Plastics Technology

OCT 2018

Plastics Technology - Dedicated to improving Plastics Processing.

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Page 27 of 75

In last month's column we discussed the fundamental connection between variations in the condition of the polymer at different locations in a molded part and the level of internal stress in that part. These variations are influenced to a significant extent by processing conditions. Differences in pres- sure within the mold cavity will produce variations in the way the material shrinks. This is one source of internal stress. The other consideration is differ- ences in the rate at which the polymer cools. Knowledge of these factors allows a processor to establish molding conditions that reduce internal stress in a part. It is not possible to mold a part with zero internal stress. Even under the best of circumstances, a well molded part will still have a stress level of 500 to 600 psi. But poor molding practices can result in stress levels that exceed 3000 psi. This will result in reduced impact resistance, dimensional instability upon exposure to elevated temperatures, and greater sensitivity to failure mechanisms such as environmental stress cracking (ESC). The objective should be to minimize internal stress while still making the part cost-effectively. Mold filling and packing have a significant influence on the pressure distribution in the cavity. The pressure drop in a flowing polymer system is proportional to the viscosity of the material. Viscosity is influenced by two processing conditions: the melt temperature and the shear rate applied to the material. Higher shear rates result in lower viscosities and can frequently achieve this objective as an alternative to raising melt temperatures. This is the reason that the principles of scientific molding dictate that most of the cavity should be filled at a relatively rapid rate during what is typically referred to as first stage. The switchover to pack, or second stage, A Processor's Most Important Job should take place when the cavity is almost full by volume. The velocity of the polymer during second stage is usually much slower than on first stage, and this will result in a much higher viscosity. For some polymers, the viscosity on second stage can be more than 10 times greater than on first stage. Therefore, switching over too early means that it is more difficult to pressurize the polymer uniformly. Switching over too late will result in an overpressure condition that can cause flash and may even damage the mold. The relationship between mold-filling strategy and the pressure distribution across the cavity can be readily demonstrated if the cavity contains pressure transducers at the beginning and end of the flow path. The pressure will never be exactly the same at these two locations, but good mold-filling practices can reduce the difference. The more uniform the pressure in the cavity, the more uniform the shrinkage of the material as the polymer cools. Cooling rate is the other critical factor. Rapid filling introduces a high level of orientation in the polymer. This orientation is at a maximum in the layer of polymer that is just below the mold surface. This is also the layer of material that cools most rapidly. While some level of orientation can be beneficial, if it is excessive it will repre- Mold filling and packing have a significant influence on the pressure distribu- tion in the cavity. How to establish molding conditions that minimize internal stress in a part. Get more insights on Materials from our expert author: Learn more at KNOW HOW MATERIALS By Mike Sepe Mechanical properties and dimensional stability of a part are greatly affected by the levels and uniformity of molded-in stresses. Mold-filling velocity and switchover point, as well as cooling rate and flow-channel dimensions are molders' tools for controlling stresses in molded parts. (Photo: Teknor Apex) 26 OCTOBER 2018 Plastics Technology PART 9 K now How MATERIALS

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