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What The Heck Is Sprue Bushing Exactly?

If you have found yourself here, then you are likely wondering, “what in the world is a sprue bushing?”. In short, sprue bushings are an essential component of the injection molding process. While they are crucial, there is much to learn about sprue bushings and their benefits. Please continue reading to learn more about these molds and how they apply to injection molding.

First Thing First: What Is A Sprue?

Essentially, a sprue helps to introduce the liquid into a plastic mold. It is a vertical passage that is large in diameter and provides a necessary channel that allows the desired material to enter a mold. In many cases, the sprue controls the flow of a liquid into its mold. It is tapered with a larger end at the top to receive the liquid or material adequately. It allows the fluid to exit through a smaller opening connected to the runner.

So, What Is A Sprue Bushing?

Sprue Bushings are a component used within the sprue. They are made of hardened steel and help to accept an extrusion nozzle. Sprue bushings provide the necessary opening to transfer the molten plastic into its respective mold cavity. These components help to produce a more rigid sprue, as well as reduce cooling times. Because pickers and other industrial robots pick up molded parts when it is time for removal, sprues need to contain a certain amount of rigidity.

Sprue bushings resemble a bolt in shape and size. As they are designed to assist in the connection of the sprue, they are fixed with two bolts to ensure that the sprue does not detach during periods in which high injection pressure is experienced. Made of hot working steel SKD61, sprue bushings contain properties of both heat resistance and wear resistance.

Sprue bushings are usually made with either hardened steel or a copper base alloy. In some cases, they may be lined with carbide to assist with wear and corrosion resistance, as well as faster heat transfer rates. When using abrasive resins, carbide-lined sprue bushings can be especially beneficial.

Their most essential function is to prevent the leakage of plastic materials during an injection mold process. To do this, the sprue bushings must be appropriately sized to work correctly with the feed throat. The feed throat is typically designed into a chamfer R. The spherical radius, or SR, of the mold sprue needs to be one to two millimeters larger than the spherical radius of the injection nozzle.

How Sprue Bushings Are Used

Sprues are an essential component of the injection mold process. They work with runners and gates to successfully transport a plastic’s molten material from an injection unit’s hose directly to its specific cavities. Because production processes require different specifications regarding machine parts, sprue bushings can be designed based on particular calculations or purchased directly from a part provider. Sprues with two bolts are used for smaller molds and injection machines weighing less than 350 tons. However, sprues with four bolts are used for larger molds, and they may be used with injection machines weighing more than 200 tons.

Two Kinds Of Sprue Bushings

There are two most commonly used types of sprue bushings – hot and cold. However, many production floors are switching to heated sprue bushings. Because of this, it is vital to understand the differences between hot and cold sprue bushings and why many production facilities are switching to heated sprue bushings.

Cold sprue bushings are not heated and result in a sprue that must be removed typically during a secondary operation. This bushing type is inserted into a mold and helps create the channel between the molding machine nozzle and its respective mold cavity. The two most important dimensions of the cold sprue bushing’s nozzle are the spherical radius and the O-shaped hole opening.

Like cold sprue bushings, hot sprue bushings are also inserted into the mold. However, these sprue bushings provide a hot channel between the mold cavity and the mold nozzle. In addition, a heating element contained within the bushing helps keep the melted resin or plastic hot as it moves through the bushing.

Why The Switch?

Many reasons can be considered to explain why the switch from cold sprue bushings to heated ones is occurring. However, four significant benefits arise when switching to hot sprue bushings. These include reducing regrind, reducing cycle time, eliminating sink, and reducing trimming or machining in the gate area.

Product and Performance Specifications

Most sprue bushings have a spherical radius between .50” and .75”. However, another kind of sprue bushing available is flat-type bushings with a flat surface rather than a nozzle radius. Three dimensions are essential for buyers and production teams to specify when using sprue bushings. These include:

· The Shank Length

· The Overall Length

· The Tip Hold Diameter 

The Shank Length

The shank length can be measured by starting from the underside of the sprue bushing head to the end of the bushing. This measurement does not include the nozzle, much like the measurement for the overall length.

The Overall Length

When measuring the overall length of the sprue bushing, measure it from the start of the length to the end without including the measurement of the nozzle.

The Tip Hold Diameter

The tip hold diameter, also commonly referred to as the “O” diameter, describes the diameter of the inlet hole located on the nozzle seat. Other words to describe this measurement include the ‘gate diameter’ or possibly the ‘melt passage diameter.’ Whichever term refers to this measurement, it is essentially the hold that connects the nozzle tip to where the material enters the sprue bushing. 

