What are plastic assembly techniques?

When designing a product, you naturally want to avoid too many parts that need assembling. At the very least, you try to minimise the parts needed, which can save costs. First, you need to consider several factors to produce a cost-effective product that performs at the level it needs to.

Material

Before deciding on your plastic materials, you need to think about their performance characteristics. Will your parts be under constant load? What chemicals, if any, will come into contact with them? If the parts are made of different materials, how do those materials work together? This is where working with an experienced moulder can help. A good one will know which plastics will work best for your application.

Cross sections

Design your cross sections as thinly and uniformly as you can. You can avoid cross-section thickness by using ribs, which will give you rigidity and strength. Ribs can help, too, when you must have a thick cross-section, as they can minimise the distortion that will invariably occur. However, if you’re using complex shapes that rely on both thick and thin cross sections, you'll need to get a handle on dimensional stability and tolerance changes, as these will happen during and after the moulding process.

Shrink

Perhaps this should come under ‘Cross sections’, but it’s worth noting on its own. Materials shrink at different rates, which impacts part integrity. For instance, thermoplastics are heated at high temperatures and injected into the mould cavity. As it cools, it shrinks. Thick cross sections cool at a lower rate than thin cross sections. Keep in mind that press cycle time depends on the cooling rate of the thickest cross section. This could reduce the number of parts you’re able to produce per part, driving up your costs.

The different rate of cooling between thick and thin cross sections is likely to cause distortion of the part after removal from the mould – a distortion that means the part will fail to meet specifications.

Assembly

How will you actually assemble the part? You have lots of choice here but in general, they fall into three categories:

• Mechanical: Press Fit and Snap Fit are two examples. If a product has to be dissembled at some point in its life – think of a removeable battery pack, for example – then this is the perfect choice. Manufacturers tend to use this method for low-volume applications when the cost of capital equipment is more than the cost of the consumable.
• Adhesives : When applications contain materials that are not compatible, adhesives are usually the go-to method for a permanent solution. You’ll need a dispensing machine and most likely a device to hold the parts together as the adhesive solidifies.
• Friction and heat methods: Ideal when you need a permanent or tamper-proof seal. Methods include Ultrasonic Assembly, Vibration Welding and Spin Welding. As none of these use consumables, the only costs you’re looking at is the initial equipment investment and the electricity needed to operate them.

There are advantages and limitations to every assembly method. Not every method is right for every project, so first define your application. Think of the properties and necessary requirements, from geometry to end-user needs. From there, you’ll find the right assembly method for your needs.