Injection molds are composed from a variety of different mechanical features which form the detail found in injection molded, rubber molded, and plastic molded parts. The style and location of the gate is also integral to the strength and robustness of the subsequent part. The mold can also vary in the method of the melt flow.

Slide Core Mold Base
Stripper Plate Mold
Standard Mold
Three Plate Mold
Thread Forming Mold

Runner Systems
Cold Runner
Hot Runner
Hot Runner Manifold and Fixed Nozzle System
Hot Runner Manifold and Valve-Gated Nozzle System
Typical Hot Manifold Configurations
Hot Manifold Arrangements

Custom Injection Molding

Injection molds are composed from a variety of different mechanical features which form the detail found in injection molded parts. The style and location of the gate is also integral to the strength and robustness the subsequent part. The mold can also vary in the method of melt injection. Mold Components include:

1. Slide Core Mold Base. The Slide Core is used to create the general shape and is withdrawn before the part is ejected from the mold. It is used when an undercut is necessary which cannot be done though a standard mold opening.
2. Stripper Plate Mold. A Stripper Plate Mold is used when blades or pins are insufficient for removing parts from a mold.
3. Standard Mold. The Standard Mold is generally used with an ejector plate when line runners, ejector pins, sleeves and/or blades are sufficient to remove the part from the mold.
4. Three Plate Mold. The Three Plate Mold is used when the gate edge position is unacceptable or would be difficult to fill. It allows for uniform part filling without part-weakening weld lines.
5. Thread Forming Mold. The Thread Forming Mold is used with a hot manifold when internal threads are needed and the part cannot be ejected by a standard stripper plate mold. The manifold is present to eliminate the runner.

Cold Runner Systems - Plastic Molding

In a cold runner mold, the runner is cooled and subsequently ejected with the part. In each cycle, both a part and a runner are produced. The clear disadvantage of this system is the excess plastic generated. Runners are either disposed of or reground and reprocessed with the original material, which adds a step in the manufacturing process. Also, adding a regrind step will increases variation in the injection molding process, and could decrease the plastic's mechanical properties. Finally, material in the runner has the tendency to lose its heat on the way to the gate potentially requiring higher temperatures or pressures from the molding source. Despite these disadvantages, there are many noteworthy advantages to using a cold runner mold. The mold design is very simple, and much more inexpensive than a hot runner system. The mold requires less upkeep and less skill to set up and operate. Color changes are also very straightforward, since all of the plastic in the mold is ejected with each cycle.

There are two major types of cold runner molds: two-plate and three-plate. A two-plate cold runner mold is the simplest type of mold. It is called a two-plate mold because there is one parting plane, and the mold splits into two halves. The runner system must be located on this parting plane; thus the part can only be gated on its perimeter.

Hot Runner Systems

Hot Runner systems are typically used to alleviate the need for runners, which need to be removed with the molded parts of an injection mold. Removing the need for runners creates a simpler, cleaner system and increases efficiency.

By removing the runners, the entry point to the mold cavity will have a hot drop (or nozzle) balanced in the gate area. This will keep enough controlled heat to allow continuous molding in the drop area adjacent to the mold cavity. If runners are reduced but not entirely eliminated, multiple parts will be fed by individual runners. The runners are designed to evenly supply all parts within each drop zone. In both cases, a heated manifold is placed behind each drop in order to maintain heat evenly within the manifold assembly. The system is designed to be efficient with heat, and not transfer it into any unintended areas. The mold will disperse heat predictably to solidify the molded parts as efficiently as possible to keep cycle times to a minimum.

Advantages of a Hot Runner System Over a Cold Runner System include:

• no runners to disconnect from the molded parts
• no runners to remove or regrind, thus no need for process/ robotics to remove them
• having no runners reduces the possibility of contamination
• lower injection pressures
• lower clamping pressure
• consistent heat at processing temperature within the cavity
• cooling time is actually shorter (as there is no need for thicker, longer-cycle runners)
• shot size is reduced by runner weight
• cleaner molding process (no regrinding necessary)
• nozzle freeze and sprue sticking issues eliminated

Hot Runner Manifold and Fixed Nozzle System

Hot Runner systems are typically composed of a heated manifold and a drop. The drop area is where we generally find the most variety of design and flexibility. Some of the designs incorporate multiple pin point gates on the drop. Others have multiple edge gate drops that allow the flow to be perpendicular to the drop center line. There is often variation in the pin point types, and some of the hot drops with screw-in tips are available in differing shapes. Some hot drops have flat faces that allow material to enter the cavity through a small opening within the drop, as opposed to off center and into the cavity gate.

