Robust Process Development and Scientific Molding

[Walther Maas]

On this moment we have formed a group of procesengineers in our company who were willing to study and discuss for understanding the content of the book "Robust Process Development and Scientific moulding". Now after 3 chapters we have already several questions were we couldn't find the answers for. We have asked our material suppliers but no reaction is comming from them.
During my search I found this forum and if the knowledge is somewhere it should be here.

My current questions:

1) In chapter 2.1 there is a talk about polymersmolecules.
To get a better understanding we were asking ourselves how long is in general the fysical length of a polymermolecule for plastics?

2) In chapter 2.2 page 11 it's written that there is a relationship between the residence time of the plastic in the barrel and a narrow MWD
It is saying: "When the residence time in the barrel of the injectionmolding machine reaches the upper limit, the possibility of molecular breakdown or degradation, resulting in the loss of properties in the
final product, increases. This is another reason why a low MWD is desired".
How can be explained that a narrow MWD is more sensitive for degradation then a broader MWD?

3) In chapter 2.4.5 page it's written that crystalline materials can also be heat sensitive and cannot stand high temperatures for long periods of time.
It is also saying that amorphous materials can tolerate longer residence times in a heated barrel.
It seems this has a relationship with the question before , but we are not sure.
How can this difference between cristalline and armorfe materials be explained?

4) In the Graph 2.8 on page 17 we see at the crystalline Tm suddenly a decrease of the heatflow and when melted a return to the level before.
What is the definition of the heatflow and what is it's dimension. How can the graph be explained.

Hopefully some of you are willing to support us during our excercisings

[Suhas]

Hi Walther,
Thanks for these questions and also discussing the book.

Here are my thoughts -
1) In chapter 2.1 there is a talk about polymersmolecules.
To get a better understanding we were asking ourselves how long is in general the fysical length of a polymermolecule for plastics?
Answer: Polymer molecules are characterized with their molecular weight. The physical length is not usually taken into consideration since it is never a straight ‘rod’ molecule. Think of a sample as a bowl of cooked spaghetti where all the spaghetti are entangled with each other. The molecular weight decides how ‘long’ the molecule is. Generally, as the molecular weight increases, the melt viscosity goes up and the mechanical properties also increase. For the physical length, you will have to refer to a polymer chemistry book. I will try and look at this for you.

2) In chapter 2.2 page 11 it's written that there is a relationship between the residence time of the plastic in the barrel and a narrow MWD. It is saying: "When the residence time in the barrel of the injectionmolding machine reaches the upper limit, the possibility of molecular breakdown or degradation, resulting in the loss of properties in the final product, increases. This is another reason why a low MWD is desired".How can be explained that a narrow MWD is more sensitive for degradation then a broader MWD?
Answer: With a broad MWD, the lower end of the distribution can start to degrade and in some polymers, the degraded products act a catalyst to upzip the polymer molecule and speed up the degradation of the plastic.

3) In chapter 2.4.5 page it's written that crystalline materials can also be heat sensitive and cannot stand high temperatures for long periods of time. It is also saying that amorphous materials can tolerate longer residence times in a heated barrel.It seems this has a relationship with the question before , but we are not sure. How can this difference between cristalline and armorfe materials be explained?
Answer: There is no connection with MWD and this question. The answer lies in the fundamental melt processing difference between amorphous and crystalline materials. Amorphous materials can be melt processable over a broader range and so typically processors set the temperatures to the center of the window where the melt can have a longer residence time. In case of the crystallite materials the crystallite has to melt to be processable and so it needs a specific amount of thermal energy. This energy is on the higher side where the molten molecules can’t reside for a longer time. I am not very sure if this is a sufficient answer, but again, it should be referred to a polymer science expert.

4) In the Graph 2.8 on page 17 we see at the crystalline Tm suddenly a decrease of the heatflow and when melted a return to the level before. What is the definition of the heatflow and what is it's dimension. How can the graph be explained.
This is a DSC graph. If you google Differentail Scanning Caloriemeter and its working you should get the answer.

Regards,
Suhas

[Walther Maas]

Dear Suhas,

Thank you for your quick replay. We appreciate this very much.
Concerning question 1 we had the discussion if a shredder was able to influence the viscocity of the melt by breaking / cutting the molecule chains.
Often we see diffrences in flow between virgin and re-used (close-loop) mixed with virgin material. We are looking for the main root-cause of this.
Is the main reason for this difference the degradation of the polymer after once used or has the shredder a bigger influence than we think.

For the questions 2, 3 and 4 we will follow your advice and will contact a polymer science expert for better understanding of the principle.

[ronaldduck]

Shredder influences the viscosity of the melt on wide scale. I hope now you are clear with question #1.
_________________________
injection molding information (http://www.actionmold.com/)

[Pilot]

Hello, I will add some more informations:

Molecular weight is of polymers depends on many facts. For example for PE it could be between 5000 grams/mol (LLDPE like wax) , 100 000 grams/mol (HDPE) 1 000 000 g/mol (UHDPE).
If you will count all basic elements (C-C) in the main chain of PE you ger something between 100-100000, on the base of MWD. This is Typical for synthetic polymers. for natural polymers, you can get 10^9 and special DNA isa giga molecul.
Cooked spagethi model, like Suhas wrote, is based on ball, where the radius is described like gyration radius with distance between ends of molecule.
Viscosity of polymers is depending not only on temperature and molecular weight, but you can imagin this like resistance of melt under shear stress to flow. Big influence on this has a "shape" of the chain. If the long chain has only -C- or -C-C- segments , will have smaler viscosity than shorter chain with big atoms or groups (Nitro, Cl ..)

Generally for better understanding of polymer physics you have to "forget" on temperature. A lot of things, where the temperature is mentioned you have to understand like ENERGY. - higher temp = higher energy, better molecular movement, bigger radical amount.
This will explain to you behaviour of amorphous and crystalic polymer.

[rickbatey]

With a truly robust process and load compensating press, you should see the actual injection pressure change but not the fill rate or cushion. Impact and some dimensions will vary, but again with good machines and settings, you still get good parts.
Rick.