Optimizing Mold Runner Design: Key Considerations for Feeding System Design

mold runner and sprue

Understanding the terminology associated with the runner system can be challenging, especially for newcomers. Let’s establish the definitions of each component involved to clear up any confusion. By gaining clarity on these terms, it becomes easier to comprehend and discuss the different elements of the runner system effectively.

The runner system, also referred to as the feeding system, consists of multiple components, including the runner, sprue, gate, and cold slug. The runner, a vital part of the system, can be further classified into two sections: the primary runner, the main runner, and the sub-runner. These components work together to ensure the smooth flow and distribution of molten material throughout the mold cavity during the injection molding.

In our previous discussions, we explored the concept of sprues as an integral part of the feeding system. Today, let’s delve into the next component of this system – the mold runner. Understanding mold runners’ role and design considerations is crucial for optimizing the injection molding process and achieving desired product quality. Let’s explore the world of mold runners and their significance in the overall feeding system.

Do all molds have sprue and runner?

In a hot-runner system, the traditional sprue and runner are not present. Instead, the molten plastic is directly injected into the mold cavity through heated channels called hot runners. The hot-runner system eliminates the need for a separate sprue and runner system by keeping the plastic in a molten state within the channels, allowing for more efficient and controlled injection of the material into the cavities.

The hot-runner system typically consists of heated nozzles, manifold systems, and individual hot runners delivering molten plastic to each cavity. The temperature of the hot runners is precisely controlled to ensure proper flow and distribution of the plastic material. This system offers advantages such as reduced cycle time, minimized material waste, and improved part quality, particularly in applications requiring high precision or multiple cavities.

Design Guidelines for Mold Runners

A well-designed runner can significantly impact the overall performance and efficiency of the injection molding process:

Design Principles of mold runners

· The melted plastic should be fed into the cavity quickly and smoothly to minimize heat and pressure loss.

· The melted plastic should be able to be fed into the cavity from each gate simultaneously under the same temperature and pressure conditions.

· A larger cross-sectional area of the runner is beneficial for filling the mold and ensuring sufficient holding pressure.

· However, from the perspective of saving plastic material, the cross-sectional area should be as small as possible.

· A larger cross-sectional area will increase the cooling time, so minimizing the cross-sectional area can aid in efficient cooling.

· To save material and facilitate cooling, having a small surface area to volume ratio for the mold runners is advantageous.

Shape and size of mold runner cross-section

The shape of the mold runner cross-section

Rectangular cross-section: Rectangular-shaped runners are commonly used in mold designs. They offer advantages such as ease of manufacturing, simple tooling design, and uniform flow distribution. The dimensions of the rectangular cross-section can be adjusted based on the specific molding requirements of the part.

Trapezoidal cross-section: Trapezoidal-shaped runners are another option in mold runner design. This shape helps promote better flow and reduces pressure drop, resulting in improved filling of the mold cavities. The wider end of the trapezoid is typically connected to the sprue, while the narrower end is connected to the gate.

Circular cross-section: In some cases, circular-shaped runners may be used. These runners offer excellent flow characteristics and are particularly suitable for parts with complex geometries or when a balanced flow is required. The diameter of the circular cross-section should be carefully determined to ensure optimal flow and minimize pressure loss.

Semi-circular cross-section: A semi-circular-shaped runner features a half-circle profile. This shape promotes smooth material flow and helps minimize pressure drop. It is often used when balanced flow and reduced pressure loss are critical. The diameter of the semi-circular cross-section should be appropriately sized to accommodate the flow requirements of the specific injection molding process.

U-shaped cross-section: A U-shaped runner has a curved bottom and two vertical walls that form the shape of a “U.” This design facilitates efficient material flow and allows for easier separation of the runner system from the molded part. The U-shaped cross-section is commonly employed when easy removal of the runner system is desired or when gating is located at the bottom of the part.

types of mold runner cross-section
Types of mold runner cross-section

The selection of the cross-sectional shape depends on factors such as material properties, part design, mold layout, and production requirements. Each shape has its advantages and is chosen based on the specific needs of the molding process.

