MORE TIPS

Every molding machine has two water fittings at the barrel throat. These fittings connect to a small heat exchanger that cools the throat and that end of the screw. Most molding machines also have a thermometer mounted in this area. When you read the processing specifications for various materials you will usually see a callout for barrel throat temperature. However, what do you do if you do not know what the temperature should be? Consider this: if the temperature is too high you may find that your material bridges at the end of the screw and the screw will not recover. This can be caused by excessively high rear heats but it can also be caused by too high of a throat temperature. In either case, the feed zone of the screw becomes so hot that it actually melts the material (the feed zone is designed simply to pick up material, not melt it!). What can happen if the throat is too cold? You can actually run moisture splay even with non-hydroscopic materials. The air around the inside of the throat can get so cold that the temperature reaches the air's dew point and condenses out to become water! (yes, there is air there too: air and raw material) I have seen this happen when people run chiller water to the throat. So, what temperature should you run if you do not know the processing spec?. I would suggest anywhere between 80 Degrees F to 120 Degrees F. There are not too many materials where you would need to run it higher than that. Some companies install closed loop throat temperature controllers. This will keep the throat at a constant temperature automatically. However, most presses use a simple manual valve to control the temperature. Remember too that on any given press the temperature will run cooler with low heat materials and hotter with high heat materials. You may have to adjust the water flow as you change tools.

When a mold closes, the force that holds the mold together is also applied to the tie bars. When a press is set up properly, the force will be equally applied to each of the tie bars. Upon clamping, the tie bars actually stretch in the tenths of thousandths of inches. Very serious problems can arise when the tie bars do not stretch evenly. In the worse case, a tie bar can actually snap and break. Tie bars are very expensive to replace and are very labor intensive, not to mention the loss of machine time and production. Even if unevenly stretched tie bars do not break they can cause excessive wear on the toggle mechanism, pins and bushings. Moreover, the mold will not feel equal clamp tonnage across its face and will tend to flash on one side. There are specialized devices like the one depicted below to measure toggle stretch. Machines can also be retrofitted with permanent strain gages. However, all it really take is a few dial gages and the appropriate hardware to mount them. At the end of each tie bar is a large threaded nut or threaded gear. This is what is used to adjust the stretch. Never touch them if you do not know what you are doing! You could cause serious damage to the press.

The dial indicator and electronic digital indicator accurately reads stress on tie bars so they can be adjusted for equal stress and maximum safe locking tonnage. The magnetic bases attach firmly to the tie bar when aligned with the tie bar axis. A simple on/off lever on each magnetic base controls the attachment between the base and the tie bar being tested. A zeroing lever on the dial gage (zeroing button on an electronic gage) sets the indicators to "0" prior to usage and between tests.

Hot runner molds should be provided with compression resistant insulating plate between back plate and machine platen. This is to prevent the heat flow from the mold to the machine platen, which can create an unbalanced heat flow in the mold. With out an insulating plate the machine platen will act like a large heat sink, there by destabilizing the balance between heat given to the mold by the hot melt and electrical heaters, and heat taken away by circulating water through mold. This can affect the stability of your molding process. The lack of an insulating plate will also make the manifold heaters cycle on and off more often (which will reduce their life) and it will take the mold longer to reach homeostasis (a constant even temperature).

