TROUBLESHOOTING

Bernards' Injection Molding Tips, Techniques and Technology for Shop Floor Technicians and Supervisors | home

Basic Troubleshooting of the Injection Molding Process

There are three ways to mold. The first way is to mold by "the book". In other words, follow the material manufacturers' guidelines and do exactly as they specify. Some molders swear by the book and will do it no other way. The second way is to follow your gut instincts and go by experience. Some molders never pick up the book but rely, instead, upon their vast knowledge and experience. The third way and the best way is to read the book, consider your experience and then combine both into a scientific approach to Injection Molding. Material manufacturers provide guidelines based upon laboratory tests, but lab tests are not the same as running production in the plant especially when you have to ship parts that day! However, "the book" gives you a good starting point for job set ups and problem solving. From that point on it is up to you to apply your experience and knowledge to get a job up and running at its' maximum efficiency and capability. Sometimes you may throw your hands up in frustration and throw "the book" away. Just remember that the information in that book is meant to help you and if understood and applied properly, can make your job less difficult.

Someone should provide you with processing documentation from the raw material manufacturer of every material you run. This will be "the book" you refer to when faced with new jobs of faced with new molding problems. Every material manufacturer publishes processing data for every material they manufacture. They will suggest, for example, temperature ranges, velocities, pressures, problem solving, etc. Below I will provide you with a list of typical, generic molding problems and suggested solutions. Many of these solutions will seem obvious but realize I am keeping this basic for the benefit of the novice molder. However, even if you have years of experience it might still help you to read it anyway. How often have you faced a seemingly difficult problem only to realize later that you over looked the obvious? Simply consider these solutions as problem solving reminders.

Flash is an excess of material that may extend out past the parting line, down ejector pins, under slides, etc. Generally it is caused by excess pressures. The following suggestions may help you determine the root cause.

1) Is your mold clean and well lubricated? A dirty mold will tend to hold the mold faces apart and thereby reduce the effective clamp pressure. Also look for debris or even crushed runners, etc. smashed and stuck on the mold. What about the slides? Sometimes material will flash under a slide and although you may not see it there, it can cause flash to appear around other areas of the mold.

2) Is your clamp tight? How do you know? Are you injecting before the clamp pressure has time to reach its' maximum? If you are running a toggle machine, you really need to check your clamp lock up. Are you absolutely certain that the high pressure limit switch or set point is making soon enough? If it is not, it will appear that your clamp is tight when in reality, although tight it is not clamped at full tonnage. If you are running a hydraulic clamp, check your clamp tonnage on a pressure gage. Yes, you can read a numerical value on the newer computerized machines but nothing will ever really replace a good pressure gage. The problem with pressures on a computer screen is that they get their data from a transducer. If the transducer is not calibrated properly or if it has a defective wheatstone bridge leg, the numerical values you see will be erroneous.

3) Look for mold damage in the area of the flash. Did someone fail to set the mold protect properly (let's hope it wasn't you) and damaged the mold when it closed on a runner or part? If the tool is damaged there is little you can do until it is repaired.

4) Is the flash just on one side of the mold? If so, this can indicate uneven tie bar stretch. When stretch is uneven, one side of the platens will receive more clamp tonnage than the other side. Obviously, the clamp pressure should be even across the entire platen. Uneven stretch can even break a tie bar. If you do not know what you are doing and if you do not have the proper gages, do not mess with the stretch. You could cause serious and very expensive damage!

5) Are you shooting the tool with too much injection velocity? As you will recall from reading the Rheology section, excessive injection speed can cause flash even before or when the tool is not filled. The pressure behind the melt flow increases in direct proportion to melt flow resistance.

6) Are you certain that the boost (first stage) pressure is cutting off before the tool is filled? To find out, reduce the hold pressure to zero or 50 pounds and then take a shot. The part should not be fully filled. If it is, then adjust your cut-off.

7) Is there a problem with your stock temperature? When is the last time you took a purge temperature? Although your barrel pyrometer temperatures are fairly accurate, they really do not read material temperature. They read the temperature of the steel barrel.

