An injection molding port design helps companies produce the best quality plastic components. This sprue design controls the plastic melting machine from the sprue channel. If there are errors in the design of the injection molding port, it is likely that there will be defects in the molded plastic components.
Therefore, you need to understand the fundamentals of injection molding design to get the best results from the manufacturing process. This article will explain what an injection molding sprue is, including everything you need to know before designing injection molding sprues.
What is injection molding?
An injection molding port is a constructed opening, usually small, that controls the flow of molten plastic into the mold cavity. There are different types of injection molding ports, and using the correct port will ensure the success of your injection molded parts.
Parameters such as port type, location, dimensions, material and mold type influence the quantity, pressure and temperature of molten plastic in the injection molding process. Several industries use injection mold designs when producing complex plastic parts.
Why do you need an injection molding gate design?
The design of the injection molding port directly affects the outcome of a plastic mold. A well-designed gate controls the volume and direction of molten plastic flowing into the mold. This is because you want the melted plastic to flow toward the mold rather than the channel/nozzle.
Additionally, the mold opening design ensures that the molten plastic reaches all areas of the mold before cooling occurs. This prevents the molten plastic from hardening unevenly and prematurely. This also ensures that the plastic parts do not suffer any deformations, such as breaks or stress cracks.
Additionally, injection molding generates heat through dissipation. An ideal gate design increases the polymer temperature to prevent the formation of flow marks and weld lines. The simplified injection molding process allows manufacturers and product designers to eliminate channels, facilitating post-treatment.
Importance of gate position in injection molding
The port in injection molding is a small opening, usually between the runner and the mold cavity. While gate design covers aspects such as gate size in injection molding, the location of the gate determines the success of the process and the quality of the finished parts.
The location of the injection molding port can help prevent or mitigate problems that may arise due to port size errors. Incorrect sprue placement will result in molding defects such as injection molding burrs, weak spots, breaks, uneven thickness, etc. Determining the most appropriate sprue position during injection molding will help ensure you use the correct sprue size and timing.
Sprue Types for Injection Molding
Different types of injection molding sprue affect the quality of the plastic mold. These sprues have different sizes and dimensions and can determine the design of the molded plastic product. Here are some of them:
1. Direct or channel openings
The direct/sprue gate type is simple and widely used in injection molding. The sprue moves and melts directly into the mold cavity, allowing large quantities of plastic to be injected quickly. Typically, less injection pressure and a short delivery time are required. The direct channel is easy to construct and provides high tension around the channel.
Due to its simplicity, this sprue is an economical option. However, these sprues can leave marks on the finished parts as the technician must manually remove the sprue from the molded parts.
Direct casting is particularly suitable for bowl or box-type molds with deep individual cavities and unsightly parts. It is widely used in household appliances and consumer products such as washing machines, trash cans, televisions, printers, etc.
2. Perimeter goals
Edge gate is the most popular and simplest form of injection molding. As the name suggests, it is usually positioned along the edge of the piece, forming a visible mark at the parting line. It has larger cross-sectional areas that allow sufficient flow of the molten plastic into the cavity.
The edge gate is relatively inexpensive to construct and is ideal for flat parts and medium or thick sections. Hoop casting does not require a specific type of resin, making it an ideal choice if you can simplify your project for injection molding.
3. Undersea Gates
The underwater injection molding gate or tunnel is generally placed below the mold parting line and facilitates automatic cutting during component ejection. This uses a narrow channel that connects to the cavity near the dividing line and fills the cavity below the dividing line. The release angle also makes it easier to eject finished plastic parts without breaking them.
This high shear channel allows only a small amount of molten plastic to enter the mold cavity. Therefore, subsea/tunnel sprues are ideal for molding small components. Its use for larger parts results in unnecessarily long cycle times and poor surface finishes due to shear heating.
4. Cashew Gates
The cashew sprue is shaped like a walnut. Manufacturers use this type of channel for products that may become stained when the channel is removed. The cashew sprue has a curved structure, which makes it difficult to remove the molded parts without damaging or deforming them.
The cashew channel can reach other difficult areas of the mold that cannot be connected by the normal tunnel channel. Therefore, injection molding manufacturers install removable fittings that can be easily released during the ejection process.
This type of sprue is not limited to or best suited for any specific plastic resin. Therefore, the options remain open when considering this type of gate based on the needs of your product designs.
