Hope everything going fine with everybody.

The reason to write is because we're now running PE compound in the plant and we're experiencing some cracks in the insulation in one of the mechanical tests that we need to perform, On the cold bend test the heavy gauges are failing due some circular or longitudinal crack. We're using a core preheater at 120C as well as a Thermolator (for "cooling" the cable slowly - Gradient cooling in the water trough at 60C The extruder profile temp that I'm using is 210 C average on a 80mm extruder with a 24:1 barrier screw.

Any advice to improve this condition will be much appreciated,

Regards,
Angel

Hello xzfxhg,

The one important thing you haven't told us is the kind of PE compound this is. For example LLD, LD, MD mixed, MD polymerized or HD.

Please let us know the details ASAP.

Thank you.

Regards
Peter J. Stewart-Hay
Principal
Stewart-Hay Associates
www.Stewart-Hay.com
519 641- 3212

Hello Peter,

The cracks come on PE with 18% EVA blended in it and the profile is 210 C and the other compound that I'm using is HDPE at 150 C and sometimes I see some leak sparks on it

Thanks for your support

Regards,


Angel

Hello Angel,

The first thing is the EVA blend:

The mixing of poly(ethylene-co-vinyl acetate) EVA and semi-crystalline polymers such as low density polyethylene (LDPE) gives a blend with better flexibility, toughness and high resistance to environmental stress cracking. The latter is due to increased adhesive strength at the matrix-rubber particle interface.

The second thing is your extruder profiles:

EVA Blend (LDPE?) - A flat 210C (410F) temperature profile I guess is OK but I am not used to flat profiles. Melt temperature please.
HDPE - A flat temperature temperature of 150C (302F) seems quite low to me. Again, I need to know your melt temperature.
The 24:1 L:D and a barrier screw are fine as is the gradient cooling in the first section of the water cooling trough.

If I were you, I would get hold of the polymer manufacturer(s) and get their chemical engineer(s) quickly out to your plant for a visit. They should be providing you with the recommended extrusion temperature profiles for the compounds in question. Not the salesmen cannot help you with technical problems.

Are you cooling the screw?

What are you using for screen packs? Describe.

Are there any other things I need to know about these compounds such as the percentage of color concentrate, other fillers in the compounds or cooling water on the screw?

Regards
Peter J. Stewart-Hay
Principal
Stewart-Hay Associates
www.Stewart-Hay.com
519 641- 3212

Hi Peter,

On the EVA Blend let me ask to know exactly what kind of PE is also the temp profile that I'm using is
Z1 185C
Z2-Z4 200C
Z5 (Clamp) 215C

The melt temperature for this is 190C (374F)

On the HDPE The profile is
Z1 135C
Z2-Z4 140C
Z5 Clamp 150C

The melt temperature for this is 170C (338F)

We're not cooling the screw and for the screen packs we're using 20/40/60 mesh plus breaker plate.

It depends on the color that I need to run I'm putting between 1-4% of color concentrate at base and 1-2% on the stripes.
The information for the fillers or plasticizers I don't have and don't know if the compound supplier will share.

I'd really appreciate all your support

Regards,
Angel

Hello again Angel,

OK and thank you for the information. Don't worry about the fillers and plasticizers. I only asked that question in case you were using a heavily filled fire retardant compound or something like Megalon. It is very easy for us the be caught out because we cannot see what you are seeing.

Think of these pieces of extruder hardware as follows:

- A screen pack is not a filter but rather a method of maintaining the melt pressure inside the extruder. I am used to using symmetrical screen packs such as 20/40/60/40/20. This effectively contains the finest mesh screen.

- A breaker plate not only holds the screen pack but also changes the motion of the melt flow from say rotational to linear along the axis of the extruder.

- A barrier screw is designed to ensure a very stable melt pool (Very homogenous or well mixed and of uniform temperature.) by forcing the melt over a shallow barrier into a brand new screw section. It is not really designed to be a method of heating the melt by shear although there will always be some amount of heat added.

- The thermal heaters and fans are to melt the plastic and effectively control the temperature of the melt.

Please note that in the HDPE case, you are using the barrier section of your screw to add the final heat to your melt by shear. (Your melt temperature is much higher than the zone and clamp temperatures.) I would rather see you adding most of the heat by conventional means and not by shear in the barrier section of the screw.

In both instances I believe your melt temperatures are low and thus I suggest a bit of experimentation to get those temperatures up. (Contact your polymer manufacturer (Through the rep) and learn the extruder temperature profile and melt temperature they recommend for their compounds. They will likely have data sheets for their compounds with this information on it.)

Regards
Peter J. Stewart-Hay
Principal
Stewart-Hay Associates
www.Stewart-Hay.com
519 641- 3212