The purpose of draft is to first provide release from the cavity side of the mold upon tool opening. Then upon ejection, draft allows instant release of the plastic part without dragging. If plastic parts have completely vertical walls, drag marks will occur on the plastic as it scrapes along the metal tool face. If money is no object vertical faces may be obtained however with the use of slides and lifters.
What is the correct draft angle? Industrial designers want 0.0 degrees, mold designers want 45.0 degrees... use the following as a guide.
0.0 degree --- Very small details under 0.040 tall that will get polished. The act of polishing will apply some draft. Faces To be 100% relieved with side actions.
1/4 degree --- Emergency use only. Deep ribs, one internal side of a box where the other sides have good draft, bosses ejected by sleeves.
1/2 degree --- Use sparingly and for good reason. Ribs, one internal side of a box, snaps, hooks, etc..
1.0 degree --- Standard draft, all features.
2.0 degree --- Standard draft, very light texture, cavity side to ensure good release.
3.0 degree --- Textured faces, faces that are in common with a shutoff.
The term parting line is a bit of a misnomer. Perhaps the term parting surface would be better. Many plastic part designers seem to have difficulty understanding that parting line is not simply a vector that may be randomly moved about on the plastic part. When done well, parting line indeed appears as a faint vector upon the face of the plastic part. This vector however is simply a witness of the two parting surfaces perfectly contacting each other when the mold is closed. Draft must fall away from the parting lines on ALL vertical faces in the plastic part. The widest point on the part is thus the parting line. The location of parting line and subsequent application of draft to the plastic part are design decisions that affect both the aesthetics and functionality of the part. These decisions should be made before the part is sent to the mold maker if time to market is critical.
-The failure to apply draft to a CAD file before sending it to the mold maker forces the mold designer to guess about what the part designer intended. Frequently the mold designer does not even know what your part is.
-Time is consumed by someone unfamiliar with your parts applying draft arbitrarily without knowledge of your mating parts, sheet metal or components.
-If your mold designer is conscientious he will attempt to alert you to what he is changing, however more time is being sucked away by this process. Frequently your mold designer is using a different CAD package than you are so that even if he sends back a drafted model, it may be difficult for you to do a good interference analysis.
-Many toolers will simply slap some draft on and if the parts don't fit, its your problem.
-When the mold is finished and your parts don't fit, time is lost reworking the tool.
-Bottom line if you are one of those "toss it over the wall" designers who can't draft your parts you are costing your company time and money and should get a job doing something that you CAN do.
Once you understand the basics of draft and shutoffs it is very simple to apply these to your model and save time to market. If you still have questions, get the mold designer to review your parts BEFORE tooling release.
CAUTION: Do not depend upon the opinions of molders sales people. They will tell you whatever they think you want to hear just to get your parts in the door.
Would you like to be absolutely positive that your part is 100% moldable? E-mail: email@example.com