Tapped holes and threaded holes play crucial roles in joining plastic parts or plastics to metal components. While both involve threads for fastening, their formation, strength, cost, and applications differ significantly.
In plastic injection molding, holes such as through, blind, locating, and threaded holes are commonly formed using core pins or inserts embedded in the mold cavity. These features, while necessary for assembly, add complexity to the mold design and can influence both cost and strength of the molded part. An adequate spacing should be maintained between holes and edges: typically, the distance from a hole to the part edge should be at least three times the hole diameter to avoid stress concentration and cracking. Holes can be directly molded or machined later (secondary operations). However, during ejection, parts with holes are prone to distortion or stress, so draft angles and carefully placed ejector pins are essential.
Tapped Hole vs Threaded Hole
Threaded holes can be created either during the molding process or machined afterward, whereas tapped holes are always machined after molding and cannot be formed directly in the mold. The figures below illustrate the molded threaded hole formed directly during molding (Figure 1), and a tapped hole created later using a tapping process (Figure 2).
Table 1 below shows a comparative analysis of incorporating threads using self-tapping screws (Tapped Hole) and directly molding the threads or using inserts (Threaded Hole).
Table 1: Comparison of tapped and threaded holes in plastic injection molding.
| Criteria | Tapped Hole | Threaded Hole |
|---|---|---|
| Formation | Threads formed after molding using self-tapping screws or tapping tools. | Threads molded directly or formed using inserts. |
| Strength | Moderate; dependent on plastic stiffness. | High; reinforced by insert or in-mold precision. |
| Cost | Lower tooling, low per-part cost. | Higher initial tooling cost. |
| Production Volume | Ideal for low to medium runs. | Best for high-volume production. |
| Repair / Rework | Easy to modify post-mold. | Complex to repair if thread is damaged. |
Design Recommendations: Tapping vs Threading
To achieve reliable fastening performance in molded plastic parts:
- For high-volume production → prefer molded-in inserts or in-mold threads.
- For cost-sensitive or prototype builds → use self-tapping screws with optimized boss geometry.
- Maintain hole-edge spacing ≥ 3× diameter and include draft angles (1–2°) for easy ejection.
- Select plastics with a higher flexural modulus for load-bearing threaded regions.
- Always validate screw joints with torque-to-strip testing during design verification.
Threaded Holes in Injection-Molded Plastic Parts
Threaded holes are integral features molded directly into the plastic part or formed using inserts. They provide permanent, reusable threads that allow screws or bolts to be repeatedly assembled without degrading the plastic substrate. A “threaded hole” simply means a hole that contains threads, whereas “tapping” refers to one of the techniques used to create those threads. Therefore, all methods in Table 2 represent different ways of producing threaded holes in plastic parts.
There are three main methods to create threaded holes during injection molding explained in Table 2.
Table 2: Comparison of threaded-hole formation methods in injection-molded parts.
| Method | Description | Advantages | Limitations | Recommended Use |
|---|---|---|---|---|
| Self-tapping threads (direct molding) | Smooth holes are molded, and threads are formed later by self-tapping screws. | Simple mold, low cost, quick assembly. | Lower strength, limited reusability. | General plastic-to-plastic joints. |
| In-mold tapping | Threads are formed in-mold using integrated tapping mechanisms. | High precision (ISO 4H/5g), durable threads. | 30–50% higher mold cost. | High-volume mass production. |
| Secondary tapping | Threads cut post-molding by CNC or tapping machines. | Flexible sizing, high accuracy. | Additional machining step. | Low-volume or custom parts. |
In some cases, metal threaded inserts are placed into the mold cavity before plastic injection (insert molding). As plastic cools, it bonds mechanically around the insert’s knurled features, providing superior pull-out and torque resistance. Figure 3 below is the visual representation of how a molded-in metal insert integrates within a plastic boss.
Tapped Holes in Injection-Molded Plastic Parts
Tapped holes are a type of threaded hole created after the plastic part is molded. They can be made in two ways:
- By using traditional tapping tools (manual or CNC taps) that cut threads inside a molded pilot hole, or
- By using self-tapping screws, which form or cut threads during assembly. This removes the need for pre-molded threads or metal inserts and is common in electronics, consumer products, and automotive components.
Figure 4 below illustrates common taps for “tapped holes” used to cut internal threads in molded plastic holes during post-molding tapping operations.
Types of Self-Tapping Screws Used to Create Tapped Holes
Different types of self-tapping screws are:
1. Thread-Forming Screws
They deform the plastic to form threads, ideal for soft polymers.
2. Thread-Cutting Screws
They have sharp flutes that cut material, suitable for glass-filled or stiff plastics.
Design Factors for Tapped Holes
- Pilot Hole Diameter: Determines engagement and torque. Typically, 75–80% thread engagement gives optimal joint strength.
- Engagement Length: 2–2.5× screw diameter ensures sufficient holding force.
- Boss Design: Should be 2.5–3× pilot hole diameter; include draft angles to ease molding.
- Strip-to-Drive Ratio: Ideally around 5:1 for automated assembly to prevent over-torque and thread failure
Self-tapping screw joints are economical, fast to assemble, and do not require threaded inserts. However, they can sustain only about 10 re-use cycles, as repeated tightening may strip the threads. The crucial mechanical performance of a self-tapping screw joint is often analysed using a torque-turn graph, which illustrates the relationship between applied torque and penetration depth, defining the critical Strip-to-Drive Ratio.
Thread Design Considerations for Plastic Tapped and Threaded Holes
When addressing tapped versus threaded holes in plastic injection molding, it is critical to note that standard sheet metal screws (such as Type B or AB) are generally unsuitable due to their wide flank angles and shallow threads. Figure 7 below compares standard metal fasteners and specially designed plastic fasteners, highlighting their main difference in how thread geometry affects stress distribution and boss design.
The comparison shows reduced radial stress and smaller boss requirements for plastic applications.
Technical Features of Tapped and Threaded Holes
The technical standard for screws used in thread-forming applications, either for tapped holes or threaded holes in plastic, requires specialized features.
Table 3: Key Technical Features of Fasteners for Thread-Forming in Plastics
| Feature | Technical Purpose/Benefit |
|---|---|
| Optimized Thread Flank Angles | Creates minimal material displacement, ensuring strong engagement, and reducing stress on the boss to prevent cracking. |
| Deep, Steep Threads | Maximizes length of engagement to significantly increase pull-out resistance (ultimate torque). |
| Variable Cross-Sectional Geometry | Reduces contact area during installation (lowers required torque) and allows material relaxation for enhanced thread retention and stability. |
| Specialized Drive Systems | Ensures maximum torque transfer and minimizes the risk of cam-out during automated assembly. |
Conclusion
Both tapped and threaded holes are fundamental in designing injection-molded plastic components, but their selection should balance strength, cost, and manufacturing feasibility.- Tapped holes using self-tapping screws provide economical and flexible fastening for moderate loads.
- Threaded holes, especially with molded-in metal inserts, deliver superior durability and reusability, ideal for repeated assemblies or high-torque joints.
