Blush

Blush

Blush can be defined as a clouded discoloration normally found at gate locations, but they can be found anywhere on the part surface. These visually defective areas have very weak physical properties due to loose molecular structure.

MACHINE
EXCESSIVE INJECTION FILL SPEED
Explanation: The speed and pressure of the melt as it enters the mold determine both density and consistency of melt in packing the mold. If the fill is too fast, the material tends to ``slip'' over the surface and will ``skin'' over before the rest of the material solidifies. The slipped skin area does not faithfully reproduce the mold steel surface, as does the material in other areas, because it has not been packed tightly against the steel.Solution: One solution is to adjust the fill speed rate until the optimum has been achieved. This will help eliminate blushing.

MELT TEMPERATURE TOO HIGH OR TOO LOW
Explanation: Although this may sound contradictory, either condition might cause blushing. If the injection barrel heat is too high, the material will flow too quickly, resulting in slippage of the surface skin, as mentioned above. If the barrel heat is too low, the material may solidify before full packing occurs and the plastic will not be pushed against the mold steel, especially in the gate area because that is the last area to pack.Solution: Melt temperature must be adjusted to the optimum for a specificmaterial and specific product design.

LOW INJECTION PRESSURE
Explanation: The plastic material must be injected into the mold in such a way as to cause proper filling and packing while maintaining consistent solidification of the melt. Injection pressure is one of the main control variables of the machine and must be high enough to pack the plastic molecules against the steel of the mold while the plastic cools. Low pressure will not achieve this packing and the material will appear dull in local areas that do not have enough pressure.Solution: Increasing the injection pressure forces the material against the mold surface, producing a truer finish that replicates the steel finish.

NOZZLE DIAMETER TOO SMALL
Explanation: The nozzle diameter controls the time during which the material fills the mold. If the diameter is too small, the material may begin to solidify before the mold is filled. Then, packing cannot occur because the material is already rigid. Blush will occur because the plastic has not been forced against the steel surface.Solution: Enlarging the nozzle diameter will minimize the condition. The nozzle tip is interchangeable and a tip with the opening the same as, or slightly smaller than, the sprue bushing opening is recommended.

LOW NOZZLE TEMPERATURE
Explanation: A nozzle that has a low temperature will cause the material going through it to cool off too soon and not be allowed to pack out the mold. The non-packed molecules will form blush because they cannot replicate the steel finish.Solution: Increasing the mold temperature will allow the material to flow easier, and for a longer time, thereby packing the mold and replicating the steel finish. Normally, the nozzle temperature should be set the same as, or 10 degrees hotter than, the front zone of the barrel.

MOLD
LOW MOLD TEMPERATURE
Explanation: A low mold temperature may cause the molten material to slow down and solidify before the mold is packed out. This will cause dull areas where the plastic was not forced against the steel finish.Solution: Increasing the mold temperature allows the material to flow farther and pack properly. The material temperature could also be raised to accomplish the same effect.

IMPROPER VENTING
Explanation: Trapped air can cause blushing if the air is trapped in an area that does not compress the air enough to ignite it. The air takes up space where the plastic should be, so the plastic is not forced against the steel finish.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. Add vents in local areas that show blush. Vent the runner, too. Any air that is trapped in the runner will be pushed into the part.

SMALL SPRUE BUSHING DIAMETER
Explanation: A small sprue-bushing diameter will keep the material from packing because the small opening reduces the ability of the plastic to flow far enough to fill the mold. An unpacked mold will cause blushing where the material is not forced against the mold steel.Solution: Size the sprue bushing major diameter so its cross-sectional area is equal to (or greater than) the sum of the cross-sectional area of all the runners leading from it. Then, taper the sprue diameter to match the nozzle. That will ensure proper pressure drop adjustments to pack the mold.

IMPROPER GATE LOCATION
Explanation: If a mold is gated such that the thinnest areas fill first, those areas will begin to solidify before the thicker areas are packed. Blush will form in the thicker areas because there is no pressure left to pack the plastic against the steel surface.Solution: Make sure the gate is located so the thicker sections fill first. The material should flow from thick section to thin section. That ensures equal packing of all areas.

SHARP CORNERS
Explanation: If the product design contains sharp corners, the material tends to slip by those corners without fill them in. The corners are not packed with material and blush occurs due to that non-packing.Solution: Radius all sharp corners, especially in the gate area, as that is the last place to pack. Sharp corners should not be allowed on any molded part.

MATERIAL
EXCESSIVE MOISTURE
Explanation: In some cases, excessive moisture in a melt will accumulate at the gate area. The reason for this is that the gate area is the last place the pressure builds up. Moisture trapped in other areas may be forced into the gate area due to this pressure buildup. The gate area will appear dull due to the moisture that gets screened out. Usually, this type of blush is accompanied by splay.Solution: Dry the material to the supplier's recommendations and make sure it is used within two hours of that drying activity.