Some of the forces that can affect the manifold assembly are: force from the molding machine nozzle, injection pressure, and heat expansion. These forces are alleviated by a design that limits contact by the inner and outer areas (inner being at the drop head diameter and outer as a pressure disk in line with the hot drop across from the manifold). There is also a support across from the sprue to keep pressure from adversely affecting the manifold. Manifold assembly components are designed to have around 1/10 inch between them to act as an insulating gap. This helps separate the high molding heat in the hot runner manifold from the cooler temperatures in the cavity plates and mold base.

Hot Runner Manifold and Valve-Gated Nozzle System

In some Hot Runner systems, the heated manifold and drop assembly also contains an air or hydraulically-actuated gate valve pin. This adds a moving valve pin where the previous conformation used a stationary drop.

The advantages of a moving valve-gate system are numerous. Cycle times can be improved, giving the press more time to solidify. In some cases the molded part does not allow for a standard gate remnant in the preferred area and can be substituted with a valve pin with an identical finish. When the thickness of the material creates a reduced flow, a valve gate can often improve the quality by allowing lower injection pressure and more rapid part filling. Finally, should the material have a sticky or stringy composition, the valve gate may make it possible to inhibit manifold plastic from entering the cavity area between shots, thus preventing overlap that could compromise the mold.

Valve gates can be activate hydraulically or by air depending on the molding material are the force expected to open and close the gate. Generally, hydraulic systems are integrated when increase pressure is expected. Activation of the gates must be in sync with the overall injection cycle of the system and are generally CPU-controlled.

Typical Hot Manifold Configurations

Hot Manifold Arrangements

A hot manifold system can have as many as eight successive drops. This illustration shows possible conformations for these systems. Manifolds are typically placed in the centerline of the runners. Heat sources are then positioned and the entire system blasted into a single unit.

Runners

Runner channels can have different shapes and properties.

Round runners are beneficial in that they provide even flow through the channel. They have the negative effect of not keeping the runner on one specific side of the parting line. It also requires both sides of the parting line be machined. Extended runs have the tendency of forcing parting line steel into the runner area. The illustrated 10 degree angle slows the process.

Trapezoidal runners provide a less even flow, but are confined to one side of the parting line. These runners will stay with the mold half if it is cut into unless puller pins are integrated into the channel from the opposite mold half. The angled walls allow for release from the mold.

Runner undercuts are used to release the sprue from the nozzle and then pull it until the sprue tail is flush with the parting line. This allows for the runner to drop from the mold without the sprue tail attaching to the extension nozzle sprue bushing.

Gates

A gate creates the opening for plastic to enter the mold cavity. All of the illustrated gates, with the exception of the Sprue Gate, are of a conservative nature. Restrictive gates can be used when the smaller entry point is simpler to remove from the plastic part and would not create a visible mark. The restriction occurs when plastic crosses the gate and creates friction in the plastic (which subsequently causes the plastic to effectively re-heat). When traveling long distances, this re-heating it essential to an even flow, and helps reach the complete mold cavity. Manifold drops are individually heated and thus do not depend on this friction to reach more remote regions of the mold. Therefore, plastic parts that are derived from hot manifolds are more structurally sound.

Gates Styles defined:

An Edge Gate is used when multiple parts are attached to a runner for orientation or control. It would also be used in a situation where marks on the part walls would be more objectionable than on the P/L. It is also the most commonly used gate.

A Sub Gate is another popular gate style. It is often used in high cavitation molds as it provides for both the plastic part and runner separation within the molding cycle. Sub gates are not suggested for shallow parts.

A Sprue Gate is often used on larger molded parts in a single cavity. It allows for a parts that has less stress and higher strength. The disadvantage is that the sprue remnant is noticeable regardless of whether the sprue is cut or machined after molding.

A Fan Gate is used to deliver plastic to a wide area to minimize backfilling and reduce imperfections and stresses in the part. The gates are then clipped from the runners after molding.

A Flash Gate is a more extreme version of the fan gate. The difference is that a longer gate can be integrated as the runner is cut parallel to the edge of the part and the gate is cut perpendicular to the runner. Like the fan gate, it must be then clipped from the runners after molding.

The Ring Gate is used when a round part needs to be molded without weld lines. For maximum effectiveness, the part must require a hole in the middle. Gases form at the center of the part and must be removed through a center hole.
degrees off vertical to help alleviate these issues.