keep up with the latest design standards.
Industry Guides: Companies like Protolabs and Fictiv have handy design-for-manufacturability (DFM) charts. These charts show things like tolerance and wall thickness for many different resins.
Peer Advice: Online forums, such as Reddit's engineering groups, are effective for solving everyday problems. People share tips on topics such as using glass-filled nylon or designing threads that won't break when you assemble things. A popular tip is to use threaded inserts instead of just molding plastic threads, especially if you plan to take the part apart a lot.
Final Design Checklist:
- Are the walls the same thickness all around?
- Do all vertical walls have a draft of at least 1.5°?
- Are all inside corners rounded?
- Is the material entry point in the thickest part?
- Have you picked the right material, considering shrinkage?
Using these tips can help you avoid costly mold changes and make your production process smoother.
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