Screw and Barrel Tolerances for Injection Molding

Whether you're assembling an injection molding machine or simply doing some maintenance on it, you'll want to make sure that you're familiar with the different screw and barrel tolerances that are used in the process. Knowing these details will help you get the most out of your machine and prevent unnecessary stress on it.

Dimensional differences due to non-uniform deformation

During the injection molding process, there are several factors that may influence dimensional differences between molded parts. One of the most important factors is the polymer used to produce the part. Different polymers exhibit different behaviors that affect the molding process. The properties of the molded part can be degraded resulting in brittle parts. There are several ways to reduce the effects of this degradation.

For glass-reinforced nylon resins, a high injection speed is essential. Increasing the speed may also decrease the melt temperature.

The flow length of the melt is limited by the length of the walls in the mold cavity. To enhance the flow length, the mold temperature can be increased. In addition, the temperature of the cooling water can be adjusted.

Thermal expansion and contraction

During the injection molding process, thermal expansion and contraction can lead to significant problems. If not controlled, these effects can lead to damage to expensive components and downtime. In addition, they can lead to non-uniform deformation.

There are several ways to combat thermal expansion and contraction. These include pressure stability and support design.

First, the screw must be designed to compensate for thermal expansion. This is achieved by changing the screw's parameters to increase the length-to-diameter ratio. It also involves changing the extruder's length to accommodate the thermal expansion.

Working life of a bimetallic barrel

Using a bimetallic barrel is a good option if you need to process corrosive materials or add fillers. It is also a great option for manufacturing or construction.

These bimetallic barrels are typically used in most of Europe and the Americas. They are based on metallic carbides, such as titanium or tungsten. These alloys are very abrasion-resistant and are suspended in a corrosion-resistant matrix.

Using a bimetallic barrel will increase your productivity and help you meet new industrial requirements. The working life of a bimetallic barrel is generally three to five times longer than a standard nitriding steel barrel. This type of barrel is very economical and helps you to control your processes.

Ejector pin marks

During the injection molding process, a molded part will usually have ejector pin marks on the outer surfaces. These marks are caused when the ejector pins try to push the molded part out of the mold.

Injection molding machines are designed to force material at high speed and pressure. This is done to collect the material into shot volume. The shot volume provides a cushion to prevent the material from sticking to the mold cavity. However, when the material is held at high pressure for a long time, it causes the material to shrink. Then, it is more difficult to remove the material from the mold cavity.

Double-flighted screws help heat transfer

Adding an extra flight to an injection molding screw can help boost heat transfer. The extra flight forms two parallel channels. While the volume of the channel may be small, the total volume of the screw may increase. This increased volume can be used to cool partway down the barrel.

In general, there are three major ways that heat is transferred in an extruder: by screw, by barrel, and by pellets. Screws provide the largest part of the heat. Pellets and the barrel help to produce frictional heat, which is a significant contribution to the melting process.

Draft angles

During the design phase of an injection molding machine, many variables are considered to determine the best method to produce a part. Some of these variables may change without negatively impacting the quality of the part. This is why it is important to consider the process as a whole.

The screw and barrel play a large role in the success of an injection molding operation. For instance, the screw has to rotate at a sufficient speed to produce a desired molded part. During the process, the plastic flows through the barrel and into the mold cavity through channels called runners. These channels are made of a material that is compatible with the base resin.

Parting line

During injection molding process, material is forced at high pressure into part forming cavity. This pressure can be generated on either side of the cavity. This pressure is known as back pressure. Experts recommend a back pressure not to exceed 20% of the machine's maximum rated injection molding pressure.

The screw and barrel of an injection molding machine are important for the process. They are also responsible for forming parting lines in injection molded products.

The parting line refers to the area around the perimeter of the molded part. It is usually perpendicular to the direction of opening of the mold. There are various types of parting lines, including vertical parting, sprue parting, gate marks, and ejector pin marks.