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Thin-wall injection packaging: What you need to know

Injection molding demonstrations at the K show will cover the gamut of end-use markets, but packaging production lines, especially for thin-walled packaging, are among the biggest drawing cards. May of us stop and stare at these cells, marvel at the speed and the slick automation, and snag parts to inspect as they roll off of conveyors. Here's what you need to know to shop smart.

October 12, 2010

3 Min Read
Thin-wall injection packaging: What you need to know

Knowing there will be so much interest in molding of these parts, we asked Paulo Gomes, business manager, packaging, at Husky Injection Molding Systems (Stand 13/A59) to share his tips on what processors need to consider when shopping for machines for these applications. As he notes, thin-wall packaging parts are among the most demanding applications in the injection molding world. The combination of fast cycles, small shot sizes, high plasticizing rates, heavy molds, and extreme injection pressures and speeds can push a standard injection molding machine beyond its limits.

Given that, here are the five points Gomes considers critical to consider when selecting a thin-wall packaging machine:

1. Injection speed: Thin-wall applications are more about speed than pressure. Faster plastic flow eases the filling of the part. Many processors tend to shy away from high injection speeds, driven by the fear of high injection pressures. In fact, high pressure on the injection unit resulting from a high injection speed can reduce the pressure inside the mold.

2. Throughput capacity and shot stroke: While high throughputs or plasticizing rates are proportional to the screw diameter, shot stroke is inversely proportional. Plasticizing drive selection, together with the right screw design, helps to achieve the best repeatability without compromising the cycle time. Alternative injection unit solutions like two-stage (extruder + shooting pot) help to meet extreme performance demands.

3. Clamp design: The degree of platen flexing all depends on how clamp forces are applied to the mold. This will always happen regardless of how good the design is (it’s just physics!). And if the platen flexes, so will the mold. Minimizing this deflection through optimal platen design allows for:

Reduced part weight—a more even wall-thickness distribution allows for a lower nominal thickness. Mechanical properties like top load can also benefit from an improved distribution. Failure is driven by the weakest section.

Decrease of clamp tonnage—clamp units with more even platen deflection do not need to “over-clamp” the mold to ensure that a minimum clamp tonnage is reached on the maximum deflection point.

Increased uptime—excessive mold maintenance to tapers, venting, or parting line wear can be a result of an uneven force distribution. More time repairing, less time producing.

Increased output—Mechanical strength achieved through thicker platens tends to increase the overall weight of the moving masses. This will slow the clamp down.

4. Stack mold capability: Using stack molds is an excellent way to improve the output per capital invested. Ensure that the clamp unit is long and strong enough to take the additional opening stroke and mold weight. Preferably, the weight of the stack mold center section should be loaded onto the machine base, instead of hanging on the tie bars. This will allow for faster, more accurate movements or heavier tools.

5. Automation integration: A good clamp design allows you to achieve fast but accurate movements at the same time. This is not a tradeoff as the lack of clamp accuracy will increase the time the mold is open for part removal. This is especially important for applications like IML, but also applicable to any system running automated part removal.

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