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What is the Difference Between Sprue and Runner?

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In the world of plastic injection molding services, three terms are often confused: sprue, gates, and runners. Many people use these terms interchangeably, but they have different meanings.

In this blog post, we’ll discuss the difference between gates, runners, and sprues. Gives you a better understanding of these important components of the runner system.

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The difference between gates and runners

A runner system consists of the main flow path, a manifold, a gate, and a cold material well.

The molten plastic enters the mold cavity from the injection molding machine nozzle through the main flow path, runner, and gate.

The entrance to the mold cavity is called the gate. To prevent cured cold material from entering the mold cavity at the end of the nozzle, a cold well should be designed at the end of the runner.

The sprues runners and gates are the important parts of the cold runner system and hot runner system. Computer simulations (mold flow analysis) help avoid some difficulties by suggesting either different molding parameters or different gate locations during the injection molding manufacturing process.

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What is a Sprue?

A sprue, also known as an inlet, is the passage from the manifold to the injection mold cavity and is the smallest and shortest part of the pouring system.

A sprue is a vertical passage through which liquid material is introduced into a mold and it is a large-diameter channel through which the material enters the mold.

It is the smallest and shortest part of the casting system. The sprue can be understood as the last “door” through which the molten plastic enters the cavity through the casting system.

The function of the gate

Firstly, it plays a controlling role in the flow of plastic melt into the cavity;

Secondly, when the injection pressure is withdrawn, the cavity is blocked so that the plastic in the cavity which has not been cooled and cured will not flow backward.

Avoiding the cavity pressure from falling too fast to produce shrinkage or depression in the product, and facilitating the separation of the casting condensate from the product after molding.

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Gate shape and size

The choice of gate type depends on the requirements of the injection molded product appearance, the wall thickness, the size and shape constraints and the type of plastic to be used.

The gate shape and size have a great influence on the quality of the molded part.

The gate location on the part is usually where the thickest wall section is. It is important visually the plastic flowing into the cavity from the gate location.

The gate is in most cases the smallest part of the runner in terms of cross-sectional size (except for the gate of the main runner type), and the ratio of its cross-sectional area to the cross-sectional area of the manifold is about 0.03-0.09.

The cross-sectional shape is mostly rectangular or circular, the gate step length is about 1-1.5mm, and generally small gates are used.

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Advantages of small gates

First, small gates can increase the flow rate of material through them. There is a large pressure difference between the two ends of the small gate, which can reduce the apparent viscosity of the molten plastic and make it easy to fill the injection mold.

Secondly, the small gate can increase the temperature of molten plastic and increase fluidity.

The friction resistance at the small gate is large, when the molten plastic passes through the gate, part of the energy is transformed into friction heat and heats up, which is good for improving the quality of thin-walled plastic parts or plastic parts with fine patterns.

Thirdly, small gates can control and shorten the time of replenishment, reduce the internal stress of plastic parts and shorten the molding cycle.

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In the injection mold making, the pressure-holding stage continues until the gate condenses.

The small gate condenses quickly and the replenishment time is short, which reduces the condensation orientation and condensation strain of macromolecules and greatly reduces the internal stress of the replenishment.

The closure of small gates can also correctly control the replenishment time and improve the quality of plastic injection molded parts.

Fourth, the small gate can balance the feed rate of each injection mould cavity. Only after the flow channel is full and has enough pressure, the multiple cavities can be filled at a similar time, which can improve the imbalance of the feeding speed of each cavity.

Fifth, it is easy to trim the plastic injection parts. Small gates can be quickly removed by hand. Small gates leave small traces after removal, which reduces the trimming time.

However, too small a gate will greatly increase the flow resistance and prolong the mold filling time. High viscosity molten material and molten plastic with a small shear rate effect on apparent viscosity should not be used.

Gate form and application

(1) Direct gate
the melt enters the cavity directly from the nozzle through the gate, with a short process, fast feeding speed, and good molding effect.

