There are a few variations of two-shot molding, including rotary platen, movable core, and overmold. The first two require a second injection unit, two runner systems, and two processes. The latter, in most cases, involves two separate molds&#;one for each material&#;but can also be run with one mold, two runner systems, and pick-and-place part transfer, either manually or with a robot.
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It&#;s important to understand what materials are being used and their ability to bond to each other. A mechanical bond is necessary&#;with details and holes in the part design&#;but some materials do not like to adhere to each other. Most of my experience is with using TPE or TPU as the second shot and PP, PC/ABS, or ABS as the first shot (substrate). There are also cases were the same material with different colors or additives is used for two-shot molding.
Rotary two-shot molding, from my perspective, is the simplest version of the options, But there are still critical things to consider, such as tooling, crush, support and the processes for both shots. In this article, I will focus on the version of the process that uses a rotary platen. The tool itself can have a rotary plate built in, actuated by hydraulics or a rack system. This makes the tooling a little more complex and typically is used when the molder does not do a lot of two-shot molding.
The rotary platen option itself may have two very different meanings to some molders. The simpler one&#;which I am addressing here&#;is a vertical platen on one face of the clamp that rotates on a horizontal axis. The other&#;far more complex&#;is often referred to as a &#;cube mold&#; or &#;spin stack.&#; It is a center stack with two or four faces that rotates on a vertical axis. This creates a multi-daylight stack-mold configuration, which is the most complex and costly version of a rotary two-shot mold.
With the simpler rotary platen, as defined above, the tooling itself is not that complex; you just have a second runner system and two sets of ejector plates. The movable half is the half that rotates. If you are producing one part per cycle, there will be two ejector cavities (movable half) and two cover cavities (stationary half). The two ejector cavities will be identical, and the cover cavities will have one for the first shot and one for the second shot. So the pattern continues with multiple parts: Two parts in each cycle would have four ejector cavities and four cover cavities. Again, the four ejector cavities would be identical and the cover cavies would have two cavities for the first shot and two cavities for the second shot.
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With a rotary-platen mold, it is critical to have locators on the mold and the machine platens to ensure that it is exactly on center of the rotary platen. If the mold is not exactly on center, you will have alignment issues; when the mold rotates 180° it can damage the leader pins, bushings, and shutoffs. It is also important to consider tonnage with part surface area. In most cases your cavities will be off-center&#;first shot on one half and second shot on the other half&#;so you will not have full advantage of the machine tonnage to compensate for the cavity plastic pressure.
As I mentioned, the movable/rotating half contains the first-shot cavities and the stationary half contains cavities for the first shot and the second shot. After injection, the mold rotates to align the first shot with the second-shot cavity. So in one cycle you are molding the first shot with the base material and molding the second-shot material over the first-shot parts from the previous cycle. There are two sets of ejector plates because you are only ejecting the parts with both materials molded, leaving the others to have the second shot molded. There are two important things to consider in the tool design for the first shot when it is in the second-shot position: crush and support.
Crush would be defined as the shutoff to prevent the second-shot material from flashing or bleeding out onto the first-shot plastic where it doesn&#;t belong. Crush is a raised area of cavity steel that presses into the first shot to prevent flash, typically 0.003-0.005 in. Also, in areas with thicker wall stock on the first shot, you may have to add extra crush to compensate for the extra shrinkage. You can crush by adding steel to the second-shot shutoff or adding first-shot material by removing steel.
You also need to make sure the first-shot part is supported in the second-shot cavities to prevent the second-Shot cavity pressure from deforming or compressing the first-shot material. I have seen many times where the first-shot part was not supported properly, which contributed to flash and deformed parts. Remember that the second shot is applying thousands of pounds of plastic pressure against the first-shot plastic, not just the steel.
The other version of two-shot molding is the movable core, which does not require a rotary platen. The cavitation will be like a normal mold: one part, one cavity. The movable cores move internally to adjust for the second shot. The mold closes and injects the first shot; then the first-shot core/slide is pulled, the second-shot core/slide is set, and then the second shot is injected. There are cases where there is just one core/slide, which pulls back after the first shot and creates the cavity for the second shot. In either case, the setup and process are the same.
Concerning tool design, you need to make sure the first-shot cavity has support against the second-shot cavity pressure. At times, ribs or undercuts are needed to hold the first shot in place to prevent flash and deformation. Make sure your hydraulic cylinders actuating the cores are robust enough to resist the cavity plastic pressure or you will be struggling with flash and narrow process windows.
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Over the years, I have preached about robust process windows. In two-shot molding, this becomes a greater focus with another variable involved. People often will process the first shot to address issues with the second-shot process, or the opposite. Crush, support, deflection, and inadequately sized cylinders can all contribute to issues that many molders will process around, putting them at risk for quality issues and scrap.
ABOUT THE AUTHOR: Randy Kerkstra has been in the plastics industry for more than 26 years, occupied frequently with troubleshooting injection molding. He is currently a tooling manager for a large, multi-plant molding and manufacturing company. Contact: .
As many of us in the machine industry are aware, Plastic Injection Molding is just as relevant and important as any other machining process. There are many things to consider when considering purchasing plastic injection molding equipment, such as: Shot Size, Tie Bar Spacing, Ejector Stroke, Platen Size, and Tonnage.
Shot Size
Shot size is best defined as the maximum amount of plastic that the injection molding machinery is capable of injecting into the molding cavity during one molding cycle. The amount is rated in ounces of &#;general purpose polystyrene&#; (GPPS) for U.S. machines, and cm3 for European and Asian machines.
When searching for a machine, it's always a best practice to look for a machine that is capable of producing 30-40% greater shot sizes than what your parts require.
Tonnage
In this case, I mean &#;clamp tonnage or clamping pressure.&#; This could be considered the same concept as when you're examining press brakes. With press brakes, tonnage capabilities are measured by how many tons of downforce the machine is capable of directing towards a workpiece. However, with plastic injection molding machines, tonnage is measured by how many tons with which the machine is capable of pressing together the platens, which hold the mold cavity and form the plastic that is injected into said cavity to produce the desired part.
Platen Size
The platen is the "table" or tool that holds the mold cavity. Spaced apart, the mold is inserted and clamped securely to the platens. Once the two platens come together and are held under the pressure of tonnage, the plastic is heated, injected under pressure, and then allowed to cool through a molding cooling cycle. Here, the plastic takes on its hardened, final shape while continuing to be kept in place with many tons of pressure to retain the desired shape.
Tie Bar Spacing
Tie Bar Spacing is defined as the space between the horizontal tie-bars on an injection molding machine. Basically, this measurement, along with the platen max spacing, determines the maximum size of molds that can be placed in the molding machine.
Ejector Stroke
Simply put, Ejector Stroke is the action of the machine pushing out (ejecting) the final workpiece from the platens using ejector pins. The ejector in the molding machine pushes against an ejector plate on the mold, Ejector "Pins" or rods are attached to this plate and perform a pushing operation on the molded part after the plastic has hardened and the mold has opened.
Once the action of molding the workpiece is completed, the mold opens and then the ejector pins slowly extend outwards, pushing (or ejecting) the finished part out of the mold cavity. Pin movement is timed with the mold opening&#;because pins that move too quickly could impact the plastic workpiece and damage the final product and pins that move too slowly may not fully eject the molded part.
At Southern Fabricating Machinery Sales, Inc. we have many Plastic Injection Molding Systems available and can assist you in selecting the right system for your part or application. Also, with equipment appraisers on staff we can help you evaluate and sell any excess plastics machinery you may have. Please call me directly at 813-444- X113 to discuss your machinery needs.
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