Plastics Technology: October 2009 Archives

As a general rule, the best cooling is one which provides the most consistent part cooling. This means your entire part should cool down consistently. 

You can often encounter many complications when you try to run different temperatures on the same half of the tool (unless the tool is relatively large such as automotive applications). The additional cooling in one zone will often cool the zone next to it. This can often result in erratic mold temperature fluctuations in the warm portions as it will often reduce the return coolant temperature causing the controller to act as though the mold is cooler than it really is. This can be done... but it requires extensive monitoring over coolant flows and coolant temperature coming in and out of the thermolator, and it is unlikely to get the result you desire.

In truth, areas in the part such as cores, bosses, and thick sections often need additional cooling. Ultimately, adjusting the temperatures in any specific region of the tool will have minimal affect on the transfer of heat.

The best way to provide additional cooling is through increased heat transfer. This can be done in many ways... (1) You can increase the coolant flow to a particular coolant line, which should be verified through the use of a flow meter (2) You can use more heat conductive steels or materials such as beryllium copper in the regions which need more cooling (3) You can additional cooling lines in the areas which require more cooling (4) You can also use more efficient cooling techniques such as water bubblers, baffles, and thermal pins.

When optimizing a process, always look at the entire process and experiment with different temperatures, speeds and pressures. The material suppliers recommendations are just that 'recommendations' and can be bypassed when necessary.

-Andy

What is the proper rule for safety glasses? Some say they are not always needed, others say they should be work at all times, what is your opinion?

Before I relate my opinion, I first want to bring up three important factors...

1) Virtually all machine and equipment manufacturers suggest that eye protection be worn at all times.

2) Virtually all safety regulatory groups recommend of require safety glasses to be worn at all times on the production floor.

3) Virtually all safety training recommends safety glasses to be worn around any piece of production equipment.

First... anyone violating a safety guard, servicing a machine, purging, grinding, or opening the safety gates for any reason must wear safety glasses.

Second... I strongly feel that anyone on the production floor should be wearing safety glasses.

Third... I also feel that the tool room, maintenance area, quality lab, and warehouse should also be included. 

In all stages of my professional development, I have been taught the importance of personal protection equipment and specifically... eye protection.

I have personally seen safety hazards such as fragments and fluids travel hundreds of feet across the production floor... all being serious hazards to everyone on the production floor.

As always, I am interested in hearing your opinion on this matter.

-Andy

Can high clamp tonnage actually cause flash?

Yes, it actually can. High Clamp Tonnage can block the vents, causing gases to become trapped in the mold during 1st stage injection. Although, these gasses most often heat and cause burning or short shots... these gasses can also force the parting line to open allowing the gases and some polymer to escape.

This rare flash resulting from high clamp tonnage tends to be very thin and wispy since it often tightly clamped immediately after it is formed. Often, this flash remains with the mold and builds up on the parting line.

Excessive clamp tonnage is never a good practice as it will often cause damage to the vents, parting line, and may even stress the mold components.

-Andy

A frequent blogger asked this question the other day...

I know, the rules for DII process as follows:

 

•  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

 

Do you have something like this for DIII process?

As with above, the following items are also rules of thumb, and are not set in stone.

A DIII process requires cavity pressure transfer capabilities, and should only be used if you and your technicians have a high level of competence. Such a process gives you excellent control, but may be overkill for many applications with good processing windows.

-Andy

One of our customers brought up this question...

With respect to shear during first stage injection, virtually all the shear occurs in the nozzle, runner, gates, and mold cavity. The diameter of the barrel, in comparison, is inconsequential.

If you used the same rotational speed (RPM) with a larger diameter screw, then you would create more shear during recovery since the circumferential speed would increase. For this reason... you should try to match the recovery times to help balance the shear caused during screw recovery.

What is usually overlooked is the ID and length of the nozzle which may change from machine to machine as well as the resulting melt temperature due to recovery… These oversights often make people think it's the barrel that is causing the difference.

-Andy

An employee confronted his management with this proclamation he found on a website… I was asked to explain why this was an incorrect philosophy for a 21st century processor. 

This is the philosophy behind the older, pressure controlled, machines. This process theory focuses on, and requires, a fully pressure limited process.

Aside from the reciprocating screw, velocity-control is the most important advancement in the injection molding of thermoplastic polymers… The theory above requires that you neglect this feature completely.

In 99% of the cases, such a process will be significantly less reliable and more machine dependent than a robust, velocity-controlled process with a short shot during 1st stage fill.

There are always opposing positions… but you would be hard-pressed to find any successful consultant or educator who would subscribe to the theory espoused above.

-Andy

In a recent webinar, I received this short question...

Does the use of in-mold labelling have a significant effect on polymer flow?

Basically, there should be no significant change in the behaivor of the polymer. Since the polymer exhibits fountain flow, the polymer touching the label surface is stagnant, while the moving polymer passes through the center.

Many injection molders forget that the polymer does not flow through the mold with a straight plug flow as water does. In fountain flow, the polymer comes in contact with the mold surface and begins to freeze off.

The polymer behind the flow front passes through the center of the channel until it comes in contact with the mold surface. As a result, All the polymer that fills the mold comes from the center of the flow channel as is passes by the polymer it distributes onto the mold surface.

-Andy

Conveying in the feed zone will only occur if the plastic grips the barrel and slides on the screw.  Whenever it grips the screw surface and slips on the barrel, it will not move forward. Consistent feeding requires a consistent balance of friction between the barrel and the material, between the screw and the material, and between the particles of material themselves. Because a consistent balance of friction is difficult to achieve, many problems with surging, uneven output, and size variations actually begin in the feed zone.

Feeding and conveying take place a little differently in a grooved feed extruder. With a grooved feed throat, slippage between the plastic and the barrel is effectively zero. It must move forward as the screw turns, regardless of any variations in friction between the plastic and screw.

Therefor, companies who purchase grooved feedthroats to improve material conveyance. Please discuss the specific application with the machine manufacturer to ensure the feedthroat grooves accommodate the size of your pellets and regrind. 

One interesting variation on this is with a micro-extruder. Many of the small bench-top extruders will taper and groove the feedthroat since it is the only way for the pellets to fit into the barrel and convey down the screw to be properly melted.

Another area of interest for grooved feed throats is in the newer, high capacity shot-pot style injection molding machines. Many of these use the shot-pot two-stage configuration to increase melting capacity... manufacturers are testing more traditional extruder configurations including twin screw extruders and grooved feedthroats.

-Andy