Plastic moulding methods often require post processing or additional fastenings to assemble parts together once they have been moulded. The injection moulding process however, lends itself to the mass production of more detailed and complex shapes, allowing designers to incorporate fixing methods directly into the part itself.
Advantages: Reduces the required number of components and materials such as adhesives, therefore reducing assembly time and cost.
Disadvantages: Increased tool complexity and initial investment required.
A variety of snap hook designs exist that can be adapted to suit most applications. They can be permanent or removable depending on the shape of the hook and utilise the flexibility of the material to temporarily deform as the part is assembled. This allows assemblies to be constructed quickly and without need for jigs or adhesives.
By their nature snap hooks create an undercut which can require a side action in the mould for the parts to eject successfully, although this can be avoided by placing the hook along an edge or above a through hole.
When moulded into the part, hinges remove the need for many additional components by attaching two plastic parts together either directly or with a simple metal pin. These hinges can lock to each other permanently or with a removable clip. They can usually be moulded easily, with a tool that opens and closes in one direction, depending on the design of the part.
Plastics such as Polypropylene can incorporate living hinges where the material thickness is reduced to enable a single part to bend back on itself.
Tabs and pins, or a combination of both, create guides for cables and are particularly useful for organising complex assemblies. By including a few simple shapes, the need for additional cable ties or glands can be reduced and potentially removed altogether. These features can also provide built in cable retention for electrical products that are required to have cables fixed in place by market safety standards.
Screw bosses are commonly included in parts to provide a location for other fixings, often nuts and screws, to hold two components together securely. They can also provide a housing for thread forming or thread cutting screws, facilitating the use of self-tapping screws instead of a nut to simplify and speed up assembly time.
Bosses are not solely for use with screws. They can also help parts to locate in an assembly interlocking with a pin or a round snap hook that is built in to a corresponding part.
Inserts such as metal threads or alternative plastic parts can be placed into the tool between each cycle so that the plastic is then moulded around it. The insert will usually have undercuts or holes to fix it in place permanently. This slows the production cycle time for each component but allows parts to incorporate features that otherwise would not be possible. At Paul Norman Plastics’ UK based plastic moulding facility, this type of assembly is performed at the machine by an operator and as such moulded-in inserts are more suited to smaller volume production runs.
All the ideas that have been covered here can be achieved with some straightforward design alterations. Prior to investing in an injection mould tool, replacing fastenings by including built-in fixing methods has the potential to save considerable amounts of time and money in the long-term, particularly for larger production volumes.