8) If all else fails and you absolutely have to get parts, try starve feeding the screw. This means giving the screw just enough feed to fill the tool. This is a poor solution and you can expect poor consistency but it might work under poor circumstances. Watch out for sinks because they might appear erratically. Add a little back pressure and this might help.

A short shot is the incomplete filling of the mold cavity (cavities). There are numerous possibilities as to the cause and particularly if it is sporadic.

1) Do you have sufficient feed? Obviously, if the screw is not given enough material shorts will result.

2) Do you have adequate boost velocity to fill the tool? Velocity needs adequate pressure behind it. If your velocity is wide open but you pressure is limited, you could have shorts. Clearly, you will have inconsistency.

3) Is your cut-off set too far back for de-coupled molding? Take a shot with the hold reduced to zero or 50 pounds. The tool should fill between 90% and 95% on first stage alone.

4) Do you have adequate hold pressure speed? The newer computerized machines allow you to adjust the hold pressure on the screen. Start out at 50% and then work your way up. The older presses need to be adjusted at the main hydraulic valves. Your maintenance department can do that for you. Realize that the hold pressure is provided with a separate low volume pump. In some cases the volume may need to be increased up to 100%.

5) Do you have adequate back pressure? Fifty or even one hundred pounds is not a lot regardless of what your material manufacturer (the book) says (unless you are running a heat sensitive material. Even two hundred pounds on a large press is not usually excessive. Back pressure helps assure that the screw will pick up the same amount of material each shot.

6) Check out your stock temperature. Take a purge temperature shot. What temperature does the material manufacturer recommend? Check your nozzle temperature. Is it so low that you are getting cold slugs?

7) Do you have any blocked gates? If it is a hot manifold job, are all the drops open? Might you be getting sporadic freeze offs?

When a mold fills with material, the air in the cavity (cavities) must escape somewhere. Tools have vents to allow the gas pressure to be relieved. Some materials even "out gas" while being injected and these gasses must also escape. When gasses cannot escape, they become compressed. Have you ever pumped a bicycle pump? If you have then you noticed that as you pumped, the pump became hot. The reason is that you compressed the air (gas). As air is compressed, it becomes hotter because more gas molecules are forced into a smaller volume. When gasses are compressed in a mold, they are under tremendous pressure and the temperature can become so high that the gasses will actually ignite. Years ago Robinson Plastics built a mold with a clear quartz window so they could photograph the material as it was being injected. They actually filmed burns as they took place and as they ignited. Those black deposits you see in burns are actually the products of gasses igniting. Burns reach such high temperatures that they can eat away at steel and cause permanent tool damage.

1) Are your vents clean? All tools should be cleaned at least once a shift. Simply wiping down a tool with a rag may not be adequate. Pay particular attention to the vent areas. You may need to use some solvent (mold cleaner) and elbow grease.

2) Can you slow down the injection speed. Sometimes injection speeds can be so great that although the tool does not flash, the gasses do not have adequate time to escape. Consequently, internal gas pressure increases and burns result.

3) Try cutting back on the cut off position. This will allow a little more time for the gasses to escape before the tool is fully filled.

4) Can you reduce the clamp pressure a bit? Less clamp pressure reduces the seal on the parting line and on the vents. This may help in releasing trapped pressures. However, be very cautious and do it in small, incremental steps. Otherwise, you risk flashing the mold.

5) If all else fails and you are under the gun to produce parts, you can create your own temporary vent. Place a small width (for example, 1/4 to 1/2 inch) of masking tape near the gas trap (burn) area and near the cavity edge and run it out all the way to the edge of the mold. Be absolutely certain that the tape is not placed on the cavity edge. If you do place it on the cavity you could cause parting line damage. This tape will act as a temporary vent and allow gas to escape. You may need more than one thickness. Also, as the tool runs, the tape will deteriorate and will need to be cleaned off and replaced. Do not keep placing new tape over the old. This is only a temporary measure until vents can be ground into the steel.

