In two-shot molding, this becomes a greater focus with another variable involved. Over the years, I have preached about robust process windows. 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. At times, ribs or undercuts are needed to hold the first shot in place to prevent flash and deformation. In either case, the setup and process are the same.Ĭoncerning tool design, you need to make sure the first-shot cavity has support against the second-shot cavity pressure. 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. 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.
The movable cores move internally to adjust for the second shot. The cavitation will be like a normal mold: one part, one cavity. The other version of two-shot molding is the movable core, which does not require a rotary platen. Remember that the second shot is applying thousands of pounds of plastic pressure against the first-shot plastic, not just the steel. I have seen many times where the first-shot part was not supported properly, which contributed to flash and deformed parts. 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. You can crush by adding steel to the second-shot shutoff or adding first-shot material by removing steel. Also, in areas with thicker wall stock on the first shot, you may have to add extra crush to compensate for the extra shrinkage. Crush is a raised area of cavity steel that presses into the first shot to prevent flash, typically 0.003-0.005 in. 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.Ĭrush 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. 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.
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. After injection, the mold rotates to align the first shot with the second-shot cavity. This creates a multi-daylight stack-mold configuration, which is the most complex and costly version of a rotary two-shot mold.Īs 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. 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. 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 rotary platen option itself may have two very different meanings to some molders. 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 tool itself can have a rotary plate built in, actuated by hydraulics or a rack system. In this article, I will focus on the version of the process that uses a rotary platen. 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. There are also cases were the same material with different colors or additives is used for two-shot molding. 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). A mechanical bond is necessary-with details and holes in the part design-but some materials do not like to adhere to each other. It’s important to understand what materials are being used and their ability to bond to each other.
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