When it comes to the angles used in the plastic part design of injection molded parts, you’d be surprised to learn what a big difference as little as half a degree can make. In particular, the draft of a part can play a major role in everything from how easy it is to eject the part from the mold to how smooth the surface of the finished part is.
People who are unfamiliar with the importance of draft sometimes find it hard to think outside the box—literally. For parts that are envisioned as being roughly square or rectangular, they want all the corners to be 90 degrees. What they don’t understand is that just one degree of draft can improve the molding and ejection process. Have you ever wondered why the compartments in your ice cube trays aren’t perfectly square? Well now you know… the draft makes it easier to remove the cubes.
Modeling Draft
Often, you will conceive your design and draw it with sharp & square features. That's okay -- you're focused on what the overall design. Next, you probably will add fillets and round corners to reduce stresses. Now, it is time for draft. Draft, a term used in many 3D modeling programs, will offset a sidewall feature from another plane by an offset angle. It seldom serves a design purpose, so why do we need it?
Purpose of Draft on the Cavity
Typically, the cosmetic side of your part is in the cavity side of the tooling, also called the "A-Side" or "stationary side". This is typically the side of the tool that is textured to your style. The purpose of draft is to help the part release when the mold opens without creating a vacuum and without scuffing your texture. A good rule of thumb is 1 degree draft for many textures for every inch of cavity depth, with a minimum of 1 degree. Two would be better (twice as good, you could say), if your design can accept it. Heavier textures may require more.
Purpose of Draft on the Core
The "B-Side", or "moving side", needs draft too. Once your part moves across during mold opening, it is time to eject the part. If there is no draft, the advance of the ejector pins will create a fierce vacuum between the part and the steel. The struggle will continue until the vacuum breaks, distorting and stressing your part along the way. Using features such as poppet valves, the vacuum can be broken with reduced draft, but these are less common.
Purpose of Draft to Reduce Tool Wear
Your tool's purpose is to control where plastic goes and where it doesn't. Controlling "where it doesn't go" is all about shut-offs, or interfacing components of your tool that prevent plastic flow. If you have two metal features sliding past each other, they should be drafted at about 3 degrees to mitigate tool wear. Without it, your surfaces will wear prematurely and you risk galling surfaces or flash as they are no longer able to keep the plastic out.
Putting It Together
When making an enclosure with two mating halves, ensure that your case goes together. You can do this by checking your reference dimension where the two halves meet.
Getting It Right
If draft isn't considered fully before the tool is released for build, it can be enormously expensive to correct later. In some cases, it can approach the cost of a new tool. Take some time and get an opinion on whether you have adequately drafted your part for molding.
Get the Details on Draft
Draft is an often-overlooked aspect of part design. If you’ve got questions about the design for your next project, we’re happy to give you our perspective. Don’t hesitate to contact us.
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