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Combine Multiple Components in a Single Part with Plastic Insert Injection Molding
Historically, the only known benefit of molding parts together, as opposed to assembling them, was that the parts would stay together better.
Although that is very true, the evolution of injection moldable materials has made it possible to apply the insert molding technology in many new ways.
Inserts once were limited to simple things such as pins, or threaded fasteners. Today, an insert is considered anything that can withstand the injection molding process.
In most cases, inserts are conductive metals such as copper, brass, or steel, and usually in the form of metal stampings, screw machined and cold headed parts, wound coils, and wires. Also, items made of ceramic, die castings, and even molded parts are often used as inserts.
Components unique to their application can possess molded features. For example, a hypodermic needle may be an insert in a medical application, a flexible circuit could be overmolded for a printer component, or an electronic subcomponent can be converted to surface mount technology. New ideas for inserts emerge all the time.
The process itself begins with the same process used in injection molding. Solid pellets of raw material are melted and extruded into a mold, the plastic solidifies, the press opens, and the molded part is ejected.
Insert molding also uses the same materials used in the injection molding process. Specifically, thermoplastic resins that have been developed in recent years can be used in environments previously off limits.
For extreme applications, high heat engineering thermoplastics are used. These materials withstand temperatures beyond 600F, and display excellent physical, electrical, and chemical properties for use in harsh environments (i.e. engine components).
Mid-range materials come at a lower cost and are still suitable for some high heat applications (i.e. soldering processes, high current). And commodity resins (nylon, polypropylene, etc.) are now compounded in ways to satisfy tough requirements, while providing even more economy.
In insert molding, the insert(s) is placed into the mold before the raw material is injected. Then, as the molten plastic fills the mold, it flows into undercut features in the insert, such as holes, grooves, or bosses. The insert is anchored much more securely than if it were assembled to a previously molded component.
In the injection molding technology, the molding press brings two halves of a mold together in a horizontal direction. When the mold opens, the molded part is ejected, and it falls out of the mold. However, for the insert molding process, the horizontal press can be a disadvantage, since the inserts are prone to fall from of the mold before the press can close.
This fact prompted the development of the vertical clamp molding press. Using gravity, the vertical press helps the insert stay in position during mold closing.
Most vertical presses also incorporate a "shuttle" feature, which allows two bottom mold halves to be used with one top half. While one bottom half is molding with the top half, the other bottom is available to be loaded with inserts. Then, when the press opens, the bottom half that was just loaded with inserts shuttles into location with the top half, while the bottom that was just molding shuttles out for part ejection and insert reloading.
With a single top and bottom half, the operator waits when the mold is closed, and the mold waits when the inserts are being loaded. Since multiple bottoms allow the inserts to be loaded in one bottom while the other is molding, press time is reduced.
The same principle of multiple bottom mold halves applies to the vertical rotary press. Indexing in a rotary manner, (as opposed to shuttling) this equipment can use up to four bottom mold halves.
Insert molded components often serve as substrates that provide electrical circuitry through their inserts. The insert provides electrical interconnection, and can offer features such as clips, spring contacts, pin or blade terminals, contact or surface mount pads, rivets, threads, and so forth.
Presently, most insert molded components are designed to provide housings for electromechanical devices. Applications are found in the automotive, computer, consumer electronics, industrial, telecommunications, medical, defense, and various other industries.
Recently, insert molding has been used to combine multiple technologies in a component. One example is a component used in a computer disk drive.
Here, an injection molded component a trapezoidal wound coil, and a CNC machined metal component are molded within a single part This part resides in a high vibration environment where insert molding is used to insure the reliability of the component.
In a continuous strip form, the insert is transported to the molding process on a reel. In much the same way a reel-to-reel tape deck works, the strip comes off of the first reel, passes through the molding process, and is either taken up on a reel or blanked into individual components on the opposite side.
Molded parts left on the strip can be fed directly into additional automation processes. With parts blanked from the strip, the insert can be formed to provide various features.
For example, the parts can be blanked from the strip so that "legs" are formed down, or down and under the part. Legs formed down can be used for pin insertion, and down and under for surface mount applications. One molding system can provide both designs by using an interchangeable forming station.
Todays products continually strive to pack more functions in less space. This has created a need for components (such as three-dimensional circuit boards) that consolidate circuitry within a molded housing.
Since the circuitry and molded part are one, the component can rely on the combined strengths of the plastic and the insert for its structural integrity, and size reduction is obtained.
As a matter of fact an insert molded part requires no more space than an injection molded component without circuitry. These more streamlined, functional, and secure components also cost less than assemblies using molded components.
For example, an application that requires eight circuit paths can be produced by insert molding over a single copper stamping. Then, by blanking after molding, we separate the insert (within the molded part) into the eight circuit paths.
As opposed to assembling the circuits individually, we now provide eight circuit paths in two steps; insert mold then blank. Assembly operations are eliminated, along with their costs.
The size and cost of the component has been reduced, but an equally important benefit of the encapsulation process is increased quality. The insert is very secure. Proper circuitry is inherent since there are no wires to be crossed. And the oven molding process demands precise mold tooling, which insures the circuitry is accurate in its finished location.
The end result is a reduction in the size and cost of the product with an increase in quality.
The technology does have some disadvantages. First with few exceptions, mold and insert tooling costs begin at five thousand dollars and, depending upon complexity, a fully automated system can get into the hundreds of thousands of dollars. Often, minimum volumes of around five thousand annual parts are required to justify the expense of this tooling, and simple parts are likely to be more expensive in lower volumes than if assembled after molding.
Another area to be aware of is the ability of the insert to withstand the oven molding process. The injection temperatures and pressures of some materials can be extreme and can damage delicate inserts. However, there are design solutions that can shield the insert from the turbulence and heat while still providing the full benefits of the insert molding technology.
Finalizing the design of an insert molded product requires experience, but developing the concept for a component requires only a basic understanding of the principles of the molding and insert technologies. There are some fundamentals to keep in mind.
For example, try to provide some means of holding the insert during the oven molding process. Bosses or undercut features can provide additional retention strength in the molded part. When possible, inserts should be held to close tolerances. But bear in mind these are guidelines, not rules, and there are many solutions that can be used to overcome design blocks in these areas. Working with an insert molding specialist the design engineer can determine specifically which components or technologies can be combined into a single component. That is insert molding today.
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