The Gates of Plastic Mold

The gates of plastic mold are a method or orifice linking the runner to the cavity. They feature a small cross-sectional region, which contrasts with the remainder of the feed method. This small cross-sectional region is necessary for several reasons:

• Prevention of Voids: The entrance freezes right after the cavity is filled, allowing the injection plunger to be withdrawn without the risk of voids being created within the molding due to suck-back.
• Ease of Degating: It enables easy degating, and in some molds, this process can be automated.
• Minimal Residue: After degating, only a small observation mark remains.
• Better Filling Management: Improved control over filling in multi-impression molds can be achieved.
• Reduced Packing: Packing the cavity with excess polymer to compensate for shrinkage is minimized.

Dimensions of the Entrance

The dimensions of the entrance can be considered in terms of the entrance cross-sectional area and the entrance length, the latter being known as the entrance land. The ideal dimensions depend on several factors, including:

1. The flow characteristics of the polymer to be cast.
2. The wall thickness of the molding.
3. The volume of polymer to be injected into the cavity.
4. The temperature of the melt.
5. The temperature of the mold.

No theoretical dimensions exist for the ideal entrance. The dimensions selected in practice for a specific part are usually based on past experience. However, for those lacking this knowledge, a guide to the measurements for each entrance type is provided. This guide applies, unless stated otherwise, to standard situations involving a molding with a wall thickness between 0.76mm and 4.1mm.

Types of Entrance

To achieve optimal filling conditions, the type of entrance must be carefully selected. In most cases, the choice is straightforward, as only one type of entrance will meet the specific requirements for the molding at hand. The commonly used types of entrances include:

• Sprue Entrance (f): A straightforward entry point that allows for direct injection into the cavity.
• Edge Entrance (n): Positioned at the edge, facilitating efficient flow into the mold.
• Overlap Entrance (a): Designed to overlap with the runner, enhancing material flow.
• Fan Entrance (b): Spreads the material evenly across the cavity for better filling.
• Diaphragm Entrance (g): Utilizes a flexible barrier to control flow rates.

Other types include ring entrance (h), film entrance (c), pin entrance (d), submarine entrance, and tab entrance (e).