This blog post will provide you with a comprehensive guide on plastic screw boss design, which is an important aspect of injection molding. You will learn about the purpose and applications of screw bosses, the essential design guidelines for screw bosses, and the challenges and solutions in screw boss design.
Table of Content
- What are Screw Bosses and Why are They Important?
- Essential Design Guidelines for Screw Bosses
- Boss Size and Shape Recommendations
- Boss Location Recommendations
- Guidelines for Material and Thickness Selection
- Draft Angle Recommendations
- Wall Thickness Considerations
- Spacing Between Bosses
- Height to OD Ratio
- Standalone Screw Boss
- Challenges and Solutions in Screw Boss Design
What are Screw Bosses and Why are They Important?
Screw bosses are cylindrical protrusions on plastic parts that serve as key components for assembly and attachment. They have holes that accommodate threaded inserts, screws, and other mechanical fasteners. They can also act as positioning aids, bearing surfaces, or levers, facilitating gear rotation.
Screw bosses are needed to align parts during assembly and create a seamless connection between parts. They also improve part strength, design moldability, and mold life while reducing manufacturing costs. Screw bosses are very common features in injection molding, as they allow fixation with other parts using self-tapping screws.
Essential Design Guidelines for Screw Bosses
Designing screw bosses requires adherence to specific guidelines to ensure accuracy and optimal performance. Various injection molding design principles are recommended for screw boss design. These guidelines take into account factors such as the size, shape, and location of the boss. Additionally, considerations are given to draft angles, material thickness, and type. Below, you will find a set of recommended screw boss design guidelines to follow.
Boss Size and Shape Recommendations
The boss should be the same diameter and size as the injection-molded parts’ wall thickness and size. The reason is that selecting a smaller screw boss will result in inadequate strength while choosing a larger one could lead to an unnecessary increase in the cost of production. The length, pitch, and thread diameter are typically used to calculate screw boss size.
It is worth noting that incorporating a screw boss into the design of a plastic part typically results in an extended cycle time. This is because the presence of bosses increases the time required for the molten plastic to fill the mold completely.
Boss Location Recommendations
The boss should be located as close as possible to a rib or wall to provide support and prevent cracking or deformation. The boss should also be aligned with the direction of mold opening to avoid undercuts and facilitate ejection. If possible, avoid placing bosses near corners or edges, as this could cause stress concentration and warping.
Guidelines for Material and Thickness Selection
The material and thickness of the boss should be compatible with the material and thickness of the part. Different materials have different properties, such as ductility, thermal expansion, and clamp retention. These properties affect how well the fastener can hold the part together without causing damage or loosening.
For example, brittle materials require thicker bosses than ductile materials to withstand stress. Similarly, materials with high thermal expansion require larger clearance holes than materials with low thermal expansion to accommodate dimensional changes.
Draft Angle Recommendations
The draft angle is the angle between the side wall of the boss and the vertical axis. A draft angle is necessary to facilitate mold release and prevent sticking or dragging. The recommended draft angle for screw bosses is between 1° and 3° per side, depending on the material type and surface finish.
Wall Thickness Considerations
The wall thickness of the boss should be uniform and consistent with the wall thickness of the part. A uniform wall thickness ensures even cooling and reduces shrinkage, warping, and sink marks. The wall thickness of the boss should also be sufficient to withstand the torque applied by the fastener without cracking or stripping.
Spacing Between Bosses
The spacing between bosses should be at least twice the diameter of the boss to prevent interference and stress concentration. If multiple bosses are required for a part, they should be arranged symmetrically and evenly distributed to balance the load and avoid distortion.
Height to OD Ratio
The height to outer diameter (OD) ratio is the ratio between the height and the outer diameter of the boss. A high height to OD ratio could cause the boss to bend or buckle under load, while a low height to OD ratio could reduce the strength and stability of the boss. The recommended height to OD ratio for screw bosses is between 0.5 and 1.0.
Standalone Screw Boss
A standalone screw boss is a boss that is not connected to a rib or wall. This type of boss is usually used when there is no space or need for a rib or wall. A standalone screw boss should have a larger diameter and wall thickness than a regular screw boss to provide adequate strength and rigidity. A standalone screw boss should also have a tapered base to reduce stress concentration and improve mold filling.
Challenges and Solutions in Screw Boss Design
Screw boss design can pose some challenges that could affect the quality and performance of the part. Some of these challenges are:
- Radial stress: Radial stress is the outward force exerted by the fastener on the boss. Radial stress can cause cracking, deformation, or expansion of the boss, especially if the boss is too small, too thin, or made of brittle material. To reduce radial stress, use special fasteners for plastics that have narrow thread profiles and shallow threads. These fasteners reduce the contact area and the friction between the fastener and the boss, thus lowering the radial force.
- Pull-out: Pull-out is the failure of the fastener to hold the part securely due to insufficient engagement or resistance. Pull-out can occur if the hole size, thread depth, or thread pitch of the boss is not compatible with the fastener. To increase resistance to pull-out, use fasteners with coarse threads, large thread depth, or self-locking features. These fasteners increase the engagement and friction between the fastener and the boss, thus enhancing the holding power.
- Stripping: Stripping is the failure of the fastener to grip the threads of the boss due to excessive torque or wear. Stripping can occur if the material, wall thickness, or hole size of the boss is not suitable for the fastener. To prevent stripping, use fasteners with low torque requirements, high wear resistance, or self-tapping features. These fasteners reduce the stress and damage on the threads of the boss, thus improving the grip.
Plastic screw boss design is an important aspect of injection molding that requires careful consideration of various factors. By following the essential design guidelines for screw bosses, you can ensure a strong connection between parts and mitigate cosmetic defects. By understanding the challenges and solutions in screw boss design, you can avoid common failures and optimize performance.
If you need plastic parts with screw bosses, you can contact RapidDirect, a leading online manufacturing platform that offers high-quality injection molding services. RapidDirect can help you with screw boss design, selection, testing, and production. RapidDirect also provides other manufacturing services such as CNC machining, 3D printing, sheet metal fabrication, and more.