Sport Mouth Guards

Hi,

you don't say much about the grade of EVA (e.g. shore A hardness) nor about the "welding" process (thermoforming, hot plate welding, or ...), it's therefore difficult to guess the root cause for your "bubble problem". Switching the material is of course always an (easy?) option, but as long as you don't know where those bubbles come from you may be getting them also with an other resin, from time to time.

Question: do you always get those bubbles, or only sometimes? If only sometimes, what might be the difference? You ought to analyse your complete "chain of production".

Bubbles during welding could be related to outgasing (decomposition, moisture, additives, etc.) and can sometimes be eliminated by applying a (or more) vacuum during the thermoforming or welding process.

Resinwise you could switch to something completely new, like silicon for example, or you could consider blending your grade of EVA with another compatible resin, like TPU or SEBS or ...! I am sure the supplier of the EVA might come up with an idea or two ...

"SASS at ODS" schrieb im Newsbeitrag news: snipped-for-privacy@posting.google.com...

I don't have any answers at this point, just more questions. Having worked with adhesive for 13+ years, the first question that is always asked if there is an adhesion problems is: are the parts clean? Given the irregular occurrences in your situation, I would ask: are the parts consistently clean? How do you know? After cleanliness, the three key variables for adhesion are time, temperature and pressure. How do you ensure that your production process is constant? Specifically, that the time, temperatures, and pressure are not fluctuating? Since the problem is sporadic, is there anything that you can identify that might be associated with this, such as start-ups, certain types of the second layer(sticker vs. paper), the first/second/third shift, operator A vs. operator B, certain times of year, certain weather conditions (rain/humidity/heat/smog), long production runs vs. short runs. EVA is an old standby adhesive, so there probably isn't anything really bizarre going on here. Some good critical thinking will go a long ways. John

Sorry all,

I can't seem to access the previous thread.

In any event, for those that responded would you be so kind to reply to snipped-for-privacy@personainternet.com I would appreciate it. Thanks to John for the reply. Here is my response. In terms of cleanliness, we usually use methyl alcohol to clean. We haven't actually used the alcohol on the final strata and will try next time. Temperatures are not monitored but we gage it by whether the guard is tacky or not?We're also toying with the idea of using a small oven to heat the model (or first layer) for evenness of heat prior to dropping the final strata. Is methyl alcohol the best method of cleaning the EVA?

Also would anyone know of a product that can be welded to the outer unit at the molar (biting) surfaces to reinforce. The product need to be fairly thin (no thicker than a dime lets say) in order to maintain comfort. I'm not to fussy about whether its clear or alters the color as this section is seldomly seen.

Thanks again John.

Rick,

The cleaning solvent depends on what the dirt is. Methanol is a good option. Other possibilities are rubbing alcohol (isopropanol) and heptane, which is flammable - it's in gasoline. Some people don't care for the smell. There are low-odor mineral spirits as another option. Some of these could start softening the EVA, which would generally not be a good idea unless you are looking to do a solvent weld.

Consistent temperature from part-to-part as well as within each part is going to be critical. You could use a thermocouple to measure this, or stick some thermochromic indicators all over a part as well (they change color with temperature). While measure each and every part will assure you the consistency you desire, a more practical approach is to measure a lot of the parts early on, then as you get more comfortable that this is overkill because nothing is changing, you can cut back the testing. When a problem occurs, go back to more extensive testing.

John

John's right monitoring your temperature is critical in a thermoforming process. Most thermoformers will use non-contact IR pyrometers to monitor the temperatures of both the plastic sheeting and the thermoforming mold. Doing so may help you get a better handle on your process and minimize and bubbles due to offgassing or degradation.

Larry Effler