The importance of designing draft angles for injection molding

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.

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.

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.

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:

• Minimizes the warping to your geometry
• Increases the tool life of the mold
• Improves the surface finish on your plastic parts
• Cuts down cost by reducing the cooling time

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.

*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:

• Integrate the draft into your design early. If you want to prototype the part with 3D printing, make sure it already contains the draft to avoid design re-work.
• Draft both Cavity and the Core sides of your part. The core (inside surfaces) would require a slightly larger draft due to the shrinkage direction of the material
• All vertical faces need to have a draft - that includes ribs and gussets, bosses and stand-off features.(learn more about injection molding design features here)
• The larger the draft angle - the better. Of course, it’s not always possible but even a 0.5 angle (absolute minimum for most materials) is better than nothing.
• Be mindful of the reference face you draft from and whether the draft impacts functionality or fit of mating parts.
• The location of the parting line is not always obvious. Be sure to consult your supplier early on in the process to deliver the end design on time.
• Draft the features that would require a side-action (side cavities and bosses)