Draft Angles: Solving Vacuum Lock in Injection Molding
Have you ever stacked two plastic buckets together and couldn't pull them apart?
That frustrating vacuum lock is exactly what happens inside an injection mold...
Unless you use a simple design trick called a "Draft Angle." 👇
When molten plastic cools inside a steel mold, it shrinks.
It grips onto the inner steel core like a vice.
If the walls of your part are perfectly straight (0 degrees), the part scrapes against the steel for the entire ejection distance.
This causes terrible scratch marks, warping, or ruined parts.
But by angling the walls just slightly (adding a Draft Angle), you change the physics.
The moment the ejector pins push the part even a single millimeter, a gap forms between the plastic and the steel.
The friction drops to zero instantly. The vacuum is broken. The part falls off safely.
If you are designing plastic parts, follow these 3 rules of Draft:
𝟭. 𝗧𝗵𝗲 𝟭-𝗗𝗲𝗴𝗿𝗲𝗲 𝗥𝘂𝗹𝗲
As a baseline, you must add a minimum of 1 degree of draft for every 25 mm (1 inch) of part depth.
𝟮. 𝗧𝗲𝘅𝘁𝘂𝗿𝗲 𝗠𝗲𝗮𝗻𝘀 𝗠𝗼𝗿𝗲 𝗔𝗻𝗴𝗹𝗲
If your outside surface has a rough texture (like a leather or sand finish), 1 degree isn't enough. The plastic will drag on the microscopic bumps. The math: Add 1.5° of draft for every 0.025 mm (0.001") of texture depth.
𝟯. 𝗗𝗼𝗻'𝘁 𝗙𝗼𝗿𝗴𝗲𝘁 𝘁𝗵𝗲 𝗥𝗶𝗯𝘀
Even the hidden internal structural ribs need draft (usually 0.5°). If you don't draft them, they will stick to the tool and rip right off your part.
What is the deepest plastic part you've ever had to design, and what draft angle saved you? Let me know below! 👇
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