You probably take good care of your car and change the oil every six thousand miles. Why? Because you want your car to last! Certainly hydraulic oil on a molding machine cannot be changed often but it can be monitored and filtered. A number of things happen to hydraulic oil over time. When oil breaks down it oxidizes. This means it combines with oxygen from the atmosphere. Oxidation changes the oil's viscosity, decreases it's lubricating qualities and causes a gum like substance to form. This substance can cause valves to stick or become sluggish. Once it forms, it deposits itself on every hydraulic component in your machine. Oxidation also settles in the sediment at the bottom of the tank. Oxidation is accelerated when hydraulic oil exceeds 55-60 degrees C (131-140 degrees F). It is also accelerated by frictional heat generated in pumps, valves, etc. Moisture enters the oil from the atmosphere. Water does mix with oil under terrific hydraulic pressures (actually it forms a very fine suspension). This moisture can cause fine rust deposits to form in the hydraulic components. In fact, the compounds formed by water are acidic (meaning acid!). For that reason special additives are added by the refinery to reduce the influence of water. Oil also becomes contaminated with very small particles of metal. As you know, in any mechanical device where metal rubs against metal there is wear even if the metal is lubricated. There is wear in the hydraulic system as well. Were does the metal go? A molding machine has a closed hydraulic system and the only place the worn metal can go is back into the oil! These particles are very small, from say, 5 to 50 microns (a micron is one millionth of a meter). This contamination can wreck havoc with a molding machine and especially with the newer variable volume pumps and proportional valves. Any of the above mentioned contaminates can block the small orifices and passages.

A sample of hydraulic oil should be taken from each press and sent to a lab for testing at least twice a year with 5 day/24 hour production. The people who sell you your hydraulic oil can do the tests for you. One test is called a gravimetric analysis. The sample is placed in a test tube and subjected to very high G's (gravity's) and this causes the suspended contaminates to settle out. There are also other tests they can perform for oxidation, etc.

As you just read, excessive hydraulic temperature can seriously degrade your hydraulic oil. This, in turn, will affect the useful life of your machine and your process could become very erratic. Oil temperature is controlled by a heat exchanger. Look at the lower rear portion of your press and you will see a long tubular object that is connected to a large water delivery and return line. That is the oil heat exchanger. Located inside the heat exchanger are a series of long copper tubes. Cooling water runs thru these tubes and the hydraulic oil flows across the outside of the tubes. With this arrangement the oil gives away its heat to the water and the water carries the heat away. Sometimes these tubes can become blocked with sludge from your central cooling water system. When this happens the hydraulic oil can overheat. In such a case the heat exchanger must be taken apart and the cooling tubes cleaned. Never attempt this yourself if you do not fully know what you are doing. Those tubes have very thin walls and are easily damaged. If damaged, water will leak into your hydraulic oil and could very easily damage the press. Repaired heat exchangers should always be tested before being put back into service.

Some plants simply run water pipes or flexible lines directly from the central water cooling system with out using a "water saver valve". This is not a good idea, especially for newer molding machines. The problem is that although the oil is cooled, its' temperature is not controlled. As the press runs the temperature will vary over time because of varying machine loads caused by the molding process. This is because there is a steady stream of water flowing thru the exchanger regardless of the cooling waters temperature and regardless of the molding machine's load. Oil viscosity decreases as temperature increases. Higher viscosity will affect pump suction and can result in erratic behavior. Pump efficiency is also reduced, the effectiveness of the lubricating film is reduced and wear will accelerate. You will notice too that you might have to make continuous process readjustments as the oil warms up. This is why it is a good idea to use a water saver valve. The term "water saver" is really a misnomer. Yes, it does save water but it does something much more important than that. It controls the oil temperature. There are two main components to a water saver: the sensor and the valve. The sensor screws into the oil tank and it senses the temperature of the oil. As the oil temperature rises above a preset value the sensor causes the valve to open. This allows water to flow thru the heat exchanger and the oil is cooled. As the oil temperature falls, the valve closes and the oil temperature can rise again. Water saver valves can hold the temperature within +/- 5 degrees F.

Most molding machines today have a central, automatic lube system that lubricates virtually every moving part of the machine. Some use oil, as in the case of Bijur units and some use grease, as in the case of the Trabor units. Do not let them run out of lubricant! Molding machines are subjected to terrific stress because they often run 24 hours a day and for days and sometimes weeks at a time. How long would your car last if you drove it that way. For this reason molding machines have finely engineered lubricating systems. However, when they run out of lubricant extreme wear begins immediately. Lubricating systems sometimes have small manifolds that direct the oil to various points and sometime have small fittings that control the rate of lubricant flow. These should be checked periodically. All lubricating systems have a multitude of lube lines that can become plugged. An indication of this is a lack of oil at the distal portion.