EJECTOR PIN MARKS

Ejector pin marks are blemishes located directly behind ejector pin locations on the mold. Unfortunately, to save costs, molds are often built with an inadequate number of ejector pins. There is a number of things you can do in an attempt to work around this problem.

1) The most obvious thing to do is slow down the ejector speed. Unfortunately, this can affect your cycle time particularly if you need to use dead slow speeds. Does your press have programmable ejection speeds? If it does, then set it so the initial ejection begins slowly but then increases just as the part breaks away from the cavity. Also, you may be able to start ejection while the mold is still "on the fly" and is still opening. However, be certain that all slides, cams, etc. are cleared first.

2) You may be using excessive pack pressure. Try backing off. Of course, you will need to watch for sinks to appear. As an alternate, you can try decreasing the hold time. This will allow the final cavity pressure to be decreased and may eliminate your pin marks. Again, watch for those sinks!

3) Is your ejector plate cocking? This would cause an uneven pressure on the ejector pins. The most common cause of a cocked ejector is the use of ejector rods that are of unequal lengths. Watch the plate as it ejects. Does it move straight and parallel to the tool? Cocked ejectors can damage a tool so correct the problem immediately.

4) The first thought that usually comes to mind is to increase the cycle time. That will allow the part to set up better and become stiffer. Right? Yes, but it will also do something else. The part will shrink more around the steel! Indeed, this might make it more difficult to eject. With that understanding in mind, it is worthwhile to try to decrease the cycle (mold close) time. Be aware, though, that this could also affect your part dimensions.

5) Is the back half of the tool too cold? This could cause excessive shrinkage on the core. On the other hand, it could also be too hot thus preventing the material from setting up. There is no set rule for tool temperatures so you may need to experiment. Remember too that there is a big difference between tool temperature and the temperature of your chiller, tower of mold heater. You need to take actual steel temperatures on the core and cavity. When you do this, do it immediately when the mold opens. That temperature is in flux and will rapidly change as the tool sets open.

6) Is this a mold that must be sprayed with mold release? If it is, does your operator know the proper technique? Does she hold the spray can back and simply mist it on or does she make the mold wet with release? Does she spray consistently, every X number of shots or just when she feels like it or when she sees pin marks? Is the mold dripping with release or does the operator keep it wiped down?

7) Again, and as with most molding problems, you should check your stock temperature. At the very least, it should be within the range as recommended by the material manufacturer (the book).

Splay is characterized by light colored streaks in the part that usually follow the direction of the melt flow. There are two kinds of splay: moisture splay and heat splay. Some materials are hydroscopic. This means they absorb moisture right into the pellets. Non-hydroscopic materials cannot absorb moisture but they can get moisture on the pellet surface. When material contains moisture and then is melted in the barrel, that moisture turns to a vapor commonly known as "steam". This vapor then causes streaks in the parts. Heat splay has a similar appearance but an entirely different cause. When polymers are over heated they begin to degrade. Many of the products of degradation are gasses. These gasses displace the material in the melt flow and tend to migrate to the mold surface. Thus, they take on the appearance of moisture splay.

1) If this is hydroscopic material, is it being dried at the proper temperature and for the proper amount of time? All materials typically call for a minimum of two hours residence time in the hopper at the required temperature. Inlet temperature alone is not a proper indication of proper drying. Is the dryer properly sized for the hopper? Does the dryer blower have enough pressure to assure a good flow thru rate? Is the hopper large enough for the required residence time in relationship to the molding machine thru put rate in pounds per hour? You really should also have a thermometer probe mounted on the hopper to read interior hopper temperature.

2) Check the dryer intake filters. If they are plugged with fines, airflow circulation will be severely reduced and the material may never properly dry. Be aware to that even with plugged filters, the temperature indicator on the dryer will probably still read normal.

3) Is the spreader located all the way down to the bottom of the hopper. Is it plugged with material? Are there kinks or leaks in the dryer hoses or are clamps missing? Are there two hoses attached: one for delivery and one for return?

