Cars, refrigerators, portable video games and submarines all have two things in common. One: Each contains electrical and electronic devices that are essential to keeping it working properly. Two: Each is exposed to a wide variety of environmental rigors that can disturb this essential, intricate electrical configuration, causing inconvenient and dangerous problems to arise.

Exposure to water, dust, oil, chemicals, extreme temperature changes, and hostile jostling can damage circuitry, threatening malfunction. This problem has lead manufacturers to search for ways to seal electrical/electronic components from these harmful elements to help ensure the functional reliability and stability of their products.

Upper and lower parts of an aluminum mold used for low-pressure injection overmolding.

Traditional methods of attaining a defensive environmental barrier include potting (the encapsulating of an electronic component in a protective shell-like housing with an epoxy or urethane filler), and conformal coating (the painting or spraying of protective chemicals onto the surface of an electronic component).

These methods, however, can be costly and wasteful, and ultimately, do not provide the type of sustainable protection desired by manufacturers and consumers alike. The costs and time associated with tooling, manufacturing and assembly of either injection-molded cast housing or conformal coating can run high, and in some cases even exceed the cost of the components these processes are applied to.

Several assembly steps are involved in the potting process, which lead to added assembly line length and higher equipment and labor costs. The process for conformal coating can produce toxic fumes, causing possible work place environmental hazards, and may require long cure times, thus dragging out the assembly process. When these processes are applied to particularly small components, the quantity of sealing materials wasted in the process is often greater than what is used in the application. In many cases the value of wasted material is greater than the value of the finished component.

Even if the chemicals involved in the process are properly mixed and the component survives the application of the protective seal undamaged, the protection provided is not necessarily adequate to resist day to day wear. If the components get wet enough, or if they are shaken hard enough, the protective barrier can be breached. The result is the inevitable malfunction and failure of the essential electrical/electronic component.

There is, however, an alternative to potting and conformal coating. Low-pressure overmolding is a more cost effective protection method that involves fewer assembly requirements, is safe and easy to implement, and provides a a level of protection that is virtually impervious to environmental stresses.

Low-pressure injection overmolding was initially developed for the automotive industry in Europe in the late 1980s. The process embeds, or molds, and seals an existing part with molten thermoplastic polyamide hotmelt. There are four parts critical to the process: the material, the mold, the machine, and the application technology.

The thermoplastic hotmelts used are flame-resistant materials based on adhesive polyamides. As a one-part material, it eliminates the need for chemical mixing. The thermoplastic material is supplied and stored in solid form, either as beads or flakes, unlike the chemicals used in potting and conformal coating that are liquid and often toxic. This eliminates any concerns about toxic spills creating environmental or safety hazards.

Moreover, when the hotmelt materials are used, no chemical reactions take place during application, and no solvents are released. The materials produce no toxic fumes either in storage or during the short cooling/curing process. And, as an added environmental benefit, the thermoplastics can be produced from renewable raw materials.

Thermoplastics cure by air cooling at room temperature in a matter of seconds. Therefore, there is no need for waiting between the application of the material and the next step in the product assembly. The materials adhere tenaciously to most substrates, including PVC coated wires and PCBs. Material grades are available for a variety of applications.

The mold sets used in the low-pressure injection overmolding process can be made of either aluminum or steel. The use of aluminum molds offer excellent heat transfer for short cycle times, and steel may be used where durability over extremely long runs could be necessary.

The process begins with the melting of the thermoplastic. The thermoplastic is melted in a tank at 370 DegF to 450 DegF, then fed by a gear pump or rod system to an injector head. The device builds up pressure, typically regulated by a bypass, from 20 psi to 200 psi. A nozzle, located at the injector head, is opened and closed by a timer, allowing the thermoplastic to flow into the mold, encasing the electrical/electronic component in the protective thermoplastic.

Most application pressures range from 80-100 psi. However, the low viscosity of the molten thermoplastic, 1,000 cP to 10,000 cP, makes it possible to mold fragile electronic components at pressures as low as 20 psi. These types of components could be easily damaged by the high pressures involved in traditional high-pressure injection molding.

Waste produced by the low-pressure overmolding process is miniscule in comparison to traditional encapsulation methods, as the amount of purging under the lower pressure application is kept to a minimum. In addition, the process eliminates the need for shell casing, while retaining the capacity to add legs or grommets to the finished form for attachment purposes.

The total cycle time (injection and cooling/curing time) is completed in less than 60 sec. for approximately 90 percent of all applications. This production rate far exceeds that of the traditional protective processes such as urethane or epoxy potting that can take up to 24 hours.

In terms of effectiveness, the barrier created by low-pressure overmolding is virtually impervious. In addition to being water tight and dust tight, it has an extremely high resistance to most chemicals and hydraulic fluids. And because the thermoplastic used is also an electrical insulator, the molding also provides ESD protection for electronics, in some cases eliminating the need for anti-static packing materials.

The hardness of the encasing material also protects components from vigorous mechanical vibration. And the molding possesses high-temperature resistance and low flammability, helping to maintain the functional integrity of the component in a range of temperatures from –40 DegF to 320 DegF.

Low-pressure injection overmolding can be used to protect sensors, motors, switches, fiber optic components, connectors, PCB’s, relays, electrical components, and fuel cells and is ideal for temperature-sensitive parts and complex shaped parts. The process has been successfully implemented by companies in a number of industries, including appliance, automotive, electronics, marine, medical, and telecommunication.