Here are the some basic rules to be followed:

Holes

• ALL holes create a weld line.
• Any depression that passes two-thirds of the way through the wall will encompass a weld line.
• Irregular and squre holes are not recommended.
• A basic rule of thumb to remember is that the space between two holes should be the greater of two times the wall thickness of two times the diameter of the hole.

Cavity identification

• Multicavity molds should always have cavity identification.

Bosses

• Boss height should be around 2.5 times the inside diameter of the boss.
• Separate the bosses from the wall to maintain a uniform wall.
• The receiving hole diameter of a boss should be equal to the self-tapping scew pitch diameter.
• The boss outside diameter should be equal to twice the major diameter of the insert.

Walls

• Walls should have a uniform wall thickness throughout the design; core out thick sections.
• Walls MUST have draft. A minumum of at least 1 degree of draft for every inch of wall. Any draft angle is better than no draft angle.
• Texture requires 1.5 degrees of draft for 0.001-inch texture depth.
• If the nominal wall must varie, the change should be gradual if possible and should not exceed 15 percent of the wall.
• Expect a flow line on a smooth surface when the plastic flow came from a textured surface (opposite the gates and holes).

Radius

• Avoid sharp corners; radius outside and inside corners.
• The radius of an inside corner of a part should be 50 percent of the nominal wall thickness.
• The radius of an outside corner of a part should be 150 percent of the nominal wall thickness.
• Radii can be as large as the design will allow.

These are listed to be a starting point for designers. Each plastic part is different so some of these rules might not apply.

Being creative is knowing when and where to "bend" the rules...

Check here for an illustration of a "typical" injection molded part. Many of the above points are illustrated in this drawing.

Feel free to send us your comments, tips, tricks and hints for injection molding to us in the Core77 Discussion Boards.