One of the vital tools for any successful injection molding procedure is the gate. The operation of these structures is straightforward – allow molten plastic to flow into the mold cavity.
However, you must select the right gate design for your fabrication to reduce molding costs and enhance product design.
An error in the gate design may result in significant molding defects or result in failed fabrications. Therefore, you need to familiarize yourself with the different types of gates and understand the importance of these structures and how they can affect molding outcomes.
Read ahead to understand these details.
What Is a Gate?
Gates in plastic injection molding refer to openings through which molten plastic passes to enter the mold cavity.
As the name suggest, they serve as gateways – passage – for liquid plastic components to enter the mold cavity. Injection molding gates occur in different forms, taking distinct sizes and shapes – appearing narrow, wide, tapered, etc.
Each gate type has specifications that significantly affect the end product’s structure and appearance.
The type of gate for manufacturing often depends on the material used and the parts’ shape. During injection molding, plastic polymers are heated to liquid under extreme temperatures, then forced through a mold where they solidify to form the desired part’s shape.
The liquefied plastics move through a series of channels – spruces and runners till they reach the gate where they enter the mold cavity.
Besides solidifying in the mold cavity, the molten plastic may cool down and solidify at different points, including the gate. When this occurs, it may cause protrusions in the final product.
Therefore, you should ensure the trimming of the gates (the protruding parts) to enhance the product’s appearance. The gate trimming process is called degating, performed automatically – by the injection molding machine or manually – after ejecting the part.
Why Do Gate Type And Gate Placement Matter?
Gates type and placement significantly affect the end product of injection molding. The placement, for example, determines when the molten polymer enters the mold cavity, and the type influences the angle, speed, and force at which the flowing plastic penetrates.
The combo of both features affects the forming part. In fact, the reduction in the flow rate may affect the temperature of plastics that enter the cavity.
An ideal gate regulates the volume and direction of liquefied plastic as they enter the mold. If the opening is not wide enough, some of the molten plastic may flow back to the spruces and runners, preventing them from entering the mold cavity and resulting in a failed fabrication.
In addition, these small-sized gates may cause pressure build-up at the gates, which speeds up the flow into the mold. This may result in jetting, a phenomenon used to describe the presence of wavy imperfections on injection molded material.
Furthermore, obstruction of molten plastics as they pass through the gates may affect the temperature of the material that enters the mold cavity. This may result in stress buildups, affecting the structural integrity of the final product.
The position of gates also plays a significant role in the fabrication. Gates tend to leave marks and blemishes, so the presence of flaws in visible parts of the final product often results in more post-processing to improve the aesthetics.
Below are some gate placement tips to improve fabrication.
- Create allowance in design for degating.
- Increase gate size to slow the flow of molten plastic into the mold, preventing jetting.
- Place gates at a distance away from cores and pins; this helps to prevent defects and imperfections like weld marks.
- Regulate injection speed, and reduce it if necessary
- Minimize flow path length
- Consider using multiple gates.
Different Injection Molding Gate Types
Each gate type possesses specific features that affect the flow rate, temperature and pressure at which molten plastic enters the mold. These structural differences, in one way or another, affect injection molding parts.
Let’s examine each of the seven main gate types in injection molding.
The Edge gate is one of the most common gate types for injection molding operations. Their simplicity and effectiveness is the main reason for their wide acceptance.
As the name suggests, the gates are placed along the sides (edges) of the part. When creating molds for injection molding, edge gates are easy to produce and support easy modification.
They are suitable for vast applications, for fabricating flat components, including medium or thick sections. Compared to other gates, edge gates may have a broader cross-sectional area, which allows the easy flow of plastic material, reducing molding defects.
Edge gates may occur in two categories: fan gates and tab gates. Fan gates are usually rectangular, having broader widths than the runners leading to them.
The wide fan gate configuration allows for easy plastic flow and dimensional stability, which helps reduce molding defects. Fan gates are more suitable for fabricating large components.
On the other hand, tab gates are narrower, having uniform thickness with the runner. This may cause an increase in shearing heat as the molten plastics to pass through the gates.
However, the design is such that the gates can absorb most of this heat, preventing it from disseminating into the cavity. Therefore, like the fan gates, there are reduced molding defects with edge gates. This gate configuration is suitable for molding thin flat components.
