A Deep Dive into the Various Types of Injection Molding Gates

In injection molding, each cavity within a mold requires a gate—a small opening that directly impacts the quality and precision of the final product. The design of these gates is critical in shaping high-quality plastic components and ensuring efficient manufacturing. A deep understanding of this aspect is essential for achieving optimal product outcomes and maximizing production efficiency.

This article will guide you through the key considerations for effective injection molding gate design.

Why Injection Molding Gates are Important for Molded Parts?

Injection molding gates have a direct effect on a plastic mold’s eventual outcome. With a well-designed gate structure, there is proper management of the direction and volume of molten plastic flowing into the mold. This prevents the flowing of molten plastic towards the nozzle/runner rather than flowing into the mold.

Furthermore, mold gates push the molten plastic to touch all areas of the mold before it undergoes cooling. This helps to prevent the molten plastic from hardening untimely and unevenly. It also prevents plastic parts from deformations like stress fractures or breaks.

In addition, through dissipation, this gate produces heat. A proper gate design raises the polymer temperature thereby preventing weld lines and flow marks from forming. With a simplified process for injection molding, product designers and manufacturers can get rid of runners to ensure an easier post-processing treatment.

There are different types of gates in injection molding. We will categorize them under manually trimmed and automatically trimmed types.

Manually Trimmed Mold Gates

Edge Gate

The edge gate is a simple and cost-effective mold design that proves highly efficient in injection molding. Its straightforward design makes it easy to produce and modify as needed. Edge gates are particularly useful for parts with thicker wall sections or larger dimensions, where a consistent flow of plastic is crucial.

One advantage of edge gates is their ability to accommodate a wider cross-sectional area, allowing for increased plastic flow into the mold. Additionally, their design supports longer hold times due to delayed gate freeze, enhancing the overall molding process.

Moreover, edge gates do not require specific resin types, making them a versatile option when streamlining the injection molding design is a priority.

Fan Gate

As the name implies, fan gates feature a fan-like shape, beginning with a wide opening that gradually expands into the mold cavity. This design ensures consistent thickness across the part, making fan gates ideal for large components where stable flow is crucial.

Fan gates also help prevent defects in injection molding by promoting dimensional stability. Their ability to minimize flow marks and directional stress makes them particularly effective for producing thin, flat products. Additionally, they are well-suited for use with polycarbonate plastics.

Tab Gate

Tab gates are well-known gate styles containing auxiliary tab sections. They are suitable for flat and thin parts requiring low shear stresses. They are also great for materials like acrylic, ABS, and PC. This tab-like feature helps to confine the stresses within the gate region, still ensuring the part maintains its quality.

Direct or Sprue Gate

Direct gates are a straightforward and widely used option in injection molding. In this design, the sprue directly feeds the molten plastic into the mold cavity, enabling the injection of large volumes with reduced feeding time and lower injection pressure. The simplicity of direct gates makes them easy to design and allows for high tensile strength in the molded parts.

However, due to the manual removal of the sprue, direct gates can leave marks on finished products. Despite this, their economic efficiency makes them a practical choice, particularly for non-aesthetic, deep single-cavity molds like those used in consumer products and household appliances, including TVs, bins, and washing machines.

Disc or Diaphragm Gate

The disc, or diaphragm gate, shares a similar tapering design with the sprue gate but is particularly suited for angular-shaped molded parts. This gate type effectively reduces warping and minimizes weld lines, although factors like pressure, speed, and temperature during molding can still influence the quality of the final product.

Disc gates are suitable for larger parts that require significant amounts of resin. Additionally, diaphragm gate designs are versatile, working well with most resin types, making them a reliable choice for various product designs.

Ring Gate

Ring gates sit around the outer edge of the mold cavity, making them a perfect choice for long, thin-walled parts like pipes. This arrangement, similar to the inner disc gate, allows the material to flow evenly into the mold, helping to prevent welding marks on the final product.

Despite their advantages, ring gates can be challenging to remove, often leaving visible marks on the outer surface of the plastic part. They are most commonly used in multi-cavity molds for smaller components.