Because the cross-section of the direct gate is generally larger, therefore, the pressure and heat loss are smaller, and the pressure retention and shrinkage are strong. Moreover, the mold structure is simple, easy to manufacture, and the cost is low.

Its disadvantage is: the cross-sectional area of the straight gate is large, so it is difficult to remove the gate, and the trace is obvious after the gate is removed, which affects the beauty of the product.

(2) Side gate, also called the rectangular gate
It is generally located on the parting surface and feeds from the outside of the cavity.

Since the size of the side gate is generally small, the relationship between cross-sectional shape and pressure and heat loss is negligible.

The shape of the side gate is simple, easy to process, easy to accurately control the size, and easy to change quickly, suitable for all plastic materials except polycarbonate (PC).

The disadvantage is that the product surface has obvious sprue defects and the sprue needs to be cut manually.

It is not suitable for thin and transparent products, and it is also not suitable for thin and long barrel-shaped products.

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(3) Point gate, also called needle gate
It can be used for all kinds of products. The residual stress near the gate is small, and it can pull off the gate by itself, which can realize automatic production.

However, the injection pressure loss is large, and the injection molding pressure is larger than the direct gate, most of them have to adopt a three-plate mold (also called double parting surface) structure, the mold structure is more complicated, and the molding cycle is longer.

(4) Submerged gate, also known as a tunnel gate
The submerged gate can be machined to the exact size and there is no fit problem in the shape.

When the tunnel gate is demolded, it can be automatically removed from the product, which is suitable for automated production. However, it is not recommended for PC, PMMA, and SAN products.

(5) Ear gate, also called ear gate, wing gate
It is suitable for all plastic injection molded products that do not allow the appearance of gate marks on the surface and can reduce the flow marks produced by the side gate. It is not suitable for PVC and PU plastic products.

(6) Fan gate
A Fan Gate will disperse the plastic material outward, thus preventing jetting. Other gates include Sub gates, Cashew gates, and Hot Runner Systems.

Plastic Injection Molding Gate Types Edge Gates Edge gates are the preferred gate for injection molding services,as they offer a wide range of flexibility for injection molding designed.

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What is a flow path?

A runner system is a pathway through which fluid flows within a component in a hydraulic system.

A common runner system, also known as a sprue system or pouring system, is the channel through which molten plastic flows from the nozzle of the injection machine to the injection mold cavity.

The runner system includes PrimaryRunner, Sub-Runner, and Gate, Flow channel system composition.

(1) Folded Main Runner
Also called the main sprue, sprue, or vertical sprue, it is the flow path from the part where the nozzle of the injection machine contacts the bushing of the main flow path to the manifold. This part is the first part where the molten plastic flows through after entering the injection mold.

(2) Folded Runner
It is also known as a split sprue or secondary sprue and can be further differentiated into First Runner and Secondary Runner depending on the mold design.

The runner is the transition area between the main flow path and the gate, which enables the flow of molten plastic to be smoothly transformed; for multi-cavity molds, it also has the function of distributing plastic evenly to each cavity.

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(3) Folded gate
Also known as the inlet. It is the narrow opening between the manifold and the injection mold cavity and is the shortest and thinnest part.

The function is to use the tight flow surface to accelerate the plastic. The high shear rate can make the plastic flow well (due to the shear-thinning characteristic of plastic); the warming effect of viscous heating also has the effect of raising the material temperature and reducing the viscosity.

After the injection molding process, the gate is the first to be cured and sealed, which prevents the plastic from flowing back and prevents the mold cavity pressure from dropping too quickly, causing shrinkage and depression of the injection molded product. After molding, it can be easily cut off to separate the runner system from the molded part.

(4) Folded Cold Wells
Also called a cold cavity. The purpose is to store and collect the colder plastic before the initial stage of filling to prevent the cold material from entering the injection mold cavity directly and affecting the filling quality or blocking the gate.

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