Injection molding lends its name to one of the most prolific manufacturing processes in the world today. Millions of plastic components for all industries are produced by this method. Its speed and capability to create complex parts in large volumes make it a precious tool.
However, consistently attaining high quality requires a precise balance of many different process factors. Among those critical considerations is the sprue, through which the molten plastic initially enters the mold cavity.
The placement and shape of the sprue can have a dramatic effect on part quality, cycle time, and material consumption. In this post, we’ll examine the considerations of sprue placement and design and why they are so important to the process.
Mandrel Sprue Former for Injection Molding
Injection molding brings plastic components to life in a very literal sense. Plastic pellets are inserted from a hopper into a heated barrel, where a rotating screw mixes and melts the material. Under exacting pressure, the molten plastic is forced through a point called the sprue into a mold cavity.
It has been shaped to mirror the final component. This main cavity is connected to thinner channels called runners. This, in turn, feeds the plastic to even smaller points called gates, located in strategic spots on the part itself.
Once in the mold, the plastic sits until it cools and solidifies. Then the mold opens, and ejector pins pop the new plastic part out to the waiting worker or conveyor belt.
In each step of the process, the sprue, runners, and gates must be performed. This helps create efficient flow rates, optimal part quality, and the least material waste.
Sprue: What is it, and what does it do?
In injection molding, the sprue is the opening through which the molten plastic first enters the mold cavity. It is the main funnel that guides the hot material from the machine nozzle into a vein-like network of runners inside the mold. Think of it as a plastic heart pumping substance through the internal blood vessels of the mold.
Channeling Molten Plastic
The shape of the sprue is very important for the efficient flow of plastic. Typically, it is tapered, being larger at the end, attached to the machine nozzle, and decreasing in size where it joins the runner.
This reduces the turbulence and the bubbles in the molten plastic flowing through the sprue. The sprue diameter and length are designed for proper pressure and velocity to ensure a good fill of the part.
Connection to Other Mold Parts
The sprue part of the mold joins with the other parts to function as a complete system. It attaches directly to a vein-like network of runners that distribute the molten plastic to individual gates for each cavity of molded parts.
Once the plastic has cooled and solidified, the sprue part of the mold is separated and attached to the completed molded part. In the design of the mold, the sprue diameter, length, and location are critical to proper flow, efficient material use, and part strength.
Integration with Other Mold Components
The sprue is formed and designed as part of a system with other mold components. It connects directly to an interconnecting runner network. This network feeds molten plastic to each of the individual gates that lead to the cavity that will form each part.
Once the plastic has cooled and solidified, the sprue will be a distinct entity that is separate from the finished molded part. In the design of the mold, the diameter, length, and location of the sprue are carefully considered. This is to optimize flow, minimize waste, and ensure the structural integrity of the part.
Sprue Design
There are several variables that need to be taken into account when optimizing sprue design for efficient plastic flow. It is also important for low-shot waste and quality parts. Here is a list of the major design factors:
Diameter
The sprue diameter must be large enough for the plastic to flow through with little pressure drop. But if it is too big, it will take longer to cool, and you will have more material waste and longer flow times. Typically, the sprue diameter is kept a bit larger than the machine nozzle diameter for the initial flow out.
Then, it is reduced in size for the final flow-through. It is to be at a rate of about 0.030″ to 0.045″ per inch of nozzle diameter. This helps to control the flow rates and reduce swells in the part.
Length
The sprue length has a direct effect on the pressure drop and, thus, cycle time. Keeping the sprue as short as possible will reduce the pressure loss due to fluid friction. However, it also allows less time for the plastic to cool, so the flow can hesitate or stop if the part is not ready.
On the other hand, a longer sprue will control the flow rate and pressure build up better. But it takes longer for the sprue to cool. Iit may need to be ejected if the machine is a fixed plate machine and the plate has cooled.
Taper Angle
The taper in the sprue allows for easier mold ejection of the solid sprue. It also helps to reduce drag marks on the finished part.
Sprues usually have tapers between 1 and 3 degrees. However, for longer sprues, a slightly higher taper (3-5 degrees) makes for easier ejection.
Waste Reduction
Minimizing material in the sprue is another important consideration in sprue design. “Pin-point” sprues use a small diameter sprue connected to a larger diameter runner to reduce the material volume in the sprue itself, or “open sprue” designs where the sprue is left partially open during the molding process. Both methods help to reduce the visible scar of the sprue in the finished part.
There are many other sprue designs besides the simple tapered style above. Depending on the application, submarine sprues are designed to enter the cavity from the bottom to reduce the aesthetic defects at the part surface. Vented sprues have channels that allow any trapped air to escape during the filling process and help prevent voids and sink marks.
Where Should the Sprue Be Located in the Mold?
Proper sprue design can help ensure even flow, reduce part stress, and aid in defect prevention. Here’s an overview of the major considerations:
Flow Rate
Proper placement of the sprue will ensure balanced plastic flow across the mold cavity. Typically, this means the sprue should be located in the heaviest part of the section.
Natural tendencies of plastic flow is from thick to thin. This ensures even fill of the entire cavity and prevents areas of under fill and venting.
Part Stress
The location of the sprue should avoid concentrations of stress in the finished part. Thin walls are generally more prone to warping and checking due to non-uniform cooling and subsequent shrinkage. Locating the sprue close to a non-critical thicker section will help alleviate stress concentrations in those thin walls.
Mislocation and Defects
There are a number of ways incorrect sprue location can affect the finished part. Locating the sprue in a thin wall section can cause that area to cool prematurely. This will result in “sink marks,” which are depressions located on the surface.
On the other hand, locating the sprue near a critical parting line or corner can create “weld lines“. These are imperfections located near the edge of the part where the two opposing fronts of molten plastic merge. This ultimately reduces the structural integrity of the part.
Air “traps” can also be introduced into the part due to improper sprue location. These are areas of air pockets trapped inside the part due to imbalanced flow. This is just another type of imperfection that can result in part rejection and/or structural weakness.
These considerations, among others, must be examined by the mold designer when positioning the sprue. Proper location will establish optimal flow patterns, alleviate stress concentrations and prevent these defects from occurring.
The Verdict
In summary, an effective injection molding process starts with a well-designed sprue that is placed in the optimal location.
It is important to understand how the sprue diameter, length, taper angle, and position affect flow, stress, and cavity defects. It allows manufacturers to create a more efficient and effexctive process for producing quality parts. This, in turn, reduces cycle time and material consumption which lowers costs.
For those ready to fine-tune your injection molding operation and achieve maximum efficiency, contact Hitop Industrial. Let Hitop fine-tune your next injection molding project.
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