What is draft angle injection molding? Simply put, a draft angle is a taper that’s applied to the walls of injection molded plastic parts. Draft angles help with ejecting parts from the mold after the injection molding process.
In this complete guide, as a professional plastic injection molding manufacturer, I’ll explain everything you need to know about draft angles for injection molding, including what they are, why they’re important, and some best practices for designing them into your parts.
What Is a Draft Angle?
A draft angle refers to the slight taper or angle added to the sides of a part’s features. The position and direction of draft angles are designed to assist with smoothly and easily ejecting parts from injection molds.
How Draft Angles Work
Draft angles basically create more space between the part and the mold walls to reduce friction during ejection. This prevents defects on the part’s surface finish.
Without a draft angle, the molded part would scrape along the mold as it’s ejected, likely causing scratches or other cosmetic defects. The material shrinkage that occurs when the part cools would cause it to tightly grip the mold if the walls are completely vertical.
Why Are Draft Angles Essential?
Now that you know what a draft angle is, let’s talk about why they’re so important for injection molded parts and molds.
Enable Smooth Part Ejection
The number one reason draft angles are added to parts is to enable smooth, easy ejection without friction or scraping. The looser tolerances created by the angle facilitate this.
Prevent Part Damage
Dragging on the mold walls often damages parts during ejection. Draft angles pretty much eliminate this, protecting your part’s appearance and integrity.
Reduce Cycle Times
Draft angles also speed up cooling times and reduce ejection times since the parts “pop” right out of the mold more cleanly and easily. This brings down overall cycle times.
Protect Mold Finish
An inadequate draft would lead to more friction and wear on the mold over time during ejection as parts scrape across the cavity surfaces. Draft angles prevent this mold damage.
Minimize Warpage
Drafts reduce warping risks by preventing vacuum voids from forming as parts are ejected. The space created allows air pressure to equalize smoothly.
Best Practices for Draft Angle Design
When designing draft angles, keep these rules of thumb and tips in mind:
Apply 11⁄2-2 Degrees Draft
Most parts will require 11⁄2-2 degrees of draft to offset material shrinkage. This works for mold depths around 2 inches.
Increase Draft For Deeper Parts
Add about another degree of draft for every inch deeper that the part’s features go beyond 2 inches deep to account for more surface area.
Draft Towards Top of Mold
Make sure draft angles follow the path of the mold opening and components moving upwards/outwards when separating along parting lines.
More Texture Needs More Draft
The more intricate the texture, the more draft you’ll need to preserve surface quality – add additional 11⁄2 degrees per 0.001-inch of texture depth.
Draft All Part Geometry
Any ribbing, bosses or other part geometry that contacts the mold should also be drafted just like walls.
Use Core-Cavity Approach
Draft inner/outer faces slightly differently to end up with required finish only where needed if certain part appearance matters.
Add Draft Everywhere Possible
Even just half a degree is way better than no draft angle at all to meet basic requirements.
Following those tips helps ensure effective, high-quality part production from your injection molds when you factor draft angles into your design correctly.
The little upfront effort pays off with faster cycles, lower scrap rates, and smoother manufacturing down the line. And understanding exactly how draft angles benefit your specific parts helps inform smarter design choices.
Hopefully this gives you a clearer picture of why draft angle injection molding matters and how to best implement it! Let me know in the comments if you have any other draft angle questions.