Most plants use a tower water system to cool their tools, molding machines, etc. This is an evaporative system. Water from the plant is pumped to the top of the water tower where it is sprayed from nozzles. The water then cascades down baffles into the basin and is pumped back out into the plant. As the water is being sprayed by the nozzles air is blown up thru the spray. This causes a portion of the water to evaporate. As water evaporates it takes with it some heat from the water. Thus the water becomes cooler.

The above drawing represents a typical cooling tower water system. As water evaporates it is replaced with either city water or well water. As water mixes with the air various contaminates also enter. This can include dust, dirt, atmospheric gasses, algae, fungus, microbes, bacteria, viruses, etc. City water and well water contain minerals. When water evaporates the minerals are left behind. They do not evaporate. This means that as time goes on the concentration of dissolved mineral becomes greater and greater. If unchecked, the minerals will begin to deposit on the inside of the system pipes, mold cooling channels, heat exchangers, etc. This severely decreases the water's ability to cool. And worst, channels can become completely blocked and water will not flow at all. The dissolved minerals can also be corrosive and actually eat away at metal. Moreover, this provides a perfect media for micro biological agents to grow. For these reasons a chemical treatment regime should be initiated and adhered to. The purpose of treating the water is to protect your expensive equipment. Additives cause the minerals to stay in suspension or solution and prevent them from settling out or depositing on the metal and hoses they contact. Cooling water also tends to become alkaline (like drain cleaner!) and corrosive. Chemicals are added to counteract this and make it neutral ( determined with a Ph test). Anti-rust agents are also added as well as anti-microbial agents.

Injection molding machines have two pressure settings and set points designed to protect the mold from damage. One is called "low pressure" or "low pressure close" and the other is called "high pressure" or "high pressure clamp". These are two distinct settings and it is imperative that you understand how they function and how they should be set. Most molding machines begin to close under high pressure until they pass the low pressure position set point (this may be a limit switch or a numerical value on the computer screen). When that position is made the mold continues to close at the low pressure value you have set. Finally, just as the mold halves contact, the clamp goes into high pressure and full clamp. The purpose of both low pressure and high pressure position is to protect the mold. When should the mold closing go into low pressure? In an ideal set up this should happen just before the guide pins come into contact with the other half. However, sometimes this will slow down the cycle and affect productivity. In that case, at a minimum the mold should go into low pressure just before the slides come into contact with the other half. This is to protect the slides from possible damage. If there are no slides then it should energize before the thickness of the largest part. The position of low pressure close is only part of a proper set up. The amount of pressure is also adjustable. This is very important. In general, it should be adjusted as low as possible for any given mold. Granted, there is a certain amount of pressure necessary for the mold to close on the guide pins (be sure they are lubricated). If a tool has slides or other mechanical movements then it may take even more pressure (and they should be lubricated too). The point is that you want to use as little pressure as possible to protect the mold. A simple test is to place a runner section on a guide pin and close the mold (be absolutely certain the runner does not touch a cavity). The runner should not be crushed when the mold closes (assuming high pressure is also set properly ).

As explained earlier, high pressure is set by position and it should energize when the two mold halves touch. This is accomplished by placing the control on manual, closing the mold, and while you keep the mold on close (it will be in low pressure), move the high pressure position setting back until it just makes high pressure. Use this method with both hydraulic and especially toggle clamp. If you simply close a mold on a hydraulic clamp machine and then let go of the mold close switch, the mold could drift back a bit. In that situation when you set high pressure it will actually activate a little too soon. If you do not hold the mold on low pressure close with a toggle clamp the linkage will definitely drift back and your high pressure set point will be way off. A simple test is to close the mold on a small piece of cardboard (again, be careful of the cavities). It should not go into high pressure.