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 fill at a faster speed and cause slippage as explained earlier. Slippage causes blush.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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Contamination

Contamination

Contamination can be defined as an imperfection in a molded part caused by the presence of a foreign object or material that is not part of the original molding compound.

MACHINE
OIL LEAKS AND GREASE DRIPS
Explanation: It is common for most molders to allow small oil leaks to become big ones before they consider repairing them. This oil can find its way into unusual locations, such as a container of raw plastic being prepared to go into the machine hopper. Also, when equipment is greased, it is usually overdone and grease drips can fall into material containers. These contaminants are not compatible with the base resin so they are very evident in the molded part.Solution: Fix oil leaks as quickly as possible. Clean up grease drips as they occur and do not use the same cleanup rags to wipe out the hopper between material changes.

UNCOVERED HOPPER
Explanation: When the molding machine is first built, it is supplied with a cover for the hopper. It is there to keep contaminants from getting to the molding compound. If that cover is not present during production runs, the material can be contaminated with such things as dust from the ceiling, drops of condensation from overhead waterlines, and even bird droppings in large facilities.Solution: Use the hopper lid. Do not improvise with flattened cardboard boxes, as the paper particles will cause contamination. If the original lid is lost buy a new one. They are designed to do a specific job well.

IMPROPERLY CLEANED HOPPER 
Explanation: When a material change is required, the hopper must be cleaned out. It is not good enough to simply remove the material present. The sides of the hopper must be wiped to remove any material dust or ``fines'' that stick to the sides due to static charges. If not removed, these fines will get picked up by the new material and, because of incompatibility, will appear as contamination in the molded parts.Solution: Clean the hopper thoroughly between material changeovers. This may require wiping the inside with a cloth slightly dampened with denatured alcohol to remove all traces of fines.

MOLD
EXCESSIVE LUBRICATION
Explanation: Molds with cams, slides, lifters, and other mechanical actions need periodic lubrication. Sometimes there is a tendency to overdo this and the lubricant may find its way into the cavity of the mold. This is especially true for ejector pin lubricants. The oil-based lubricant is not compatible with the base resin and is evident as contamination on the molded parts. In addition, excessive mold release acts as a contaminant and appears on the molded part as blotches, dark spots, smears, and streaks.Solution: Optimize the use of lubricants and minimize the use of mold release sprays. Clean up any excess lubricants and use only the amount needed for a specific application. A little lubricant goes a very long way. Investigate the reason for using mold release. Usually it is a temporary approach to a more severe problem, and the problem should be solved to eliminate the need for the release agent.

MATERIAL
CONTAMINATED RAW MATERIAL
Explanation: The most common cause of contamination in molded parts is molding compound contamination. Such contamination is usually the result of dirty regrind, improperly cleaned hoppers or granulators, open or uncovered material containers, and poor quality virgin material supplied by the manufacturer.Solution: This type of contamination can be minimized by dealing with high quality, reputable suppliers and by using good housekeeping practices. Properly trained material handlers will also help reduce contamination.

IMPROPER REGRIND
Explanation: Regrind can be defined as any virgin material that has seen at least one heat excursion through the molding machine. It can be in the form of molded parts or runner systems.Solution: Grinding the plastic in a granulator that is specially designed for the purpose creates the regrind. The material is ground until it falls through a screen with specifically sized openings. The larger the openings, the larger the regrind pellet. If the granulator is not cleaned out between changes (including the screen), pellets of incompatible materials may be mixed with the next material and this causes contamination. Granulators must be completely taken apart to perform a thorough cleaning between uses. Do not assume that the granulator will always be used only for a certain material. Plans have a way of changing. As soon as a run is completed, the granulator should be removed from service and cleaned. Use of compressed air, combined with wiping with clean rags will suffice, as long as every component is cleaned, including rotor, blades, container, screen, sidewalls, and feed throat.

EXCESSIVE MOISTURE
Explanation: Excessive moisture should be considered a contaminant. It doesn't belong in the molding compound. Moisture turns to steam when heated in the injection unit, and this steam interferes with molecular bonding. This causes splay, which is a visual defect, but also creates a weak part due to brittleness.Solution: Although it is commonly understood that non-hygroscopic material does 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: The machine operator may be the source of contamination in a variety of ways. First, if the operator is allowed to have food or drink at the work station, these may accidentally get spilled into containers holding fresh material ready to go into the hopper. Second, the operator may have been instructed to keep the area clean and sweeping dust into the air may result in contaminating raw material or freshly molded parts. Third, a lack of concern or outright sabotage could be the incentive to purposely add contaminants to the molding compound.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 contaminant-free areas.