Pros of Edge Gates
- Simple and effective
- Vast application for carious design specifications
- Relatively cheap to design and manufacture
- Easy to modify to suit design requirements
- Allows for long holding time of plastic material
Submarine or Tunnel Gate
The tunnel or submarine injection molding design allows the flow of plastic material to the cavity edge below the parting line.
This gate configuration allows easy automatic trimming (degating) before ejection. It is the go-to gate design for small components and high cavitation molds, requiring automatic trimming.
Unlike edge gates, these gates are usually narrow, limiting the amount of molten plastic that enters the cavity. This may result in high shearing and pressure build-up.
Consequently, you should use this gate type only when fabricating small structures. Using this gate for large components will increase production costs due to large production cycles, increased molding defects or products with poor finishes needing more post-processing operations.
Pros of Submarine or Tunnel Gate
- The gate placement below the parting line allows you to conceal gate marks and blemishes easily.
- The narrow configuration of these gates makes them suitable for molds with multiple cavities, considering they only take up a little space.
Like the tunnel gate, cashew gates are placed below the parting line, making it ideal for automatic trimming. Its design is such that it forms an arc, smoothening the point of contact between the gates and the part. Therefore, they are suitable for smooth and consistent surface finishing components.
However, because of their size, they share the limitations of tunnel gates. Also, it is always difficult to maneuver the gate region during ejection because of the curved (arc) configuration.
Therefore, you must be extra careful when using cashew gates as they are prone to breakage during ejection. An easy way to reduce this risk is to use removable gate inserts – they are easier to remove, even when broken.
Pros of Cashew Gate
- Produces products with smooth and uniform finish
- Like tunnel gates, they are suitable for molds with several cavities.
Pinpoint gates are usually machined below parts near the ejectors. After the plastic cools, solidifies into the desired shape, and is ready for ejection, it detaches from the pinpoint gates with little to no marks or blemish.
This feature makes it ideal for components that require fine finishing and automatic degating.
Like other gates that produce a fine finish, pinpoint gates have a narrow dimension of not more than 2 mm. Their thin design makes them an excellent fit for molds with several cavities.
Also, pinpoint gates are suitable for molds that require multiple gates for better results. Consider using polymers with fast-flowing properties to prevent high-shearing heat.
Pros of Pinpoint Gate
- Products have a fine and consistent finish
- Suitable for molding that requires multiple gates
Diaphragm gates are designed for fabricating cylindrical components with a hollow diameter like a pipe. The gates are positioned at the center to allow for uniform flow and distribution of molten plastic into the mold.
Placing this gate at the center helps to reduce the chances of shrinkage, allowing the hollow to form. The placement also gives the plastic enough stability as it cools off into the open-ended cylinder.
Without diaphragm gates, fabricating pipe-like structures with injection molding will be almost impossible, as most gates tend to cause the filling of the center with plastic.
In addition, this gate is suitable for fabricating large structures because it allows a large volume of molten polymer to flow into the mold.
Furthermore, since the gate is usually at the center – the hollow, there are little to no markings or gating imperfections on the end products.
Pros of Diaphragm Gates
- Produces smooth plastic parts with little to no imperfections
- Allows uniform flow of molten plastic material
- Prevents shrinkage of plastic components to the center
Hot Runner Valve Gate
The hot runner mold system has specific temperature and pressure settings designed to keep molten plastic in liquid form till they enter the mold.
Hence, the hot runner gate valve further maintains temperature and pressure settings as the plastic passes through it. The gates have similar diameters as the runners and spruces.
However, this gate design contains a movable pin in the hot runner tip. Upon pushing back the pin, the flow of molten plastic in the hot runner mold system pauses, forcing those in the gates into the mold.
This design feature helps to control the flow of plastic during injection molding. This prevents materials from accumulating at the gate and improves molding efficiency.
Consequently, hot runner valve gates are for fabrication with tight tolerance and high demand for precision and accuracy. Also, they are ideal for design specifications that require intermittent filling of mold, as opposed to filling at once.
Pros of Hot Runner Valve Gate
- It gives exact and accurate parts
- Eliminates the need for trimming
- Suitable for controlling multiple gates simultaneously
- Provides control of the injection speed
- Reduces material wastage
Hot Runner Thermal Gate
The hot runner thermal gate is another suitable design for the hot runner mold system. Like the hot runner valve gate, it permits temperature and pressure modulation.
This gate is usually placed at the top of the mold parting line. However, it has no pin that allows total control of plastic flow.
