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Introduction of Plastic Die Casting Process

Posted by: Alan Xia 2021-10-21 Comments Off on Introduction of Plastic Die Casting Process

In the die-casting process, the thermosetting resin material is added to a separate material cavity, often called a material tank, and then forced into one or more closed molds for polymerization (curing).

The sprue, also called sprue and runner, allows the material to flow from the trough to the cavity, and pass through the restrictor or gate before entering the cavity. Many cavities have a single trough. The air in the material cavity is replaced by the incoming material and discharged through a specially placed exhaust port.

When the material is placed in the trough, the amount of material is measured in a compact measuring device, and then preheated to close to the polymerization temperature. Only add enough injection volume at a time.

The force of sending the preheated raw material out of the trough sends it to an injector, which is mounted on the trough immediately to prevent leakage from the gap between the piston and the edge of the trough. The sealing sleeve is usually clipped into the injector to further prevent leakage.

The surface of the chute, injector, gate, runner and mold cavity maintain a certain temperature that can quickly solidify the raw material. The temperature is 280-380°F according to the nature of the material, the design of the mold and the geometry of the part.
When the die-cast product reaches the end of the curing period, the complete die-cast product is demolded, including removing the gate, runner, sprue and trough to form a solidified mat (called residual material).

In die casting, preheating of the material is very important. The cold material flows slowly, and the material that enters the cavity first has not yet reached its end point, that is, it may polymerize. If the above situation occurs, the product quality is inferior, not only in appearance, but also in mechanical properties. There are some exceptions, such as a small injection volume or some low-viscosity materials. It can be heated by a heating lamp or a furnace, but an effective and common method is to use a dielectric heater specially made for plastic molding.

Screw-type plasticizing and heating of thermosetting materials are also commonly used nowadays.

This equipment can be combined with the molding equipment, or it can be independent. It has the advantages of reducing the volume and measuring the feed amount accurately. It must be combined with the pre-forming in other systems.

Types of die casting mold

The term “integral trough die-casting” was first used because the trough and the injection rod are made together as a whole of the mold. The most commonly used is the circular trough, and other shapes may also be used to meet the requirements of special gates to reduce waste.

A simple compression molding press can be used with a die casting machine. The integral trough mold frame is a three-plate type with the trough in the middle. The feeding rod is installed at the top of the mold frame, and the cavity is at the bottom.

The area occupied by the trough is at least 10% larger than the total clamping area (the horizontal plane in contact with the plastic material) in the cavity section. This prevents excessive mold clamping force from causing mold flash.

After the material is solidified, the part is demoulded by moving the pressure of the ejector rod, but the waste and sprue residue are still held at the bottom of the injection rod by one or more molded “dove tails”.

Use a wooden pole or soft hammer to remove waste and clean the mold.

When using a single trough to produce large waste, sometimes double troughs can be used to feed multiple cavities. At this time, although the weight of the two troughs is slightly different, a horizontal plate is still needed to balance the pressure in each trough.

The press rod type die-casting mold is also called the injection rod type mold. It uses an auxiliary pressure plunger to force the injection rod into the trough (or barrel), and remove the material from the trough and move it into the cavity. Die-casting pressure and die-casting speed are easy to control and have nothing to do with clamping pressure.

The size of the trough in the plunger die-casting mold (determining the size of the waste) only needs to be large and deep enough to meet the exact feeding amount.

The maximum trough area is determined by the force (in tons) given by the auxiliary plunger of the die-casting machine and divided by 3.5. This ensures that a pressure of 3.5 tons per inch 2 is used as the molding pressure, which is sufficient for most die-casting grade formulations.

The auxiliary plunger is usually installed on the upper fixed pressure plate and acts downward. The punching plunger moves the lower die plate upward to close the die. When the mold is clamped, the raw materials are loaded into the trough, and the auxiliary plunger exerts force. The ratio of the force applied by the clamping plunger and the auxiliary plunger is generally 3:1 or 4:1.

After the material is solidified, the auxiliary plunger is retracted and the die casting machine is opened. Molded parts, waste materials, and runner cold materials are sent out by the ejector rod at the same time.

A variation of this type of casting mold is a three-plate injection rod die-casting mold, in which a floating runner plate distributes the material to the material channel directly feeding the mold cavity.

This method is suitable for places where the parting line cannot open the runner, or moving the core on the parting line plane, and the parts are very irregular and disturbingly arranged.

The specially designed die casting machine has the following types

  • 1) The bottom extrusion rod die-casting mold is installed in the die-casting machine, and the auxiliary plunger is installed in the main clamping plunger. If there is enough space between one or more die-casting units, the auxiliary plunger is installed on the lower platen .The advantage of this method is that the chute can be filled with material when the mold is opened.This design is slightly faster than the design that waits for the mold to close before filling the chute. And the stroke of the auxiliary plunger can be shortened, saving several seconds for each cycle.The above-mentioned die-casting machine can be used for the top plunger injection rod type die-casting mold, but the pre-pressed ingots must be aligned to ensure that the material enters the trough well, otherwise it will be difficult to close the mold.
  • 2) The die-casting machine with multiple auxiliary plungers feeding multiple troughs has a long and curved flow channel that can fill multiple cavities, does not generate waste, and does not require low efficiency.
  • 3) The small powder die-casting machine can automatically die-cast, and can be operated horizontally or vertically. There is an auxiliary plunger perpendicular to the clamping plunger, which is used for the injection of raw materials in the clamping line.
  • 4) For prefabricated integral molding compound (BMC), usually glass fiber-filled polyester, there is a horizontal die-casting machine with a feeding accessory to compact the material, and then send it to the die-casting barrel.
  • 5) Die casting machine is the prototype of thermosetting resin injection molding machine. This machine combines screw plasticization and preheating molds together.

The pre-heated material is sent to the mold clamping line or the Qing casting barrel under the tight grid.

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