4) Do you know the dryer dew point. It really should be checked with a portable, calibrated dew point meter. Do not necessarily trust the meter built into the dryer. What is the condition of the desiccant? Do you know when it was last changed? If the dryer is operated with internal pneumatic valves, is the compressed air connected? Does it have a regulator and is the pressure set above the minimum?

5) Has the hopper been running low? If so, then you probably have wet material! It is absolutely imperative that drying hoppers always remain topped off. If they run low, the material may then not have adequate residence time to dry.

6) Contaminated material can often take on the appearance of splay. Although it is very time consuming, you can rule out contamination by molding a known sample of non-contaminated material.

7) The products of material decomposition cause heat splay. Excessive barrel temperatures can certainly cause this. Another likely source is hot manifolds. Excessive barrel residence time can also cause decomposition. Is the splay evenly distribution thru the part or is it seen in just certain areas. Even distribution indicates the degradation is taking place in the barrel. If you are running a relatively small part in a relatively large barrel, the material has a long residence time. This means that by the time the material makes if from the throat to the mold it has been in the barrel so long that it begins to degrade. There are a few things you can do to slow the degradation if you cannot pull the tool and place it in a smaller press. Obviously, you will want to lower the barrel heats. However, take caution that you do not lower them so much that you seize the screw or worse yet, snap the screw tip. You must keep the front zone at a temperature where the material is at a molten state. This means you go by "the book" and set your front zone as recommended by the material manufacture. However, you will set your rear and center zones much lower. For example, if the normal temperatures are in the range of, say, 550 Degrees, you might set your front at 550, your center at 500 and your rear at 450. Note that your front zone is still at the normal operating temperature but you profiled the barrel lower towards the rear. The material still has the same residence time but it resided at a lower average temperature.

8) If you notice that the splay effect is not evenly distributed through the part, this tells you that the cause may be upstream from the barrel. For example, you may have metal in your nozzle tip. This can cause tremendous shear effects on the material as it passes by and can generate so much heat that it actually begins to degrade the material. Take an air shot and observe if the stream comes out evenly. If it does not, this is a clear indication of an obstruction. However, if the stream is steady and even, this does not necessarily rule out an obstruction. The only certain way to know is to pull the nozzle.

9) Leaking or cracked check rings can also cause splay effects. As with an obstructed nozzle tip, a leaking check ring can cause shear heat. If excessive, the material will degrade and out gas and cause splay like defects. Does the screw turn backwards while injecting? Does the screw drift forward through the cushion. These are two clear indications of check ring leakage.

10 Splay effects can also originate from a hot manifold. You cannot always trust the indicated temperature on the controller. This is because sometimes the thermocouples are poorly located. Also, if a heater burns out the other heaters may cycle on more often and cause isolated hot spots. Do you notice some zones constantly overriding? Do you see some zones that are always on? If so, this is a clear indication that there are problems in the manifold. Manifold drops (tips) can also cause splay. Excessive drop temperatures or metal in the tip can easily cause this effect. If you notice that the splay originates directly at one or more tips, this clearly indicates the cause of the problem is located there.

11) Excessive screw RPM can also cause splay. When a screw rotates there is a thin material boundary layer between the flights and the barrel wall. Typically, this layer is only a few thousandths of an inch thick. As the screw rotates the material in the layer is subjected to severe friction. High screw RPM exasperates this condition and can generate so much frictional heat that the material begins to degrade. The solution, of course, is to slow the screw. In fact, in most cases, it is a good practice to slow the screw to the point that it finally reaches its feed set point just a few moments before the mold close timer times out.

