Injection Moulding Design for Assembly - Part 1/2

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’ll cover each of these in more detail over the next two articles and how they fit into the design process.

 
When designing for plastic injection moulding, the aim is to reduce the overall number of parts for any given product. There are several advantages to this aim, not only will it reduce costs but it will make assembly easier. When designing, designers should lookout for any opportunities to simply the assembly process through a more efficient design.

Mechanical Fastening

Types of mechanical fasteners include:

• self-tapping screws
• clips
• studs
• bolts
• rivets
• press-in & mould-over inserts

When choosing fasteners consider:

• Screws with a suitable shaft-to-head diameter ratio and a head style that is designed for automated feed during assembly.
• Screws that have washers fixed under the head to avoid dealing with loose washers
• Self-tapping screws to skip unnecessary assembly time

 It is worth noting that using mechanical fastenings can add to the assembly costs but also to the disassembling for repairs or end-of-life recycling if permanent. Other considerations should be how the fastening will be inserted into the part, will this cause stress within the moulding and lead to product inefficiency. When designing using metal fastenings remember the differences in material properties when exposed to various environmental changes, such as temperature or moisture. 

Press Fit

Press fits rely on continuous stress and friction to work. In metal, if you press an oversized pin into a hole, they’ll stay together indefinitely, but plastic on plastic will not perform under constant strain and eventually the stress and thus the friction holding the parts together will disappear. It is worth noting that metal to plastic parts can provide a high strength joint.

It is crucial when designing plastic parts using press fits to ensure the degree of interference between the two parts is 100% accurate. This type of assembly is only an option where the required tolerances can be achieved and more importantly maintained over time. Press fits are usually the best option to gain maximum pullout force on plastic parts.

Snap Fit

Snap joints are a very simple, economical and rapid way of joining two different components. Snap fits work by using an undercut in one piece and on the other, a lip or hook which “snaps” into the undercut. Although assembly costs are relatively low, mold costs for snap-fit parts may be higher because of the complicated cross sections required.

After joining the parts together, the snap-fit features should return to a stress-free condition. The joint may be separable or inseparable depending on the shape of the undercut; the force required to separate the components varies greatly according to the design.

It is particularly important when designing to consider the mechanical load and force required during assembly. Snap-fit joints that are intended for automated assembly and not hand assembly should join with a single unidirectional motion. For hand assemblies, designs should be kept simple to avoid complex hand motions or more than two hands to lock or unlock joint.

  There are three types of snap fit:

• Cantilever Snap Joint – These are spring applications which are subjected to high bending stress during assembly.

• Cylindrical Snap Joint - These can be designed to separate with ease, with difficulty or not at all, depending on the shaft lead and hub return angle.

• Spherical Snap Joint – These are much stiffer structures than cantilever snap fits and a good alternative to the cylindrical snap fit. This type of snap fit is not suited to stiff materials as it relies on the expansion of the counterpart.

Hot Staking

Heat staking is another option at the point of assembly to permanently fasten plastic components together. The design will include holes that have been created during the plastic injection moulding process and using a heat stake device, heat is applied to soften the plastic allowing the fixture to be inserted and remain in place.

Benefits to hot staking:

• Low cost option – this method is part of the moulding process and therefore adds little to no additional cost to the productions
• No fasteners required
• Recycling Friendly – no foreign materials so much easier to recycle whole

When designing be sure to include multiple stakes for assembly rather than a few large stakes, this will deliver better results.

In our next article (part 2) we’ll discuss welding, solvent bonding and adhesive bonding, detailing what they are, how they work and how to incorporate these methods into your design.