In the development and design stage of new products, the selection of plastic material is the most important link, but in addition to plastic, in fact, there are a lot of parts that need to be taken seriously, but there is a small part that is often ignored, that is screwed.
From a manufacturing point of view, a poorly designed screw and the stud are often the last straw to kill a design. Screw really such minor parts? This article will try to discuss first what a self-tapping screw is? What are the advantages and disadvantages of tapping screws? We’ll try to talk more about screw torque later in the article.
What is a self-tapping screw?
A self-tapping screw is a type of fastener that creates its own threads as it is driven into a material (such as metal, plastic, or wood), eliminating the need for a pre-tapped hole.
Tapping screws the conception of good, in fact, really can greatly improve the efficiency of the industrialized mass production, because want to advance the creation of a “female” screw holes, need to spend more time and effort, and plastic injection parts must be commonly used embedded value screws (Insert) can produce the effect of the female, both when waste and work, Self-tapping screws do not need to make a “female thread” screw hole first.
What is Difference Bewteen a self-tapping screw and common Screw?
Most of the ordinary screws refer to the machine screws that have to first make the so-called female thread on the parts so that the screw can be locked in. Just like the nut, there are already threads in the nut hole, so the screw can be easily locked in the thread;
However, self-tapping screws do not need to make threads on the locked parts in advance because it can lock in the locked parts and tap threads on the locked parts at the same time, and then make itself fixed to the locked parts.
Factors affecting the torque of plastic self-tapping screws
The following are the factors that I think may affect it:
Screw diameter:
The larger the screw diameter, the greater the torque required for the electric screwdriver. This is because a larger-diameter screw typically needs to engage deeper into the plastic material.
Screw length:
The longer the screw, the greater the torque required for the electric screwdriver. The torque is essentially proportional to the contact area between the screw and the plastic—the deeper the screw is driven into the hole, the more surface area engages with the plastic.
Screw pitch
While this may seem counterintuitive given the earlier discussion about contact area, it occurs because a screw with a larger pitch advances deeper into the plastic material per rotation compared to a finer-pitch screw—thus demanding greater torque.
This principle mirrors gear mechanics: when driving the same gear with two different pinions of equal diameter, the pinion with more teeth (smaller pitch) requires less effort—just as a finer-pitch screw reduces torque demand.
Screw shape
The torque requirement isn’t directly related to the screw’s tip shape itself. However, different tip shapes typically indicate different thread designs. Empirically, screws with pointed tips tend to have shallower threads, consequently requiring less torque from the electric screwdriver.
Screw hole diameter
The smaller the screw hole diameter, the greater the torque required from the electric screwdriver.
Screw hole draft angle
The larger the draft angle of the screw hole, the greater the required torque. This is because the hole diameter progressively decreases toward the bottom of the screw hole.
Additionally, it’s important to note the accompanying screw boss draft angle on the outer side—both draft angles directly affect the wall thickness of the screw boss.
Screw hole chamfer
The bevel helps the screw stay aligned in the screw hole, preventing it from slipping sideways during assembly.
However, if the bevel is too large or too deep, it may reduce the required torque from the electric screwdriver—but this also decreases the remaining engagement area between the plastic screw boss and the screw, potentially weakening the clamping force.
Plastic resin material
The harder the plastic material (such as reinforced or glass fiber-filled plastics), the greater the torque required from the electric screwdriver.
Limitations of self-tapping screws
1. Stress Concentration Risks in Self-Tapping Screw Applications
The fastened component is prone to stress concentration due to the forced insertion of self-tapping screws, which may lead to cracking or fracturing around the screw hole and ultimately compromise product reliability.
For this reason, the mounting area for self-tapping screws must maintain sufficient wall thickness to withstand the tightening stress. As a general rule, the outer diameter of a plastic screw boss should typically be 2.5–3.0 times larger than its inner diameter, though the exact minimum wall thickness requirement varies depending on the resin material used.
2. The Pitfall of Copy-Paste Design in Plastic Components
Some designers blindly replicate legacy designs without considering differences in plastic materials, resulting in screw holes with insufficient wall thickness that fall below the required safety factor.
3. Draft Angles Secretly Erode Screw Boss Strength
In some cases, designers correctly specify the required wall thickness for screw holes but fail to account for the impact of draft angles—particularly for taller screw bosses. Since the draft angles of the inner and outer diameters are oriented in opposite directions, the remaining wall thickness progressively diminishes toward the top of the boss.