Sink Mark

Sink Mark

A Sink Mark can be defined as a depression, resembling a dimple or groove, caused by excessive localized shrinking of the material after the part has cooled.

MACHINE
BARREL TEMPERATURE TOO HIGH
Explanation: If the barrel temperature is too high, the resin absorbs an excessive amount of heat and this increases the size of the voided area between the plastic molecules. Upon cooling, the skin of the material solidifies first and the remaining resin closes up the excessively large molecules and voids as it cools, pulling the solidified skin with it. The larger the molecules and voids, the greater the amount of shrinkage. This is displayed as sink marks.Solution: Decreasing the barrel temperature allows the molecules and voids to expand and contract normally and provide consistent shrinkage values. Consistent, uniform shrinkage minimizes the condition that causes sink marks.

INSUFFICIENT INJECTION PRESSURE OR TIME
Explanation: Injection pressure must be high enough to push molten material into the mold, through the runners and gates, and into the cavity image area. It should be used to force material into every part of the mold until it is packed solidly. The proper amount of pressure held for the proper amount of time ensures that all the resin molecules are the same size and are held closely together while they cool and solidify. This creates uniform shrinkage after the part is removed from the mold. But, if inadequate pressure is used or if it is applied for too short a period of time, the molecules will not be constrained during the solidification phase and the entire part will not shrink uniformly, resulting in sink marks.Solution: Increase the amount of pressure or the time applied. Upon initial startup the mold should be filled incrementally starting with intentional short shots (if the mold design allows) and progressively increasing pressure shot-by-shot until the mold is filled and packed properly. If injection pressure and time are adequate the shrinkage should be uniform and consistent resulting in parts without sink marks.

INADEQUATE COOLING TIME
Explanation: The cooling phase of the total molding cycle determines how long the molten material is held in a constrained condition until a strong skin is formed. After that, the part can be ejected from the mold and the next cycle can begin. If that cooling time is too short, the skin will not be of sufficient thickness and strength to hold the part together after ejection from the mold and continued cooling may cause sink marks due to inconsistent and non-uniform shrinkage.Solution: Increase the cooling time portion of the cycle. It is true that longer cycles make the part more expensive but there is a minimum amount of time required for the resin to form a proper skin. It depends on what material is being molded. A general rule-of-thumb for a part with a wall thickness of 0.100'', the cooling time should be 20 seconds. The overall cycle would then be 25-30 seconds.

INSUFFICIENT CUSHION AND/OR HOLD TIME
Explanation: A cushion (pad) is required at the end of the injection stroke to maintain steady pressure against the molten material after it has filled the cavity image. This pressure (and the time it is applied) keep the molecules packed together and constrained while the product skin forms and solidifies enough for the part to be ejected. If the cushion is too small or the time is too short, the plastic molecules are not constrained and some will actually pull back out of the cavity. This allows the remaining molecules to move when the part is ejected and they will cause excessive shrinkage and sink marks as they contract beyond their normal values.Solution: Maintain a cushion that is somewhere between 1/8'' and 1/4'' at the end of the injection stroke. This provides something against which hold pressure can be applied. The amount of time for holding pressure should be long enough for the gate to freeze, normally 3-4 seconds.

EXCESSIVE NON-RETURN VALVE CLEARANCE
Explanation: The non-return valve, found in the front section of the screw and barrel assembly, keeps molten plastic from slipping back over the injection screw when the screw is pushed forward during the injection phase of the process. The valve lies between the outside diameter of the screw and the inside diameter of the barrel and creates a seal between the two. If there is too much clearance (due to wear) the sealing effect is lost and slippage occurs. This results in a reduction in volume of plastic and a bottoming out of the injection stroke, eliminating the required cushion.Solution: Inspect the non-return valve mechanism and replace worn or damaged components. This wear is normal but is accelerated by molding materials that have reinforcements (such as glass) in them. The valve should be inspected at least every three months.

MOLD
MOLD TEMPERATURE TOO HIGH OPPOSITE RIBS
Explanation: Generally, a hot mold will allow a material to stay molten longer than a cold mold and cause the molecules to stay fluid longer before they cool and solidify. Upon ejection from the mold the material will be allowed to contract more than normal and excessive shrinkage will occur. This condition often occurs in the area of ribs because of the extra plastic in those areas, which requires more extensive cooling to maintain consistent shrinkage. Inconsistent shrinkage will result in sink marks.Solution: Decrease the mold temperature to the point at which the material has the proper flow and packs out the mold without shorting. Start with the material suppliers recommendations and adjust accordingly. Allow 10 cycles for every 10-degree change for the process to re-stabilize.

SMALL GATES AND/OR RUNNERS
Explanation: Gates and/or runners that are too small will cause excessive restriction to the flow of the molten plastic. Many plastics will then begin to solidify before they fill the cavity. The result is a material that is not fully contained within the metal mold surfaces and is allowed to shrink beyond normal expectations. The extended shrinkage causes sink marks.Solution: Examine the gates and runners to optimize their size and shape. Do not overlook the sprue bushing as a long sprue may solidify too soon. Use a heated bushing or extended nozzle to minimize sprue length. Ask the material supplier for data concerning gate and runner dimensioning for a specific material and flow rate.

