In injection molding, the molten plastic enters the mold cavity through a gate. While this process may sound simple, it is quite complex. There are many types of gates, and the placement is crucial to ensure a quality product can be manufactured. In this post, we uncover the importance of gate placement and dive into the types of gates that are most often used.
What is a Gate?
In injection molding, the mold cavity is filled with molten plastic. It doesn’t happen directly, like filling a balloon with a faucet. To create a controlled and even distribution, the nozzle feeds molten plastic into a sprue in the mold. From there, it encounters a system of runners that feed the plastic into a gate or gates, which connects to the mold cavity, creating the part shape. The gate builds pressure by constricting the material flow as it enters the mold cavity. Without the gate, there would be a pressure drop as the material flowed from a narrow runner into a wider cavity, resulting in a slower flow and potential defects. Often, when the mold cools, the sprue and runners are still attached to the part and have to be cut off.
The Importance of The Injection Gate Location
Gate placement isn’t just a matter of convenience; it’s integral to the product’s quality. The gate manages the volume and direction of the plastic entering the mold cavity. Poorly placed gates can lead to a variety of issues, like uneven material distribution, injection molding flash, weak spots, and fractures. Additionally, the gate’s location and size can impact the production cycle time.
The goal is to find the optimal position that allows for uniform filling and efficient cooling. Engineers must carefully consider the gate location in relation to the part’s geometry, material properties, and production requirements.
Generally, gate placement should be:
- Located in a thicker section of a plastic product so that the molten plastic can flow from the thick section to the thin section to ensure sufficient filling.
- Designed to reduce travel distance, directional changes, and energy loss of the melt flow to minimize pressure loss.
- Placed in an area where the witness mark will not affect the product’s appearance.
- Located in an area of the product that will not be subjected to high tensile loading during use since gate locations generally have inferior mechanical properties.
- Considered carefully when using more than one gate, as the weld line where the flows meet must not interfere with the part’s structural integrity.
Simulation tools and Moldflow analysis can assist in optimizing the variables and show the engineer how the part will fill, allowing them to make changes to get the desired outcome.
Types of Injection Molding Gates
The gate style and geometry are as critical as the gate location. Even experienced designers can underestimate the importance of gate geometry. The gate style and geometry chosen can affect the material’s force, angle, and temperature and the appearance of the part. Gate geometry refers to the physical size and shape of the gate and includes length, width, depth, taper, and orifice shape. For example, a round orifice will have a different flow than a thin, wide rectangular orifice – the round creates a straight jetting stream, and the rectangular a fanned-out stream.
The gate style refers to the specific type of gate that is used.
- Direct or sprue gates directly channel the melted plastic from the machine nozzle into the mold cavity. This is a simple style, but it leaves a large mark on the part where the gate is removed. These are ideal for single-cavity molds and large parts.
- Edge gates, commonly used for flat, rectangular parts, are positioned on the edge of the part and inject material from the side. These are versatile and suitable for many shapes and sizes but may lead to uneven filling and weld lines.
- Fan gates spread the material over a wide area, facilitating the even filling of the mold cavity. It’s ideal for thin, flat parts that require uniform wall thickness. However, they consume more space than other gate types.
- Submarine gates are installed below the parting line, eliminating the need for post-mold trimming. They’re useful for automated operations and are common in multi-cavity molds. They are best for complex geometries where gate marks are not a concern.
- Cashew gates are curved and allow for more discreet placement. Typically used for round or curved parts, they result in cleaner seams and reduced stress points. This gate works well for parts where appearance matters.
- Pin gates are small, cylindrical gates that minimize the gate mark on the part. They’re generally used in multi-cavity molds and are ideal for small, intricate parts that require minimal post-processing.
- Diaphragm gates are best for parts with a large, flat surface, allowing material to spread evenly from the center outward. They offer even filling and minimize sink marks but are more complex and expensive to implement.
- Valve gates in a hot runner system use a mechanical valve to control material flow. They offer precise control over filling, making them suitable for high-volume, high-precision applications.
- Thermal gates in a hot runner system rely on temperature control to open and close the gate. These are generally easier to maintain than valve gates but may offer less precision. These gates are ideal for high-volume production where tight tolerances are not critical.
Injection mold gate design and placement are crucial for ensuring the efficiency of the molding process and the quality of the resulting part. Optimizing the injection molding gate helps reduce production costs and optimize cycle times.
Putting It All Together
We started this blog by saying that putting molten material through a gate and into a mold cavity was not as simple as it sounds. So many variables impact the success, including part size and geometry, the gate size, the gate orifice size and shape, the number of gates, and the gate(s) placement. Finding out there are issues with the gate once it gets into production can be a costly mistake. What looks good theoretically may not transfer practically in actual use on the shop floor.
To avoid costly mistakes, work with us early in the design phase. At SEA-LECT Plastics, we have a team of experts who understand the polymer science, rheology, and thermal dynamics involved in producing a quality part. We offer design and engineering support to help you make critical decisions to ensure your part can be manufactured to your specifications. Let’s get started.