Low Pressure Overmoulding versus Potting or Encapsulation

In electronics, encapsulation or potting is the process of pouring liquid resin over electrical and electronic components, circuit boards and completed electronic assemblies for electrical insulation and to protect the product against thermal shock, vibration, moisture and corrosive substances.

The process typically consists of 7 -8 steps including a curing period. Depending on the product, higher temperatures can be used to reduce cure times; but this increases the risk of thermal damage. Potting compounds cure exothermically, and these cure reactions can generate heat to damage electronic components. It’s not uncommon, for example, for potting compounds to heat up by 200°C or higher, as they cure.

Aside from this risk of damage to components, potting can be a messy process that can have many other problems associated with it, not least of which is wastage!

A fast, cleaner and lower cost alternative to encapsulation or potting  is Low Pressure Overmoulding.

Low Pressure Overmoulding is an injection process where the PCB assembly or electronic part is placed in a specially manufactured aluminium mould tool, and completely surrounded with a liquid compound that effectively forms the housing for the device.

Low Pressure Overmoulding is a 3 step process rather than an 8 step process for potting. This removal of process steps reduces manufacturing time, labour and material costs and improves throughput, resulting in increased ROI. There is zero waste, scrap is often recyclable and the process is environmentally friendly, giving off no toxic fumes. The end result is a tamperproof and environmentally sealed product.

Here are answers to some of the most frequently asked questions about the overmoulding process:

How long is a typical injection cycle?
The moulding cycle is dependent on the size and material section thickness of the component. Typical cycle times range from 15 to 45 seconds. Multiple cavity mould-sets can be used to reduce the actual moulding drop rate.

What is a typical injection temperature for these materials?
Melt reservoir temperature ranges from 190 to 230°C (370 to 450ºF). However, depending on mould-set design, this would typically result in cavity temperatures of around half those figures, which then rapidly falls to half that again at completion and before ejection.

Do the moulds need to be heated?
No, mould-sets are typically thermally controlled, (cooled / heated) to anywhere from 15ºC to 35ºC. The Moldman® series of machines require no direct mould-set cooling since the mould platens have coolant circulating through them.

What is the lowest pressure that parts can be moulded?
2 Bar (30 psi) is typically the lowest practical pressure. For anything lower the runner and gate design become very critical. Injection pressures from 4 bar to 20 bar (60 psi to 300 psi) are typical.

How do you control the filling of the cavity?
The Moldman® machines offer programmable injection profiles. A typical profile would consist of filling 90% of the volume into the cavity at relatively high flow rate and then packing at low pressure. Packing pressures are typically from 50 to 150 psi and such pressures will not over-pack a cavity. Packing is critical to prevent sink and voids as the Technomelt® material will shrink when it transitions from liquid phase to a solid.

Are any special de-gating tools required?
No.

How are the moulds cooled?
The Moldman® machines have permanent coolant connections on the side of the machines. This feature allows you to run chilled or heated coolant through the mould platens. Mould-sets are then cooled via the liquid-cooled mould platens. Typical mould temperature is around 20°C (68°F).

How do you ensure sealing of the component?
Selecting the appropriate Technomelt® material best suited to the application is the key to sufficient sealing of the component. Typically, the best surface wetting of the component occurs with lower viscosity material at relatively high temperatures. Higher packing pressure may also help ensure surface wetting and sealing. The component design is also crucial. Low-Pressure moulding works best when encapsulating around a component versus filling into the housing.

Can a conventional heated screw and barrel type machine be used for moulding?
This has been accomplished a few times and unsuccessfully a lot of times. Technomelt® has very low viscosity at the correct processing temperatures compared to engineering plastics. This low viscosity can cause the material to bleed backwards through the screw and essentially glue it together. Moldman® machines use positive displacement gear pumps designed for lower viscosity fluids to inject the molten material into the cavities.

How much waste do you get with this process?
Typically the material in the runner and gate is waste. This can represent anything from a few percent to 35% for very small components. When runners can be recycled for less critical applications, the waste is zero. With hot runner equipment, most applications will have zero waste.

To summarise, LPO encapsulates the electronic assembly, but without the need for long set up times and the curing process and so provides a fast and cost-effective alternative to conventional encapsulation or potting.

Volumes in the 100’s, 1000’s and 10,000’s are handled easily and devices can rapidly be turned into robust products and shipped within minutes!

If you’d like to find out more about overmoulding visit www.overmould.com or contact Overmould today.

Download the Overmould low pressure overmoulding brochure.

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