Automatically Trimmed Mold Gates

Hot Tip Gate

Hot tip gates are specifically designed for use with hot runner systems, where heated nozzles are employed. Unlike gates positioned at the parting line, hot tip gates are typically located at the top of the part, making them ideal for conical and round shapes where uniform flow enhances concentricity.

These gates function as an extension of the molding press’s barrel and screw, allowing for a smaller gate opening due to the higher resin temperature. The absence of a runner and the increased heat can also help push the material into thin features.

Hot tip gates leave a small raised nub on the part’s surface, which can be easily covered with a decal or may require minimal trimming.

Pin Gate

Pin gates sit on the B-side of the mold, near the ejector pins. They work well for three-plate molds, where runner channels distribute across different plates. This design allows the mold flow to split and reach the cavity through multiple gate locations.

The small size of pin gates makes them easy to trim when the mold opens. However, one drawback is the higher scrap rate due to the larger runner system.

Submarine Gate

Submarine gates, also known as tunnel gates, are positioned below the mold’s parting line, allowing for automatic trimming during ejection. These gates use a narrow channel that connects to the cavity near the parting line, filling the cavity from underneath. The draft angle helps ensure the smooth ejection of the molded parts.

Submarine gates are designed to allow only a small amount of molten plastic into the cavity, which is why they are suitable for molding small components. However, using them for larger parts can lead to poor finishes due to shear heating and extended molding times.

Optimizing Gate Design for Better Quality and Efficiency

Gate Location

The placement of gates in injection molding is crucial, as some locations are more challenging for separation and can impact the quality of the final product. Incorrect gate placement can lead to the formation of weld lines and deformities in the molded part, so careful consideration is essential during the design process.

The location of a gate significantly affects the part’s quality. For optimal results, place gates away from cores or pins to avoid disruptions in the flow of molten plastic, which can cause weld marks. Positioning gates near thick-walled areas of the part ensures the cavity fills. Additionally, since gates can leave blemishes, avoid placing them near visible surfaces.

Different gate types require specific placements. Some gates are positioned near the mold parting lines, while hot tip gates are located at the top of the part. Submarine gates can be moved away from parting lines, offering more flexibility in mold design. Pin gates are typically located on the mold’s B-side, near the ejector pins.

Gate Size

Gate size is key to optimizing gate design. The mold cavity’s volume typically determines the gate’s width, while the part’s nominal wall thickness influences the depth.

Different materials have varying flow rates, making the type of plastic an important consideration. For instance, materials like polypropylene and polyethylene, which flow easily, may require a gate depth equal to just 50% of the wall thickness.

Proper gate sizing ensures that the mold experiences the correct amount of shear as the material flows through the machine, facilitating proper filling. However, smaller gates are prone to higher rates of shear heating, and incorrect sizing—whether too large or too small—can inadvertently increase flow pressure. Therefore, selecting the appropriate gate size is essential for achieving optimal results.

Matching Gate Types with Plastics

Here, we will be matching the different gate types with the plastics they are suitable for. Let’s explain this using a table.

• PVC: Polyvinyl chloride
• PE: Polyethylene
• PP: Polypropylene
• PC: Polycarbonate
• PS: Polystyrene
• PA: Polyamide
• POM: Polyacetal resins
• ABS: Acrylonitrile butadiene styrene
• PMMA: Poly (methyl methacrylate)

Conclusion

Injection molding gates don’t just affect the fill rates, they drive part quality, cycle times, and tooling costs. Engineers and designers must understand ways by which injection molders can optimize these gates, which connect the runners and sprues to mold cavities. Gate type, size, location, geometry, and trimming method also determine performance and quality.

But navigating these complexities doesn’t have to be a challenge.

Turn to RapidDirect. We have the expertise and experience to offer custom injection molding services that ensure the production of high-quality molded parts, all while meeting your deadlines. Whether you’re just starting or are a seasoned professional, RapidDirect is your partner in driving success.

FAQs