IMPROPER GATE LOCATION
Explanation: If certain materials are injected directly across a flat cavity surface they tend to slow down quickly as a result of frictional drag and cool off before the cavity is properly filled. The material is not held under proper pressure while solidifying and excessive shrinkage will cause sink marks as the part cools after ejection from the mold.Solution: Relocate or redesign the gate so that the molten plastic is directed against an obstruction such as a core pin. This will cause the material to disperse and continue to flow instead of slowing down.

EXCESSIVE THICKNESS AT MATING WALLS
Explanation: When a wall meets another wall, or when a boss is located on a wall, the area where they form a junction becomes a larger mass of plastic than the area surrounding it. The surrounding area cools and is already solidified while the larger mass continues to cool and shrink. Because the surrounding area is solid, non-uniform shrinkage occurs as the large mass area shrinks in on itself, causing sink marks to appear.Solution: Although it is good design practice to maintain all walls at a uniform thickness, in areas where a junction is formed, one of the walls should be between 60% and 70% of the mating wall thickness. This will minimize the mass at the junction until the shrinkage is equal in all areas and sink marks will not develop.

MATERIAL
IMPROPER FLOW RATE
Explanation: Resin manufacturers supply specific formulations in a range of standard flow rates. Thin-walled products may require an easy flow material while thick-walled products can use a material that is stiffer. It is better to use as stiff a flow as possible because that improves physical properties of the molded part. But the stiff material will be more difficult to push and this may result in a less dense material filling the cavity image. The lower this density, the higher the amount of shrinkage that will occur after ejection, and sink marks may occur due to an imbalance of shrinkage factors.Solution: Utilize a material that has the stiffest flow possible without causing sink marks. Contact the material supplier for help in deciding which flow rate should be used for a specific application.

OPERATOR
INCONSISTENT PROCESS CYCLE
Explanation: The machine operator may be opening the gate too soon, thereby effectively shortening the overall cycle time. This would cause the part to be ejected before the skin has formed properly and the resultant excessive shrinkage may cause sink marks to form.Solution: If possible, run the machine on the automatic cycle, using the operator only to interrupt the cycle if an emergency occurs. Use a robot if an ``operator'' is necessary. In addition, instruct all employees on the importance of maintaining consistent cycles.

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Cracking and/or Crazing

Cracking and/or Crazing

Cracking and/or Crazing can be defined as a fracture or surface breakage in the material of a molded part, usually found in weld line areas, but also on the surface in general.

MACHINE
MOLDED-IN STRESSES
Explanation: Although stress cannot be eliminated, excessive stress can be molded into the parts by using too high an injection pressure, too high a holding pressure, or too fast a filling rate. The molten material is forced into the mold and held there under great pressure until it partially solidifies. When the mold opens and the part is ejected, it is still cooling but is no longer constrained by the mold. Some of the molded-in stresses are allowed to release and a ``splitting'' of the plastic occurs, usually in the weakest area.Solution: Reduce packing and fill rates by adjusting until the part is properly filled with minimum stresses. Maintain a holding pressure that is no more than 1/2 the primary injection pressure.

COOLING TIME TOO SHORT
Explanation: If the cooling time is too short, the part is ejected before the material has formed a skin solid enough to constrain movement of the remaining plastic material while it cools. The surface will split open and form crazing, or cracks will form throughout the part.Solution: Increase the cooling time portion of the cycle. This holds the mold closed longer and allows a thicker skin to form on the molded part. The skin will be strong enough to keep crazing or cracks from forming.

MOLD
UNDERCUTS OR PARTING LINE BURRS
Explanation: For the molded part to eject there must be no restrictions to a straight push out of the cavity. An undercut, reverse draft, or burr will cause such a restriction. This will try to keep the part in the mold while the ejection system tries to push it out of the mold. The conflict that arises will cause the part to fracture or crack.Solution: Inspect the sidewalls and edges of the cavity. Make sure there is adequate draft (see the next paragraph) and that there are no burrs or other undercut conditions. If there are they must be removed by stoning or machining.

INSUFFICIENT DRAFT ALLOWANCE
Explanation: A draft angle is simply a tapered side wall that is used to allow easy removal of the molded part from the cavity of the mold that forms it. Without this taper the vacuum that is created (when plastic displaces air) in the cavity cannot be overcome and the part will simply crack as the ejection system tries to push it out of the mold.Solution: As a general rule-of-thumb, a draft angle should be at least 1 degree per side to facilitate easy ejection. This does result in a dimensional change in the part and must be considered in the mold design phase. To minimize future problems, the product designer should be made aware of this requirement.

USE OF MOLD RELEASE
Explanation: Mold release will interfere with the molecular bonding of the plastic. Material enters a cavity in layers and these layers must be allowed to bond together. Mold release interferes with that bonding and will cause crazing to occur on the surface of the part.Solution: The remedy is to keep the mold as clean as possible and make every effort to eliminate the use of external mold releases.