I cannot over stress the importance of these two mold safety controls. Who can even guess how many molds have been damaged in the Molding Industry because someone either did not understand or was not diligent. Certainly accidents will happen but when a mold is damaged and the mold safeties were not properly set, it was not an accident. It was carelessness or worse. Note too that if a mold is running and then inexplicably will not close, do not turn up the low pressure or move the high pressure set point back to force it! Something went wrong so find out what instead . For example, a tool may just need lubrication. I do not have pictures of damaged molds to show you but if you fail to heed my words about low and high pressure mold protection, you will see one soon enough! Remember that.

All three interlocks are actuated by the operator gate. When the gate is open, the cam moves off the hydraulic interlock and pilot pressure for mold close is interrupted The mold should be prevented from closing. Likewise, when the gate is open the electrical signal to the mold close solenoid is interrupted, the mold should be prevented from closing. Lastly, when the mold is open the drop bar plate is actuated by a cam and if the mold tries to close the plate should block movement. How often should they be checked? In an ideal situation, once a shift. Be aware too that with the older molding machines when the mold open distance is changed the drop bar must be readjusted. If the mold open distance is reduced the drop bar plate may ride on the bar and then become useless. Many of the newer machines do not need adjustment.

A job is running properly all shift long and suddenly the operator calls you over to the press. "My mold will not open" she tells you. You look and see the screw rotating happily but not going anywhere. You tap your hand on the hopper and hear that familiar sound. You yell out " Material handler get over here, the hopper just ran out!". You had been running, say, nylon or polycarbonate or ABS or any one of the hydroscopic materials that have to be dried. The problem now is that you have no dried material. You send the operator away to sweep floors or whatever and the material handler refills the hopper. Now you have to wait two or more hours for the material to dry. Meanwhile the Shipping Manager comes up to you and asks for his parts. He has to make a JIT shipment in an hour. You tell him you can't make the parts because you have to wait for material to dry. Sound familiar? OK, it wasn't a hydroscopic material you were running. But the job had been running well all day and then ran out of material. So you refill the hopper and restart it again. Everything seems fine until the inspector comes over and tells you the parts are out of print or are running black specks or any one of a number of things. You remind yourself that everything was fine until the job went down and now it has to stabilize all over again. You are wondering how long this will take just as the Shipping Manager comes over and tells you...

Yes, hoppers that run empty are costly. It costs efficiency, machine utilization, production, labor, Quality and even Customer Satisfaction. The solution could be additional "training" for your material handler or perhaps a good "talking to" or maybe you could even replace him! However, the fact is that this is a human error type of problem and it is going to happen again and again. Wouldn't it be great if you could remove the human element and solve the problem forever? You can, simply install Electronic Hopper Alarms on your presses. They cost about $300.00 (IMS) and are an extremely worthwhile investment. When installed they will sound an alarm before the hopper is low. This gives you time to top off the hopper and simply forget about it (again). Think about that the next time a job goes down because of an empty hopper. The cost of just one hopper running empty one time could pay for a hopper alarm! How many hoppers run out on your shift in a month?

So often technicians and supervisors will let their tools run shift after shift with out cleaning them until trouble develops. This is a sure way to damage those tools and run rejects. Consider what happens to a tool in production. For example, often strings form from the sprue and fall across the mold face (turn down your nozzle of apply a little decompression!). Sometimes they even fall across the mold face. As time goes on they build up and eventually pien right into the metal. How many molds do you have right now that have those tell tale indentations? Look closely and you will also see little bits of plastic or flash that got stuck in the mold. Over and over it is clamped up and compressed right into the steel. What about the vents. All materials out gas and this gas builds up in the vents and on the mold face. Do you wait until you get burns or can't fill cavities before you act? Do your tools eventually drip with mold release? Take a look at your guide pins. Are they dry and galled up? What about the slides and other moving parts? At a minimum every mold should be cleaned and lightly lubed at least once a shift! Those molds are extremely expensive and can cost hundreds of thousands of dollars. Even that little mold in your 50 ton press can cost many tens of thousands. Toolmakers spent hundreds or thousands of hours painstakingly building them. Do you let them run filthy and damage them? Set up a schedule where at least once a shift (preferably during the first two hours) every shift cleans and lubes each tool. They will run better and last much longer!