Bernards' Injection Molding Tips, Techniques and Technology for Shop Floor Technicians and Supervisors | home
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MORE TIPS STILL
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MORE TIPS STILL
20) Do you check the water circulation after you set a mold? Do you verify that all valves are open and all auxiliary equipment is on?
When is the last time a job went down on your shift because it was watered wrong or because someone forgot to turn on a valve? These are common mistakes on the shop floor and they can be so easily prevented. Do you have a check off sheet to remind you and to verify compliance?
Every tool you set should have a watering diagram that pictorially shows you where every loop should go and where every delivery and return hose belongs. You need this not only to prevent errors but to assure that each time the mold is set, it is watered in exactly the same way. Obviously, water flow affects the way a tool will mold. Looping it one way this time and another way the next time will cause you nothing but headaches and problems! Once a tool is watered you should first leave the valves turned off, then one by one pop off a return hose from the press manifold. Then hold the end of the hose in a bucket and turn on the appropriate delivery valve. You do this to check water circulation. If a hose of water line is blocked then water will not flow out of the hose you are holding. Do this for every delivery line. There is an added benefit too. This helps purge the tool and lines of air. It is not enough to have a set up sheet that says, for example, "Back half Thermolator 135 degrees, Front half tower water ". I will be writing about set up sheets in great detail a little later.
DON'T FORGET TO CHECK WATER CIRCULATION!
21) Do you have problems with material contamination from your granulators? Do you "flush" them after switching materials? Do you have problems with material contamination from your hoppers or do you have a hard time figuring out where the contamination comes from?
OK, you just had a tool change and you went from nylon to polypropylene. Your material handler cleaned out the hopper, the grinder and even cleaned out the chute beneath the mold. He did a nice, quick job for you, didn't he! You then start up the new job. This is a ten cavity, subgated mold. The start up goes well and within 10 minutes you are running production. You go to the lunch room for a coke and pat yourself on the back for a job well done. Oh, No, here comes the Quality Inspector. "You know that job you just started up..." she says. "Well, we are only getting five cavities!" You put your coke down with a little anger in your heart and you go over to the press. The operator then tells you "Well, right after you left I lost two cavities and then I lost three more. Right now I am only running two!" You open the gate and stop the press. You pick up a runner and look at it. At the end of each sub gate on the runner you see a blunt spot instead of a nice point. "Damn", you say to yourself. "The gates are blocked off" So you get out your propane torch and paper clip. You heat the paper clip and thrust it into a sub gate. What comes out? A little white colored piece of plastic material. Then you light the piece of material with your lighter and smell the smoke. It is obvious by the odor, it is nylon! Ten minutes later you are back up and running. Ten minutes after that gates are blocking off again. This goes on for an hour or so until you call over the material handler. "Drain the hopper" you tell him. "Empty the grinder too, we have contamination from the last material!" So he empties the hopper and grinder bin and puts it all into a drum. What is it, about 200 pounds? He asks you what he should do with it and you tell him to dump it in the dumpster because it is no good now (secretly you are hoping no one will find out. Probably the material will fall to the bottom and no one will see it). But, I forgot. You do not throw it away. Instead you tell him to dump it back into the gaylord (that still had 800 lbs of virgin) and you tell him to label it "contaminated"! ( To add to this typical story, the gaylord gets stuck in the back of the warehouse for months until a new inventory is taken. Someone then find the gaylord, sees that it is contaminated and throws the whole gaylord away!)
Back to our story... So the material handler recleans the hopper and grinder, albeit, a little slower and more carefully now and you restart the job. It runs fine. You take the material handler off to the side and give him "hell" for having done a poor job.
Does this story sound familiar? If you have worked in the plant for any period of time I know it has happened to you. Maybe it wasn't nylon and PP. Maybe it was ABS and polyethylene. Maybe it wasn't a subgated mold. Maybe it was a hot manifold. Maybe it wasn't blocked gates. Maybe it was splay and lamination instead. What ever it was, it was CONTAMINATION.