Injection Mold Sprue Bushing Issues TechTalk SCTools

Injection Mold Sprue Bushing Issues
An essential part of understanding sprue bushings and their function is knowing what may go wrong with them and any issues that can potentially cause. Avoiding processing problems and improving cycle times are two primary goals for any production process, which is why it is crucial to know how to identify and resolve sprue bushing issues that occur on the production floor.
Surface Finish
When inspecting your sprue bushing, keep an eye out for rust, scratches, pitting, or other machine marks. If the bore of sprue bushing displays any of these abnormalities, the sprue may start to stick in a bushing. While pitting caused by corrosive or abrasive materials may be responsible, most of these conditions are caused by human error or poor machine maintenance. 
A common cause of internal scratches and rolled edges occurs when someone tries to remove a sprue stuck within a bushing. The usual method to drive the sprue out from the nozzle seat side by using a brass rod can pose a potential challenge as it is challenging to maneuver the rod in that position.
In this case, the problem can be resolved by heating a brass screw with propane or a MAP gas torch and then inserting it into the sprue through the mold’s parting line. After the sprue has cooled down enough, it can be pried out with a pair of brass pliers. As sprues can be very expensive, it is advised to steer clear from using hardened steel tools like screws, screwdrivers, other kinds of pliers, and gate cutters.
Molded sprues must be allowed to shrink to negate its taper lock in the bushing slightly. How well it shrinks is directly related to the type of material it is constructed with and how tightly it is packed out. Achieving the right shrinkage should not be an issue if you are not using a molding material with a shallow shrinkage factor. These materials can include amorphous materials that are heavily loaded with filler and liquid-crystal polymers, or LCPs.
However, shrinkage can become an issue when the molding process packs to the sprue too much and prevents it from adequately shrinking. This is something that often occurs when the packing pressure continues long after the gates have frozen. In addition, if the sprue orifice is still molten during a hurried sprue recovery or when high back pressure is used to mix in a colorant, then the sprue can also become overpacked. Another situation that can cause slow shrinkage occurs when the mold’s cooling of the bushing is inadequate while the cycle time is comparatively fast.
Nozzle Seat Repair
It is crucial to inspect the sprue bushing nozzle at the beginning of every startup. While conducting this inspection, personnel should look for rolled edges, chips, cracks, burrs, dents, plastic, residue buildup, and any other sign of potential malfunction. It is also essential to inspect the nozzle seat after a production run. Doing so helps to identify if the part needs to be repaired while the mold is available.
Refacing tools and carbide cutters are the two most commonly employed methods of repairing a nozzle seat. It is essential to check the radius of carbide cutters before using them for the first time. Cutters are not always manufactured with the proper tolerance, but it is necessary to ensure that the dimension of the sprue bushing nozzle seat radius is correct. Using a rotary EDM to machine the sprue bushing seats can help to lower the chance of leaking molten plastic or carriage blowback. Another excellent way to ensure that any future damage is easily detected is by using a stipple finish.
Design Considerations For Sprue Bushings
Improving cycle time is an essential consideration for any production process. While there are many different ways to achieve shorter cycle times, it is vital to understand how sprue-bushing design can impact the cycle time. Here are a few ways to improve the cycle time through the sprue bushings.
Commonly, a sprue bushing needs to be keyed or oriented in a specific position to align the runner into its front face. An economical orientation method is achieved by adding a dowel pin through the head of the bushing. However, dowel pins only orient bushings and don’t help to retain them. A common and successful way to retain a sprue bushing is to add one or two ¼-20 screws through its head.
Sprues do not need to solidify entirely before opening a mold. Instead, it just needs to shrink enough to negate the taper lock and maintain enough physical integrity to stay adjoined to the runner. Cooling the sprue bushing is critical, which is why parts should be ejected at, or even slightly below, the recommended ejection temperature.
Hand-held thermal imaging cameras and infrared temperature guns are two excellent methods of checking the temperature of a part. Each of these instruments helps to indicate both hot and cold spots on a part. Identifying these spots helps to ensure even wall thickness and the evenness of cooling.
An easy and effective way to improve the cooling of a sprue bushing is to replace any existing steel bushings with ones manufactured with a copper alloy. However, it is essential to note that a trade-off does occur between thermal conductivity and hardness when switching to a copper alloy bushing. 
Surface Finish
Off-the-shelf sprue bushings are usually very well polished. However, it is essential for those who make their sprue bushings to remember to polish the sprue in a draw. Polishing in draw means that the sprue is polished in the direction in which the sprue is removed. Polishing a sprue may also be necessary when changing the size of an existing bushing or repairing a damaged one.
The Bottom Line
Because sprue bushings help molten materials travel from the injection hose to the mold cavity, their function is essential for the production process. Though they are only one part of a multi-part machine, they require a significant amount of inspection, maintenance, and expertise. Therefore, understanding the many factors and aspects that contribute to their proper function is critical to ensuring that the production process is successful.
TechTalk SCTools

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