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Injection Moulding Design for Assembly - Part 2/2

When designing plastic injection moulded parts to be used in conjunction with other parts, the assembly must be considered. There are several assembly techniques, mechanical fastening, press fit, hot staking, snap fit, welding, solvent bonding and adhesive bonding. We’ve cover some of these in part one of this article, read it here. In this article, we’ll be looking at welding, solvent bonding and adhesive bonding.


There are eight welding techniques that are worth mentioning in this article, we have identified these below:


  1. Ultrasonic plastic welding

This is achieved by converting high-frequency electrical energy to high-frequency mechanical motion. It is this mechanical motion and applied friction that creates the heat at the plastic components joint area. This causes the plastic to melt and create a bond between the multiple parts. Ultrasonic welding is suitable for most thermoplastics and therefore a popular option.


  1. Electromagnetic Welding

More commonly known as the “Emabond Process”, it is a method that provides a simple, rapid and reliable assembly technique to produce structural, airtight or high-pressure welds on most thermoplastic materials. This method uses inductive energy to achieve fusion temperature which heats the electromagnetic sensitive material, causing the parts to soften and with the application of pressure, join together.


  1. Resistance Welding

This method follows the same process as above, apart from the electromagnetic material is replaced with an electrically conductive wire or braid. The wire or braid heats to soften the plastic on the areas to be joined which with applied pressure can be welded together. Resistance welding is best suited to large parts, being quick and simple, however the wire remains which in time may weaken the finished component.


  1. Vibration Welding

Vibration welding can be divided in two, linear motion and orbital motion, this leads to surface friction, resulting in heat generation and the formation of a melted layer at the joint. Vibration welding typically has weld times from one to five seconds and compared to ultrasonic welding, operates at much lower frequencies, higher amplitudes and with a much greater clamping force.


  1. Hot Plate Welding

Hot plate welding is a direct thermal welding technique that sees the facing surfaces of the two parts heated through conduction, convection and/or radiation from an actual heated platen. This technique allows a wide latitude in designing parts with complex, irregular shapes – the joints can be curved in all planes, and welding of internal walls is possible. This method can be used for parts of all sizes and suitable for almost any thermoplastic material. 


  1. Spin Welding

This process welds parts together by applying pressure in a circular, spinning motion, it is this rotational friction that generates the heat required to melt the plastic to join together. Durring this process, one part is held stationary while the other is rotated against it, it is this action that generates the friction and therefore the heat. Spin welding produces high quality permanent joints, no additional materials are required and it therefore presents a cost-effective option. It is worth noting that this method is most effective for joining parts with circular cross sections and can be suitable for larger parts.


  1. Hot Gas Welding

Hot gas welding uses a stream of heated gas, to heat and melt both the thermoplastic material and the thermoplastic welding rod. To ensure welding takes place, sufficient temperature and pressure must be applied to the rod, along with the use of the correct welding speed and gun position. The quality of the weld is therefore dependent on the skill of the individual welder.


  1. Extrusion Welding

Extrusion welding is used for thick-sections, such as pipes, where it is required to produce large volume, regular seams in a single pass. This process has developed from hot gas welding and is the preferred technique for joining materials over 6 mm thick.


Solvent Bonding

Solvent bonding is typically used with unstructured materials and can only be used with certain types of thermoplastics. It is similar to the welding process, because the solvent softens the surfaces of the parts to be joined. At this point they are fastened together and the solvent evaporates.

Solvent bonding is a simple and relatively low-cost method but it does require the parts to be moulded to a high tolerance and not to be warped.  


Adhesive Bonding

There are many advantages to using adhesives:

  • It enables the joining of different material types
  • It provides design flexibility
  • Adhesives can give a tight seal
  • They can provide electrical and thermal insulation
  • Adhesives deliver consistent stress distribution


However, there are also disadvantages:

  • Adhesives are still susceptible to the environment (temperature, moisture etc)
  • Adhesives will weaken over time
  • Adhesives take longer to bond and reach full strength compared to other methods detailed in these articles.



Over the last two articles we’ve covered how the assembly of parts should be considered when designing for plastic injection moulding. We’ve detailed the various options when the assembly of parts is required and how these differ and offer solutions for more complex components. We advise clients to consider the assembly of a product at the point of design rather than further down the process to ensure all considerations have been made. It is far more cost effective to make changes at the point of design, which is why we take this approach. Speak to a member of our design team today and discuss your project with a specialist.


Get in touch today and talk to the specialists, click here.

Our team will be more than happy to help, advise and quote for your requirements.

Looking for more information? Check out our blog, News & Views for useful articles, tips and tricks on plastic injection moulding.