Inserts in plastic: "more than a hole for a screw"

Inserts for the assembly of plastic components

“The plastic housings and other products that we make at Pekago are often attached to other components. This often requires inserts”, Jan continued. “Our engineers consult with the customer to determine which inserts can best be used as early as the product design phase.

Compact injection moulding or thermoplastic foam casting

We look at the product’s wall thickness, and, of course, at the plastic from which it is made. For example, an ordinary insert in a TFC product can rotate easily as thermoplastic foam casting yields a more porous material than, for example, ordinary injection moulding. So for TFC products, inserts are often used that are fused with the plastic. We have, in principle, the most choice when it comes to inserts for injection moulding”.

Tensile force and repeated assembly

The amount of tensile force that an insert must be able to withstand is another criterion when choosing an insert. What forces will the product ultimately have to withstand after assembly? Is the product assembled just once, or must it be disassembled and reassembled on a regular basis? This places additional demands on, for example, the insert’s material and its assembly. Pekago uses three different methods to insert inserts into plastic. “For all inserts, the required hole diameter in the plastic is already taken into account during the (injection) moulding process - i.e. the recess in which the insert will be inserted. The size of the recess also depends on the insert to be used”, said Jan. 

Different attachment, different insert

According to Jan, Pekago primarily uses the following three methods for inserting inserts in plastic: mechanical fastening (self-tapping), fusion welding and ultrasonic welding. “Mechanical fastening is, as I said, not suitable for TFC products, but it does give the greatest tensile strength in other materials. With mechanical fastening, we use inserts made of stainless steel, steel and brass. With mechanical fastening, the insert is ‘simply’ screwed into the recess in the plastic. The insert is slightly larger than the hole diameter, which forces the thread on the outside of the insert to ‘cut’ (self-tap) itself a groove in the plastic”.

“With fusion welding, the insert is heated by a hot pin placed in the middle of the insert. In its turn, the insert then melts the plastic material in which it has been inserted. A melt-insert is provided with ridges; the insert is anchored in the plastic once the material has cooled down and hardened. Fusion welding takes more time than mechanical fastening, since the material has to be heated and then cooled down again. This, of course, affects the costs of inserting inserts in this way”.

Jan knows: “For TFC products, fusion welding is almost always chosen. However, these products must first be allowed to shrink and off-gas for 24-36 hours before inserting the inserts. After all, the plastic goes into the injection-moulding machine as a liquid, i.e. hot, and it will then shrink as it cools and the gasses that form escape from the plastic. If the inserts are already in the plastic during the shrinkage, the product may simply tear at the inserts. Of course, the long cooling period required before we can insert the inserts using fusion welding results in a longer production turnaround time”.

“Ultrasonic welding is used when the plastic is too heat-resistant to melt. For this purpose, the plastic product is placed in a mould especially developed for that product. Each product requires its own mould, because the tight specs determine how the insert will be placed. The insert is manually placed in the designated spot on the welding machine, after which the ultrasonic welder stamps the inserts and product.

High-frequency ultrasonic vibrations then anchor the insert into the plastic product. These inserts also have ridges. Ultrasonic welding is the quickest assembly method, but the mould required comes at a cost - these moulds are made using 3D printing or are milled from aluminium. Incidentally, ultrasonic welding makes a terrible noise! This is why this is done in a separate room and why hearing protection is always used”, added Jan.

The material used for the insert is also important

The material used for the inserts is also a point of attention and the choice depends on the desired method of assembly and the application requirements. “Pekago mainly works with stainless steel and brass inserts. Brass conducts heat much better than stainless steel. That is why a brass insert heats up and fuses faster. Nevertheless, stainless steel is often chosen when the final product will be exposed to moisture. Before we melt stainless steel inserts, we often first warm them on a hot plate. This gets the job done more quickly!

That's quite a bit of work for placing an insert in plastic. But we have been doing this for a very long time and very often. So surprises do not occur very often”.

Can our inserts ensure a better fastening of your plastic products? We would be only too happy to look into it with you. Call Irma de Graaf (+31 (0)13 5308077) or send an e-mail to make an appointment.