Injection molders are under increasing pressure to create parts with ultra-thin walls–and the molds themselves are under even greater pressure.

The trend towards thin-walled parts in increasing in the automotive industry, where part walls are often only 2 millimeters thick, and the portable electronics industry, where part walls can be one millimeter or less. The injection molding machinery that is currently standard in most shops can handle a few thin-wall molding applications, with the newer machines generally out-performing the older models. Yet, as the demand for thin-walled parts escalates, many shops will find that they have to invest in more specialized machinery.

The new breed of thin-wall molding presses is generally equipped with greater speed and pressure capabilities. These high speed/high pressure machines may be hydraulic or electronic, or a combination of the two. Incredible injection speeds and pressure levels are needed when one considers that a millimeter-thick wall requires that the mold be filled in less than half a second and have an injection pressure in excess of 30,000 pounds per square inch.

Levels of pressure on this order require a minimum clamp force of 5 to 7 tons per square inch. Extra heavy platens should also be used to reduce flexure. In addition, closed-loop control of injection speed, transfer pressure and other process variables can help to control filling and packing. Considering shot capacity, thin-wall molders should be careful to avoid using large barrels. A shot size that takes up only 40% to 70% of the barrel’s capacity should be sufficient. On the other hand, a shot size that is too small runs the risk of increasing the material’s barrel residence time, a condition which can degrade the mold’s property.

The speeds that thin-wall molding requires are necessary in order to minimize any premature “freezing off” of the molten material as it’s injected into the ultra-thin cavities of the mold. Reducing the wall of a mold by even a quarter of its current thickness can mean having to reduce its injection time by half. The good news is that as mold walls become thinner, there is less material to cool, which can trim cycle times by up to 50%.

The materials that work best in thin-wall molds are the harder varieties of steel, such as H-13 or D-2. So be prepared to upgrade from the more conventional P20. Harder steels are better suited for higher pressures and are safer as a result. Also, be sure to avoid molding materials that encourage mold wear when injected at high speeds.

Thin-wall molding machines, and the tougher materials that these machines require, are more expensive than their thicker-wall counterparts. Having said that, their cost is usually offset by the increase in production that thin-wall injection molding brings. Doubling production can mean less molds that need to be built, resulting in a significant overall savings for the shop.

Source: Secrets of Successful Thin-wall Molding
Plastics Technology, Feb. 28, 2002
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