Tool Design -Fusible core injection molding...a intro...

                                              Complicated parts with pronounced undercuts or those whose cavities must be molded with coresare in many cases manufactured by injection molding with lost metal (metal with low melting point) cores (below)..The fusible (lost, soluble) core injection molding process illustrated below produces single-piece, hollow parts with complex internal geometry. This process molds a core inside the plastic part. After the molding, the core will be physically melted or chemically dissolved, leaving its outer geometry as the internal shape of the plastic part.Strictly speaking the term "fusible core injection molding refers to the use of a fusible alloy as the core material; when the core material is made from a soluble plastic the process is known as soluble core injection molding or Fusible core injection moulding.

                                               This process competes against multi-shell technology . For producing parts with complex smooth internal geometries of high-dimension stability, with or without lost cores, a number of different processes have been developed since the end of the 1980s:

• Fusible-core technology (mental core)
• Soluble-core technology
• Salt-core technology
• Multi-shell technology

The aim is to produce injection moldings with lost cores that can be removed from the almost closed mould. The fusible core processes principally consist of three steps:

• A temporary metal core is cast
• It is inserted into an injection mould and over-molded with plastic
• The metal core is melted to remove it from plastic

The surface of the core forms the inner counter of the part. Typical parts for fusible-core molding are automotive intake manifolds or pump bodies.

An automated production line for, e.g., intake manifolds consist of the following components:

• Line for producing the metal cores
• Injection molding machine for over-molding the cores
• Melting unit
• Conveyors and handing system
• Washer system

    For producing relatively small parts, a simplified design is suitable . A separate core-melting station is not necessary. Melting a carried out by feeding the parts with core directly to the melting tank along with the casting melt. The disadvantage of the approximately tenfold melting time (from 3 to 5 to about 45 minutes) is offset by a larger tank volume

This process is often used for automotive parts, such as intake manifolds and brake housings, however it is also used for aerospace parts, plumbing parts, bicycle wheels, and footwear.

The most common molding materials are glass-filled nylon 6 and nylon 66. Other materials include unfilled nylons, polyphenylene sulfide, glass-filled polyaryletherketone (PAEK), glass-filled polypropylene (PP), rigid thermoplastic urethane, and elastomeric thermoplasticpolyurethane.

Process

The process consists of three major steps: casting or molding a core, inserting the core into the mold and shooting the mold, and finally removing the molding and melting out the core.

                                                  

Core

First, a core is molded or die cast in the shape of the cavity specified for the molded component. It can be made from a low melting point metal, such as a tin-bismuth alloy, or a polymer, such as a soluble acrylate. The polymer has approximately the same melting temperature as the alloy, 275 °F (135 °C), however the alloy ratios can be modified to alter the melting point. Another advantage to using a metal core is that multiple smaller cores can be cast with mating plugs and holes so they can be assembled into a final large core.

One key in casting metal cores is to make sure they do not contain any porosity as it will induce flaws into the molded part. In order to minimize porosity the metal may be gravity cast or the molding cavity may be pressurized. Another system slowly rocks the casting dies as the molding cavity fills to "shake" the air bubbles out.

The metal cores can be made from a number of low melting point alloys, with the most common being a mixture of 58% bismuth and 42% tin, which is used for molding nylon 66. One of the main reasons its used is because it expands as it cools which packs the mold well. Other alloys include tin-lead-silver alloys and tin-lead-antimony alloys. Between these three alloy groups a melting point between 98 and 800 °F (37–425 °C) can be achieved.

Polymer cores are not as common as metal cores and are usually only used for moldings that require simple internal surface details. They are usually 0.125 to 0.25 in (3.2 to 6.3 mm) thick hollow cross-sections that are molded in two halves and are ultrasonically welded together. Their greatest advantage is that they can be molded in traditional injection molding machines that the company already has instead of investing into new die casting equipment and learning how to use it. Because of this polymer core materials are most adventitious for small production runs that cannot justify the added expense of metal cores. Unfortunately it is not as recyclable as the metal alloys used in cores, because 10% new material must be added with the recycled material.

Molding

In the second step, the core is then inserted into the mold. For simple molds this is as simple as inserting the core and closing the dies. However, more complex tools require multiple steps from the programmed robot. For instance, some complex tools can have multiple conventional side pulls that mate with the core to add rigidity to the core and reduce the core mass. After the core is loaded and the press closed the plastic is shot.

Melt-out

In the final step, the molded component and core are both demolded and the core is melted-out from the molding. This is done in a hot bath, via induction heating, or through a combination of the two. Hot baths usually use a tub filled with glycol or Lutron, which is a phenol-based liquid. The bath temperature is slightly higher than that of the core alloy’s melting point, but not so high that it damages the molding. In typical commercial applications the parts are dipped into the hot bath via an overhead conveyor. The advantage to using a hot bath is that it is simpler than induction heating and it helps cure thermoset moldings. The disadvantage is that it is uneconomically slow at a cycle time of 60 to 90 minutes and it poses environmental cleanup issues. Typically a the hot bath solution needs cleaning or replacement every year or every half year when used in combination with induction heating.

For thermoplastic moldings induction heating of the core metal is required, otherwise the prolonged heat from a hot bath can warp it. Induction heating reduces the melt-out time to one to three minutes. The disadvantage is that induction heating does not remove all of the core material so it must then be finished off in a hot bath or be brushed out. Another disadvantage is that the induction coils must be custom built for each molding because the coils must be 1 to 4 in (25 to 100 mm) from the part. Finally, induction heating systems cannot be used with moldings that have brass or steel inserts because the induction heating process can destroy or oxidize the insert.

For complex parts it can be difficult to get all of the core liquid to drain out in either melt-out process. In order to overcome this the parts may be rotated for up to an hour. Liquid core metal collects on the bottom of the heated bath and is usable for a new core.

Advantages and disadvantages

The greatest advantage of this process is its ability to produce single-piece injection moldings with highly complex interior geometries without secondary operations. Similarly shaped objects are usually made from aluminium castings, which can weigh 45% to 75% more than a comparable molding. The tooling also lasts longer than metal casting tooling due to the lack of chemical corrosion and wear. Other advantages include:

• Very good surface quality with no weak areas due to joints or welds
• High dimensional accuracy and structural integrity
• Not labor intensive due to the few secondary operations required
• Little waste
• Inserts can be incorporated

Two of the major disadvantages of this process are the high cost and long development time.

Another disadvantage is the need for a large space to house the injection molding machines, casting machines, melt-out equipment, and robots