Steinwall -Design Rules of Thumb

DESIGN RULES OF THUMB FOR PLASTIC PART DESIGN

Each plastic part is unique. To follow all the "Rules of Thumb" that the plastic industry develops would probably not result in the desired design. The rules listed below are simply a starting place for designers.

This listing is not complete or thorough. "Rules of Thumb" spell out the ideal world for plastic molding. This could result in a very expensive mold. Trade-offs might be necessary to bring benefits in-line with the project's budget. Steinwall engineers are available for early design meetings. It is at this beginning stage that the trade-offs can be effective. Call on us to help.

WALLS

Should have a uniform wall thickness throughout the whole design: Core out thick sections.
Must have draft - a good starting point is at least one degree of draft for every inch (.0175 per 1"). Any draft angle is better than no draft angle.
Texture - 1 1/2 degree draft for .001" texture depth.
If nominal wall must change, the change should be gradual and should not exceed 15% of the nominal wall.
Expect a flowline on a smooth surface when plastic flow came from a textured surface. (Opposite the gates and holes.)
Typical wall thickness ranges from .060" to .125". However, molding is possible with wall thicknesses of .020" to 1.50" depending on material.

RADIUS

No sharp corners; Radius all inside and outside corners.
The radius of an inside corner of a part should be 50% of the nominal wall.
The radius of an outside corner of a part should be 150% of the nominal wall.
Radii can be as large as the design can allow.
Radii are impractical in certain mold locations such as parting lines or side-cored shutoffs.
Radius a minimum of 25% of nominal wall at the junction of wall and a reinforcing rib.

HOLES

All holes will create a weldline. (Quality concern)
Any depression that passes 2/3 of the way through the wall will create a weldline.
Irregularly shaped or square holes are not recommended.
The space between two holes should be the greater of two times the wall thickness or two times the hole diameter.

BOSSES

Boss height should be 2.5 times the I.D. of the boss.
Separate the bosses from the walls in order to maintain a uniform wall.
The receiving hole diameter of a boss should be equal to the self-tapping screw pitch diameter.
The boss O.D. should be equal to twice the insert or screw major diameter.

RIBS

Reinforcing ribs should not be thicker than 2/3 the nominal wall thickness. Any thicker will create sink marks.
The distance between two ribs should not be less than two times nominal wall.
A strengthening gusset is typically two times the wall thickness deep, four times the wall tall, and half the wall thick.

The definition of a successfully molded part is - "a part which performs its intended function within the limits of the achievable properties of the chosen raw material."