So what went wrong. Lets start from the top. Did your material handler completely blow out the hopper or did he simply clean it with a rag? Did he clean the magnet? Sometimes raw, virgin material can have fine pieces of tramp metal inside the pellets and the pellets stick to the magnet. This can come from the raw material manufacturers' equipment. Did your press have a magnet drawer? Did your man clean every nook and cranny there and use an air hose? Did your press have a drying cone? Did your material handler check to be certain it wasn't damaged and had material inside. Did your press have an automatic loader? Did your man blow it out to get rid of the old material? If the loader motor is on top of the hopper, did he clean out the filter? Was the gaylord next to the press uncovered? Did your man blow off the press to clean it and contaminated the gaylord? Did your man remove the screen from the grinder and clean the pieces of material that were stuck in the holes? Did he clean every nook and cranny with an air hose? Who taught him to clean a grinder in the first place? You, or did he learn on his own. And lastly, did you flush the grinder? (I will explain that momentarily)
I know this has happened to you and I cannot over stress the importance of thoroughly cleaning hoppers, grinders, etc. The extra few minutes that it takes to do it right is paid back many fold. The question too is "Does your material handler have a check off sheet for material change overs and does he use it religiously? Or, does he do the change over and then just check off everything "OK" one check mark after another?
Now, about flushing granulators. The fact is that no matter how good a cleaning job is done, there is always going to be a little bit of material and material fines left behind. This is a fact of life. Certainly it is not much but nevertheless it is just waiting there to give you trouble. So what can you do about it? Flush the granulator! After the material handler has cleaned the grinder, have him affix a sign to it that states "GRINDER NEEDS TO BE FLUSHED" As you start up the new job there are always a few shots that you throw in the trash. Keep a few clean ones and GRIND THEM UP. Once they are ground, empty the granulator into the trash and remove the sign. Those few runners and parts you ground will shake loose those last little bits of fines and material and will flush out the grinder. I have used this technique for over twenty years and it WORKS.
22) Do you blow the water out of the mold every time you do a tool change? Have you noticed that some of your tools are running hotter or won't come up to heat like they use to?
Another story,,,,, You set up a job and it starts out running fine. An hour later the Quality Inspector comes up to you and tells you the parts are warping. Of course, you do not believe her so you go over and fiddle with the controls to make her happy and you tell her it's OK now. She goes on with her duties and makes finishes her two hour rounds. Then, almost two hours later she walks up to you and says that she has to put all those parts on hold! They won't fit the fixture. So you go back over to the press. You feel the tool. Boy, that back half is sure warm! You look at the water lines and valves. Every thing seems fine. Now you don't know what to think. As a last resort you start pulling off the return lines to check the circulation. Nothing comes out. You check the watering against your very detailed watering diagram and everything checks out. "Must be plugged", you tell yourself. So you get yourself an air hose and blow into the line. "Splat" and a big chunk of reddish, gooey stuff comes out. You pick it up and rub it between your fingers. "Damn" you say out load (but not load enough for the Inspector to hear) it's rust! You blow out more lines and more rust goes "splat". An hour later you are back into production but you have two hours of scrap. The Inspector is mad at you too.
Does this sound familiar? Well, if you and the other guys always blow out the water lines when you pull a tool, maybe it hasn't. On the other hand, if you don't it will happen sooner or later. Everyone knows that steel rusts. Everyone also knows that wet steel rusts faster. Do you know what rust actually is? Rust is "oxidation". When metal rusts it combines with the oxygen in the air. Why does wet metal rust faster? Water is what is known as a "catalyst". A catalyst speeds up a chemical reaction. Water speeds up the chemical reaction of metal combining with oxygen. In fact, water can make the reaction work hundreds and even thousands of times faster. So what do you think that water is doing while it is setting in the cooling channels of the mold that was not blown out? Sure it is evaporating, very slowly. It is also causing the steel to rust, very rapidly. Remember that the next time you pull a mold!
Rust does something else too before it completely blocks the cooling channels. The rust that lines the channels decreases their ability to transfer heat. This means that a tool you want to cool will not get as cold. The tool you want to heat may not be able to come up to temperature. Obviously this will affect the process. Remember that too the next time you pull a tool!
23) How many safety limit switches are defective or bypassed on your molding machines, grinders or other equipment? Have you ever defeated a safety switch yourself?
