X-melt is a patented “expansion molding” technology available from Engel Machinery. It is a process that enables part designers to cross old design barriers in small, detailed part designs. The x-melt technology will consistently fill micro sized parts, thin wall parts, or small parts with minute detail, with accuracy and repeatability unattainable with traditional processes.
Typical discussions around x-melt focus on why x-melt is repeatable, why it enables plastic to flow easily, why it reduces flash and short shots, etc. But my focus here is on what applications x-melt is best suited for, and when you should look at x-melt. There are three distinct steps in getting to a point where you can really take advantage of this process and move ahead of your competition with an aggressive part designs that cannot be consistently molded otherwise.
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The first step is to understand the chemistry and physics behind what is happening so you can move on to the next step with confidence. CLICK HERE for more detailed analysis of the x-melt process.
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Next, understand the type of applications that are well suited for the x-melt process and, more importantly, why x-melt is the process of choice for certain types of part geometry. This article is an attempt to help make this more clear.
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The third step is moving forward: Look at new, more challenging part designs. Consult with Engel, they can help with mold and part design, and run trials for you.
Lets look at a typical small part design problem below, but first, look at the following diagram that gives you a quick explanation of what the x-melt process is all about.
Shows the ready, compress and expand sequence for x-melt
Here is a typical good x-melt application:
Problem:
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A new part design with a thin wall and a relatively high flow length to wall thickness ratio
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Your prototype testing shows that it is difficult to produce a consistently good shot. Variations range from flash to short.
Causes:
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Because of the high flow length/wall thickness ratio, high injection pressures are required. However, due to the small volume of the part, the margin of error between a good part to flash, or good part to short is tiny. The high injection pressures cause flash at a slight overfill, and the part is short with a slight under fill.
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Conventional molding techniques have difficulty dealing with this because of minor inconsistencies from screw position, check ring movement, and even if those factors are minimized with a given approach, minor variations in viscosity will cause the problems.
The complete solution that x-melt brings to the table:
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There is no mechanical “noise” in the system from screw position or check ring. The melt is pressurized to a developed pressure, and everything except for the valve gates remains stationary during fill which is done with the melt expansion only.
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Since the melt is brought to a set pressure before each shot (and not a screw or plunger position), viscosity variations are compensated for in the process.
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There is so much shear during the initial expansion of the melt, that it flows easily with an extremely fast flow front in the range of 2600mm/sec, a flow front speed unattainable with conventional processing.
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With flow front speeds in the range of 2600mm/sec, the shear time is a fraction of what is achieved with the traditional process. This short time in shear has been shown in studies at GE to have little or no effect on the material. It is the length of shear time and not the amount of shear that effects the melt. With traditional methods, the screw may start off moving fast, but the melt is mostly compressing and slowly shearing while picking up speed over time as it expands.
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The expansion volume is repeatable because it is based purely on the physical properties of the plastic. No flash, no shorts, just consistent expansion to the correct fill volume. The pressure is reducing from start to finish as the melt is expanding, which, if you think about it, is exactly what you want for accurate filling, and is very comfortable for a delicate process.
Recommendation:
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If you have a part design that cannot be molded consistently with conventional molding, supply Engel with part drawings, material specifications, yearly volume requirements, scheduled run time (24/7, 24/5, etc.), Engel can propose a mold and machine combination with guarantees.
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If you have a production or prototype mold, Engel can test the x-melt process on that mold in one of their Tech Centers in CA, PA or Austria.
