Sprue Bush

What Is a Sprue Bush? 

The sprue bush is an essential component in the modern plastic injection molding. It is made of metal and is linked between the mold and the injection machine. It is like a gate, as it allows the molten plastic materials to be injected into the molding machine through a component called nozzle. Regarding the process of molding, the cavity of the sprue bush is also very important. If the cavity of the sprue bush is small, there will be very little place for the molten plastic materials to flow, and will in terms  create a resisting force. As a result, it is often advised to keep the area of the sprue large when possible. 


What’s the Importance of a Sprue bush?

Plastic can be seen everywhere in our life. When we are thirsty, we go buy a drink which is very possible filled in the plastic bottle. When we want to travel, we take out our plastic suitcase, and start filling in. Thousand and thousands of plastic products that come in all shapes and different colors are produced every day. But it seems that we have taken this for granted. All of these products will not exist without the modern molding technique. With the help of the molding machine, we can now produce a lot of plastic products within a very short time. While the molding machine consists of a lot of different components, and the one that makes itself indispensable is the sprue bush.


How Sprue Bushes Work in the Runner System?

The sprue bush can also be described as the part of the runner. The runner system is the passage in which the molten plastic materials travel from the nozzle to the mold. It is generally categorized as three main parts, the primary runner, the sub-runner, and the gate. There are currently two types of runner system, the cold runner method and the hot runner method. 


● The cold runner method

In the cold runner molds, there are two or three plates which are held within the base of the mold. Through the sprue, the molten plastic materials are injected from the nozzle to the mold. This process will fill the route of runners which in terms lead to the mold cavities. In the cold runner method, the runners act as a delivery system that dispenses the molten plastic materials to the individual molding cavities. The runner in the cold runner mold is not heated. The system will subsequently cool down the components like the sprue, runner, and gate along with the molded part.


● The hot runner method

In the hot runner molds, there is a system which is capable of distributing the fluid or gas from a lot of junctions into one place. This system is called a manifold system. In the hot runner molds, the two plates which comprise the mold are heated by this system. The manifold system comes with the advantage of keeping the temperature consistent. In order to achieve such effect, the temperature of the molten plastic materials and the heating cylinder are to keep at the same temperature. The molten materials are later delivered to the core mold through the nozzle, for the final part to be formed.


What Are the Pros and Cons for Each System? 

Generally speaking, the cold runner system is more cost effective than the hot runner system. Besides, it is also much more flexible and capable of dealing with a wide range of commodities. However, it is the production of the runner itself that holds it back. 

In the two-plate cold runner systems, it is important that after each run, the runner needs to be separated from the finished part manually. If this process is not achieved, a lot of the plastic material will be wasted. Furthermore, it is a very time consuming work, when it comes to the removal of the sprues and runners and the recycling of each runner. It usually produces much slower than the hot runner system. 

In comparison, the hot runner system can process in a relatively faster time. It also does not have to do the sprue removal, the regrinding and recycling process due to the fact that it does not have a runner. This is also helping the system save a lot of waste materials. The downside of the hot runner system is that the tool it utilized is very expensive. Besides, the system is very complex, which means it requires close monitoring and good maintenance. These all increase the potential cost for the production.

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