DFMA - Types of Plastics Joining Methods

Plastic materials are increasingly being used in many important applications, including packaging, building, appliance, electronics, automotive, aerospace, and more. Plastics already have a range of applications far exceeding any other class of material available. Joining is a critical step in manufacturing components from polymers and plastics composites. As the requirements for the component increase, so do the requirements for joining, especially in structural applications. 

Why do we require the Plastic Joining Process? 

Whenever the finished assembly is too large or complex to mould into one piece, we need to join the components to make it functional. Also, breaking down the bigger components into smaller ones will help reduce the production costs; joining will be cheaper than creating the big tool moulds. Thus with functional aspects of critical components, we need to join processes for Plastics.

Plastic joining is the method of joining semi-finished products of plastic materials together or with other materials as a fabrication process or damage repair. Joining methods can be classified into three categories:

• Mechanical fastening, 
• Adhesive and solvent bonding,
• Welding. 

There are various welding methods. Among all these, welding is an effective method of permanently joining plastic components. By reviewing and comparing all commercially available welding processes, this article helps the reader select the best-suited process for the application and provides an understanding of the limitations and benefits of each process. The focus on practical aspects such as weldability and testing makes this article an indispensable tool for everyone involved in welding plastics. At the same time, it also conveys the basic theoretical principles of plastics welding, such as squeeze flow, molecular diffusion, and heat transfer mechanisms. 

 

 

Plastic is a common name for thermoplastics and thermosets. The name “plastics” refers to their easy processability and shaping (Greek: “plastein” means to form, to shape). Polymers are raw materials for plastics (thermoplastics and thermosets), elastomers, and many other products (thickeners, varnishes, binders, etc.). Therefore, plastic and polymer are not synonyms. 

Now we will see each method in detail for the joining process.

Mechanical Fastening: 

Mechanical fastening is used when precision bonding is not required, and it is the simplest way of joining plastics. Mechanical fastening is more suited for joining stronger plastics. Mechanical fastening involves joining plastics together with simple fasteners like latches and nails. Mechanical fastening presumes additional parts (fasteners) like polymeric or metallic screws, bolts, washers, and rivets, or it relies on integrated design elements such as snap-fit or press-fit joints. Mechanical fastening can be used to join both similar and dissimilar materials. 

For example, mechanical fastening is commonly used when joining plastic to metal, producing permanent joints or connections that can be opened and sealed again. 

The advantages of this approach are that no surface treatment is required, and the disassembly of the components for inspection and repair is straightforward. This method has some drawbacks, which cause stress concentration on materials, galvanic corrosion, mismatch due to thermal expansion coefficients, etc. The main limitations are increased weight, large stress concentrations around the fastener holes, and subsequent in-service corrosion problems. 

The typical mechanical fastening applications are in the aerospace, automotive, and construction industries.

 

 

Adhesive & Solvent Bonding:  

In adhesive bonding, an adhesive is placed between the parts to be bonded (adherents), where it serves as the material that joins the parts and transmits the load through the joint. In solvent bonding, applying a solvent at the bond line induces sufficient mobility for the polymer chains to interdiffusion. Because the solvent must strongly plasticize the polymer surface, this joining technique is primarily applied to glassy amorphous thermoplastics, such as polycarbonate (PC), acrylic (AK), and polystyrene (PS) resins. Solvent bonding involves coating plastics with a solvent and clamping them together. The solvent softens the plastics, and the plastics are bonded together when it evaporates. These techniques have found wide use by virtue of their low cost and adaptability to high-speed production. In addition, adhesive and solvent bonds provide a uniform distribution of stresses over the assembled areas and a high strength-to-weight ratio. Solvent bonding applies only to joining amorphous thermoplastics, whereas adhesive bonding can be used with almost all plastics.

Welding : 

In welding, also called fusion bonding, heat is applied to melt the polymeric material at contacting surfaces enabling intermolecular diffusion and polymeric chain entanglement processes to form a joint. The scope of this text is polymeric welding technology only. Welding largely eliminates these shortcomings, but its applications are restricted to thermoplastics. Welding is the most widely used process to join Plastics. The major benefit is to join dissimilar types of plastic grades together. 