12) Excessive back pressure can also cause splay. Do you understand what back pressure actually is? As you know, the screw assembly is physically connected to the injection cylinder. When hydraulic pressure is applied, the screw moves forward. When the screw rotates back, it actually screws itself back out of the material towards the end of the barrel. As it is doing this the hydraulic oil in the rear of the injection cylinder flows through a valve and through oil lines back into the oil tank of the press. Now imagine that we completely closed the valve so the oil could not flow back to tank and then we rotated the screw. What would happen? Obviously, although the screw is rotating and trying to go back, it cannot move at all. If we placed a pressure gage on the front of the injection cylinder, we could measure how hard the screw is pushing when trying to go back. This is exactly what a back pressure gage measures. We would also notice that as we increased the screw RPM, up to a point the back pressure got even higher. Thus, this experiment demonstrates the meaning of back pressure: it is the force exerted as the screw moves or tries to move back. All your presses have back pressure controls. These controls meter the volume of oil flowing from the back of the injection cylinder to the tank. If the valve is wide open, the oil flows freely and the back pressure gage will read zero. If the valve is completely closed, the pressure will rise to the maximum value the material viscosity and screw RPM will allow. It is this oil flow that you control when you adjust back pressure.

Back pressure does several things. It allows the material to mix well, particularly when you are molding regrind mix or color concentrate. It also packs the material in front of the screw and gives a more consistent shot load. Lastly, it generates heat. Briskly rub your hands together and you feel warmth. Rub two sticks together fast enough and they can catch on fire. Add back pressure to your screw and a great deal of heat will be generated into the screw, barrel and material. In each case, the heat is generated by friction. In the case of the screw and barrel, you can add so much heat from back pressure that the material begins to degrade and causes splay! I once worked on a press that had a high compression screw and even with only slight back pressure so much heat was generated that the barrel heaters never kicked on. The frictional heat generated by the screw rotating through the material was enough to keep the barrel up to heat. In fact, so much heat was generated that we had to blow air with a fan across the barrel just to keep the temperature down. It is very unlikely that you are using high compression screws but the principle remains the same: back pressure generates heat!

13) Screw pull back (decompression) can sometimes cause splay. This is because when the screw and check ring shift back, shear effects can result. If you need to use pull back, use the smallest amount possible.

Sinks are depressions on the surface of a part caused by the shrinkage of material below the surface. They are often found beneath ribs, bosses, walls, etc. Quite often, the part designer failed to consider this type of shrinkage and you are forced into molding around a tooling problem rather than a processing problem. I can make a few suggestions that may help.

1) Are you certain you have enough hold time? Your hold time should be set long enough that the gates can freeze off before the timer times out. If hold time is inadequate, material will flow backward out of the gate and internal cavity pressure will drop. This can certainly lead to sinks.

2) Obviously, you will want to see if you can increase your hold pressure. Additional hold pressure will help compensate for the material shrinkage: you are actually increasing the amount of material in the mold (and that is why your parts will become heavier).

3) Do you have adequate hold speed? If you are using de-coupled molding techniques and your hold velocity is set too low, the cavity pressure build up will be slow and will exhibit excessive pressure drops.

4) Excessive stock temperatures increase the amount of shrink. In other words, the hotter it is, the more it will shrink. Consult "the book", see what temperature the manufacturer recommends, and then try setting them at the low side.

5) Increasing fill time can help reduce sinks. This means decreasing the injection speed. Also, a cooler mold will sometimes improve sinks.

Weld (knit) Lines

Weld or knit lines are areas where two or more melt fronts meet and join. Any time there are two or more gates there will be weld lines. Sometimes the lines are simply cosmetic. However, weld lines can also cause a reduction in part strength. Weld lines cannot be eliminated but their severity and appearance can be improved.

1) Generally, the slower a mold is filled, weld lines become more obvious and weaker. This is because as the material flows into the mold, it immediately begins to cool. Cooler material wave fronts do not weld together as well as hotter wave fronts. Consequently, you should try to increase the injection speed.

2) A hotter mold should improve weld lines. This is because the material wave front will be hotter too. However, when ever you change tool (or stock) temperatures, you also affect part size. If everything else stays the same and you increase the tool temperature, your parts will exhibit more post mold shrinkage and will be smaller. However, this can be partially compensated by increasing cavity pressure or adding additional cooling time (You should never slow down a press unless it is absolutely necessary. Slower presses decrease efficiency and profitability).

