An Important Question

You should test one important rule . Its important to hold the dosing volume stroke between one and 4 diameters maximum . This is , if your screw diameter is 40mm , the correct stroke must be , 40 to 160 mm .
If you need more that mean that you need one bigger injection unit.

Why too much cushion is not good ?

I think it is for preventing degradation. But in my situation long cushion is good, because I already want the material to get melt more. Am I right?

I have an other idea. What if I preheat PVC at 60-70C in a drying oven and in this case PVC may reach the desired melt temperature faster, and with lower barrel temperatures????

Comments pls.

Thanks for the answers. Arburg is 570S just bought at 2011 February with touch screen. I will calculate or look for mm.

The main problem is organization actually. This machine is new with new mold and this machine is sold with 3 options of screw barrel diameter; 35, 40 or 45. They have just thought about mold sizes and the distance between tie bars. It is OK. Injection volume is enough. OK. Let's buy it. There is no investigation, anylsis as you mentioned. Nobody thinks about residence time, temperatures, injection shot size, injection volume or compression ratio of screw or temperature profile. I have just seen the machine when it is in factory.

The solution was simple and it was buying the machine with 45 diameter and it has 254cc volume, 40 diameter has 201cc and the mold is 120cc. 254 was enough for material for proper residence time.

Ashwin;
We are using PVC VJ374, VJ373, VH775 clear and supplier is INEOS .

Could you please let me know the PVC grade you are using and the supplier .Is ti clear or opaque resin?
Thanks,
Ashwin Bhayani

One additional comment. If you look on the screw housing on the operators side, just below the cover for the screw motor, you will see a small window with a pointer. This is going to be in mm. So you can see where the screw stroke is prior to injection and where transfer & cushion are.

Hope this helps.

Bill28

I don’t know how old you Arburg 570 is. If it has a touch screen on the controller, it’s very simple to switch between metric & US systems. For an Arburg without a touch screen, it’s a bit more cumbersome but it’s doable. Your manual should show you how to do this.

Let me know which unit you have & I’ll try to help you. Mathematically it’s very simple to calculate cc into inches. Simple formula: Pi r2H. Where Pi is 3.1416, r2 is the radius of the screw squared, and H is the starting position of the screw.

Regarding adiabatic heating, this is a condition where you use the shear induced by the screw to heat the next shot during recovery.

Regarding Rheology curve, I would recommend you contact Bill Tobin. He has a manual as well as software that will walk you through how to set up and qualify a mold.

I have been injection molding and extruding PVC for over 50 years. If you treat it with respect and take the time to establish parameters, it’s no big deal. You’re lucky to have a PVC screw, most of my injection molding of PVC was done on GP screws in custom molding, where one does not have the luxury special screws especially when the runs are short, i.e. 1 to 2 days.

I hope this helps
Bill28

Bill 28;

I am sorry to ask this but what do you mean with "adiabatic heating". I know adiabatic but could not link to injection molding.

About recovery delay, i can't use it. Because in this mold, cooling time is already almost same with recovery time. But it is a good idea for other molds.

There is no a work to determine the proper injection speed. It is trial-and-error. Now the machine is working with 70cc of cushion. It is too much I know. But I could lower the temps 10 degree in this way and parts are substantially shiny. Diameter of screw is 45mm. How can i calculate cc to distance ? I didn't contact to supplier but I already have starting conditions (tds).

Bill;

Now, I have been told that we are already using PVC screw.

I'm glancing over the website you forwarded and newsletters engaged my attention. I wish my mother language would be English so that I could read these articles faster and without being tired :)

You've just stated why everyone is cautious (read: they HATE) PVC. It's temperamental.

The first problem is that most GP screws are 10-5-5 meaning all the mechanical melting occurs in 25% of the screw's length. PVC likes to be melted slowly in a long transition screw. Your heats are a function of the shot capacity and the residence time. This is called temperature profiling. All this boils down to two strikes against you.

WESTLAND USA (www.westlandusa) has an interesting newsletter describing temperature profiling and the significant differences between melting crystalline materials and non-crystalline materials. Check it out.

While you can run PVC in a conventional, GP machine it's tricky. Purchasing (assuming you have the ROI) a 'vinyl' screw will solve 80% of your problems.

Sorry I couldn't be of more help.

Bill

It is flexible PVC. I'm out of work now I will write again when i got home.

You did not indicate the type of PVC. Is it rigid, semi-rigid or flexible? When working with PVC, adiabatic heating is usually the preferred method to increase melt temperature. This is accomplished by increasing back pressure &/or rpm until the desired melt temperature is reached. In this manner the material for the next shot is heated without causing the entire barrel to overheat. I would also use the recovery delay on your Arburg so that the material for the next shot is not being cooked while cooling of the mold is on the way. Increasing barrel temperatures only will lead to early degradation.

Did you run a rheology curve to determine the proper injection speed? Be careful not to have too large of a cushion. For PVC I like to use no more than 0.250 inches. I don't know what 71 cc translates to inches on your screw. What is diameter of your screw?

Have you contacted the PVC manufacturer for starting process conditions?

I hope this helps.

Bill28