Commodity Molds are used to produce a variety of plastic parts for many applications

A mold (also spelled Mould) is the cavity or matrix in which a fluid or plastic substance is shaped into a finished product. Metal molds are used for casting metal products, while plastic ones are used for injection molding.

Pelletised raw material is fed into a heated barrel with a reciprocating screw. At transfer position, packing pressure is applied, which completes mould filling and compensates for thermal shrinkage.

Injection Process

Injection molding is a popular choice for creating custom plastic parts due to its high production rate, low scrap rates, and high-quality results. The process uses a precision steel or aluminum mold to form the desired shape of the finished product. Injection molding is also highly repeatable and cost-efficient, making it an ideal choice for mass production of plastic products.

In this process, thermoplastic polymers are heated to above their melting point and then mechanically forced into a plastic mold through a sprue or runners. Once injected, the polymer cools to form its final form within the mold.

Injection molding can be used to produce a variety of products, including automotive components, household items, and machine and equipment parts. It can also be used to over-mold a base or substrate part with a second, more flexible material for added durability and functionality. After cooling, the molded part is removed from the mold using ejector pins or a clamping motor.

Reciprocating Screw

Commodity Molds are used to produce a variety of plastic parts for many applications. They are often large and can produce a high volume of parts per cycle. This makes them ideal for large production runs.

The reciprocating screw in a commodity blow mold is designed to melt and extrude plastic resin, producing a hollow parison. This is similar to how a typical injection machine works. However, the difference is that in reciprocating blow machines, the barrel only moves up and down instead of back and forth like a ram machine.

As the screw retracts, melted plastic accumulates in the end of the barrel. Then, hydraulic pressure activates the screw and it is pushed forward, forcing a shot of melted resin through flow heads and dies to form a parison. This process can occur in 1.8 seconds. The parison is then closed between chilled mold halves, forming a finished plastic part. This type of machine is popular because it produces parts quickly and efficiently, while delivering high quality results.

Barrel

The barrel is the heart of the injection moulding process. The material for the part is fed into the barrel and mixed (using a helix-shaped screw). When it has been thoroughly blended, it is injected into a mould cavity, where it cools and solidifies into the configuration of the cavity. Injection moulding can be used to produce parts made from metals, glasses, elastomers, confectionery, and most commonly thermoplastic and thermoset polymers.

A barrel defect is a flaw in the polymer melt-flow that results in an uneven fill of the plastic moulded part. It can be caused by:

Nozzle

The nozzle is the key element of the injection machine, as it is responsible for both heating and injecting the plastic. It can have a variety of different designs, but the most common is a reverse taper design that has an orifice at its smallest diameter portion. This nozzle can be designed to filter, mix, and shut off the flow of polymer into the mold.

There are some problems with this nozzle design, such as the potential for a dead spot (shown in the picture above). As more and more contaminant is captured at the screen pack, the pressure drop increases, which can prevent acceptable molding.

This problem can be avoided by using a nozzle with a smaller orifice, or by changing the material. Another option is to use a pickout, which is more costly and less reliable. The choice depends on the size and shape of the part, as well as the desired production volume.