The true definition of viscosity is “the resistance to flow” and in molding this is a very important factor when trying to make a good part for your customer. There are many different ways to influence viscosity and they are all dependent on the situation at hand. It is important to note that viscosity is most affected by two things: heat and shear. This blog is intended to help you understand what to look out for when adjusting certain parameters and how the adjustments will affect your material.
In most cases when a processor is looking to increase or decrease viscosity they are usually adjusting one of the following parameters:
- Barrel temps
- Screw RPM
- Injection rate
- Back pressure
If you look closely at the options above you will notice that 3 of them are related to shear while one of them is related to direct heat. It is important to remember that viscosity is 80% affected by shear and 20% affected by heat. With that being said it is always a better practice to try and shear a material to get the desired viscosity rather than add heat. By adding heat to the barrel you are increasing your chances of degrading your material. Degradation is caused by the material being exposed to high heats over a long period of time. If you end up increasing your barrel heats then you are then exposing that material to a higher temp during the entire residence time of that material. When shearing a material that you can see the temperatures go up astronomically. As stated before, degradation is a factor of both time and temperature and because a material only sees shear heat for a split second it is not as susceptible to degradation.
Injection rate is usually the number two option after barrel temps, but it should be number one. Physics says that the faster you inject the material the less viscous that material will be. This is a true statement, but it does have a downfall. There will be a point when increasing your injection rate that you will encounter a pressure limit situation. Yes the material is easier to move the less viscous it is, but the velocity you are trying to achieve is so high that the press will reach a pressure limit trying to obtain that speed. In these cases you will need to further adjust your process. It is recommended that you take your injection rates back to a normal processing speed and then adjust back pressure and screw RPM’s. After adjusting them slightly it will then be more beneficial to try and increase your injection rate again.
There are always more parameters you can adjust to try and get your mold to fill the way you want [mold temps, for example], but remember that every adjustment has more than one effect. Adjusting your mold temps might help you fill your part, but it will also directly impact your shrink rate. A hotter mold will allow your material to flow easier. It will also cause your part to shrink more, therefore it may not be the answer to your processing issue. Adjusting mold temps will also change the way the two halves of the mold comes together so it is important that you do not make any major changes without consulting someone with knowledge on the clearances that the mold can withstand.
Overall the most important thing to remember is that there is no wrong or right answer to adjusting your process, but it is important to understand what the possible outcomes are from changing those parameters. Processing is an interesting mix between science and math so it is always good to keep an open mind to different processing styles.