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Jonathan Bowen
06-07-2014, 11:31 PM
I'm working on some ideas and thinking of getting a vacformer. The cost isn't terrible. You can build a cheap one but I need one with integrated heater. My biggest issue is transferring the formed pieces to the laser for processing. Mostly looking to put them into the machine upside down and cut them out. Looking to make things like card holders and other items for cheap. I also have some other crazy ideas. Mostly involving scale buildings and other stuff that I already make.

So I have some ideas that I have not tried but I was wondering if anyone has been down this road or not. I'm thinking I can make some sort of key or frame that is formed into the plastic and keyed to the laser. The tolerances wouldn't have to be that critical for some stuff but some of my ideas will need to be cut really close. I could also build the frame so it came out of the former and went into the laser and held it in place but that might be tricky with focus lengths and stuff. Still brainstorming and the idea is a but more difficult because my machine doesn't have a good fence on it. I built one that works for my needs but it's not very good. Just keeps my sheets square and prevents me from missing the edges when I cut full sheets.

Doug Griffith
06-08-2014, 12:03 AM
I've vacuum formed a kerjillion parts. Never used the laser to trim the parts but don't see why it wouldn't work. Materials might get you though. Typically ABS, polycarbonate, PETG and polyethlene are formed. All of the aforementioned materials don't laser cut with clean edges.

Allen Rawley
06-08-2014, 12:26 AM
In my experience, the tolerance is key issue.

One job was laser cutting out vacuum formed signs used for neon signs open and closed. The problem is that when the plastic cools, then it shrinks with a varying tolerance from part to part. So, close trimming was not possible for good results.

A slightly difference method of forming caused the same problems. The method was laser trimming injection molded parts. In this case, the mold part in 3D format was provided as a basis for laser cutting. Then, a number of the injection molded parts were accurately measured and the laser cut drawing modified. The quality was never quite there for the buyer to be satisfied, so they stuck with their manual cutting process. We spend more than 40 hours on that job, and it never got to the quality needed.

In both cases I used the lower level G-code rather than the print driver software to control the cut path. With G-code (or plotter language), there is added functionality for control. For example, the velocity of varying cuts around corners were modified to selectively control the burn back of the material. Though some control systems including power ramping on curves, in my experience, the wavelength absorption of different materials affects the burn-back rate, so there is no one-curve-fits-all for a recipe setting.

Doug Griffith
06-08-2014, 10:52 AM
In my experience, the tolerance is key issue.

One job was laser cutting out vacuum formed signs used for neon signs open and closed. The problem is that when the plastic cools, then it shrinks with a varying tolerance from part to part. So, close trimming was not possible for good results.

A slightly difference method of forming caused the same problems. The method was laser trimming injection molded parts. In this case, the mold part in 3D format was provided as a basis for laser cutting. Then, a number of the injection molded parts were accurately measured and the laser cut drawing modified. The quality was never quite there for the buyer to be satisfied, so they stuck with their manual cutting process. We spend more than 40 hours on that job, and it never got to the quality needed.

I can see how vac formed parts fluctuate a bit from part to part but shrinkage can be calculated. Good equipment and understanding all the idiosyncrasies of the process helps a lot. For example, shrinkage can be different depending on which direction the sheet was extruded. Zoned ovens, pre-draw boxes, assists, cooling fixtures, and more can all help control the finished part. I'd say 1/2 a percent over the length of the part is easily attainable. Injection molded parts are right on the money every time. It sounds like your customer was trying to achieve NASA tolerances for signage.

Richard Rumancik
06-08-2014, 6:53 PM
Jonathan, if you have some margin on the material that is always cut off maybe you could use a couple of tooling holes. Let's say you take one of your flat blanks and laser a couple of holes in two diagonal corners. Then the thermoforming fixture has two pins with the same locations. You slide the sheet onto the pins, thermoform, and then bring it back to the laser. Slide it onto the laser fixture which again has two locating pins on it.

In reality, you might need a hole and a short slot (rather than 2 holes) to accomodate tolerances but the concept remains the same.

You may need to be clever with hold downs of the formed piece.

Doug Griffith
06-08-2014, 7:12 PM
I'm guessing it's a male tool so if I was to try to hold tight tolerances to the formed part, I'd create a 2 part holding fixture. The fixture that mounts in the laser would have dowel pins that locate a removable fixture with corresponding dowel pin holes. The removable fixture would indicate to the inside of the part (tool surface side). I'd drop the part in the bottom fixture upside-down, then drop the second fixture on top of it and align to the dowel pins. Clamp the part. Lift off the second fixture. Laser away.

Jonathan Bowen
06-09-2014, 4:08 PM
I have considered a few options for a frame that then transfers to the machine. I can easily make one that drops into the hole of the machine but making one that centers itself on the platform might be tricky. I plan to basically cut upside down or maybe 2 sided. So top side and then bottom side so I need to vary the focus and watch that the gantry doesn't get hung up on any of the parts. I might have to create a rather larger platform that centers on the table and then mates with pins or something to the vac formed frame. Then I can make extensions if things get too big because the form will be upside down with the laser cutting out the bottom of the formed piece. The other trick will be getting the frame so that it can mate with the vac base the same way each time.

What it really needs is almost a 3d key system sorta thing. Were the laser hunts for divots in specific locations that reorient the zero position to meet the piece. However, that might be a bit much for my poor Hurricane Agnes to handle. I don't think the MCP controller supports any of that so it would have to be a totally different system that adjusts the code and then submits it to the machine after the alignment has been corrected for.

Doug Griffith
06-09-2014, 4:23 PM
Can you post a pic of one of the parts your attempting to trim? And are the parts formed using male or female tooling?

Jonathan Bowen
06-09-2014, 4:30 PM
The would be male tooling. It's stuff like card holders and other items. I can't post any pics because it's all theoretical now. I'm looking at getting a machine for $80. Well the parts to make a machine. He used to do Stormtrooper armor and wants to build a larger machine. He never assembled it after his recent move. I'd need to add heating elements but it's rigged for them.

Here is one of my products that I'm trying to convert to a vac formed piece.

http://corseceng.com/picture/starfighter%20template%20holder%201.jpg?pictureId= 18244684&asThumbnail=true

Doug Griffith
06-09-2014, 8:02 PM
Hmmmm. $80 for a vacuum former isn't going to get you much. Add 3 zeros to that and you can get a pretty good one. 2 zeros will get a small shrink wrap machine.

However you go about trimming, you need to index off the tool surface. Otherwise you not only have to deal with shrinkage but variation in material thickness.