4 Common Plastic Parts Assembly Methods

It’s a common sense that any of product is made from lots of parts or components, which can be divided into plastic, metal, and electronics. Of course, meanwhile plastic parts is the most essential component for nearly any of products. Therefore we would like to discuss the method of plastic parts assembly. If you are a plastic or molding industry related person, I think it will be helpful for you to understand how you can assemble a plastic part.

Picture of plastic parts assemblyMechanical assembly

Mechanical assembly is the most common and economical process to assemble plastic parts. As well it will be operated in a high efficiency. This is why it’s widely used in many industries. On the other hand, mechanical assembly is natural and environment-protective process compared to adhesive bonding.


According to our investigation, there are 60% products in the world that used screw fastening process. Since screw fasten is steady, adjustable, and recyclable when assembly a product. This is why you can see many factory pictures that shows their operators assemble products using screw. If you design a screw hole on a plastic part, you must make the hole diameter 1/4 smaller than the screw. Otherwise there will be an uneven screwing problem to happen.

Tight Fitting

Tight Fitting mean two features with protrusion and outstanding structure to match together, like holes and bosses, windows and inserts. The most important element for tight fitting is the clearance that how much two parts mesh together. If it’s too tight, that will be hard to assemble in. but it’s too loose, the two parts would not match together. So it requires the product designer have very rich experience and define the fitting tolerance appropriately.


Snap fits normally happens between clips and holes, covers and housings. It particularly requires a back-stop feature to fasten two features together. This is the most easy assembly process for workers to operate. But the risk of snap-fits is the failure of clips that might break. Normally snap fit is used for some small structures that requires less torque or strength. For example, mouse, PC frame, monitors, etc.

Ultrasonic welding

Ultrasonic welding is a quick, efficient, green to environment assembly process, it’s very popular assembly process to get two plastic parts assembled, it uses a high frequency vibrations waves to apply onto two parts by a vibrating device, then a part of material from one part will be melted and welded onto another part.  After few seconds, the melted material will be solidified bond the parts tightly.

When design a ultrasonic welding process, there is at least one ultrasonic welding line needed, which will be used to melt to other part. The weight and high of the welding line depends on melting material, structure, and use environment.

Metal Molded Fastening

When use necessary, the designer might use a metal part to mold into a plastic part when plastic injection molding process. This is to enhance the strength of plastic parts while the part works. Normally, we combine metal screw, metal sheet, bearing, support bar, and metal shaft for onto a plastic parts. But the efficiency of metal molded fastening is low, since it takes time to insert metal on a plastic injection mold when injection molding process. Certainly the cost of metal molded parts is a little bit high.

Adhesive Bonding

Adhesive bonding used for two materials are not compatible  or a permanent assembly requireemnt is needed, normally it happens between two plastic parts, metal and plastic part. It’s very quick assembly process, but one big disadvantage of adhesive bonding is contaminating, even some kind of glue is poisoning to our body. Therefore less and less process use adhesive bonding.


The above mentioned methods is the most common plastic assembly process, we hope it’s helpful for you. Once you still have any question or plastic project to run, please just feel free to contact Inno Molding company for help. and we will give you professional assistance.