5. Membrane Gates
The membrane port and sprue are somewhat identical in appearance as they both taper below the sprue. These sprues are generally used for frames with square shapes. The membrane gate effectively reduces the formation of weld lines and curled shapes on molded parts, although the temperature, speed and pressure of the injection molding process may affect the quality of the ejected part.
Membrane sprue is suitable for larger parts that require a significant amount of resin to complete the molding process and fill the part. The membrane gate design is compatible with most resin types and is an ideal option depending on the product design.
6. Hot Runner Valves
The essence of the hot runner molding system is to keep the molten plastic in its liquid state until it fills the mold evenly. There is a specific pressure and temperature setting that helps achieve this. The closures of the hot runner valves maintain the same pressure and temperature conditions in the channels. Valve closures also maintain exact channel width dimensions.
The hot runner needle port has ejector pins that provide control advantages. The molten plastic flows into the gate when you pull the pin and the flow stops when you push the pin back.
Additionally, pushing the pin back into position pushes the remaining plastic in the sprue into the mold. This prevents the accumulation of material in the inlet channel, improving efficiency. The hot runner valve nozzle provides a more reliable control mechanism in the injection process, while also allowing individual control of multiple gate openings.
7. Hot chamber heat engines
Just like hot runner valve nozzles, hot runner heat engines have a fixed temperature and pressure. The thermal gate is only used in hot runner systems and is placed on top of the parts, directly above the mold parting line. This injection molding gate design does not require assistance from the runner before filling the injection mold. With this design there is no need to separate the gate from the finished part after molding.
The hot runner heat engine works differently than other gate designs because it does not have a pin or valve as a closing mechanism. The melted plastic flows through the gate to the valve. The remaining plastic in the gate forms a cold plug called a “heat engine” when the flow stops and acts as a temporary stopper.
As the subsequent stream of molten plastic flows through the gate, it melts and pushes the temporary thermal gate into the mold. It is mainly compatible with different types of resin, which makes it an ideal option for most designs.
8. Fan goals
As the name suggests, fan cuts are shaped like a fan. These ports allow the mold to flow into the cavity through a wide opening. The door gradually widens and forms a fan from the runner to the direction of the mold cavity, maintaining a uniform thickness. They are often used to ensure stable flow on large parts.
A fan gate helps prevent injection molding errors while maintaining the dimensional stability of parts. Additionally, a fan gate is ideal for producing flat, thin products because it can limit directional stresses and flow marks. The fan ventilation channel is generally suitable for polycarbonate plastics.
9. Pen goals
Pin gates are typically located on the B side of the mold, near the ejector pins. A pinned sprue is ideal for three-plate molds where the sprue is located on another sprue plate. The mold flow is divided into several directions and leads to the cavity through several inlet locations. The tip of the gate is very small, so it can be cut off by the opening of the injection mold. Due to the large corridor, the scrap rate is high, which is a disadvantage.
Design considerations for injection molding sprues
Different types of injection molding sprue have different processes for molding different components. However, when choosing between these designs, you should consider some important factors:
Gate placement
Some gates are more difficult to separate than others due to their location. Likewise, the order of some gate openings can cause warping and lines in molded parts. Therefore, you must be careful when placing ports in your injection molding project.
Gate size
The port size in injection molding must allow for adequate shear as the mold passes through the machine. The dimensions of the gate must allow the mold to be filled correctly. Smaller sprues have high shear heating rates. However, if they are too small or too large, they can inadvertently increase flow pressure. Therefore, use sprues of sufficient size to obtain optimal results.
Shape and surface of the part
Each gate design is recommended to form parts with different shapes and achieve a specific surface area. For example, the cashew gate design is ideal for machining smaller parts and provides a smooth, consistent surface finish.
Therefore, you need to determine the perfect gate for your parts and the desired surface finish. Additionally, you must consider features such as cutouts, which can make it difficult to directly eject finished plastic parts in complex shapes.
Concluding
Injection mold design is a crucial element in ensuring the quality and productivity of plastic molds. Proper gate design can make the difference between perfect and faulty molds. Proper injection molding port design helps reduce production costs and optimize cycle times.
Common questions
Where do you place the gate during injection molding?
Gate openings must be installed at the deepest cross-section to ensure the best flow and reduce voids and subsidence. The gate must be located on a side of the mold where stress and deformation of the runners and gate will not affect operation.
What are the basic steps of injection molding?
The most important steps in the injection molding cycle include clamping, injection, cooling and ejection.
What problems commonly occur during injection molding?
Defects in injection molding of plastic components include sink marks, flow lines, warping, surface separation, short circuits, and spatter.
