Flame treating is a very important step in preparing certain types of plastic for hydrographic immersion printing. It is an essential process to achieving optimal water transfer results, but it is necessary to know why it needs to be done and it all relates to adhesion.
Primarily, when paint or film doesn’t stick to your object, it is usually due to a lack of adhesion. Regarding hydro dipping plastics, there are a couple of different factors to consider, especially the type of plastic you are using as a substrate. Polyethylene (PET or PE) and Polypropylene (PP) are probably the most common types of plastic that are both hydro dipped. These types require flame treating, whereas ABS plastic DOES NOT need to be flame treated and can have graphics directly imaged onto it.
The reasoning behind flame treating boils down to the surface and the chemical makeup of plastic. PET and PP are considered to have a low surface energy which means that it is hard to “wet out” liquids on the surface. Flame treating will chemically modify the surface by oxidizing it which promotes liquids to “wet out” better. Many plastic parts also contain “releasing agents” on the surface. In other words, if you’ve ever sprayed a bread pan with cooking spray to help bread pop out easier– molding plastic uses the same concept, except the releasing agent is the cooking spray. Releasing agents are designed to prevent bonding/adhesion. Flame and degreasers aid in removing this layer. Here you will learn when flame treating is necessary, what supplies are needed, and how to do it.
First, you must identify the type of plastic which you intend to dip. There are a couple ways to make this determination.
1.Look for Plastic Identification Codes
2 .Try the Sink or Float Method
3. Acetone Method
4. Consult the manufacturer, Safety Data
Sheet, User Manual
5.When in doubt, flame treat everything.
*ABS plastic is already in a state to be hydro dipped, but it won’t hurt it if you accidentally flame treat.
1. Plastic Identification Codes
Identification triangles, or Plastic Identification Codes (PIC), are one of the easiest ways to find out what type of plastic you have. If you can jog your memory all the way back to grade school, you might be familiar with these common recycling symbols. Who knew these would come in handy for hydrographics??….
Flame treating is required for any plastics that are stamped with numbers 1,2,4, or 5. For example, 1 = Polyethylene, 2 = High Density Polyethylene, and 3 = Polypropylene. For a full list of PICs and what they actually mean, follow this link>>. You’ll find examples of common household products and what category they fall into.
2. Sink or Float
If you don’t want to leave the shop and you’re looking for a quick estimation of the type of plastic you’re working with, shave off a little piece of plastic that is not visible to the naked eye when the part is in its final assembly phase. Drop the plastic shaving in a clear glass of water. If the plastic floats on or near the top of the water, you need to move on to degreasing and flame treating before you apply a basecoat. If the plastic sinks towards the bottom of the glass, you can simply move onto scuffing the part with a sandblaster or Scotch-Brite pad and continue to prime with a basecoat.
- Float = Flame Treat
- Sink = Scuff & Paint
3. Acetone Method
Applying a light coat or a few drops of acetone to a small portion of the material can help determine the plastic type. If the plastic becomes sticky or starts to dissolve on the surface this material most likely does not need to be flame treated.
4. Manufacturer, SDS, User Manual
Often if you are buying directly from the manufacturer, you can easily call or shoot a message to their technical department, and they should have some answers for you. If you are worried about wasting time on the phone or waiting for days for a return email, try checking the manufacturer’s website or take to Google to track down the product’s Safety Data Sheet or User Manual.
5. When In Doubt
If you are unsure if the type of plastic doesn’t fall within the Polyethylene or Polypropylene family, and suspect that there’s a possibility of it being ABS plastic, take the safe route; flame treat it anyway. ABS plastic has a melting point very close to PET and PP plastics. So, it doesn’t hurt to flame treat everything. It just means you’ll be taking an extra step that may be unnecessary and a waste of prep time.
- Degreaser Cleaner
a. Prep-All Degreaser (Aerosol or Liquid)
b.Isopropyl Alcohol (nothing lower than 70%)
- Lint Free Cloth (Scotts Shop Towels or WypAll L40 Towels )
- Propane Torch
- Flame Spreader
- Spray Bottle for Prep-All Liquid (Label it!)
- Heat Resistant Gloves (Optional)
- Disposable Gloves
- Sandblaster or Scotch-Brite Pads
- Air Blow-Off Gun
Instructions – How to Flame Treat
After you’ve identified what type of plastic you are working with, you can now move onto the cleaning process. Be sure to use protective gear when working with these toxic chemicals. Wear proper eye protection and face coverings to filter out hazardous emissions and odors. N-95 masks work, but if you already have a respirator to use for the hydrographics process, it doesn’t hurt to use it. Latex or nitrile are the most ideal disposable glove types to use when working with the degreasing chemicals suggested below. Using your degreaser, spray a moderate amount onto the surface that is being hydrodipped. Wipe off quickly using a lint-free shop towel. Acetone and Isopropyl Alcohol dry extremely fast, so be sure to spray and wipe down as quick as possible.
Choose your preferred method to scuff the surface, whether it be with a Scotch-Brite pad, Sandpaper, or Sandblaster. Sandpaper is probably the most time-consuming method because it does not flex around curved surfaces, if you’re using a sandpaper block. It can also be a little overkill for the process. Scotch-Brite pads are inexpensive and easy-to-work with when dulling the surface of plastic. Sandblasting is the fastest method, saving more time for splashing around with hydrographics. When sandblasting, be sure to have a course enough grit to roughen the surface. In the attached video, we used 120# Aluminum Oxide at 40-50psi. Ultimately, you can use any size of abrasive, however, it will alter the time spent scuffing. Be sure not to sandblast each square inch of the plastic for more than 30-45 seconds. The idea is to prevent any melting or distortion of the plastic’s shape. If you have an air compressor blow-off gun, blow off all abrasive remnants from the object. Personally, we prefer using the CrystalBlast Sandcarvers from IKONICS Imaging as many of them have pre-installed blow-off guns inside their cabinets, but you can find supplemental air guns on Amazon. Remove any left over grit or dust from sandblasting by wiping the substrate with a damp cloth.
3. Flame Treating
After thoroughly scuffing and cleaning the plastic, you are now ready to use a handy dandy blow torch for flame treating. Benzomatic is one of the more commonly trusted blow torch brands. They have attachments that feature a simple push button ignitor. We suggest using this kind vs. the manual ignitor used in the instructional video. Strictly use Propane and stay clear of using MAPP Gas. They burn differently and MAPP contains other substances that could interfere in the results of flame treating. Properly attach a flame spreader to the end of the blow torch. By making the flame fan wider, the spreader creates much more consistency when treating the plastic and decreases the chances of melting a hole through the plastic. Using the end of the flame, be sure that ¼” – 1” of the flame tip comes into contact with the plastic’s surface. Every square inch of the plastic should be flame treated for at least 1 second. Use extra caution when working with a torch and keep the flame moving. If need be, use fire resistant gloves and always make sure to keep hair and clothing kept back and out of your working area. Play with fire, DON’T catch on fire! Keep a fire extinguisher on hand. Finish up by mildly cleaning one last time with the decreasing agent. As a tip, it is suggested to use Isopropyl Alcohol or Acetone in this last cleaning cycle because Prep-All has additional substances, such as mineral spirits, which are an oily substance. You just worked so hard to remove all the oils from the plastic and changed the surface energy.
4. Prep for Hydrographics
Now that you’ve completed the flame treating process, you are ready to move onto basecoats and the hydrographics stage. Handle the plastics with care by using disposable gloves. This will keep any natural oils from your hands from being deposited on the freshly stripped plastic. And of course, don’t forget to use Hydrokon film for your customized hydrographic prints.
Keep Calm & Dip On!