The answer to both questions should be an emphatic "NONE and NEVER". If the answer to these questions is "SOME and SOMETIMES" then you, the employees and the Company are in serious trouble! Let's start with the press itself. Earlier we talked about the three basic operator gate safeties on a molding machine: Electrical, Mechanical and Hydraulic. However, there are many other interlocks as well. What about that safety guard screen that slides open next to the gate? That has an electrical safety interlock. There may be more than one safety guard on either side of the press. Behind the press is what is commonly referred to as the "rear gate". Although it is not an operator gate at all and it is not protected as is the front gate. It has only one or two electrical interlocks. Never should this be used or be modified to be used as an operator gate! Probably you are thinking "Who would ever try that!" but you would be amazed at the things I have seen over the years. What about that switch on the purge guard? Is it working? That interlock is designed to help prevent YOU or SOMEONE near you from getting burned while purging. Does the purge guard safety switch work on every press? Take a look at your granulators. Every one has a limit switch that prevents the motor from running if the hinged body is not completely closed against the cutting chamber. Do those switches work on all your grinders? Look around. Do you see any grinders where someone put a piece of cardboard under the limit switch because the grinder would not close flush? Much more will be said about PLANT FLOOR SAFETY and LOCK OUT PROCEDURES when we get to that section. But, please, start thinking about safety now and forever when you are in the plant.
24) Question: An operator calls you over to a press that is running an eight cavity tool and tells you she is getting shorts once in a while on one cavity. You notice that the screw is bottoming out occasionally. Which of the following do you do? A ) Increase the injection speed to fill the tool faster. B ) Increase the pressure to pack the tool better. C ) Increase the feed to give the tool more material. D ) Turn up the nozzle temperature because you figure the tip is on the verge of freezing off. E) Tell the operator to sort out the shorts and scrap them out. F) None of the above.
The answer is given at the bottom of this page. But, I will give you a hint directly below:
25) Have you ever seen a mold fall out of a press? I have. How many of your platen holes are stripped?
I have seen a mold fall out of a press, the front half at least! It was in a 500 Ton molding machine. The die setter used bolts that were too short and the barrel pushed the mold forcibly enough to strip every bolt hole and the front half fell into the chute! Let's hope this never happens in your plant.
There is a general rule of thumb for bolts. They must be inserted at least one and a half times their diameter into the platen. In other words, a 1/2 inch bolt must be inserted at least 3/4 inch deep and a 5/8 bolt must be inserted 15/16 inch deep. These are absolute minimums and of course, they should be screwed in deeper. This rule of thumb assumes the bolt hole threads are in good condition, none are stripped and the threads start right at the bolt hole opening.
There is also a rule of thumb for how hard they should be torqued down. With a torque wrench, a 1/2 inch bolt should be torqued to between 120 and 140 foot pounds. A 5/8 inch bolt should be torqued 200 to 240 foot pounds. Far too often people will put a five foot helper bar on the end of the wrench and crank on it for all they are worth! Not only does this over stress the bolt and bolt hole threads, it can strip the platen. Most cases of stripped bolt holes, however, are caused by using too short of a bolt. Proper washers must be used too and not the hardware store variety. The washers must be of hardened steel and of the appropriate thickness. The lack of a washer on a mold clamp can cause the bolt head to spread the clamp prongs and pull right thru .
26) How often to you have to hunt for things when you change a mold? When changing a mold how often do you have to leave the press to go and get something (hoses, loops, washers, wrenches, etc)?
It is an absolute truth that in the Injection Molding Industry, time is money! OK, but what does that mean? Consider this, no matter what your job is at your plant, there is only one thing that pays your wages . That is, of course, parts. Not just parts, but good parts that the company can sell to the customer. In a very true sense of the word, the customer pays your wages and your company's bills when they send in the check for their parts. If a molding machine is just sitting there and not running, it is not paying for anything. When your company quotes for jobs, someone there will have to figure out what to charge. They will calculate the cost of the raw material and allow for a little scrap (typically 5% or much less). They will also calculate the size of the press it will take to run the job and they will have already calculated how much they have to charge an hour to run that size press. That hourly rate for your presses is the income your company needs to pay everyone's wages and the bills. Typical hourly rates in the industry might run from, say $25.00/hr for small machines and up too say, $200.00 for very large machines. This does not mean the company makes that much from the press. It simply means that is what they need to charge to pay all the bills.
