PVC is a thermoplastic. That means that PVC becomes a viscous liquid when it is heated, and becomes solid again when it is cooled.

The role of additives is a key issue in the PVC’s processing. By mixing different additives with PVC we obtain compounds either in powder form (dry blend) or in granules (after granulation).
The compounding is a way to give to PVC tailor made properties. There is a chapter dedicated to additives.

Once we have a particular compound, we may process it by extrusion, injection moulding,
calendering, extrusion blowing, foaming, or thermoforming.

a. Mixing

1. Is it necessary to use a double stage mixing machine to mix PVC with additives?
2. At which temperature do I have to mix the PVC?
3. Is the mixing temperature important for the final compound result?

b. Granulation

4. Which kind of process, and equipment could you advise me to process rigid and soft PVC?

c. Extrusion

5. What can I do when I have "shark skin" on my profile?
6. What can I do when my profile is too brittle?
7. What is the typical extrusion temperature of PVC dry blend?
8. Do I need special equipment for cooling water of calibrator?
9. What to do to remove the stripes on the profile?
10. What can I do the increase the gloss of the profiles?
11. What are the means to avoid plate out in the die?
12. Is it possible to process PVC by injection moulding?

d. Injection moulding

13. Can I use dry blend to process PVC by injection moulding?
14. Which kind of steel is required for the moulds?
15. Any special treatment for the tools?

e. Calendering

16. How can I avoid flow lines in calendar films generated by calendaring process?

f. Extrusion blowing

17. What can I do when there is a variation in the wall thickness in blow film extrusion?
18. Is a blow film extrusion possible for rigid application?

g. Foaming

19. Is it possible to foam PVC?
20. Which technique is used to foam PVC (mechanical or chemical)? Which kind of blowing agents?
21. Could you explain the difference between Bicarbonate and Azodicarbonamide as chemical blowing agents?
22. What do you advise as blowing agents for: plates, profiles and pipes?

h Thermoforming

23. What is the best temperature range for thermoforming rigid PVC plates?

a. Mixing

1. Is it necessary to use a double stage mixing machine to mix PVC with additives?

This is not absolutely necessary but highly recommended to better disperse the additives. In a hot mixer, the lubricants and the stabilizers are melted and well integrated into the PVC grains.
In the cold mixer (2nd stage), the dry blend is cooled down to avoid the formation of lumps. The discharge temperature of the cold mixer is generally around 40°C.

2. At which temperature do I have to mix the PVC?

In a hot mixer, the final discharge temperature is between 90 and 130°C, depending on the kind of additives and the final application. To obtain a better flowability of the dry blend, a high discharge temperature is recommended.

3. Is the mixing temperature important for the final compound result?

A high temperature is better to have good dispersion and better flowability of the dry blend but with a longer mixing time. It could also give a reduction of the thermal stability of the dry blend. A too low temperature has a negative effect on the plate out.

b. Granulation

4. Which kind of process, and equipment could you advise me to process rigid and soft PVC?

The best process is a two stage machine: a co-rotating screw machine followed by a single screw extruder and a pelletizer cooled by air. The first one makes the gelation of the product and the second one put the material in pressure before entering the die.
Another process is the kneader single screw technology
Other ones are counter rotating system and two-stage planetary roller extruder.

c. Extrusion

5. What can I do when I have "shark skin" on my profile?

The shark skin surface defect is due to flow rupture at the outlet of the die. To reduce or avoid the phenomenon, you must increase the temperature of the head and the die. If necessary, you can also put or increase the amount of acrylic processing aids and/or the external lubrication.

6. What can I do when my profile is too brittle?

The brittleness could be due to very different factors:

• Low or too high gelation level.
  In the first case, increase the temperature of the barrel or use a reverse temperature profile to increase the shear in the end of the screw. The use of a breaker plate at the end of the screw is a must. You can also put some processing aid or decrease the external lubrication. In the second case, reduce the temperature of the barrel or increase the lubrication of the recipe.
• Not optimized formulation: the amount of natural filler could be reduced, the replacement of natural CaCO3 by precipitated one or by thinner NCC is interesting, the use of impact modifier is useful. The increase of the K value of the resin is also efficient.
• Presence of failure initiators: the design could be revised to avoid notches at the internal face of the external wall of the profile.

7. What is the typical extrusion temperature of PVC dry blend?

The typical extrusion temperature of dry blend is around 190° for rigid PVC and 180°C for plasticized PVC.

