Plastic part design Rules

Overview:- The following is a summary of the design rules covered in both Designing Injection Molded Parts and this course. Keep in mind that these rules are more or less guidelines intending to help you design parts that can be successfully molded. In extreme cases, it is possible to bend or even break some of these rules. You should consult a tool designer to determine the mold capabilities available for your design.

Wall Thickness:-

 - Maintain uniform wall thickness as much as possible.

- If you have not yet determined a material, a thickness of approximately 2.25 to 3 millimeters (.09 to .12 inches) is a good starting point.

- Typical wall thickness' based on material are outlined in this table. 

- When changing wall thickness, do so gradually over a distance of at least three times the change in thickness. Use a radius for a smooth transition.

- When changing the wall thickness, do not exceed 25%.

Draft:-

 - Always add draft to the walls of a part. Draft can be as small as 1/4 degree, but many materials require more.

- Add more draft to walls greater than 25 millimeters (1 inch) in depth.

- Add a minimum of 5 degrees for textured surfaces. Some texture may require more.

Undercuts:-

 - Add draft to undercut surfaces to easy operation of slides or lifters. 

- Plan for cosmetic blemishes formed by slides and lifters. 

Openings:- - Ensure there is a minimum of at .125mm, or .005in, for the contacting steel to pass for a shutoff opening. An angle of 3 degrees is preferable. 

Bosses:- 

- Add Draft to the inside and outside walls of a hollow boss whenever possible.

- Blend the outside corners using a radius of 25% of the nominal thickness.

- The thickness should be 50% - 75% of the nominal thickness.

- For use with self-tapping screws, the outer diameter should be 2-1/2 times the major diameter of the screw.

Holes:-

 - Limit the depth of a hole to twice its diameter.

Ribs:- - The thickness should be 50% - 75% of the nominal thickness. 

- The height should be limited to 3 times the nominal thickness.

- Ensure the top of a rib is at least 1mm (.04in) in thickness.

- Blend the base of a rib 25% of the thickness.

- Blend the top of a rib at least .25 mm (.010in)

- Space multiple ribs apart by at least twice the nominal thickness.

Snaps:- 

- Define the angle of the sloped surface on the back of the snap appropriate for either a non-locking or self-locking snap. 

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Living Hinges:- 

- Use a flexible material such as polypropylene or polyethylene.

- Consider secondary operations such as coining or flexing the hinge while the part is hot to enhance the hinge's strength.

Machining:-

- Select a material with a high melting temperature, good lubricity, and good hardness.

- Use the correct speed.

- Lubricate appropriately with a compatible oil.

EMI Shielding:- 

- Add conductive particles to material, metallize the part, or electroless plate the part.

- For assemblies, use overlap or tongue and groove joints to prevent leakage.

Metallization:-

- A base coat may be required to ensure good adhesion.

- A top coat may be required for protection.

Plating:- 

- Electroplating requires a base coat of a conductive material.

- Electroless plating may require roughening the surface of a part, as with chemical etching.

Painting:-

- Crystalline polymers with good chemical resistance may require a primer.

- Check heat resistance of material when oven curing is necessary.

Press Fits:-

- Minimize interference to ensure a minimum amount of stress. 

Self-Tapping Screws:-

Thread-Forming

- Use for assembly / disassembly applications.

- Most suitable for non-brittle materials with a flexural modulus of up to 1

1400 MPa (200,000 psi).

- Materials with a flexural modulus of 1400 to 2800 MPa (200,000 to 400,000 psi) may also be used.

Thread-Cutting

- Use in applications where no disassembly is required.

- Most suitable for brittle materials with a flexural modulus of 2800 to 6900 MPa (400,000 to 1,000,000 psi) and can be used for some larger than 6900 MPa.

Machine Screws:-

- Avoid countersink screws in order to minimize stress due to wedging.

- Do not apply too much torque during installation. Consider using a shoulder screw or washer to prevent compression of the plastic.

Molded Threads:- 

- The root of the thread must have a radius (.13 - .25 mm or .005 - .010 in) to minimize stress.

- A lead-in relief of .8 mm (1/32 in) is recommended.

- The pitch must be at least 0.9 mm (28 threads per inch).

Rivets:- 

- Use a washer to distribute stress.

Bonding:-

Solvent.

- Provide sufficient area.

- Use tight mold tolerances.

- Best for amorphous materials.

- Materials should be same, or equally soluble.

Adhesive

- May require preparation such as abrasion or chemical etching.

Ultrasonic Welding:- 

- Mating surfaces should lie on a single plane.

- Mating surface area should be uniform around the part.

- Hygroscopic materials (those that absorb water quickly) may need to be dried first.

- Where applicable, use an energy director.

- For materials not suitable for energy directors (i.e., some crystalline polymers), use an interference joint.

- The amount of interference depends on the size of the part. 

Vibration Welding:- 

- Mating surfaces must be parallel.
- Incorporate traps to eliminate flash. 

Staking:- 

- The standard profile is recommended for bosses 1.6 mm (.062 inch) in diameter or larger. 

- The dome profile is recommended for bosses less than 1.6 mm in diameter. 

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