In short, mold making is the most expensive part of plastic injection molding. Therefore, for particularly complex parts, it is sometimes worthwhile to build a prototype for the tool, which can be used to test functionality or produce a small series. Prototype tools are usually made of aluminum, but can also come from 3D printing of liquid metals.
Injection Molding Tool
For 150 years, plastic injection molding has been produced in essentially the same way. By heating, the liquefied plastic is injected into a hollow mold, through a piston or today a screw. A hollow mold consists of two parts and is similar in construction to a mold. It gives the plastic its planned shape. Once the plastic has reached this shape and the right temperature, the upper part opens and the molded part, called injection molding, is ejected. It is now ready to be shipped or further processed. In plastic injection molding, the hollow mold is called the tool and the two halves are called the nozzle side and the ejector side. The nozzle side is the non-movable part. The nozzle side and ejector side consist of several plates. On the nozzle side, these are, for example, the machine cleats to which the actual mold and its cleats and mold plates are attached. As on the ejector side, the cavities and inserts that give the molded part its actual shape are incorporated into the nozzle side.
In addition, this site contains a centering ring through which the nozzle is precisely pressed against the closed mold and the so-called gate sleeve, allowing the injection of plastic. The ejector side also contains cavities and inserts corresponding to it. The ejector plate also contains the ejector pin and other elements with which the molded part is demolded, i.e. ejected from the mold. This can be done in many different ways, depending on the mold design and production. A mold can simply be peeled off with a ram or pushed through with pliers. Sometimes it can only be unscrewed or pushed open with a so-called collapsible core. Does this sound complicated? Let’s wait and see, because now different temperature ranges are also needed to keep the molded parts stable. Depending on the plastic, cooling or a certain, consistently higher temperature may be required during ejection.
The More Complex the Part, the More Expensive the Mold
However, this description only applies to injection molded parts that are really simple to mold. But depending on the intended use, molds become arbitrarily complex. For example, how do you create cavities in a component that has no connection to the outside? How do you put together different kinds of plastics with very different properties and temperature ranges without glue? However, the many very different requirements placed on production tools will be beyond the scope of this article. In general, it can be said that standard molds for tools are made of stainless steel, which has minimal effect on the different plastics. Within the standard mold, interchangeable molds can be added, each on the nozzle and injector side of the standard mold only. These interchangeable molds can be used in production, for example, for producing small batches of molds that would be too costly to make yourself.
This is because the molds not only have to be manufactured very precisely and consistently so that the complex opening and closing processes and very high pressures during production can actually withstand thousands of cycles without problems. In addition, there are sometimes extremely difficult internal structures or almost inlays whose complexity can sometimes be compared to the construction of a mechanical clockwork. In such calculations, 3D CAD is used and all processes can also be tested virtually. If tool making seems so complex, how powerful and sophisticated must the tools use to perform it be? Stamping tools or five-axis milling machines that machine hardened steel to one-hundredth of a millimeter accuracy are not exactly cheap.
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Aluminum for Injection Molding
On the other hand, much cheaper is the machining of aluminum. Aluminum is a high-quality material that is easier to machine, but not as durable and stable as hardened tool steel. Therefore, injection-molded tools made from aluminum, for example, are manufactured with a high-speed cutting process for low-volume production, or accurately as prototype tools for testing before “real” tools are made from hardened steel. But this is by no means the end of the use of the term prototype tool. The term, the prototype is also used quite differently in plastic injection molding.
It is very interesting to produce prototype tools through additive manufacturing in 3D printing. For some time now, the production of very small series of plastic parts in 3D printers has been more cost-effective and does not require any tooling, and now the injection molded tools themselves can be produced in this way. Here, the metal used is first in liquid form and then hardened. Of course, the stability required for usual production is not achieved here, but for a few hundred simples, small ABM molded parts, such a long service life is simply not required.
Prototype – It’s Always a Matter of Definition
Tools are often called prototypes if they consist only of inserts, which, as mentioned earlier, are built into larger standard tools. They are produced quickly and can be mounted and dismounted again on order, and are therefore used, for example, for the production of small batches that require longer intervals. Other injection molding specialists focus entirely on the production of aluminum molds. They determine the difference between a prototype and a production tool depending on the customer’s requirements for molded parts or injection molding. For example, a prototype mold may be a component that is initially used to inject mold with only a few parts.
However, due to a sudden order change, as many molds as possible must now be delivered. The prototype must be used until the actual production mold is completed. This can take weeks or months. Prototype tools can now be used as production tools. However, additional work is now required that was not previously needed. This includes the measurement of many parts during the injection molding process, extended quality management checks, and the creation of documentation to prove the quality management process. So, in effect, prototype tools will, by definition, become production tools overnight, so to speak.
TX MOLD – Over 30 Years of Injection Molding Processes
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