3) Increased stock temperatures will also improve weld lines for the same reason that a hotter mold does. However, the same caution as in # 3 above applies.

4) Sometimes it is necessary to block off a gate (or move or add gates) to move weld lines to a different location. Sub gates can be blocked off with a little lead and a drop of super glue.

STRINGS AND NOZZLE DROOL

I would never be able to guess how many times I have walked out on to the shop floor and saw jobs running with nozzle strings all over the tool face. Likewise, I would never be able to guess how many times I saw molds closing on nozzle drool shot after shot. However, I can tell you how I felt each time: I was upset! Strings and drool damage tools. Surprisingly, these problems are normally very easy to solve. Yet, I have seen supervisors and technicians who were oblivious to the problem and oblivious to the damage happening to their tools. Strings cause parting line damage. True, one or two strings will hurt nothing. They are quite thin and simply are crushed by the tool. However, strings build up shot after shot. Eventually they build up so much that they leave their impression in the steel! This is particularly bad when they fall across the parting line or slides. Drool caused damage as well except it does it faster. Tools should never be allowed to close on strings or drool.

1) If you are getting strings are you using the proper style nozzle tip and correct size orifice? For example, when molding nylon you should really use a reversed tapered nylon tip. This will help minimize strings and nozzle tip cold slugs. The tip orifice should be just slightly smaller than the sprue bushing orifice. If it is larger, strings will tend to develop. It is true that a smaller tip orifice will help eliminate strings but there is a serious down side to this. A smaller orifice will cause a larger pressure drop at the tip and will increase shear effects. This will have negative effects on your process.

2) Sometimes the only thing that is necessary to eliminate strings and drool is to decrease the nozzle temperature. Also, check to see that the nozzle heater band is as far forward towards the tip as possible. This will place the heat where it is needed most.

3) Screw pull back (decompression) can also help eliminate strings and drool. This is because pull back relieves the pressure on the shot load in front of the screw tip. Use the minimum amount necessary.

4) There may be circumstances where you find that the nozzle either keeps freezing off (or forms cold slugs) or constantly forms strings or drool. This is a sign that the mold is drawing too much heat from the nozzle tip. Generally speaking, increasing the tool temperature will not help. There is simply too great of a difference in temperature between the nozzle tip temperature and the tool steel temperature. However, there is a trick of the trade you can try out. Take a small, thick piece of cardboard from a gaylord and insert it between the nozzle tip and the sprue bushing. This will act as a heat insulator and will be able to drop the nozzle temperature. The cardboard will have to be replaced from time to time as it wears out. It will last longer if you first wrap it in a piece of aluminum foil.

The melt flow thru and at the gate normally exhibits a fountain type flow if the injection speed is not excessive. The material near the gate and at the part wall will orient and freeze and will not slide down the wall. However, if the melt is injected too quickly, the plastic will blast through and the formation of a fountain flow will be retarded. This can be demonstrated by placing a small piece of tissue paper in the cavity right in front of the gate. If fountain flow is established immediately, the tissue paper will not move downstream. However, if fountain flow is retarded, the tissue will be pushed forward until fountain flow is established. Profiling injection speeds is ideal in this situation. The speed of the melt flow right at the gate can be slowed momentarily until the material has broken thru the gate(s) and then increased to take advantage of fast fill rates.

THE ABOVE ILLUSTRATES EXCESSIVE INITIAL INJECTION SPEED

THE ABOVE ILLUSTRATES PROPER INITIAL INJECTION SPEED

MOLD TEMPERATURE

The ability to control mold temperature is often highly dependent on tool design. If possible, all surfaces of the tool should be maintained at a consistent temperature. This will ensure consistent properties throughout the part. Below are additional suggestions for setting mold temperature:

1) When possible, use a pyrometer to verify that the mold temperature is at the desired temperature and consistent throughout both halves of the tool.

