Welcome!

Someone was preparing for some in-house training, and asked me this important question…

Although every situation is different… as a general rule, you do not want to use a mold which uses less than 2/3 of the space within the tie bars. For example, if the horizontal spacing between the tie bars is 18 inches and the vertical spacing between the tie bars is 12 inches… then the minimum mold size should be 12w x 8h.

If you go below this value, the force is will be concentrated on the center of the platen. The factor is not the amount of tonnage being used, but the centralization of force. Since the platens are cast, they do can be deformed very easily… a mold which is too small will quickly increase the concavity of the stationary platen, and possibly damage the movable platen as well.

A bolster plate can also be added to the mold to provide additional support and distribute the force more evenly across the platen. This plate bolts to the clamping plate and extends beyond the mold base; thus increasing its effective size. In some cases, bolster plates can be as much as nine inches thick and can extend beyond the mold base by as much as twelve inches.

-Andy

On some of our machines, we can adjust the hold velocity. How should this feature be used when molding a decoupled II process.

Before I respond, I want to relate the general attributes of this process:

• A process that uses one injection speed to fill – whenever possible
• The mold fills 95 to 98 percent full during first stage
• All cavities are short shot during first stage
• First stage fill is velocity-controlled and not pressure limited
• Second stage pack is pressure-controller and not velocity limited
• Process uses only 20 to 80 percent of the machine’s available shot size
• The final cushion is approximately 10 percent of the overall shot size

In essence, the purpose of a DII process is to ‘decouple’, or separate, velocity controlled fill from pressure controlled pack. To do this, you should either turn the velocity control off so the pressure control takes prominence.

Always make sure you understand the specific controls on your machines. In some cases, the 2nd stage velocity is actually a limit rather than an actual setting. In this case, you should set the speed near the maximum so that 2nd stage pack does not become velocity-limited.

-Andy

A friend emailed me this great question the other day regarding the relationship between fill speed and pressure…

The pressure required to fill will increase because the viscosity change will not completely outweigh the pressure losses… The overall energy consumption does drop considerably during fill since the pressure to fill is being appllied for only one third the amount of time!

It is very likely that a rheology curve would demonstrate that the 1 in/s fill is on the left hand side of the shear thinning transition region and the 3 in/s is on the right hand side of this region.

Think of it in vehicular terms… Your 1 in/s is like pushing a large pickup (higher resistance to movement) at 20 miles per hour, while your 3 in/s is like pushing a compact car (lower ristance to movement) at 60 miles per hour. It takes more gas per minute to move the smaller car at the higher rate, but it takes less time and you consume less gas overall getting to your final destination.

Always keep in mind…the reduced viscosity due to shear thinning will actually save you money… making your processes much more economical since the overall energy consumption to fill the mold as well as the time are reduced.

Additionally, the drop in viscosity will also reduce the pressure required to pack out the mold cavity during 2nd stage. In the long run, all these changes can make a big difference in the productivity and efficiency of your facility.

-Andy

Yes, the goal in PET processing is to (1) maximize output, (2) control shear heat, and (3) minimize melt tempertaure… yet ensure the polymer is fully melted and as homogeneous as possible.

(1) The need to maximize output is due to the fact that most PET injection molding processes are for high-cavitation high-speed molding PET preform machines.

(2) The need to control shear heat is due to the fact that a higher temperature polymer requires more time to cool… and may increase the development of semi-crystalline regions. Also, inconsistent melt temperatures will result in inconsistent cooling and crystallinity… resulting in profit losses due to scrap and troubleshooting.

(3) PET also has very poor thermal conductivity… making it great for drink containers, but terrible for part cooling. The goal in processing PET is to process the material as close to the melting point as possible, while ensuring all the pellets are melted and mixed.

To address all these concerns, different machine manufacturers have devised some great machine and screw designs.

Whether it is a reciprocating screw machine, or a two stage ‘shot-pot’ machine, most PET single screw designs incorporate a barrier screw. In such a design, the screw channel depth remains the same from the feed into the transition zone, but the channel width becomes progressively smaller. As this channel width decreases… an adjacent shallow channel, separated by a barrier flight begins. Ultimately, the melted material flows over the barrier flight into the shallow channel until it transitions into the metering zone. Such a design reduces shear heating and allows stingy pellets more time to melt.

In most single screw designs, a distributive mixing element is typically incorporated on the screw or a static mixer is placed within the nozzle to ensure a more homogeneous melt… especially if a colorant is added to the polymer.

Some of the more ambitious machine manufacturers are working with twin screw designs for two stage molding machines. These systems show great promise since a good twin screw design would provide the best control over both shear and melt temperature.

PET processing is one of the more advanced and refined fields of injection molding, making it very competitive. I strongly suggest anyone getting into PET molding do some significant research to ensure your strategy is well suited to take advantage of today’s advances.

-Andy