Short Shot Detection

[Gamble 0]

I need something to detect a "short shot" in a plastic molding process. In our system the part is molded and then an arm picks up the part out of the mold, and then drops it onto a conveyor. I've been thinking that putting some sort of detection device on the arm would work well. The defect I need to detect is pretty small, and I think that a laser with a beam no more than 1mm in diameter could do the job. The problem is that I want to avoid thru-beam devices (since getting the part in between the tx and rx would be tricky), and the idea of a laser beam shooting off of a moving robotic arm is a bit scary. To complicate the matter the defect occurs on a curve. My bosses are cheap so a nice vision system is out of the question. They would like to see it done with a $100 IR sensor. I don't think that possible though. Any thoughts?

[TConnolly 13]

If it can't be done reliably with a photo sensor then it cant be done with a photo sensor. In other words, you may have to spend the money for the right solution. An economically priced vision solution is the Cognex Checker. Once the CC is set up it would send a good/bad signal to the PLC much the same as a photo eye would do.

[TimWilborne 108]

Here are some articles Chakorules has written about vision systems that might be of help http://www.mrplc.com/kb/index.php?page=ind...;id=20&c=37 http://www.mrplc.com/kb/index.php?page=ind...;id=78&c=37 http://www.mrplc.com/kb/index.php?page=ind...;id=31&c=28

[ianbuckley 0]

It depends on what you are looking at as a defect. If it is simply looking for presence at an exact location, you could use a laser sensor. We have used Banner QS30LQD in the past with good success. It detects color changes as well as depth changes and is very configurable. The one we use has a class 1 laser, so it would take a real effort to make it dangerous (Don't get into a staring contest with it, but otherwise...). If I remember right, it is about $200 with sensor, cable and bracket. However, if you are looking at more than 1 specific location, the Cognex checker Alaric calls out is probably your best bet.

[Peter Nachtwey 15]

Why not fix the cause of the short shot instead of trying to detect it afterwards. If the mold pressure is too low during the pack and hold then you know you have a short shot. There should be a window or band of acceptable mold pressures for a good part. A controller should be able to maintain the correct pressures.

[TConnolly 13]

I'll second Peter on that. Perhaps you could spend a couple of weeks running with an SPC control chart on a clip board and manual inspection to determine the correct range of mold pressures that will relibalby detect a short shot.

[IO_Rack 178]

I'll agree with Peter as well. I like that kind of thinking. At my company we try to verify a process (meaning, create checks in the process) before creating a defect inspection. If an inspection is necessary for your application, you'll have some other things to consider like the orientation of the part and the orientation of the defect. Edited 2 Nov 2006 by IO_Rack

[Gamble 0]

Thanks for the suggestion Peter. I'll look into that.

[Gamble 0]

Unfortunately we don't have the ability to monitor pressure in the mold. The only pressure we can monitor is that at the beginning the screw feed, and I am told that this is not precise enough.

[Peter Nachtwey 15]

The injection force or pressure should be very precise. The problem that arises is that the plastic can solidify at the nozel or the sprues( spelling ) before the mold is full. Then the pressure in the mold is not the same as the injection pressure. This obviously prevents the mold from filling properly, The real problem is hard to diagnose without any graphs of what is happening. Do you know how far the screw is retracted on each stroke? Do you know how far the screw moves forward on each stroke? Can you measure the pressure or force as injection, pack and hold take place? Does this problem occur on all molds or just one mold? Are these values consistant from shot to shot? What ever the problem is you are better off fixing the problem and understanding your process better than rejecting parts. In the long run you will be money ahead.

[Clay B. 1]

We do "thin wall" molding where I am at and shorts are a constant issue. We have the causes to range from the tempature of the plastic to the amount of venting in the mold. This is never an easy problem to fix. That being said the only effective way we have found to detect shorts by automation is with a vision system. there are lots of systems out there. For us intergrating a vision system into or stacking equpment has been the most cost effective. You meentioned you are using a robot so I would assume you stage your products for packing after the robot so inspection could be doen there.

[cmoore73 2]

Banner makes an inexpensive "smart sensor". It is the Presence plus P4. We are using it as a pattern match sensor and it works quite well. Omron makes what they call the F10 series of pattern match sensors that work fairly well also.

[Clay B. 1]

Are you looking at the silhouette of the object or patterns on the object itself? How simple is this system to set up?

[cmoore73 2]

We are looking at a silhouette, but we are also picking out patterns on the part. The Banner works much better at identifyinf characteristics than the Omorn. The Omron only looks at an overall profile of the part.

[Gamble 0]

[Wiseco 0]

Why not using a cavity melt pressure transducer? Ok you'd have to modify the mold but if it could work... What machine are you running? Is it an old machine or a young machine with a good control? What king of product are you making? How many cavity? Many things could correct this issue instead of wanting to detect it. If it's not consistent, try to know why. It could be a pressure rise in the water cooling channel which will cooled too much the mold which will played against the melt from flowing forward. That was just an example but try to know why it happen and what cause that. Maybe it's a venting problem too! Which would be very easy to correct. Try to post a pics of a short-shot part with an indication of his gate location. Another thing, try different injection profile, different melt temp, and always get a cushion in front of the screw. If it make flashing on the part even with a low holding pressure, your mold need to be repair, is simple than that (I assume that if you get many short-shot it's beacause you don't want a high holding pressure for whatever reason) I know many poeple that try every thing and don't want to hear that their molds are damaged and need to be repair. But a very good, well maintained mold will makes money. Edited 17 May 2007 by Wiseco

[gtsuport 61]

This company makes mold pressure transducers that mount under ejector pins. RJG Technologies Just a thought