Here are some secrets of success to improve your effici […]
Time is money. While cycle times in the low- to mid-20-second range were state of the art in the 1980s, today’s high-speed presses deliver thin preforms at around 6 sec. Not everyone has the budget for one of these sophisticated machines, but there are a number of measures molders can take to reduce cycle times that are effective even on less expensive presses.
Cooling is foremost on the to-do list, especially with preforms thicker than 3 mm or 1/8 in., while speeding up the dry cycle of the machine is in most cases less rewarding. The tips are as follows.
Injection time: Resin suppliers suggest a fill speed of 8 to 12 g/sec/cavity. This works well for preforms up to about 4-mm wall thickness. However, thicker preforms can be filled much quicker, as it is easier for the resin to be pushed through the tool channels. For example, a 525 g preform with a wall thickness of 8.5 mm (0.335 in.) can be filled at 27 sec, resulting in a fill speed of 19.4 g/sec/cavity.
Cooling time: As soon as the preform is sufficiently cooled to avoid sink marks, the machine can switch over to cooling time. The preform now shrinks away from the cavity and onto the core and cooling is less efficient. This shrinkage is important to allow the preform to be easily removed when the cores move away from the cavities. Cooling time should be a minimum of 1.5 sec but can be as high as 20 sec for very thick preforms. It should be adjusted to get the desired demolding temperature.
Water pressure and flow: High-volume water flows are more important than the coldest temperatures. Optimal flow is achieved when the pressure difference between in and out is 5 bar (70 psi). Increasing the chiller pump size to achieve this and running at 20 C water temperature may be cheaper than running the coldest possible water with a smaller pump.
If you follow the tips we propose to shorten cycle times of pet perform mould, you will be more productive and efficient.
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