Integrating Hot Runners Into Stack Molds

Over the past few years stack molds have been a viable solution for increasing productivity for high volume parts utilizing existing molding equipment. However, if not properly integrated, stack molds can be costly and may not yield the results you want. Because of the complexity of the tooling and the amount of time involved in proper setup, it is not advisable to use stack molds in situations where part volumes are such that the mold will be taken in and out of the press every few weeks. The following information is intended as a guide to integrating hot runner technology into stack molds. You will have to determine for yourself if this fits your needs.

Stack molds have been around for quite a few years now, but they were not utilized heavily until the last few years as machine and hot runner technologies have advanced, making stack molds more user friendly. This makes the integration more reliable and easier to maintain. However, there still is no substitute for common sense and good molding practices.

Items for Consideration

The Molding Machine
You will need to determine if the existing press will suffice or a new press is required. If you are using an existing press, there are some basic considerations that need to be taken into account in order to be successful in integrating the stack molds. The following are some critical areas:

• Will there be a need for automated part removal, such as with robots? If a robot is installed, is there enough room to hang the stack mold? Does the robot have enough travel to reach both part lines? A top-mounted robot may not work, so other alternatives will have to be investigated.

   

• Is the current water supply to the press adequate for the stack mold? Stack molds require additional cooling to realize optimum performance. Remember you are putting two molds into one press - this requires plenty of water to achieve proper cooling of the mold. The tool builder or designer will have to review the GPM requirements for the stack mold and determine if the current water supply is adequate for the tool.

   

• Is there enough room for the extra water lines? Some molding machines may have frame rails or other obstacles that may make it difficult to place water lines.

   

• Does the injection press have enough shot capacity and injection velocity? The additional cavities may require more material capacity then the current machine barrel can deliver. To achieve the same molding conditions in the stack mold from a single face mold means the molding machine needs to be capable of delivering double the shot volume in the same time frame. This will result in higher injection pressures and velocity.

   

• Does the machine have the proper frame and supports for the added weight of a stack mold? Depending on the size of the stack mold, additional supports may be required on the press in order to prevent tie bar sag with larger presses and molds.

   

• Does the injection unit have a fast recovery time? In the event this may be a high-speed molding process, the machine barrel and screw may not recover fast enough for the next shot. So a special screw may be required to optimize performance.

   

• Does the injection unit have profile decompression? Some styles of hot runner systems require a profile decompression at the end of injection to properly decompress the hot runner before mold opening or drooling and stringing may result from the part gates on hot tip systems.

   

• Is the hot runner system going to be a valve gate system or a hot tip system? This is important because the valve gate system will increase the stack height of the mold, thus requiring the molding press to have more mold open stroke.

   

• Depending on the manufacturer of the hot runner, the press may or may not require a sprue break option. You may also need additional circuits for air or hydraulic activation, depending on the requirements of the hot runner system. (Refer to your hot runner supplier for specific requirements.)

Mold Construction and Hot Runner Integration
Steel integrity - depending on the size of the stack mold - is very important. The hot runner can take up a lot of real estate in the tool. Therefore it is important to design the split lines and plate thickness according to the hot runner supplier's recommendations. It doesn't pay to go skimpy here. Because stack molds' stack height is taller than single face tools, there is additional weight added to the mold itself. A good design will be self-sufficient and not rely on the molding machine to maintain squareness and support. So make sure that the press you want to run a stack in has enough tie bar spacing to accommodate a proper design stack mold. A weak design will only relate to poor performance and cost more money later.

Electrical requirements of each hot runner vary. So the electrical boxes need to be placed away from water circuits and any automated part removal systems. Also, the mounting of the boxes is critical for ease of assembly and maintenance. It is advisable to have two electrical boxes - one mounted on each mold half of the center section of the tool.

There are currently three different devices available for timing the center section opening - the harmonic arm, the rack and gear and the helix drive. The most popular is the rack and gear, which can be recessed into the mold base - room permitting - and also can be mounted on the sides or top and bottom of the mold.

Most stack mold hot runner systems are similar in design. Where they differ is how the melt is transferred from the mold machine to the center section of the stack mold.

There are three basic types of melt transfer systems, one being a sprue bar, where a long heat pipe is attached to the main manifold in the center section. When the mold is closed, the molding machine nozzle sits on one end of the pipe and material is injected through the center to the manifold. This type of system requires a sprue break on the molding machine during mold open.

The second device, called a valve-to-valve transfer, has a valve gated hot runner system mount in the stationary side of the mold base with another valve gated system mounted in the center section opposite the stationary mold base. The opposed valve gate nozzles shut off against each other when the mold is closed. The valve pins are opened, allowing the melt to flow across the parting line. At the end of injection the valve pins close. When the mold is open there is no sprue bar and a sprue break is not required on the molding machine.

The third device utilizes a valve gate system and a hot tip or sprue gate system - similar to the valve-to-valve transfer but using a valve gate on one side and a hot tip on the opposing side.

Water Circuits
As in any injection mold, good cooling and water circuits are critical to the performance of the tool. However, when integrating a hot runner into a stack mold application, cooling becomes more important because of the added heat generated in the center section of the stack mold. Also, because of the movement of the center section, the placement and strength of the water lines are important in the manufacturing environment to avoid water leaks. Some options are available for integrated water circuits within the mold carrier support.

Valve Gated vs. Non-Valve Gated Systems

Hot tip hot runners are the most common and easiest to integrate into a stack mold system because they share a common manifold and the nozzles are directly opposite each other.

Valve gated systems become more complex because of the valve pin actuators. If a common manifold is utilized, the nozzles have to be offset from each other in order to accommodate the valve pin actuators. In order to do a valve gated system back to back, two levels of manifold are required to accommodate the valve pin actuators.

As you can see, there are many areas to consider before integrating hot runner systems into stack molds. The road to advanced manufacturing technology has never been an easy one. However, when you take a good design with solid manufacturing techniques and proper implementation, you can reap the benefits of higher productivity.