Gate types in injection molding.
What is a Gate?
In the injection molding process, in the mold, there is an entry point for the molten plastic to fill the part called the gate. The gate is the transition from a mold runner system into the part cavity which separates the runner from the plastic part.
The size, shape, and placement of this access point is crucial to the quality, cycle time, and overall molding efficiency of the finished product. There are several different gates to consider during the Design for Manufacturing (DFM) process and it all depends on the part and its end use.
During the cooling process, excess plastic hardens in the gate which creates a tab of plastic that must be removed – in a process called degating – from the final part. The removal and residue of this excess plastic drives many decisions made during the design for manufacturing (DFM) stage of the part and mold.
Automatic vs manual degating
The process of degating can be done two ways, either automatically by the functions or features of the mold or manually by an operator after the part is ejected from the mold. The style of gate will determine which degating process is required.
Manual trim gates include:
Edge gate – Placed on the edge or parting line of the mold, this gate is commonly used on square or rectangular parts that are thick in design and utilize multicavity tooling.
Tab gate – Used for flat or thin parts, a tab gate includes an extra piece of plastic attached to the gate to assist with clean trimming.
Sprue gate – Used for single cavity molds, this style of gate has straightforward construction where the sprue delivers material to the mold cavity directly. It is commonly used on cylindrical molds.
Automatic trim gates include:
Valve gate – A valve gate is used to instantly cut off the flow of plastic into the mold. As a result of this, there is a nearly invisible gate mark. This gate is mainly used for cosmetic purposes when the application or proper processing requires gating onto a visible area of the part.
Hot tip – This style of gate, along with the valve gate, requires a hot runner system that includes a heated nozzle to keep the plastic in a molten state. This allows for a faster cycle time and less waste. Ideal for round or conical shaped parts, this gate is placed on the top of the mold cavity(s). Great for high mold cavitation and reduced runner scrap.
Pin – This provides similar placement options to a hot tip but utilizes a series of plates to degate a cold runner system.
Sub Gate – Like the name suggests, a part with this style of gate is filled from underneath the parting line by utilizing a tapered channel. This gate is utilized when hiding blemishes is necessary.
Hot Edge Gate –This is a hot tip style gate that simulates a cold runner sub gate. It is directed into the side of the part with very clean separation.
Factors for deciding gate type and placement
When choosing the type of gate and its location on a molded part, there are many important factors to consider. Mold flow and pressure, part design, material, and end use are factors to analyze during part design.
Additional Factors to Consider:
– Flow Distance (Used to determine how many gates are needed)
– What is cosmetically acceptable
– Location (Ensure part is packed out evenly)
– Wall thickness (Targeted toward thickest part- to achieve best flow and avoid uneven cooling)
How gate type and location affect part quality
Determines the visual vestige or residue left on the surface.
Can provide a greener environmental footprint
May affect cycle time leading to cost impact
Mold cost. A cleaner gate or less runner waste may be offset by higher mold cost.
Gate opening size restrictions will affect the molding process
- Cosmetics of the part must be taken into consideration as there will always be a remnant of gate residue
- Placement also determines the part quality via the ability to properly fill and pack out the parts
- Filling the part in the thicker section reduces sink marks and warpage
- Direction of flow affects shrinkage and warpage
The Sussex IM Difference
Gate style and placement is a complex decision process which Sussex IM reviews and recommends at the time of quotation. Taking cost, speed to market, quality, aesthetics, and all other important specifications into consideration, we can ensure your part design is optimized for high-quality injection molding and end use. Overall, this decision affects mold and part cost.
To assist in the DFM process, Sussex IM has an experienced team of tooling engineers, project engineers, and tool & die makers who analyze all aspects of the part, from molding to the finished product and provide our customers with the best options for molding.
To speak with a Sussex IM engineer and learn more about part design for plastic injection molding, contact us today.