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raystowntool
5th October 2012, 01:39 PM
I was wondering if anyone has any experience processing this type of resin.
It is a TPU (thermoplastic polyurethane) from Lubrizol. We are having issues
of excessive cure times. Our best parts are outside the max. material temp
specified by Lubrizol. For example we process material at 430 F (pyrometer
check at nozzle) and get acceptable parts. Cure time is 10-15 minutes. We
then strip parts from cavity/core. When we run at lower temps 390-400 F
parts shrink approximately .250" along length of flow. Gate sizes we tried were
.025 x .025" and .055 x .055" tab gates. Better results as far as fill and trapped
gases with larger gates. Could not deviate off of 430 F material temp though. Part
cure times still excessive. 10-15 minutes. If we stripped parts sooner they would
not keep their form.

pjhall
5th October 2012, 07:35 PM
I haven't run tecothane, but I have run Isoplast and Tecoflex, both are TPU's. The first thing to check is your drying. These materials can be tough to dry. Check moisture contant if you have an analyzer.

"Cure times still excessive. 10-15 minutes. " Are you saying your cooling time is 10-15 minutes or do you have multiple hand loaded cores and you leave the part on the core for 10-15 minutes?

I run a similar job to the one you have pictured. Its molded from Tecoflex B20 80A hardnes. We have 10 hand loaded cores. We leave the parts on the core for 9 shots. This is to prevent internal bubbles down the very small diameter thru bore. The cores are sprayed with FDA approved Lecithin spray so the parts can be unscrewed. The parts still tend to curl after they are unscrewed from the core, but this is not a problem with the application.

I have never seen that much shrink on such a small part. What is it 2 inches? .125in/in? Are you sure?

brentb
5th October 2012, 11:55 PM
Excuse me, but I don't see a wall stock thickness?

"Keep on Molding"

brent

pjhall
6th October 2012, 01:16 AM
Excuse me, but I don't see a wall stock thickness?

"Keep on Molding"

brent

Yep that might be a good piece of information!!!!!

raystowntool
6th October 2012, 02:15 AM
The shrink we measured is overall part length. We processed at 430 F stock temp and 390 F stock temp (at nozzle) and the
shrink between the two was unreal. The part processed at the cooler temp. shrank by about .25". Tecothane process sheet
lists shrink at .008" to .012" . I have 6 core/cavity sleeve assemblies I rotate in and out. I can only get a cooling time of about
4 minutes. I then strip the part and give it to the operator to put back in mold. Parts are still deforming at this cool time. I need
10 to 15 minutes before stripping parts. Wall thickness is about .050" to .060". Thinnest at small end and gets a little thicker toward
insert end.

raystowntool
6th October 2012, 02:21 AM
Also, I am working with other experienced molding personnel, 25+ years experience, and
they have never seen anything like this material. Drying time was about 2 days in hopper dryer
at 150 F, and moisture check read -40 F (dessicant dryer)

mmartel
10th October 2012, 07:47 PM
This is probably too late, but Im guessing your using 74A Tecothane. It shrinks at crazy rates and warps bad. If you can move up the scale a bit to the 85A you will have much much better results. The 95A molds even better. There is not a whole lot of difference from 75A to 85A but it processes a lot more consistent......

raystowntool
11th October 2012, 06:22 PM
Yes we are running the 74A. Thanks for the info. We are looking at adding more cores to increase our
cooling time. The longer the parts have to cool on the cores the better.