The shape and size of the mold runners depend on various factors, including the product design, mold construction, and the specific requirements of the injection molding process. While the product size and wall thickness may influence the runner design, it is not accurate to say that larger cross-sectional runners are always more effective in facilitating the filling process. Material flow behavior, part geometry, gate location, and process parameters determine the optimal runner design.

Additionally, the length of the runner does not directly affect the viscosity of the plastic. The material properties and processing conditions primarily determine viscosity.

MaterialsRunners Diameter(mm)
Recommended Diameter Of Runner For Common Plastic

Mold Runners Arrangement

There are two types of mold runner arrangements: balanced and unbalanced. In a balanced runner system, the runners’ length, shape, and cross-sectional dimensions from the sprue to each cavity are designed to be equal. This helps achieve thermal balance and plastic flow balance in each cavity, resulting in consistent part quality. On the other hand, an unbalanced runner system allows the plastic to enter each cavity at different times, leading to variations in the filling process and potentially producing different parts. However, unbalanced runner systems can offer advantages such as more compact cavity arrangements, reduced template size, and shorter overall runner length.

Balanced Arrangement of Runners
Balanced Arrangement of Runners
Unbalanced Arrangement of Runners
Unbalanced Arrangement of Runners

Whether the runner system is balanced or unbalanced, it is important to ensure that the cavities are symmetrical with the center of the mold base. This ensures that the projected center of the cavities and runners align with the center of the clamping force of the injection machine. By doing so, additional tilting moments during an injection can be avoided.

A balanced runner system is advantageous as it allows for consistent injection and holding pressure across all cavities. This is particularly beneficial for multi-cavity molds where maintaining uniformity in producing all products is desired.

The surface of the mold runners

The mold runner’s surface roughness should not be excessively low to avoid the risk of cold material being transferred into the cavity. Typically, a surface roughness value of 1.6 μm (Ra) is recommended to ensure the proper functioning of the runner system.

Form of connection between the mold runner and gate

The connection between the mold runner and gate can adopt various configurations, tailored to the specific design and requirements of the injection molding process. There are several common forms of connection:

Direct sprue gate: In this configuration, the mold runner is directly connected to the gate, allowing the molten plastic to flow directly from the runner into the cavity. This type of connection is straightforward.

Submarine gate: Also known as a sub gate, this form of connection involves placing the gate below the part’s surface. The mold runner feeds the molten plastic into the sub gate, directing the flow into the cavity. Submarine gates often achieve a gate vestige-free appearance on the part surface.

Edge gate: In an edge gate configuration, the gate is positioned at the edge of the part. The mold runner delivers the molten plastic to the edge gate, which allows it to enter the cavity. Edge gates are commonly used when a gate mark on the part is acceptable or easily concealed.

Fan gate: A fan gate involves multiple gates arranged in a fan-like pattern. The mold runner distributes the molten plastic to the various gates, allowing for simultaneous filling of the cavity from multiple points. Fan gates are beneficial for achieving balanced filling and reducing the chance of weld lines.

Tab gate: A tab gate, also known as a sub-runner gate, is a small runner connected to the gate. The mold runner supplies the molten plastic to the tab gate, which then directs it into the cavity. Tab gates are often used when gating on the part surface is undesirable.

These examples represent just parts of the various ways in which the mold runner can be connected to the gate. The selection of the specific form depends on factors such as part design, material properties, desired appearance, and manufacturing considerations.

Is it necessary to design the sub-runner in the mold runner design

A sub-runner is typically used when there is a need to divide the flow of molten plastic into different directions within the mold cavity. It can help facilitate the filling of complex or multi-cavity molds by directing the flow to specific areas or components of the part.

However, in some cases, a simple runner design without a sub-runner may be sufficient to achieve the desired mold filling and part quality. Whether to incorporate a sub-runner or not depends on the specific requirements of the part design and the injection molding process.