IMPROPER EJECTOR DESIGN
Explanation: If ejector pins are too small, or located on thin flat sections of the part, the plastic will be distorted during ejection and cracks will form due to the amount of stress being imparted. Also, if the ejection speed is too great, the plastic will not have time to conform to the normal stress being applied and cracks will form from distortion.Solution: Any evidence of cracking in ejector pin areas indicate the pins are too small, or the ejection speed was too great, or there was too much injection pressure used to fill the mold. Injection pressure was covered earlier. Ejector pins should be resized, or relocated. They need to be located such that they are close to side walls or under bosses or other strong areas of the part that can absorb the ejection stresses being produced. They need to be as large as possible in diameter to distribute the ejector forces over a large area.

MATERIAL
DEGRADED MATERIAL
Explanation: One common cause of cracking is the use of material that has become degraded. This can be the result of overheating in the barrel, but a more common cause is the use of bad regrind. Regrind that has been used over and over can easily become degraded to the continued exposures to elevated temperature. It melts at lower temperatures than virgin so the regrind can degrade in the barrel, which must be heated high enough to melt the virgin thereby degrading the regrind. Degraded material is weak and does not have a good molecular bonding of molecules. This results in cracking when the part is exposed to any stress, such as that of the ejection system.Solution: Use only high grade regrind and use it only once. Mix regrind with virgin at a level of approximately 15% regrind by weight to minimize the tendency to degrade. If this is still a problem, eliminate the use of regrind altogether.

EXCESSIVE MOISTURE
Explanation: Excessive moisture causes cracking or crazing because the water droplets actually turn to steam when heated in the injection unit, and these steam pockets erupt causing voided areas between molecules. This causes those areas to be extremely weak and brittle. The voided areas easily break apart once the mold opens and relieves constraint conditions.Solution: Although it is commonly understood that non-hygroscopic material do not require drying, do not take chances. Dry all materials. It may be that fillers used in the material are hygroscopic and they will absorb moisture. Every plastic material requires specific drying conditions. And each material should be dried according to the material suppliers recommendations. The desired moisture content is between 1/10th of 1 percent and 1/20th of 1 percent by weight. This means the dry air being used to take moisture from the material should have a dew point of -20 to -40 degrees F.

OPERATOR
POOR HOUSEKEEPING
Explanation: Machine operators who have been told to use mold release sprays sparingly, will eventually overuse the spray. The thought seems to be that if a little bit works, a lot will work better. Excessive mold release will interfere with molecular bonding of the plastic and cause weak areas that break apart easily.Solution: If possible, run the machine on automatic cycle, using the operator only to interrupt the cycle if an emergency occurs. Use a robot if an ``operator'' is really necessary. And, instruct all employees on the importance of maintaining consistent cycles.

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Warpage

Warpage

Warpage can be defined as a dimensional distortion in a molded product after it is ejected from the mold at the end of the injection molding process. Warpage is sometimes called ``potato-chipping'' because the part tends to appear wavy.

MACHINE
INADEQUATE INJECTION PRESSURE OR TIME
Explanation: If too little injection pressure is used the plastic material will tend to cool and solidify before the mold is packed out. If no packing is achieved the individual molecules are not held tightly together and have space to move while the part is cooling. Also, if the injection hold time is not long enough, the packing process is minimized and the molecules can relax before full solidification occurs. In either case, as the part cools it is uncontrolled and the plastic is allowed to move because it is not being constrained. Each area of the part cools at a different rate and warpage will occur due to the differences.Solution: Increase the injection pressure or time applied. This will ensure the total part is cooling while constrained and the tendency for warpage will be minimized.

INADEQUATE RESIDENCE TIME
Explanation: Residence time is the amount of time a material must spend being exposed to heat in the barrel. The time is determined by the ability of the specific resin to absorb heat enough to be properly processed. Inadequate residence time will result in under-heated material, which causes the material to be stiff. It will cool off before the mold is packed and individual molecules will be unconstrained while they solidify. Molecules that are not constrained during cooling will shrink at differing rates throughout the part and warpage will occur.Solution: Increase the residence time by adding time to the cooling portion of the cycle. While increased cycle time may add cost to the final product, each material requires a specific minimum amount of time to absorb heat in the barrel, and if the time is not long enough warped parts will occur.

BARREL TEMPERATURE TOO LOW
Explanation: When barrel temperatures are too low, the material will not have a chance to heat to the proper flow temperature. The cold material gets pushed into the mold but solidifies before the molecules are packed and constrained. This results in warpage as the molecules shrink at varying rates.Solution: Increase the barrel temperature. This will allow the material to come to proper heat and it will fill the mold before solidification takes place. The molecules will be packed and constrained as they cool, thus shrinking at uniform rates, minimizing the chance of warpage.