So you have a job to set and the press is down. Clearly, down time is necessary to change a mold. But, every minute that press sets idle it is actually costing money because it is not making money. Think about it, the bills are still there and everyone still gets paid no matter how many presses are down. So you change the mold. Lets say it is a large tool in a large press and it has core pulls, etc. The mold takes you four hours to change. Four hours! Instead of paying the company bills it is waiting for you. Ask your self how much time is wasted during that mold change. How much time is spent leaving the press to get things? How much of that time is spent looking for things you can't find? You definitely need a mold change cart? What is that? A properly designed mold cart will contain everything you need or might need to change a mold. It will have bolts and washers and loops and hoses (draped across a vertical upright). It will have water fittings and replacement O-rings for when fittings leak. It will have hose clamps and mold cleaner and clean rags. It will have wrenches and screw drivers and all the tools you might need. In short, it will have virtually everything you need to change a tool. The purpose is, of course, to get the jobs up and running as soon as possible. The next time you change a tool think about the machine's hourly rate and think about that when you are out in the plant looking for mold change items you need.
27) If you are a Manager or Supervisor, how often do you go out and look in the dumpster? You can learn all sorts of things looking there.
The dumpster will tell you about things you might not normally see. For example, you know you had enough material to run that job in press #3 and simply cannot understand why you ran out. Looking in the dumpster, someone may have contaminated it and threw it out! While you are looking, look for all the runners and parts you might find there. Was someone lax and did not clean up that oil. Did those parts and runners get oily that way? Was someone just cleaning up and simply threw them away instead of grinding them? Look for assembly components too. Now why are they there? How about all those cartons that cost fifty cents to a buck or so? Why are they in the trash. How many pounds of excess purgings do you see? Look deeper, what else will you find. As soon as the people know someone is checking the trash, the amount of unnecessary waste will decrease. I know this for a fact!
The correct answer to #24 is "None of the above".
Before you ever make a change in the process you should first read the Process Change Documentation Book and you should do a quick check of the process! The first thing you want to know is if anyone else had these problems and what they did about it. The second thing you want to do is check out the process itself to see if anything is amiss. The operator told you she was getting sporadic shorts and you notice that the screw sometimes loses its' cushion. You tell yourself that you probably have a bad check ring so you increase the feed. However you never checked the process set up or the Documentation Book. The change you made took care of the problem, right? It ran OK for the rest of the shift, that is, until the next shift took over. The following day you come into work and you are called into the front office. Your boss tells you that after you left work yesterday the next shift technician had to clean off the bonnet of material that worked itself half way up the barrel! It destroyed half the heater bands and the press is down still being repaired! He raises his voice and angrily asks " Didn't you read the Documentation Book at the press? I t says right there that the nozzle tip was leaking and that you have to clean it every thirty minutes until it can be replaced on days (no one could find a spare). We lost sixteen hours of production because YOU DIDN'T READ THE BOOK!" Get the hint? Of course, maybe it wasn't shorts, maybe it was warp or sinks. Maybe you did read the book but you didn't check the process. Maybe it was a Thermolator that kicked of or maybe it was you because you did not follow procedure.
Incidentally, if you ever do find yourself in a situation where a nozzle tip is leaking and you do not have a spare, try placing a thick piece of cardboard (like from a gaylord) between the nozzle tip and the sprue bushing. Shoot right thru it. It will act as a gasket and perhaps seal the leak. This also works for nozzle tips that freeze off on matter what you do to the nozzle heat or nozzle band position (which is to move it forward as far as possible).
What do you think about the person who checked (E) as the correct answer (Tell the operator to sort out the shorts and scrap them out) ? Firstly, you are costing your company money. The parts that come out of the presses pays for your wages and everyone else's. It pays the rent, and the bills and the fringe benefits the company gives you and your family. Even the loss of one or two cavities out of every eight can mean the job is losing money. In other words, money is lost each shot instead of being made. Secondly, you stand a very bad risk that the operator might miss some shorts and they could be sent to the customer. What then? The operator will get into trouble when his or her job ticket is returned with the rejects. The company will get a bad mark on its' Quality Record. The Quality Department will get into trouble because they didn't catch the defects but you will be OK, right? Wrong! Quality is part of your job, a big part, indeed. Your company's reputation is part of your reputation too. Remember all of this the next time you are tempted to tell an operator to simply sort out the bad parts. Instead of telling the operator that, do your absolute best to try to fix the problem (after first reading the documentation Book and checking the process)!
UNDER CONSTRUCTION