Insert moulding is a specialised type of plastic injection moulding process which is often used to reduce assembly steps, improve the strength of the part and to increase the cost-effectiveness of high-volume part production.
The insert moulding process involves a pre-formed component – known as an insert – that is put into the mould before the molten plastic resin is injected around it. The end result, once the plastic has cooled, is that the insert is embedded into the plastic, forming a single, integrated component.
Inserts can be made of metal, such as screws, pins, blades or bushings, they can also be made of ceramic, or plastic, or be electronic components. As a result of the flexibility of insert moulding in the plastic injection moulding process, it can be used for a variety of applications. Plastic injection moulding companies use insert moulding for the creation of components that include medical devices, consumer electronics, electrical connectors and automotive parts.
Although insert moulding services can vary between plastic injection moulders, here is a general step-by-step guide to the insert moulding process:
- Careful consideration of the design of the part and mould – the mould must be designed with insert placement and retention in mind. The mould should hold the insert securely during the injection of plastic resin, and core pins or magnets can be used if needed to hold inserts in place. Also, it is important that thermal expansion differences between the insert and part are taken into account, as well as the placement of the gate into the mould cavity from the runner system, as well as attention paid in the design of the mould to flow pattern.
- Insert preparation is key – the inserts must be free from oil, dust and rust before use in the insert moulding process. To improve adhesion and to reduce thermal shock, they may need pre-heating and application of a primer or bonding agent maybe required.
- Manual or automatic insert loading – depending on the application and volume levels, an operator may place the inserts in by hand, or more commonly for higher-volume production by robotic arms or pick-and-place units. Whichever method is used; the important thing is that the insert is correctly aligned and fully seated in the mould cavity before the plastic resin is injected.
- Mould closure and plastic injection – The mould needs to be clamped shut, with the inserts securely in place. This can be achieved via pins, vacuum suction, grooves, bonding agents, magnets etc. depending on the material being embedded. The moulten plastic is then injected into the mould cavity and flows around the insert, bonding it as it cools to the part. It is important at this point that the injection pressure, speed, temperature and cooling time are managed to optimise the long-term, robustness of the resulting component.
- Cooling and ejecting – The cooling time for the part will depend on its geometry, the material it is made from and if there is cooling in the mould tool. Once the moulded part is cool, the mould can be opened and the finished part ejected in such a way as to avoid damage to the insert and moulded part. It is at this point that it is important to check that a proper bonding and insert position has been achieved. This inspection also needs to take into account any warping, voids, flashing and looseness. Vision systems and AI can be implemented at this stage to improve the reliability of the moulding process depending on the volume of the parts.
- Finishing off – Once the finished part has been checked, secondary processes, such as overmoulding, assembly operations and testing can be done.
Common mistakes in insert moulding
Successful insert moulding begins with the mould’s design and takes into account how that design impacts the position of the insert, and the part’s filling, cooling, end functionality and look.
Some of the common insert moulding mistakes include:
- Insert misalignment through poor insertion into the mould or inconsistent placement – this can be overcome with precision location features, automated insert loading and moulds designed with tight insert seating tolerances.
- Weak insert and plastic bonding through contaminants, incompatible materials or cold insert temperatures – this can be resolved by using thoroughly cleaned inserts, compatible materials or adhesives/primers and preheated inserts.
- Uneven cooling due to metal inserts creating cracks or warping – the cooling system needs to be optimised around the insert area, cooling and pressure parameters adjusted, and the part’s design adjusted to distribute stress.
- Insert damage during the ejection process – the ejection system must be altered to avoid insert contact. Also, the bonding surface or mechanical interlock may need adjusting.
Insert moulding delivers a wide range of benefits in the plastic injection overmoulding process. From improved product strength, reliability and durability and labour and time cost savings, to reduced assembly steps and part size minimisation, insert moulding enables manufacturers to deliver complex and custom designs that have improved aesthetics.
Find out more about insert moulding and how OGM can help you achieve all these benefits and more – https://ogm.uk.com/injection-moulding-services/ or by contacting us directly – https://ogm.uk.com/contact-us/.