Ten Tips to Slash Cycles in PET Preform Molding
Time is money: The adage is certainly true for preform injection molding. 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.
Let’s review the parameters that contribute to effective cooling and cycle time:
• Injection time;
• Hold time;
• Cooling time;
• Robotic takeout or free drop;
• Demolding temperature;
• Machine dry-cycle time;
• Water temperature;
• Water pressure;
• Coolant composition;
• Tool construction.
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.
Hold Time: The purpose of hold time is to supply resin that is needed to fill out the voids in preforms as they shrink from a melt density of 1.15 g/cm3 to the solid density of 1.33 g/cm3. This difference is about 13%, and for many years we taught to move the transition or switchover point to the 13% mark of the total injection stroke. However, preforms already cool down as they are being injected. Very thin preforms do this a lot because with cavity channels as low as 2 mm (0.080 in.), almost half of the preform weight sticks to the cold mold walls and is already at or close to solid density by the time injection is finished.
For thicker preforms, the melt spends so much time inside the cold cavity that it is cooling down. In either case, the transition point needs to be set to between 6% and 8% of the total stroke for these preforms in order to be at the right position when the cavity is actually filled.
Cooling during hold time is very effective because the melt is pushed against both core and cavity. Times are dependent on wall thickness. Figure 1 can be used as guidance.
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.
Robotic takeout or free-drop: Machines with three, four, or even six post-mold cooling stations allow removal of preforms from the mold as soon as they are cold enough that they won’t buckle (around 160 C, or 320 F). Extremely fast robot speeds allow the mold open/close sequence to be as quick as those for free-drop machines. The latter machines can be adjusted to eject the parts while the mold is opening so that the only waiting time is for the ejectors to move back. This helps, but robotic machines with three servo-driven takeout stations cut cycle times easily by 40%. There are a number of companies that offer PET robots for machines that don’t have them.
Demolding temperature: This determines how many nicks and scratches will be visible on the preforms as they tumble against machine parts and each other. A temperature of 45 C (113 F) is recommended to minimize visual flaws. How these deformities will show up in the blown bottles depends to a large degree on the applied stretch ratios. High stretch ratios over 12 tend to stretch out the flaws, making higher demolding temperatures possible without affecting aesthetics. Large wrap-around labels hide visual flaws effectively, and molders should discuss this with their customers. Figure 2 is a graph for a 35-g preform with a wall thickness of 3 mm.
Machine dry-cycle time: These vary between 2 and 6 sec. Speeding up clamps and ejectors will usually not result in noticeable cycle-time reductions and should be optimized once and only touched again if really necessary.
Water temperature: For fastest cycle times, water temperature should be between 8 and 10 C (46 and 50 F). However, this can become costly, especially in hot and humid climates. Figure 3 shows a 2-sec cycle time increase for a 35-g preform when the water temperature is raised from 10 to 20 C. This difference is reduced to 0.9 sec for a 10-g preform with a wall thickness of 2 mm, while it will be higher for thicker preforms.
Processors in hot climates should weigh this against the operational cost of cooling the water down to 10 C, and the air conditioning of the molding area to prevent mold sweating, which may not be necessary at higher water temperatures.
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.
Coolant composition: Anti-freeze is necessary to prevent water from freezing in the chiller. However, only the minimum percentage should be added, because anti-freeze has less cooling capacity than water. It is also harder on the pumps because it is more viscous.
Tool construction: The key is balanced water flow to all cores and cavities as well as within them. Many molds feature water in and out ports on the same side of the mold, with the water flowing into each core/cavity in sequence, returning in a second line. This leaves the mold areas farther from the port with less pressure than the ones close to it. And that, of course, changes the cooling capacity of the water.
A better construction is having the ports on opposite sides of the mold so that the short water flow to the closest cavity is balanced by longest return water flow. Inside the cores, the area inside the bubbler tube should be the same as the area between bubbler tube and core ID to maximize flow. Since the surface area of the core is always smaller than that of the cavity, this is a crucial element in the cooling equation.
That is the size-reduction question confronting many processors today. Look here for some guidance.
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