2) Avoid a temperature differential between the halves of the tool greater than 100 Degrees F. This will reduce the risk of tool steel interference or binding from temperature-induced expansion in the mold.

3) A temperature differential between the mold halves may induce the part to warp toward the hotter side of the tool.

GENERAL HOLD PRESSURE GUIDELINES

Potential to flash parting line

Reduces gloss on grained parts

Reduces part sticking

Reduces sprue sticking

Induces surface gloss

Possibly reduces part strength

Back pressure on a screw results when its backward movement (screw recovery) is restricted. Back pressure is always recommended to ensure a consistent shot size and homogeneous melt. Higher pressures may be required for more intensive mixing but may induce higher melt temperature, glass breakage (reinforced materials), and increased cycle time due to hotter melt temperatures and slower screw recovery. Increasing back pressure as a substitute for a proper heat profile or an inadequate screw design is not recommended.

Generally speaking, materials should be injected with high velocity rates and particularly when molding crystalline materials. Fast injection provides for longer flow, improved pack condition, and better surface aesthetics. Filling the cavity at a faster rate will allow the material to crystallize or set up at a uniform rate in the tool which tends to result in lower molded-in stress. Slower fill speeds may be required when filling through gate designs where jetting or gate blush is occurring. Slow fill is also used in tools with poor venting to eliminate part burning and in parts with thick cross sections to reduce sink marks and voids. Programmed or profiled injection has been proven successful when molding parts with non-uniform wall stock to reduce voids and gas entrapment burning. It has also proved to be an advantage when molding through subgates and pinpoint gates. Slow injection speeds at the start of injection can be used to eliminate/reduce gate blush, jetting and burning of the material.

The use of a cushion of material at the end of the screw stroke is absolutely necessary for consistent molding. A cushion of .100"-.500" is recommended depending on the press size. The inability to hold a consistent cushion is usually indicative of a worn non-return valve. Maintaining a cushion will assist in the processing of the material in the following ways:

1) Helps to maintain consistent physical properties throughout the molded part.

2) Aids in ensuring dimensional reproducibility, weld line integrity and control of sink marks.

3) Assists maintaining a consistent surface quality.

SCREW PULL BACK (DECOMPRESSION)

Decompression or "suck back" is the intentional pulling back of the screw and polymer from the nozzle area to prevent drool. It is usually accomplished by time or screw position settings. However this may introduce air to the molten plastic which cools and may oxidize the plastic. Therefore, it is recommended to minimize the use of decompression.

The barrel should always be purged if the process will be shut down or idled for any length of time. For short interruptions, one need only purge several shots, but longer shut downs require a complete purging and emptying of the barrel. It is always a good idea to purge the first several shots at start up to reduce the chance of contamination from previous processing.

1) Mix regrind back into the virgin material type from which it was originally molded.

2) It is important that the material to be reground be free from oil, grease or dirt, and show no signs of degradation.

3) Regrind that contains excessive quantities of fines or dust-like particles may result in molding problems such as burning or splay. Try to select grinder screens that will minimize fines.

4) It is advantageous to try to make the particle size of the regrind as close to the initial pellet size as possible. This will allow for ease of blending and for consistent drying of the material.

5) Prior to processing the regrind, ensure that the moisture level is similar to that of the virgin material. This will help in processing.

6) If the regrind material does become "wet," it is usually better to dry the material off line in a separate dryer.

7) Some color shift may occur when using regrind. It is important not to over dry regrind in order to minimize color shifts.

8) To maintain a consistent regrind blend, it is recommended to use your regrind back into the process as it is generated.

9) If regrind is to be stored for future use, store it in a container with a moisture barrier.

A Defective Check Valve can result in the following:

Lack of ability to maintain a cushion, resulting in forward screw slippage.

Inconsistent shot size.

Dimensional inconsistency in parts.

Sink marks due to lack of pack pressure.

Surface imperfections from splay, whitening, or mineral bloom.

Potential to degrade material.

Screw rotation upon injection.

Possible override of barrel temperature settings.

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