Which systems to use for the assembly of the plastic components produced

There are plastic products that are printed or extruded individually and assembled together in successive phases, with the aim of creating a finished product composed of several parts.

The activity of assembling the various pieces, and their tightening, involves an in-depth analysis of which closing tools to use, to be compatible with plastic materials used and also functional with the use of the finished product.

The main fixing systems of the plastic components can be grouped into:

• Fixing with plastic screws

• Fixing with metal screws

• Fastening through nailing

• Fixing by welding

• Fixing through compression

Plastic Screws

The fixing of the elements to be assembled through the use of plastic screws has a limited use, as they express low mechanical strength and low rigidity.

Despite their low structural performance, plastic screws find great use in those products where continuous electrical insulation, very high resistance to corrosion and a continuity of the shades of the chosen color, to make the product chromatically more continuous.

Metal Screws

Fastening with metal screws is by far the most used method to assemble plastic elements, due to the excellent mechanical resistance and good grip between plastic and metal element.

The screws can be metric or self-tapping.

Those metrics, in some mechanical situations, may present some sagging of the flanks that are threaded into the plastic material, this can be caused in the presence of a low elastic modulus of the polymer that makes up the plastic element.

In fact, the mechanical resistance of a metric thread in plastic is generally limited, so it is advisable to use metric screws with internal threaded brass inserts.

These inserts are inserted, before molding in the mold itself or later through the use of ultrasound.

From an economic point of view it is almost never convenient to use this type of inserts, due to the waste of time in their positioning, however the use of a screw metrica makes subsequent product assembly faster.

The self-tapping ones are made up of different shapes and threads based on the mechanical work they have to perform and the type of plastic they will be inserted into.

The threads can be more or less close together, therefore with a number of different spirals, have inclination angles of the spirals from 30 to 60° and a diameter of soul of the vine and its variable spiral.

In the case, for example, of products made with thermosetting resins, it is absolutely necessary to use self-tapping screws, in fact this type of polymer does not conform, like other plastics , to the screw, but is drilled with the removal of the resulting chip.

In this case, an asymmetrical screw profile with a 30 ° spiral angle is recommended.

Nailing

Another type of assembly of plastic components can take place with the method of nailing the elements.

The moldable nails used are generally composed of brass, copper, aluminum or with hollow nails to be turned.

If we use nails to be turned, the impulse of the beating must always take into account the resistance to breaking and cracking of the plastic polymer we are working on, having the foresight to calculate the ratio between mass and typing speed well.

If the elements to be assembled are made of thermoplastic material, the end of the core of the product can be finished hot or cold on the nail head.

Welding

Thermoplastic materials can also be assembled through the welding process with friction methods, with ultrasound or with hot tools.

It must always be taken into consideration that the welding point or the welded ends, if continuous, will never have a mechanical strength comparable to the base element. Furthermore, the welding phases can create internal tensions with respect to the molecules of which the polymer is made and, when present, interact negatively on the reinforcing fibers.

Generally speaking, based on the plastics and the type of weld, experience in the field tells us that the mechanical strength of a weld can be lower between 40 to 80% of the original plastic material.

This weakening is also accentuated if the product has to withstand high loads over time or withstand particular dynamic stresses or chemical attacks on the plastic material.

Assembly by compression

The assembly system by compression of the elements is the cheapest and fastest, however some important considerations must be made.

If there are situations of interlocking assembly between plastic and metal elements, it is a good rule to plan the calculation of the compressive stresses calibrated on plastic materials and not on metal ones.

Furthermore, when the elements will be in operation, for example the parts of a fan, it is necessary to keep in mind the greater thermal expansion of the plastic compared to the metal elements and, in the case of hygroscopic polymers, also the possible swelling.

Automatic translation. We apologize for any inaccuracies. Original article in Italian.