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Rompa Group goes into detail: The flow of different feeding systems

25August 2021

Injection molding is a technology used for many years by Rompa Group and is one of our specialties where we have invested heavily in mechanical equipment and knowledge. In this week’s article we focus on the feed system for an injection molding machine. The main function of a mold feed system is to create a passage for the melted material to flow from nozzle to impression. This flow path is called the feeding system. In this article we talk about the different elements of the feed systems, what we should be expecting from these systems and more!  

What is a feed system? We can be very short about this: it’s a passage. It’s a passage where melt flows from the nozzle of the injection moulding machine into the cavity. A nozzle or nozzle tip is where the molten plastic leaves the injection unit and enters the mould. The mould is in most cases hollow and in general made from stainless steel. The plastic is inserted into the mould cavity, this is a hole in the mould shaped like the desired part.

We can divide feeding systems into two main types. A cold runner system and a hot runner system, which we will explain later in this article.

The main elements of the feeding systems

In general, feed systems consist out of sprues, runners and gates. These elements are precisely considered and investigated for an optimal result. Below we dive deeper into these three elements.

Sprue
Molten plastic is injected from the nozzle of the injection moulding machine into the mould. The sprue is the channel through which the molten plastic flows. The sprue is a separate part from the mould and is considered to be the continuation of the mould to the nozzle of our injection moulding machines. But moulds with single-cavity are said to have direct sprue gating as they are directly linked to the moulded part. The diameter of the sprue opening in the sprue bushing should be as small as possible while still being able to provide appropriate filling of the cavity.

Runner
Instead of sprues, runners follow channels cut into the parting line. This delivers the material into the center of the mould plate. The design of a runner is crucial for quality end products. When a runner is thicker it can make filling pressures too high and will cause unnecessary long cycle times. The ideal design for a runner has the perfect balance between mould feasibility and filling pressures.

Gate
Between the main runner and the mould cavity we will find a narrow intersection that we call the gate. This intersection is the smallest part of the complete mold. The gate makes sure that the molten plastic enters into the mould cavity as quickly as possible. When the filling is done the gate condenses first and then blocks the cavity. This prevents melted plastic from falling and this also avoids pressure drop.

What can we expect?

When a feeding system is well-designed we can expect many benefits such as:

  • An optimal number of cavities
  • A perfect flow delivering melt to the cavities
  • A good balance between filing of multiple cavities, but also the filing of multi-gate cavities
  • Less waste
  • An easy and smooth injection
  • Efficient in energy use
  • Control of the filling and cycle time

Cold runner systems vs hot runner systems

As mentioned above, we can divide feeding systems into two categories. We can consider the flow process through the injection moulding machine via cold channels as the ‘normal’ feeding system, but this is actually called a cold runner system. So what is the definition of a hot runner system? Obviously, hot runner systems have channels where plastic flows are being heated.

The cold runner system is an unheated channel and has the function to create a passage of molten plastic into a cavity after it is dispensed from a nozzle. The channels of cold runner systems are larger because it needs to transfer a greater flow of plastic with each cycle. A big advantage of a cold runner system is the ability to handle many different polymers and it's rather easy to switch between colors.  

The hot runner system is made out of two heated plates. Both these plates are heated internally and externally. The hot runner system is ideal if the flow needs more control and management. Some materials are sensitive to heat, in that case we can use the externally heated moulds only. The materials are kept in a molten condition throughout the process until this material flows into the mould cavity. There are many other methods available for the hot runner system such as heating pipes and heating rods.

Injection molding

Throughout the years the concept and process of injection moulding have been evolving and Rompa Group has been known to be on the front seat with innovation processes regarding injection moulding and everything around this technology. It offers the possibility to produce in large volumes, but it is also a very cost-effective manufacturing process to help our clients achieve their goals. The feeding systems might be a small part of this whole machine, but every detail is important to us and when all small details are as perfect as they can get, we can assure our customers that we will deliver a quality product.

Make sure to read more about our technologies here!

Interested in learning more?

Would you like to know more about the many different elements of the injection molding machine or specifically regarding the feeding systems? Make sure to contact us, we are always happy to explain to you more about our passion and what it can offer.