Injection molding parts screws

The design guide of injection molded parts: About screws

The design guide of injection molded parts: About screws

The screw is one of the most commonly used locking assembly methods in electronic products. Still, many designers (R&D) often neglect the design requirements of screw and screw columns, resulting in some defects in assembly. For example, the screw slides, the screw cracks or breaking, the screw breaks, or the screw head wears out…

Here are some basic requirements for screw and screw hole column design so that your screw design is no problem.

Injection molding screws01

Suppose we take an m2.6x10L screw with the following specifications:

Screw head diameter: 4.1~4.5mm,

Outer diameter of screw tooth (D) : 2.47-2.55mm, take 2.5mm.

Screw length: 10+0/-0.8mm,

Screw pitch: 0.91mm

Generally speaking, the diameter of the screw hole must be slightly smaller than the outer diameter of the Screw (X0.72-0.85); Ordinary people often ignore the outer diameter of the screw column also need to match the outer diameter of the Screw (X1.7 ~2.2), the factor multiple listed here should be the minimum value, too small outer diameter of the screw is easy to cause cracking; The depth factor is also the minimum requirement, which needs to match the diameter of the screw. If the screw’s screw teeth are not locked deep enough, external forces will easily pull them apart.

  • ① Screw hole needs to be designed guide hole (Counterbore) and locked with a straightening screw.

    ② The depth of the screw hole must be more profound than the length of the screw to accommodate the self-tapping Screw cutting pin.

    ③ Out of the plastic chips, to avoid the screw hole burst.

    ④ The choice of screws is also very important. If the depth of the screw hole is enough, choose the screw tooth distance.

    ⑤ a tiny screw can effectively prevent screw hole sliding teeth.

Materials Type Hole Factor Boss Factor Depth Factor
ABS 0.80 2.00 2.0
ABS/PC 0.80 2.00 2.0
ASA 0.78 2.00 2.0
PA 46 0.73 1.85 1.8
PA 46 GF 30% 0.78 1.85 1.8
PA 6 0.75 1.85 1.7
PA 6 GF 30% 0.82 2.00 1.8
PA 66 0.75 1.85 1.7
PA 66 GF 30% 0.82 2.00 1.8
PBT 0.75 1.85 1.7
PBT GF 30% 0.80 1.80 1.7
PC 0.85 2.50 2.2
PC GF 30% 0.85 2.50 2.0
PE-HD 0.75 1.80 1.8
PE-LD 0.75 1.80 1.8
PET 0.75 1.85 1.7
PET GF 30% 0.80 1.80 1.7
PMMA 0.85 2.00 2.0
POM 0.75 1.95 2.0
PP 0.70 2.00 2.0
PP TF 30% 0.72 2.00 2.0
PP 0.85 2.50 2.2
PS 0.80 2.00 2.0
PVC-U 0.80 2.00 2.0
SAN 0.77 2.00 1.9

This table is for reference only. In the end, the actual operation shall prevail.

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Advantages and disadvantages of self-tapping screw for plastic products

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.

First of all, let’s look at what is a self-tapping 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.

Below is a drill tail cutting self-tapping screw, its most gap in front of a similar shape and function of the milling cutter, convenient on the plastic hole cutting off redundant components, its thread is designed to be more just can put the wave out of plastic squeeze into the trough of the thread, the role of balance, also can let the screws and plastic closer together. Some self-tapping screw threads are designed to rotate to take out the plastic or wood chips that are pushed out so as to avoid excessive waste chips squeezing inside the screw hole of the locked part and causing the danger of local gouging or breakage.

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.

However, self-tapping screws also have many limitations and disadvantages, such as:

The locked parts are easy to bear certain stress due to the force of self-tapping screws and lead to the absence of cracking or rupture around the screw hole, thus affecting the reliability of the product. Because of this, the “self-tapping screw” locking place will require a certain area of flesh thickness to withstand the screw locking stress, such as plastic screw column outer hole diameter usually need 2.5 ~ 3.0 times larger than the inner hole diameter, different plastic materials (resin) need different minimum flesh thickness.

Some designers just blindly apply the previous design, regardless of the different plastic materials used, resulting in the thickness of some screw holes less than the safety factor should be.

Sometimes designers know meat thick design should put the screw holes in its size, but do not take into account the ejection Angle (draft Angle) influence on meat thick, and the stronger the screw column, the greater the ejection Angle influence on meat thick, because screw lives the inside diameter of ejection Angle just upside down, the farther, the less the rest of the meat is thick.

If the plastic injection molding factory may use regrind resin, the minimum flesh thickness needs to be increased. Many designers often wonder why the quality problem of screw crack still occurs after the mass production of plastic parts which have already passed rigorous design quality verification. In fact, a large part of this is because manufacturers add secondary recycling materials; Because of the trial production stage, injection molding manufacturers are afraid of problems and generally dare not use secondary materials, but they will secretly add them after mass production. If the safety factor of the original design is too small, as long as a little bit of secondary material will have the risk of rupture.

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