NOZZLE TEMPERATURE TOO LOW
Explanation: The nozzle, being the final transfer point between the heated barrel and the mold, is a critical area and must be scrutinized whenever splay patterns occur. If the nozzle is too cold, the plastic material may slow down as it travels through the area and the molecules will not get packed under constraint. They will shrink at varying rates and cause warpage.Solution: Increase the nozzle temperature 10 degrees F at a time until the warpage disappears. If splay still appears, reduce the temperature of the nozzle, and make sure the nozzle being used is of the proper design for the material being molded. There are many different nozzle designs and some may interfere with proper flow if they are not designed for the material in use.

EXCESSIVE STRESS BUILDUP
Explanation: The injection molding process tends to build up physical stress in a molded part due to the stretching and squeezing action that takes place on the individual plastic molecules as they are heated, expanded, cooled and contracted. They must be allowed to relax and recover in a constrained position before they solidify or the stress will be locked in. It will then be released as the part cools after being ejected from the mold and warpage will occur.Solution: Increase the barrel temperature and decrease injection pressure until the stress is minimized. It can never be eliminated but lower pressure will result in lower stress. And, higher barrel temperatures allow the use of lower injection pressures.

MOLD
SMALL GATES
Explanation: Gates that are too small will cause excessive restriction to the flow of the molten plastic as it passes through. This restriction may cause additional physical stress to the plastic molecules as they are stretched and squeezed again going through the gate area. The stress gets released after ejection and the parts will warp.Solution: Optimize gate size and shape. The material supplier can provide data on proper sizing and shape, or use a computer finite element analysis program to help make the determination.

MOLD TEMPERATURE TOO LOW
Explanation: Generally, a hot mold will allow a material to stay molten longer than a cold mold and cause the molecules to stay fluid longer before they cool and solidify. If the mold is too cold the molecules will solidify before they are packed and will shrink at differing, uncontrolled rates. This is a prime cause of warpage.Solution: Increase the mold temperature to the point at which the material has the proper flow and packs out the mold with maximum fill. Start with the material suppliers recommendations and adjust accordingly. Allow 10 cycles for every 10-degree change for the process to re-stabilize.

UNEVEN MOLD TEMPERATURES
Explanation: The plastic molecules must cool and shrink evenly to resist warpage. If the mold is not cooling the plastic in a uniform manner the molecules will have varying cooling and shrinking characteristics and this will cause warpage.Solution: Check the surfaces of the mold that are in contact with the molten plastic. Use a fast-acting pyrometer to determine if there is more than a 10 degree F difference between any two points, even between the two mold halves. A difference greater than 10 degrees F will cause too great a difference in shrink rates and warpage will occur.

NON-UNIFORM EJECTION
Explanation: It is possible that either the ejection system of the mold or the press will not be operating properly. If the part is warm enough and the ejection force is not even and exactly perpendicular to the part, stresses will be set up in the part as it tries to resist the ejection activity. These stresses will cause warpage of the part as it cools after being ejected.Solution: Inspect and adjust the ejection system(s) as required. Make sure all adjusting devices are locked down to eliminate slipping, and that all components are properly lubricated. It may be necessary to use a guided ejection system that utilizes leader pins and bushings to keep the system in line and even.

MATERIAL
IMPROPER FLOW RATE
Explanation: Resin manufacturers supply specific formulations in a range of standard flow rates. Thin-walled products may require an easy flow material while thick-walled products can use a material that is stiffer. It is better to use as stiff a flow as possible because that improves physical properties of the molded part. But the stiff material will be more difficult to push and this may result in the material solidifying before full packing takes place. The molecules will be left to shrink at different rates and warpage will occur.Solution: Utilize a material that has the stiffest flow possible without causing warpage. Contact the material supplier for help in deciding which flow rate should be used for a specific application.

OPERATOR
INCONSISTENT PROCESS CYCLE
Explanation: The machine operator may be opening the gate too soon, thereby effectively shortening the overall cycle time. This would cause the part to be ejected before the skin has formed properly and excessive, uncontrolled shrinkage may occur. The varying shrinkage rates will cause warpage.Solution: If possible, run the machine on the automatic cycle, using the operator only to interrupt the cycle if an emergency occurs. Use a robot if an ``operator'' is necessary. In addition, instruct all employees on the importance of maintaining consistent cycles.

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Non-fill (shorts)

Non-fill (shorts)

Non-fill can be defined as an incomplete molded part caused by insufficient material injected into the mold. Non-fill can be an extension of a flow line or knit line condition.

MACHINE
INSUFFICIENT MATERIAL IN HOPPER
Explanation: The most common cause of non-fill is insufficient material being injected into the mold, caused by not enough material in the hopper. Normally this is the result of employee error and the hopper was not checked often enough. It simply ran out of material.Solution: It is best to use an automated system that replenishes material in the hopper as it is used. That way the machine will never run out of plastic. If manual systems are utilized instead, the employee in charge must understand the importance of keeping the hopper filled. Alarm units can be used to emit an audible signal when the material in the hopper reaches a preset level.

LOW BARREL TEMPERATURE
Explanation: When barrel temperatures are too low, the material will not be heated to the proper temperature for adequate flow. The material will push slowly into the mold and the flow fronts that form will not be hot enough to complete the filling of the mold.Solution: Increasing the barrel temperatures will allow the flow fronts to stay hotter longer and complete the filling of the mold. Make sure the proper profile is being used and that the material heats progressively as it travels through the barrel from rear to front.

