During the die casting process, the thermosetting resin material is added to a separate chamber, often referred to as a trough, and then forced into one or more closed molds for polymerization (curing).
The feed channels, also known as sprues and runners, allow material to flow from the chute to the mold cavity and through the flow restrictor or gate before entering the mold cavity. Many mold cavities have a single chute. The air in the chamber is replaced by incoming material and discharged through a specially placed vent.
When the material is placed in the trough, the amount is measured in a compact measuring device and then preheated to near the polymerization temperature. Only add enough injection volume at a time.
The force that feeds the preheated material out of the chute is sent to a injector that is placed immediately on the chute to prevent leakage from the gap between the piston and the chute side. The gland is usually snapped into the applicator to further prevent leakage.
The surfaces of the trough, the injector, the gate, the runner, and the cavity maintain a temperature that allows the material to solidify rapidly, depending on the nature of the material, the design of the mold, and the geometry of the part. The temperature is 280-380 °F.
When the die casting reaches the end of the curing period, the complete die casting is demolded, including removal of gates, runners, sprues, and troughs to form a solidified mat (called a residue).
In die casting, preheating of the material is important. The cold material flows slowly, and the material entering the cavity first has not reached its end point, that is, it may be polymerized. If this happens, the quality of the product is inferior, not only the appearance, but also the mechanical properties. Some exceptions, such as a small amount of injection, or some low-viscosity materials, can be heated by a heat lamp or furnace, but an effective and common method is to use a dielectric heater specially made for plastic molding. Screw-type plasticizing heating of thermosetting materials is also commonly used. This equipment can be combined with molding equipment or it can be independent, with the advantages of reduced volume and accurate feed measurement, and must be combined with preforming in other systems.
Type of die casting mold
The term "integral trough die casting" was first used because the trough and the shot stem were made together as a whole of the mold. The most common are round troughs, and other shapes to accommodate special gates to minimize waste.
A simple compression molding press can be used with a die casting machine. The integral trough mold is a three-plate type with a trough in the middle. The feed injection rod is mounted on the top of the mold frame with the mold cavity at the bottom.
The area occupied by the trough is at least 10% larger than the total area of ​​the mold cavity (the horizontal plane in contact with the plastic material). This prevents excess mold clamping forces from causing the mold to overflow.
After the material has solidified, the part is demolded by moving the pressure of the stripper bar, but the scrap and the sprue residue are still held at the bottom of the shot stick by one or more molded "pigeons".
Use a wooden pole or a soft hammer to remove the scrap and carry out the mold removal.
When a large waste is produced by a single tank, it is sometimes possible to feed the multi-cavity with a double tank. At this time, although the weights of the two troughs are slightly different, a horizontal plate is required to balance the pressure in each trough.
The plunger-type die-casting mold, also called the shot-rod mold, uses an auxiliary pressure plunger to force the injection rod into the trough (or the barrel), and removes the material from the trough into the mold cavity. The die casting pressure and the die casting speed are easily controlled regardless of the mold clamping pressure.
The size of the chute in the die-cast die-casting mold (determining the size of the scrap) is only required to be large enough and deep enough to meet the exact amount of feed.
The maximum chute area is determined by the force (in tons) given by the auxiliary plunger of the die casting machine and divisible by 3.5. This ensures that a pressure of 3.5 tons per square inch is used as the molding pressure, which is sufficient for the formulation of most die casting grades.
The auxiliary plunger is usually mounted above the upper fixed platen and acts downward. The stamping plunger moves the lower die plate up and closes the mold. After the mold is clamped, the material is loaded into the chute and the auxiliary plunger applies a force. The force ratio applied by the clamping plunger and the auxiliary plunger is typically 3:1 or 4:1.
After the material is solidified, the auxiliary plunger is retracted and the die casting machine is opened. Molded parts, scraps, and runners are simultaneously sent out by the stripper.
One variation of such a casting mold is a three-plate shot-rod die-casting mold in which a floating runner plate distributes material into a feed path that feeds directly into the mold cavity.
This method is applicable to the place where the mold line cannot open the flow path, or the core is moved on the plane of the mold line, the parts are very irregular and the ten parts are arranged.
Specially designed die casting machines are available in the following types:
1) The bottom extrusion rod die casting mold is installed in the die casting machine, wherein the auxiliary plunger is installed in the main clamping plunger, and if there is sufficient space between the one or more die casting units, the auxiliary plunger is mounted on the lower pressing plate .
The advantage of this method is that the mold can be filled with the mold when it is opened.
This design is slightly faster than waiting for the mold to be closed before charging the tank, and the stroke of the auxiliary plunger can be shortened, saving a few seconds per cycle.
The above-mentioned die-casting machine can use a top plunger injection rod type die-casting mold, but the pre-pressed ingots should be aligned to ensure that the material enters the trough well, otherwise the mold closing is difficult.
2) A die casting machine with multiple auxiliary plungers feeding a plurality of troughs, having long and curved flow passages that can fill multiple mold cavities, produce no waste, and do not require inefficiency.
3) The small powder die casting machine can be die-casted automatically, and can be operated horizontally or vertically. There is an auxiliary plunger perpendicular to the clamping plunger for the filling line of the raw material.
4) For prefabricated integral molding compound (BMC), usually glass fiber filled polyester, there is a horizontal die casting machine with a supply accessory to compact the material and send it to the die casting barrel.
5) The die casting machine is the prototype of a thermosetting resin injection molding machine that combines screw plasticizing and preheating molds. The preheated material is sent to the mold clamping line or to the clear casting barrel below the mold county.
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