Insert Moulding, Overmoulding and 3D Printed Prototypes

Within this blog post we’ll have a look into both overmoulding and insert moulding as two modalities of injection moulding. Both processes are often named interchangeably due to their similarities, however, there’s a slight distinction that we’ll be clarifying.

From this, we will discuss how 3D printing can be used to give another solution. But, before let’s have a general overview of injection moulding.

What is Injection Moulding?

As one of the most important standard manufacturing methods, injection moulding revolutionized the plastics industry since its inception in 1872. Thanks to its capacity to mass-produce inexpensive plastic parts, most of the everyday products surrounding us today come in part from an injection mould.

The working principle is pretty simple to understand, resin pellets get pushed and melted through a heating cylinder by a reciprocating screw and into the mould. However, the engineering behind this is not that simple. Many important design details like fluid mechanics, screw geometry, draft angles, parting lines, heating and cooling cycles, tooling design, among many other considerations make injection moulding a costly but powerful process to implement.

Insert Moulding

Now that we have a grip on what an injection moulding process can do, the next step is to go beyond and into insert moulding. This form of injection moulding allows the insertion of a non-plastic part made through a completely different manufacturing method. Ideal for metal parts that require a fixed plastic encapsulation. Inserts must be fixed to the cavity during the moulding process, commonly the insert design requires slots and threads to achieve a firm grip.

Applications

It is common to find it in countless consumer products and other industries, the following are the most common:

• Grips: Handheld devices like screwdrivers and knifes, lever handles, screw head covers, and knobs.
• Electrical: Insulation is very important for any industry implementing electrical components into their designs. This process is ideal for wiring, power plugs, and waterproofing circuits like PCBs. In the case of the medical industry, it’s of common use for easing sterilization for defibrillators and many implants with electronic components. For the automotive and aerospace sectors, many parts need this, it’s even a requirement to achieve tight enclosures for some motors and batteries
• Piping: Performs great as insulation material.
• Flexible couplings.
• Threaded metal inserts in plastic parts.

Overmoulding

Some products require a combination of plastics and this is of course the best process for fusing them into one part. So, overmoulding is the process in which a part goes through many consecutive moulding steps to get the final result. It can either be a manual process or integrated into the same machine for a smoother automated workflow.

Applications

Ideal for combining a rigid base with rubbery parts to get a softer and slippery-free grasp.

• Consumer products: Toothbrushes are a clear example, also products that require multicolored plastic parts.
• Medical tubing.
• Multilayer joints: For optimal relation between stiffness and flexibility.
• Phone cases.

Breaking It Down

Materials

One of the best advantages of injection molding is its compatibility with a wide range of thermoplastics, thermosets, and elastomers. The most important consideration to take regarding materials is that the first material must handle the heat from the second step of overmoulding. Some common plastics for this process are:

• ABS (Acrylonitrile Butadiene Styrene)
• HDPE (High-density Polyethylene)
• PET (Polyethylene Terephthalate)
• PEEK (Polyether Ether Ketone)
• Nylon (Polyamide)
• PC (Polycarbonate)
• PE (Polyethylene)
• PMMA (Acrylic)
• PP (Polypropylene)
• Silicone
• TPU (Thermoplastic Polyurethane)
• TPR (Thermoplastic Rubber)

Costs and Efficiency

Undoubtedly, injection moulding is the best choice for high volume productions. Even though it comprises high initial and tooling costs, it greatly excels at speed and efficiency once production is set in motion.

It is still the case when involving both insert and overmoulding. Having a machine with multi-injection capabilities will inevitably increase costs even more, but at the same time, it allows the production team to skip assembly costs significantly.

Last but not least automation is key, thankfully all these processes are automatable. If you take into account the extra step of placing the inserts manually, there’s an extra cost. So the same principle applies to automation, higher efficiency at the cost of implementation.

Quality

As we stated before, injection moulding can produce strong and homogeneous parts with tightly embedded metal parts or overmoulded plastics. This allows engineers to make parts with exceptional combinations of materials. For instance, the possibility to combine the strength of steel with lightweight and flexible plastics becomes feasible. Nevertheless, there are still some downsides. If factors like heating cycles and resin flows aren’t well-tuned, there could be some quality issues like knit lines, cracking issues, and improper merging between materials.

Design Freedom

Having the possibility to insert or overmould a part is freedom of design in and of itself. Even when there are conditions in order for the insertion to work properly, design limitations are still an issue of injection moulding conditions. There are two main limitations: tooling costs and geometry. First, having high tooling costs turns it unsustainable for low volume productions, prototyping, and customization. Secondly, there are geometrical limitations like having to add drafting angles in the design, unavoidable parting lines, sprues, and wall thickness uniformity.

3D Printing as a Revolutionary Prototyping Solution

To understand how 3D printing is revolutionizing pre-production cycles efficiency, we must first perceive what tooling costs imply for a process like injection moulding.

For every part design, there must be also a  tooling implementation process, which for injection moulding can cost thousands of dollars and many weeks of work. And this is only referring to the final result, prior to that there are many prototyping cycles to check for necessary design changes. That also includes tool testing and calibrations, which also is time-consuming and adds significantly to costs.

As 3D printing technology evolves, prototyping cycles become much more affordable and faster. To better illustrate how much this applies to insert and overmoulding prototyping cycles, let’s have view on some successful methods:

Part Prototypes

One of the strongest advantages of 3D printing is its affordability regarding rapid prototyping capabilities. But what about printing a prototype that emulates overmoulded characteristics? Many industries in the last years have been relying on multi-material printing.

An ideal technology for best results is material jetting, thanks to its capability of printing simultaneous materials per part and how it can simulate properties and smooth surfaces like moulded parts, in some cases even low production end-use parts come out of this. Besides the product itself, printing accurate test tools are something that Stratasys has been doing for many years. But, leaving industrial giants aside, there are even more affordable desktop prototype solutions like the dual extrusion system that Ultimaker FDM printers offer. Ultimaker professional printer’s versatility is a very strong option for printing multi-material prototypes with smooth resolutions.

As for SLA solutions, engineers from the Dame startup company came up with a simple yet powerful methodology. Their workbench prototype consists of encapsulating a vibration device inside a silicone overmould. They did this by printing the sections of the mould with a Formlabs SLA printer while leaving a gap for injecting liquid silicone with an epoxy gun, the final result is a truly stimulating, affordable, and accurate functional prototype.

Surrogate Inserts

Another interesting prototyping solution is the methodology Google ATAP applied for their handheld electronic device. Their design basically consists of a PCB assembly encapsulated inside a TPU overmould. While testing the manufacturing process, they came up with some costly issues:

1. The high costs of losing the inserts to tests.
2. The complex supply line, where manufacturing and assembling the PCBs was time-consuming.

They came up with the ingenious idea of printing a surrogate insert with a Formlabs SLA printer. This simple and elegant solution saved them around 100000$ and 3 weeks of development.

Solid Print3D is here to help you make the right decision with your next 3D Printer purchase. For more information, please call Solid Print3D at 01926 333 777 or email at info@solidprint3d.co.uk