INADEQUATE BACK PRESSURE
Explanation: The back pressure control is used to impart a resistance to the molten material being prepared in the barrel for the upcoming cycle. This resistance is used to help preheat the material and also control the density of the melt before it is injected into the mold. If the back pressure setting is too low, the material may not be brought to the proper temperature and the flow fronts will not travel as far resulting in non-fill.Solution: Increase the back pressure to increase the melt temperature and improve the ability for the fronts to flow. This is best accomplished by starting at the minimum of 50 psi and increasing in 10-psi increments until the proper flow is attained. Do not exceed 300 psi. The higher the back pressure the hotter the plastic, and excessive back pressure will thermally degrade the plastic.

INJECTION PRESSURE OR SPEED TOO LOW
Explanation: If the injection speed or pressure is too low, the molten plastic will not be pushed into the mold cavity fast enough to fill because it cools down and solidifies too soon.Solution: Increase the injection pressure or speed. While these two parameters are related, it is not proper to adjust them both at the same time. Adjust them each independently and monitor the results closely to determine whether or not the other needs adjustment. As a rule-of-thumb, it is best to make adjustments in increments of no more than 10% of the original setting.

EXCESSIVE NON-RETURN VALVE CLEARANCE
Explanation: The non-return valve, found in the front section of the screw and barrel assembly, keeps molten plastic from slipping back over the injection screw when the screw is pushed forward during the injection phase of the process. The valve lies between the outside diameter of the screw and the inside diameter of the barrel and creates a seal between the two. If there is too much clearance (due to wear), the sealing effect is lost and slippage occurs. This results in a reduction in volume of plastic that gets injected into the mold and non-fill occurs.Solution: Inspect the non-return valve mechanism and replace worn or damaged components. This wear is normal but is accelerated by molding materials that have reinforcements (such as glass) in them. The valve should be inspected at least every three months.

BRIDGING IN FEED THROAT
Explanation: As material is fed from the hopper to the heating cylinder, it passes through the feed throat of the machine. This area must have a temperature maintained at around 100 to 150 degrees F, depending on the plastic being molded, to prepare the material for higher heats in the barrel. But, if the temperature is too high, the plastic pellets begin to get sticky and bond together. This will form a blockage in the feed throat and material will not be allowed to fall through. It forms a bridge across the feed throat opening.Solution: Decrease the feed throat temperature. The material supplier can provide the proper temperature value for a specific material. Make sure there is no obstruction in the water line used for cooling the feed throat.

INSUFFICIENT BARREL CAPACITY
Explanation: If the mold is running in a machine that does not have a large enough barrel, the material in the barrel may not be allowed to stay there long enough to absorb enough heat. The cold material will not flow as far as a hot material would and non-fill will occur. Increasing the heat may only degrade the plastic.Solution: Place the mold in a machine that utilizes the 20% to 80% rule. This states that, ideally, a barrel should be sized such that 50% of the capacity is used every shot, but based on heat sensitivity of the material being used, that ratio can be between 20% for most materials and 80% for heat sensitive materials. This formula allows enough time for the material to absorb heat properly before being molded.

MOLD
LOW MOLD TEMPERATURE
Explanation: Generally, a hot mold will allow a material to stay molten longer than a cold mold and cause the molecules to flow farther before they solidify. This results in a dense part with maximum fill. If the mold is too cold, the molecules solidify before they are packed and the flow fronts will not travel far enough to fill the cavity image.Solution: Increase the mold temperature to the point at which the material has the proper flow and packs out the mold with maximum fill. Start with the material suppliers recommendations and adjust accordingly. Allow 10 cycles for every 10-degree change for the process to re-stabilize.

SMALL GATES AND/OR RUNNERS
Explanation: Gates and/or runners that are too small will cause excessive restriction to the flow of the molten plastic. Many plastics will then begin to solidify before they fill the cavity.Solution: Examine the gates and runners to determine if any burrs or other obstructions exist. If possible, perform a computer analysis to determine the proper sizing and location of gates and runners. Ask the material supplier for data concerning gate and runner dimensioning for a specific material and flow rate.

IMPROPER GATE LOCATION
Explanation: If certain materials are injected directly across a flat cavity surface, they tend to slow down quickly as a result of frictional drag and cool off before the cavity is properly filled. The flow fronts have difficulty traveling far enough to pack out the part.Solution: Relocate, or redesign, the gate so that the molten plastic is directed against an obstruction such as a core pin. This will cause the material to disperse and continue to flow instead of slowing down.

INSUFFICIENT VENTING
Explanation: Venting is used to remove trapped air and gases from the closed mold, so molten material will be able to flow into every section of the mold. If the air and gases are not removed they act as a barrier to the flow of the plastic and will not allow filling to occur.Solution: Vent the mold by grinding thin (0.0005''-0.002'') pathways on the shutoff area of the cavity blocks. The viscosity of the plastic being molded determines the depth of the vent. Stiff materials can utilize deeper vents but fluid materials require thinner vents. In either case, the concept is to remove air from the mold as fast as possible with as deep a gate as the material viscosity will allow.

