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The importance of designing draft angles for injection molding

Written by Shak Akhrarov, Mechanical Engineer at 3D Hubs

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Injection Molding SPI surface finishes



A draft angle is a taper applied to the vertical walls of the injection molded component to assist with releasing the part from the mold.

Introduction

Draft angles are a hassle. As designers, we like to work with straight, perpendicular models that can be easily read, measured and modified. Instead, we're required to deal with trapezoidal shapes that can impact functionality, fit and aesthetics due to the addition of the draft.



draft angles manufacture

Why draft angles are important

However they are a required feature. A functional part for injection molding has to have drafts facing both cavity and core sides to reduce the risk of damage to the parts and ensure ejection. As the molten plastic flows into the closed die and fills the cavity, the material shrinks inside the mold upon cooling and as a result grips the core.



injection molding material shrinkage

Without any taper, the part would come out scratched while increasing wear and tear on the mold and shortening the tool life. Even worse - if the part does not eject, it requires a production halt to manually peel it off.

What’s also important is to overcome the vacuum forces by letting air in between the metal and the plastic for the part to release properly. Adding an adequate draft angle ensures that warpage upon ejection is prevented while maintaining a decent cosmetic finish.

Scratch marks often occur on parts with inadequate draft angles.


There is an inherent tradeoff between ejection mechanism and the cooling system - they both compete for real estate within the core. By making the part easy to release you simplify the ejection mechanism.

Therefore, instead of having numerous pins or sleeves to push the part out of the core - you leave a lot of room for cooling channels that significantly cut down your cycle time. As a result, you get a more competitive per unit price for your project which plays a huge role at high production volumes.

Note: On average around 70% of the cycle time during the molding is attributed to cooling

To sum up, adding draft angles to your design:

Design considerations

There’s no universal formula that would tell you exactly what draft angle you have to implement to your design, but there are some factors for you to consider:

Deep pockets and cavities

Deep pockets and cavities need a steeper draft angle to assist overcoming the vacuum and the friction forces as the part is pushed out from the core.

Rule of thumb: Add 1° of angle per inch of cavity

Part texture

Texture of the part dictates the minimum draft. A smoother surface, such as a mirror finish, is easy to push out and thus to cut down on scrap rates. For light textures, a 1-5 degree draft is appropriate. Complex textures like leather or snakeskin introduce micro-undercuts and would demand a 5-12+ degree draft.

Rule of thumb: Add another 1.5° per thousandth-inch (0.001” or 0.025 mm) of textured depth.

Learn more about designing draft angles in the video below, which also includes a practical demonstration in Fusion360.

Material considerations

The more the plastic thermally shrinks, the larger the draft must be. In addition - strong, brittle and abrasive materials would naturally require a larger taper than those that are soft, ductile or self-lubricating.

Nylon, for instance, is very forgiving and does not necessarily need a draft angle, although a standard 2 degree taper is always recommended.

Material Minimum draft angle* Recommended draft angle
Nylon 0 1
Polyethylene 0.5 1.5
PVC 0.5 1.5
PP 1 2
PC 1.5 2

*Assuming perfect conditions

Final considerations

At the end of the day, draft angles are a necessary evil. Designing them correctly is key to obtaining a competitive cost and potentially shorten the lead time. A few key points to have in mind:



Learn more design tips in our injection molding manufacturing & design guide

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