How to Design Screw Boss on Plastic Molded Parts

Screw boss is an indispensable feature that impacts the functionality, strength, and manufacturability of injection molding parts. It plays a vital role in assembly and attachment, accommodating screws, threaded inserts, and other fasteners. This article provides a comprehensive guide on plastic boss design, covering their purpose, applications, design guidelines, challenges, and solutions.

What Are Screw Bosses in Injection Molding?

Screw bosses are protrusions on plastic parts used to enhance assembly by guiding and positioning components, providing structural support, and securing them in place. These features are crucial for ensuring parts fit together correctly and maintain integrity during use.

Purpose of bosses in plastic design

• Provide strong attachment points for secure fastening in multi-part assemblies.
• Reinforce local areas to improve structural strength and distribute mechanical loads.
• Maintain precise alignment between mating parts during assembly.
• Function as bearing seats or mechanical pivots within the design in some cases

When to use a plastic boss

• Anchoring screws or inserts under torque
• Aligning and joining enclosure parts
• Mounting internal components
• Reinforcing thin-wall areas
• Controlling assembly tolerances

Consideration Factors for Screw Boss Design

There are several factors to consider when designing screw bosses for injection-molded parts. The design should ensure the parts perform as intended and are manufacturable without defects. Key factors to consider include:

Part Strength

Screw bosses must withstand continuous stresses imposed by screws or fasteners without cracking or deforming. It is recommended to avoid placing a screw boss close to an external wall. Reinforce the screw boss with gussets or ribs to improve the strength of the screw boss.

Material Type

For plastic boss design, materials with uniform shrinkage are preferred because they offer better dimensional accuracy. Choose materials that are soft enough to accommodate screws but not so brittle that they fracture easily. Brittle plastics such as polystyrene may require thicker walls to prevent fracture.

Assembly Requirements

The screw boss must be designed to fit the specific fastener being used, considering factors such as screw size, thread type, and required depth of engagement.

Design Guidelines for Bosses in Plastic Design

The following highlights 10 simple plastic boss design guidelines. Adhering to these basic guidelines for injection molded parts can improve the moldability of the designs and increase the operational lifespan of the parts.

Location of the Screw Boss

Position bosses in thicker wall regions or primary load paths to ensure mechanical support. Avoid placing them in thin walls, sharp corners, or zones prone to stress concentration. Tie bosses into the surrounding structure with ribs or gussets for load distribution and moldability benefits. The following areas are recommended:

• Stress areas: Position bosses within the main stress areas of the part to enhance overall strength and stability. Avoid placing them on thin arcs or weak walls.
• Distribution: Distribute screw bosses evenly across different sides of the parts to maintain balance and stability during assembly and use.
• Interior placement: Locate bosses inside the part where possible to reduce their exposure to external impacts and accidental damage.

Wall Thickness

To avoid surface sink marks and air bubbles, keep the screw boss thickness at no more than 60% of the part’s nominal wall thickness (T). Also, it’s common for the outer diameter to be twice the inner diameter, ensuring structural strength. For thin-wall parts, adjust the screw boss dimensions and supporting structure to maintain strength without excessive wall thickness and defects such as shrinkage marks. For more details, see our guide on thin-wall injection molding design guide.

Boss Height

Screw bosses require a draft angle to facilitate mold release. As the boss height increases, the draft angle creates a greater difference between the top and bottom diameters, reducing the available space at the top for screw installation. Maintaining the required top diameter may result in a thicker boss base, increasing the risk of surface sink marks and air bubbles. Therefore, the screw boss height should generally not exceed three times the part’s nominal wall thickness (h ≤ 3T).

Minimize Radius at Base of the Screw Boss

To reduce stress and ensure smooth plastic flow, the fillet radius at the base should be between 0.25~0.50 times the nominal wall thickness (R = 0.25T ~ 0.50T). Also, keep the base thickness at no more than 70% of the nominal wall thickness (t ≤ 0.7T) to avoid visible sink marks.

Draft Angle Incorporation

Typically, use a 0.25° draft angle for the inner diameter and 0.5° for the outer diameter for easy demolding. In some cases, you might skip the draft angle, for example, by using a sleeve in the mold for ejection, but this can increase the mold costs.

Standalone Bosses

To prevent the design of isolated screw bosses, it is recommended to integrate them with the part wall using reinforcing ribs, forming a cohesive structural unit. Standalone screw bosses can weaken plastic parts if not properly supported. Connecting them to the part wall with ribs strengthens them and aids plastic flow during molding.

When a screw boss is positioned near the gate area, bosses risk forming weld lines, which reduce strength, especially for self-tapping screws or metal inserts. Ribs should be added around standalone screw bosses to help mitigate these issues. Additionally, incorporating fillets at the junctions between ribs and bosses helps distribute stress evenly and enhances durability.

Maintaining Uniform Wall Thickness

A fundamental principle in plastic boss design is to maintain uniform wall thickness to avoid molding defects. When bosses are placed too close to the part walls, they can cause thick sections, leading to sink marks and air bubbles.

Solid plastic bosses can be redesigned as hollow, cross-shaped, or elliptical to maintain consistency and avoid defects. This can reduce material volume and prevent cooling-related issues.

Spacing Between Bosses

To avoid defects like sink marks, it is recommended to maintain a minimum spacing of twice the nominal wall thickness between screw bosses. This ensures proper cooling and prevents thin areas that could extend cycle times or compromise part quality.

Standardizing Screw Boss Design

For bosses used with self-tapping screws, adhering to standard hole designs is recommended. This ensures reliable assembly and reduces errors, avoiding the need for unconventional approaches that may introduce risks.

Material and Thickness Selection

Use the same material for the plastic boss as the rest of the injection-molded part to maintain uniform properties and avoid differential shrinkage. The following table lists the ideal thickness of some of the most popular injection molding materials.