MATERIAL
IMPROPER FLOW RATE
Explanation: Resin manufacturers supply specific formulations in a range of standard flow rates. Thin-walled products may require an easy flow material while thick-walled products can use a material that is stiffer. It is better to use as stiff a flow as possible because that improves physical properties of the molded part. But, the stiff material will be more difficult to push and this may result in the flow fronts not traveling far enough to fill the cavity image.Solution: Utilize a material that has the stiffest flow possible without causing non-fill. Contact the material supplier for help in deciding which flow rate should be used for a specific application.

OPERATOR
INCONSISTENT PROCESS CYCLE
Explanation: It is possible that the machine operator is the cause of delayed or inconsistent cycles. This will result in erratic heating of the material in the injection barrel. If such a condition exists, the colder particles may not flow properly and the flow front may not be allowed to travel far enough to fill the mold.Solution: If possible, run the machine on the automatic cycle, using the operator only to interrupt the cycle if an emergency occurs. Use a robot if an ``operator'' is necessary. In addition, instruct all employees on the importance of maintaining consistent cycles.

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Burn marks

Burn marks

Burn marks can be defined as small dark brown or black discolorations on the surface of a molded part, usually found at the end of the material flow path or in blind pockets.

MACHINE 
EXCESSIVE INJECTION SPEED OR PRESSURE
Explanation: Injection speed and pressure determine how fast molten resin is injected into a mold. If either is too high, the resin is forced in so fast that trapped air and gases are not allowed time to be vented. Many of these gases are pushed to the edge of the flow front and become compressed to the point that they auto-ignite, burning the surrounding plastic. The burned areas appear as char marks on the molded part.Solution: Reducing the injection speed or pressure will allow enough time for the gases or trapped air to escape through normal vent paths.

EXCESSIVE BACK PRESSURE
Explanation: While most materials will benefit from some back pressure application, there is a limit to the amount needed for a specific material and product. The whole idea of applying back pressure is to mix the material better, making it denser and more oriented for flow. However, this very act of mixing may introduce air into the melt, which may be too much for the venting system to handle under normal conditions. The excess air may be compressed at the vent locations and auto-ignite, causing burn marks on the part. There is also the possibility that shearing action from too high a back pressure setting will degrade the material in the barrel and cause burning to occur.Solution: Use minimum back pressure. All materials will benefit from approximately 50-psi back pressure, but some require up to 300 psi. The material supplier is the best source of information regarding proper back pressure settings for a specific material. When adjusting back pressure use increments of 10 psi.

HIGH SCREW ROTATION SPEED
Explanation: The turning of the screw brings fresh material into the heating cylinder and imparts a certain amount of heat to that material through rotary shear friction. The faster the screw turns, the greater the friction and shear heat created. If the speed is too high, the friction will cause the material to overheat and become thermally degraded. This causes small particles of charred material to form and these get pushed into the melt stream during injection. They will show up on the molded part as burned areas.Solution: Adjust the screw rotation speed. An average speed should be approximately 100 rpm. But, specific materials require specific rotation speeds. Consult the material supplier for the proper speed for a specific resin. When adjusting up or down, do so in increments of 10 rpm.

IMPROPER COMPRESSION RATIO OF SCREW
Explanation: Compression ratio is a value that indicates how much a screw will compress a material while it is being processed in the barrel of the machine. It is calculated by dividing the depth of the feed section (rear) of the screw by the depth of the metering section (front) and is usually less than 2 to 1. If the compression ratio is too great for the specific material being molded, the resin degrades thermally and the burned material flows into the molded part.Solution: The material supplier can recommend the proper compression ratio for a specific resin. A general-purpose screw can usually be used to provide adequate compression but specific conditions may require a screw that is specially designed for a given material.

FAULTY TEMPERATURE CONTROLLERS AND HEATER BANDS
Explanation: Temperature controllers are sensitive units and should be checked and calibrated often (every 3 months maximum). Heater bands do wear out or burn out and normally there is no way of knowing. If controllers or heater bands are not functioning properly, the other controllers and heater bands must compensate. The result is localized overheating of material in the cylinder. This material can degrade and char and enter the melt stream to become molded into the plastic part.Solution: Inspect and calibrate temperature controllers at least every 3 months. Inspect and replace damaged heater bands as necessary. In addition, look for broken or crimped wires, poor insulation, rusted areas on the heater bands, and poor electrical connections.

EXCESSIVE BARREL TEMPERATURES
Explanation: When barrel temperatures are set too high, the resin will overheat and undergo thermal degradation. This degraded material will break loose, enter the melt stream, and become molded into the finished part.Solution: Establish proper barrel temperatures and profile. The material supplier will provide accurate barrel temperature requirements. The profile should have the barrel temperatures increase progressively from rear to front.

