Plastics Technology: July 2009 Archives

These can be viewed here:

After viewing these yourself, I recommend you use it as a teaching tool to introduce some of your employees to these great technologies.

Also, feel free to forward the link to anyone who might be interested.

-Andy

In a recent e-mail, I received this question regarding crystallinity...

In essence... crystallinity is a phenomenon where segments within the polymer chain either (1) align themselves in an orderly, structured fashion or (2) align themselves with portions of adjacent polymer chains in a similar fashion. These semi-crystalline structures tend to improve properties such as strength, barrier, and impact.

These semi-crystalline structures do not exist when the polymer melt is above the melt temperature. As the polymer temperature drops below the melt temperature, these semi-crystalline structures begin to form. The faster the polymer cools, the fewer semi-crystalline structures appear. Likewise, the slower a polymer cools, the higher the degree of crystallinity within the polymer.

One if the biggest factors regarding crystallinity is dimensional stability... Since semi-crystalline regions are more dense than the polymer in the amorphous state, higher degrees of crystallinity create higher shrinkage. The more even the cooling, the more even the shrinkage, and therefor the more dimensionally stable the part will ultimately become.

Keep in mind, the same principles apply to both extrusion and blow molding. Ultimately, any process change that increases melt, mold, or die temperature will decrease the rate of cooling... resulting in an increase in crystallinity. This is one reason why it is always important to think twice before making a temperature change in your process.

-Andy

In a recent trade show, an attendee asked me this unique question...

Note: For those new to the industry, a 'walking machine' physically moves away from the original position where it was installed. This movement may be as little as a couple millimeters or as much as a few inches.

First, always level the machine by placing the level on the tie bars. Always make sure you check all 4 tie bars. Many people make the mistake of levelling the machine by the base of the platens. Some of the best systems use a rotating laser with a group of sensors placed on the tie bars to measure the tie bars relative to each other. 

Second, check the parallelism of the platens. This can be easily measured both when the mold is open as well as when the mold is closed. The best way to check this is to have an expert come in and use a computer assisted system which can accurately measure the parallelism, twist, linearity of the platens at rest, under tonnage, as well as during platen movement.

Third, check the lubrication system. Any busted or leaking seals, as well as blocked lubrication lines may cause abrupt movement of the platen. This is often evidenced by a series of rings or streaks on the tie bars. You may also feel a vibration when touching the machine as the mold opens and closes.

Over time, ANY awkward platen movement that is not linear and level may cause the machine to 'walk'. This can become very costly as it can often cause tie bar strain, platen twist, and uneven clamping. You must implement a routine schedule for checking the levelness, parallelism, and lubricaion of the machine, since any of these can cripple or ruin your molding machines.

-Andy

In a recent e-mail, I received this question regarding tool steel...

By itself, P-20 lacks the wear resistance to produce half a million cycles using a glass-filled material. A coating of titanium nitride on both the core and cavity should prevent wear on the P-20. Additionally, this coating is gold in appearance... so wear will be easy to identify when more coating is necessary.

Since the gate is a high shear location, it typically receives the most wear. I strongly recommend using a hardened gate insert to avoid welding and repairs in this area. Likewise, if you are using a hot runner gate, you should use a hardened drop rather than machining the gate into the P-20 cavity block.

The biggest caveat to this recommendation is thin wall molding. If the part has thin walls, or requires high injection pressures... P-20 is not likely to maintain its structural integrity for 600,000+ cycles.

Without knowing more about the application and your tool steel supplier, it is hard to recommend a specific tool steel. Depending on availability and price, there are often multiple steels that will suit your application. A good supplier should be able to review your application with you and recommend a metal that will meet your specific needs.

-Andy