This gate system is quite different from any other. Here as liquefied plastic flows through the gate, there’s a valve that pauses flow allowing the molten polymer to form a cold slug – the thermal gate.
It acts as a temporary stopper till the slug melts and continues the flow of plastic polymers into the mold. This process continues for all batches of molten plastic that pass through the gate.
Unlike the hot runner valve gate, the thermal does not give you complete control of the plastic flow, but it is still quite efficient. It is also suitable for complex structures with fewer tolerances and precision specifications.
It is the go-to gate design for fabricating conical and circular-shaped plastic components.
Pros of Hot Runner Gate
- It provides a modicum of injection molding control
- Reduces shear temperature, reducing molding defect
- Injection flow control is almost automatic
- Produces structures with reduced degating requirements
How Injection Molding Gate Design Can Affect Your Part’s Quality
The function of injection molding gates is to force and permit the passage of molten plastic into the cavity. This may seem relatively straightforward.
However, the gates tend to speed up the flow, causing heat build-up and affecting the fabrication’s end product.
If the plastic flow speed is not adequately regulated, it may result in significant defects, such as jetting, reducing the quality of injection molded parts.
Let’s take a more detailed overview of injection molding gates’ effects on fabrications.
When gate sizes are too small, molten plastics accumulate at the opening, leading to pressure build-up and causing a rapid flow of the polymers into the mold cavity. This scenario results in jetting – liquefied plastic not sticking to the mold surface, caused by increased injection speed.
Jetting affects the appearance of molded parts as they develop wavy distortions and folds of jet streams instead of smooth flowing.
Manufacturers often refer to such distortions as worming. To reduce jetting and worming from your fabrication, increase the gate size or reduce pressure to reduce injection speed. Also, optimizing the gate design may effectively promote contact between molten plastic and the mold cavity.
Overheating may result from friction or a high injection flow of molten polymers as they pass through the gates. Indeed, the process takes place under high temperatures, but when the heat build-up becomes excessive, components of the plastics may begin to degrade. The degradation may reach molecular levels, destroying chemical bonds.
Therefore you must regulate the injection flow speed of molten materials through the gates, as if overly reduced to reduce overheating, it may also result in other molding flaws.
Moreover, it may affect the mechanical strength of the final product. Also, reducing injection speed may slow production, limiting the number of parts created per hour. A slower production cycle may increase production costs.
Using multiple gates is an effective way to prevent overheating without necessarily overly reducing injection speed. You get to spread the building pressure across each injection molding gate, reducing any risk of excessive heat build-up.
However, you must also exercise care, as having molten polymers flowing through multiple openings may lead to gas trappings, multiple weld lines and markings at the junction between the gates and the molds.
Factors to Consider When Choosing a Gate
Every injection gate type varies in one way or another, offering a specific function to suit the design of your part. Regardless, you should consider the following factors when selecting a gate for your fabrication.
The gate placements determine the ease of degating – that is, separating gate components from the parts. Also, some placement may cause blemishes and leave scars on the product.
Therefore, you must carefully decide on the gate placement of your injection molding design.
The size of injection molding gates must be adequate to allow proper shearing and flow of the molten plastic. We have reiterated the adverse effects of gates with overly small openings.
They result in high shearing rates, causing an increase in pressure, and hastening the wear and tear of gate components, leading to injection molding imperfections. You just have to regulate the gate size, as when they are too large, it may also result in molding defects.
Besides the mold cavity, molten plastic cools and solidifies at the gate, resulting in attachment and protrusions on the end product. Therefore, the need for degating – trimming off such bumps.
You can trim and degate manually after the part solidify or automatically by the injection molding machine. The kind of gate utilized for your fabrication will determine the process of degating.
Part Shape and Post Processing
Before selecting a gate type, you need to consider the shape, structure and configuration of the part you are fabricating. Each gate design has certain specifications that suit a particular shape and finish.
For example, a tunnel or submarine gate is best suited for small-part fabrication, producing a smooth, consistent surface.
Choosing the best gate design for your fabrication reduces post-processing needs. Also, consider inputting features like an undercut into your plan when fabricating structures with complex shapes.
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Gate type and design are integral components of injection molding. The difference between a perfect injection molded part and a defective one may be the choice of gate design.
Understanding the difference between each gate will help you make the right choice for your product. Thus, reducing the risks of imperfections, markings and blemishes in your end product.