MOLD
IMPROPER SPRUE BUSHING-TO-NOZZLE SIZING
Explanation: If the sprue bushing diameter does not match the nozzle opening (or vice-versa) molecular shearing will occur at their junction and some of the material flowing through that area will degrade. The degraded material will enter the melt stream and be molded into the finished part. Solution: Using bluing dye or thick paper, press the nozzle against the sprue bushing, and check the impression of the openings of each. They should be close to the same and not be off center. Replace the nozzle tip or the sprue bushing if they do not match. Re-center the heating cylinder to the mold if they are off center.

IMPROPER VENTING
Explanation: Air is trapped in a closed mold and incoming molten plastic will compress this air until it auto-ignites. This burns the surrounding plastic and results in charred material in the form of burn marks.Solution: Vent the mold by grinding thin (0.0005''-0.002'') pathways on the shutoff area of the cavity blocks. Vents should take up approximately 30% of the perimeter of the molded part. Vent the runner, too. Any air that is trapped in the runner will be pushed into the part. Blind pockets can be vented using flush core pins or fake ejector pins and grinding a flat down the entire length of the pins.

UNDERSIZED GATES
Explanation: Gates are used to determine the flow of the material into the cavity. They are intended to cause a slight restriction and impart shearing heat to the plastic, as well as to control the speed at which the plastic enters. If the gate is too small, the restriction is too great and the material will overheat causing degradation. The degraded material shows up as burned resin in the finished part.Solution: Size the gate according to the material supplier's recommendations. The gate should be as thin as possible to minimize cycle time, but as thick as necessary to reduce the tendency to degrade material. Gates should be installed in removable inserts so they can easily be altered or replaced.

MATERIAL
EXCESSIVE USE OF REGRINDExplanation: Regrind melts at a lower temperature than virgin, and a regrind/virgin blend must be heated high enough to melt the virgin, which may degrade the regrind. For this reason, regrind use should be minimized if mixed with virgin material. However, regrind by itself can be used successfully by lowering the melt temperature.Solution: Use 100% regrind, or, if mixing with virgin, limit the amount of regrind to 15% by weight. It may be necessary to use no regrind at all, especially in some medical and electronic products.

OPERATOR
INCONSISTENT PROCESS CYCLE
Explanation: It is possible that the machine operator is the cause of delayed or inconsistent cycles. This will result in excessive residence time and erratic heating of the material in the injection barrel. If such a condition exists, materials may degrade, resulting in locally burned resin.Solution: If possible, run the machine on automatic cycle, using the operator only to interrupt the cycle if an emergency occurs. Use a robot if an ``operator'' is really necessary. And, instruct all employees on the importance of maintaining consistent cycles.

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Bubbles

Bubbles

Bubbles can be defined as a voided area trapped within a molded plastic part.It differs from a blister in that there is no surface protrusion with a bubble.Bubbles are usually caused by trapped gases or air pockets, but can also be caused by differential shrinking.

MACHINE 
LOW INJECTION PRESSURE OR HOLD TIME 
Explanation: If injection pressure or hold time are too low, the molten material is not forced into the mold cavity and trapped gases and air will form voids because the gases will not be forced out of the mold through vent paths. Solution: Increase the injection pressure and/or the hold time to help force the gases out as the plastic is pushed into the cavity.

INSUFFICIENT MATERIAL 
Explanation: Too little material feed will have the same effect as low injection pressure. The material will not be forced into the cavity and gases will be trapped, forming voids due to a lack of molecular packing. Solution: It is important to establish a feed setting that allows a 1/8'' to 1/4'' cushion of material at the end of the injection stroke. Without this cushion, there is no material against which holding pressure can be applied to force material into the cavity.

IMPROPER INJECTION TEMPERATURE PROFILE 
Explanation: The injection temperature profile addresses four heating zones of the injection barrel. These are commonly known as rear, center, front, and nozzle. The rear is also know as the feed zone, the center is known as the transition zone, and the front is known as the metering zone. The purpose of the feed zone (rear) is to start the material through the heating process. The heat is kept lower at this point but high enough to begin softening the plastic. The transition zone (center) heats the plastic higher and begins to compress it, squeezing out the trapped gases. In the metering zone (front) the material is brought up to the final, ideal temperature and is further compressed and sheared, which also introduces more heat. In the nozzle zone the material is simply kept at the upper temperature as it is injected into the mold. Any imbalance in the temperature values of these zones may result in plastic particles that are not properly melted at the right time. This will not allow gases to escape and voided areas (bubbles) will appear. Solution: Maintain a proper temperature profile. This is readily obtained from the resin supplier, but a rule-of-thumb sets the temperature controls at increments of 50 to 100 degrees F from rear to front, and the nozzle at the same temperature as the front zone. An air shot from the nozzle should produce a bubble-free stream of plastic that has the approximate consistency of warm honey. Remember that the temperature control settings are not the same as the actual temperature of the plastic. They are usually 50 to 100 degrees higher than the actual plastic temperature to accommodate the rapid travel of material through the barrel.