Plastics like polypropylene that are difficult to weld are usually bonded by induction welding. Vibration welding is efficient but is often only employed when other bonding methods are impractical. Vibration welding involves joining two plastics and vibrating one of them. The vibrations create friction, which heats the plastics and welds them together. To perform an induction weld, plastics are placed around a metal object and run through a magnetic field, which causes the plastics to heat and weld together.

Now we will see some commonly used Joining methods in detail. The most common methods used are - 

         1, Ultrasonic Plastics Welding 

         2, Vibration Welding

         3, Rotation/Spin Welding 

1) Ultrasonic Plastics Welding:

This method uses the heat generated from high-frequency mechanical motion. It is accomplished by converting the high-frequency electrical energy into high-frequency mechanical motion. The mechanical motion along with the applied force creates the frictional heat at the mating surfaces of the plastic component. This results in melting the face material and forms the molecular bonds between parts. Look out for the attached image for more understanding of this principle. The horn is nothing but the upper part of the machine setup, also sometimes called a Booster.

 

 

In general, the frequency range is between 20 to 40 kHz. As the frequency range is of Ultrasonic bandwidth, this method is named Ultrasonic Welding. The image attached below shows the setup of Ultrasonic Welding. 

 

 

Suitable Plastics grades for this method - All Thermoplastics are eligible for this high-frequency friction method. Teflon with a low coefficient of viscosity is also suitable. 

DFMA for Ultrasonic Welding  Joining Method - 

• The surfaces of plastic joining should be free of warpages and distortion. 
• Step joints are always recommended to reduce the unwanted flash and to increase the joining strength. Even greater strength is achieved by the “Tongue & Groove joint.” 

 

 

 

•  Beads or narrow raised sections called energy indicators are kept on one of the surfaces of the workpiece for better performance during frictional heat.

• Butt joint with the bevel edge is always avoidable as it will create the very poor type of Weld along plastics joined together.

Applications of Ultrasonic Welding Method - 

1. In the automotive industry, it is used for fabricating Headlamp Parts, Dashboard, Buttons and switches, Fuel Filters, Seat Belt locks, etc. 
2. The packaging industry is also highly impacted by this method for creating Blisters, Pouches, Tubes, Storages Containers, etc.

Vibration Welding of Plastics:

Here in this method, the weld joint is produced at the interface of Plastics due to heat generated by high magnitude vibration (3-5 mm) at low frequency. The frequency range in this method is around 120 Hz. The melton material flows along with pressure and a weld is formed along with cooling. 

The direction of vibration is parallel to the plane of the joint. [Where the Direction of vibration is perpendicular in Ultrasonic Welding] 

Find the below-attached image to understand the axis of Vibration.

 

 

Suitable Plastics grades for this method - Most of the thermoplastics can be vibration welded. Only polymers with low frictional coefficients named Fluoropolymers are difficult for vibration weld.

Spin Welding Method:

If the line of joints is Circular between the two joining surfaces then the method is famous with the name Spin Weld Method.  There is no other significant difference between these two methods. Once the melting temperature of the Material is achieved then the rotations of parts are stopped to avoid extra damage. 

DFMA for Ultrasonic Welding and Spin Joining Method - 

• One part of the assembly must be free to move relative to the other in the plane of the weld in both vibration and spin welding. 
• Adding a flange [thickness (t) equal to 2 to 3 times the part thickness (w)] to the joining surface of the parts improves the rigidity that limits flexure, applies uniform pressure close to the weld joint and improves the strength of the weld joint. 
• Special joint designs are required to contain the flash that is squeezed to the outside of the part during the welding process

 

 

• A breakaway stud or socket can be incorporated into the part halves for easy assembly alignment. 

Applications of Ultrasonic Welding Method - 

1. It is widely used in connecting ventilation pipes of fuel tanks and assembling structural components. 
2. This method is mainly used for joining HDPE pipes and Polyethylene pipes. 

In this way, we have seen some of the different types of joining the Plastics and Covered some widely used methods of Plastic Joining and DFMA rules for them. Do watch out for our other articles to learn more about Plastic joining methods and their design constraints.