8. Do I need special equipment for cooling water of calibrator?

Depending on the ambient temperature, it could be necessary to have the water cooled. The preferred temperature of cold water for calibrators is 14°C.

9. What to do to remove the stripes on the profile?

The first solution is to increase the die temperature. If it doesn’t work, increase the content of external lubricants.
But the stripes on the profiles can also be due to die plate out. In this case, decrease the amount of external lubricants or replace typical external lubricants by internal/external lubricants.

10. What can I do the increase the gloss of the profiles?

First, you can increase the extrusion temperature. If this is not successful, you can perform the following actions:

  - Replace partially or totally the natural CaCO3 by a precipitated one.
  - Increase the amount of processing aid.
  - Reduce the Kvalue of the resin.
  - Increase the external lubrication.

11. What are the means to avoid plate out in the die?

The possible solutions are:
  - To increase the mixing temperature.
  - To increase the gelation level of the profiles to better disperse and fragment the additives.
  - To reduce the external lubrication and the amount of all low compatible additives.

d. Injection moulding

12. Is it possible to process PVC by injection moulding?

It is possible even if the viscosity is higher than for HDPE or PP. The use of low MW resin (K50 – 60) is highly recommended. The formulation must be adapted to achieve a high thermal stability.

13. Can I use dry blend to process PVC by injection moulding?

The injection moulding requires pre gelated granules. It is not recommended to use dry blend.

14. Which kind of steel is required for the moulds?

Stainless steel is required.

15. Any special treatment for the tools?

Chromiumsteel is recommended for its corrosion and abrasion resistance.

e. Calendering

16. How can I avoid flow lines in calendar films generated by calendaring process?

The parameters having a positive influence on the apparition of flow lines are:

- Use our special grade SolVin 460SL.

- Reduce the temperature differences between film on the roll and rolling bank.

- Increase the size of rolling bank.

- Increase the level of processing aids (need very well balanced, too much shows a reverse effect).

- Increase temperature differences between the two last rolls.

- Increase rotating calendar nip (very important rotating regular).

- Reduce calendar speed.

- If possible, increase the thickness of the film.

- Reduce the friction on the take off system.

- Take a very balanced recipe (lubricants, processing aids and MBS/Acrylic Modifier) for the best melt flow. 

f. Extrusion blowing

17. What can I do when there is a variation in the wall thickness in blown film extrusion?

A solution is to put an automatic controlling of air introduction.

18. Is a blown film extrusion possible for rigid application?

It is recommended to use K60 which gives a good melt flowability and a strong mechanical stability (the use is mainly for hygiene and sweets packaging).

g. Foaming

19. Is it possible to foam PVC?

With chemical blowing agents, it is possible to foam PVC and to reach a density of 0.4 – 0.9. With special physical agents, the density can even be much lower (0.05).

20. Which technique is used to foam PVC (mechanical or chemical)?  Which kind of blowing agents?

The most used technique is a chemical one: the addition of blowing agents introduced in the recipe. The main blowing agents are the Azodicarbonamide and the Sodium Bicarbonate.

They are often used together to obtain regular bubbles. A physical technique can also be used (supercritical CO2).

21. Could you explain the difference between Bicarbonate and Azodicarbonamide as chemical blowing agents?

The Bicarbonate is endothermic and has a slow decomposition speed. It’s recommended for the internal expansion (“celuka process”) and it gives hard skin in this process.  It gives a wider distribution of the cells diameters.

The Azodicarbonamide is exothermic and has a quick decomposition speed but a slow diffusion speed in the melted flow. It gives better cells and narrow distribution of cells diameters but a less hard skin.

22. What do you advise as blowing agents for: plates, profiles and pipes?

For plates, the expansion is external and you need a hard skin. The Bicarbonate is preferred, eventually with a low amount of Azodicarbonamide.

For profiles, the expansion is internal. The best is to use a combination of Azodicarbonamide (0.2 – 0.5 phr) and Bicarbonate (1 – 2 phr).

For pipes, usually, the one pack suppliers include both additives.

h. Thermoforming

23. What is the best temperature range for thermoforming rigid PVC plates?

The range of suitable temperatures is between 100 °C and 140 °C and is linked to the composition of the material (Vinyl acetate copolymer will need lower temperatures compared with PVC homopolymer) and with the type of equipment (presence of Infra-Red heating, power of the vacuum system, …).

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