EXCESSIVE INJECTION SPEED 
Explanation: The injection speed determines how fast the material is injected into the mold. If it is too slow, the material tends to cool off and solidify before the mold is fills, which results in a short shot. If it is too fast, the material tends to tumble and become turbulent, which traps air and gases in the resin. These gases then show up as bubbles because they were not able to reach the vented areas of the mold. Solution: Start with the supplier's recommendations for injection fill speed. Adjust up or down according to the results. If bubbles appear, slow down the rate. If short shots appear, speed up the rate.

INSUFFICIENT BACK PRESSURE 
Explanation: Back pressure is used to help mix the material and homogenize it. It also helps remove trapped air and densifies the melt. If back pressure is insufficient the gases and trapped air are not allowed to escape and remain in the plastic melt as bubbles that can be molded into the finished part. Solution: Increase the back pressure. Most materials will benefit from a back pressure that is approximately 50 psi. But, some materials require higher settings: in some cases up to 300 psi. However, be cautious, because too high a back pressure will degrade any material. The material supplier is the best source of information regarding proper back pressure settings.

MOLD IMPROPER VENTING
Explanation: Most molds do not have adequate venting. Usually the moldmaker elects to ``wait and see'' where the venting needs to be located and then assigns an arbitrary size. While size is not necessarily as important as location, there is a tendency to use a minimum number of oversized vents rather than an adequate number of properly sized vents. If improper venting is used (or no venting), any trapped air or generated gases cannot escape. This will result in voids, bubbles, shorts, and burns.Solution: Vent the mold even before the first shot is taken by grinding thin (0.0005''-0.002'') pathways on the shutoff area of the cavity blocks. Vents should take up approximately 30% of the perimeter of the molded part. Vent the runner, too. Any air that is trapped in the runner will be pushed into the part. Another rule-of-thumb is to place a vent at every 1-inch dimension around the perimeter of the cavity. You cannot have too many vents.

SECTION THICKNESS TOO GREAT
Explanation: Most plastic parts are not of one continuous wall thickness. There is usually a need to change the wall thickness for such reasons as additional strength. Unfortunately, when that happens, there is a pressure loss in the thicker section as the molten material shrinks more there as it solidifies. The material pulls away from the cavity wall leaving a voided area. If the void is captured below the part surface, the void will appear as a bubble.Solution: A good rule-of-thumb is that any wall thickness should not exceed any other wall thickness by more than 25%. There will be little tendency for bubbles at that ratio. Metal inserts can be used to core out sections that do not meet that ratio, or ``overflow'' wells might be used to move the voided area off the primary part surface. However, the overflow would then need to be removed from the molded part.

IMPROPER RUNNERS OR GATES
Explanation: Runners or gates that are too small will restrict the molten material in the flow pattern and may cause non-packed parts. If gates are placed to flow material from a thin section into a thicker section, the restricted flow in the thin section will keep the thicker section from packing. Both of these conditions can result in a loss of filling pressure and cause sinks to evolve in the molded part. These sinks can take the shape of bubbles and voids if they are trapped within the part rather than on the surface.Solution: Gates should be of a depth that is equal to at least 50% of the wall they are placed at and should always be located to flow material from the thickest section to the thinnest. Runner diameters should be adequate to avoid a pressure drop as the material fills. Thus, the farther the travel, the larger the initial runner diameter should be. Gates and runners should be machined in the mold to be ``steel safe'' so they can be increased by removing metal. It is a good practice to place gates and runners in individual inserts so they can be easily replaced and/or reworked.

LOW MOLD TEMPERATURE
Explanation: A mold that is too cold for a specific resin or product design will not allow the molten material to fill and pack all of the mold properly before the resin starts to solidify. Any air or gases present in the plastic at the time will be trapped under the surface as bubbles.Solution: Raise the mold temperature in increments of 10 degrees F until the bubbles disappear. Allow 10 cycles for each 10-degree adjustment (up or down) for the mold temperature to stabilize.

MATERIAL EXCESSIVE MOISTURE
Explanation: Excessive moisture is one of the most frequent causes of bubbles. Moisture causes bubbles because the water droplets actually turn to pockets of steam when heated in the injection unit, causing voided areas between molecules. If the voided areas are trapped beneath the surface of the part they appear as bubbles.Solution: Although it is commonly understood that non-hygroscopic materials do not require drying, do not take chances. Dry all materials. It may be that fillers used in the material are hygroscopic and they will absorb moisture. Every plastic material requires specific drying conditions. And each material should be dried according to the material suppliers recommendations. The desired moisture content is between 1/10th of 1 percent and 1/20th of 1 percent by weight. This means the dry air being used to take moisture from the material should have a dew point of -20 to -40 degrees F.

OPERATOR
INCONSISTENT PROCESS CYCLE
Explanation: It is possible that the machine operator is the cause of delayed or inconsistent cycles. This will result in excessive residence time of the material in the injection barrel. If such a condition exists, materials may flow more easily and be injected too quickly, resulting in trapped air and gases being held in the resin and not being vented as required. The gases will form bubbles if held under the molded part surface.Solution: If possible, run the machine on automatic cycle, using the operator only to interrupt the cycle if an emergency occurs. Use a robot if an ``operator'' is really necessary. And, instruct all employees on the importance of maintaining consistent cycles.

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