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Jetting

Jetting

Jetting can be defined as a ``snake-like'' pattern on the surface of a molded part, usually emanating from the gate area. It is indicative of an undesirable turbulence and splitting of the flow front.

MACHINE 
EXCESSIVE INJECTION SPEED 
Explanation: Excessive injection speed (fill rate) will cause the molten plastic to form jet streams as it is pushed through the gates instead of the more desirable wide "tongue'' of material. These snake-like streams cool independently from the surrounding material and are visible on the molded part surface. Solution: Reducing the injection speed will allow the plastic flow front to stay together and not form the individual streams that cause the jetting patterns on the part surface.

BARREL TEMPERATURE TOO HIGH OR TOO LOW
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 front will break up into individual streams as resistance builds up. This will cause jetting patterns. On the other hand, if the barrel temperature is too high, the material is pushed into the mold too quickly. This causes the flow front to split apart as it enters the cavity and the jetting patterns will develop. Solution: Decrease or increase the barrel temperature accordingly. The material supplier can recommend the best starting point for barrel temperature and it can be adjusted from that point. Make changes in 10 degree F increments and keep the profile so it is heating progressively from back to front.

SMALL NOZZLE OPENING 
Explanation: If the nozzle opening (or sprue bushing opening) is too small for the material being molded, the restriction may cause the material to flow too slowly and solidify early. The flow front may break apart as it travels through the gate (due to sidewall friction) and jetting patterns may develop. Solution: Increase the nozzle opening. As a general rule, the nozzle opening should never be less than 7/32'' in diameter. The stiffer the material flow, the larger that opening should be. Make sure that the sprue bushing opening diameter matches or is 1/32'' larger than the nozzle opening.

LOW NOZZLE TEMPERATURE 
Explanation: As material is transported through the heating barrel, it is gradually brought up to the ideal processing temperature by absorbing heat from the heating bands and frictional heat, which is created by the shearing action of the rotating screw within the barrel. In the last heating zone, the material is exposed to is the nozzle. By the time the material gets to the nozzle, it should already be at ideal molding temperature and only a small amount of heat needs to be applied at this point to keep the resin flowing. If the nozzle is not hot enough, however, the material will begin to cool off too quickly as it leaves the barrel and the flow front will not flow properly, breaking into many streams and causing jetting. Solution: Increase the nozzle temperature. As a rule-of-thumb the nozzle temperature should be set at 10 degrees F higher than the setting for the front zone of the barrel. This helps compensate for heat loss due to metal-to-metal contact between the nozzle and the sprue bushing and keeps the material hot enough to flow properly, eliminating jetting.

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 pack together properly before they solidify. This results in a dense part with no separation of layers. If the mold is too cold, the molecules solidify before they are packed together and may break up into separate units. As they travel through the gate these units split up and form jetting patterns on the part surface.Solution: Increase the mold temperature to the point at which the material has the proper flow and packs out the mold without jetting. 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 an unpacked condition and the flow front may break into separate streams, causing jetting patterns to develop.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. As the material tries to flow through the gate, it is pulled apart into several streams and this forms a jetting pattern on 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.

EXCESSIVE GATE LAND LENGTH
Explanation: The area that surrounds the gate itself is called its land. It determines the distance a material must travel in a restricted state immediately before it enters the cavity. The length of this travel (land) should be no more than 1/8''. The land acts like a tunnel when the mold is closed and if the tunnel is too ling the material begins to cool off before it can get to the cavity. This causes the material to split into streams that create the familiar jetting pattern on the part.Solution: Decrease the gate land length. It is best to construct the mold so that the gates are located in replaceable inserts. That way they can be replaced easily at times when adjustments are needed. The insert should include the land area. This land length should be no less than 0.030'' and no greater than 0.125''.

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 breakup of the flow front as the material enters the gate. The breakup appears as a jetting pattern.Solution: Utilize a material that has the stiffest flow possible without causing jetting. 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 fill the mold before they fully solidify. Jetting may be caused as these colder areas attempt to push through the gate and are torn apart due to sidewall friction.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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Knit lines

Knit lines

Knit lines can be defined as the inability of two or more flow fronts to ``knit''together, or "weld", during the molding process. This normally occurs around holes or obstructions and causes locally weak areas in the molded part.

MACHINE
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 around obstructions will not be hot enough to reunite after they travel around those obstructions.Solution: Increasing the barrel temperatures will allow the flow fronts to stay hotter longer and knit better when they reunite. It is practically impossible to eliminate knit lines once they are formed but they can be minimized.

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 knit line areas will be more evident due to their inability to reunite.Solution: Increase the back pressure to raise the melt temperature and improve the ability for the fronts to unite. This is best accomplished by starting at the minimum of 50 psi and increasing in 10-psi increments until the knit lines improve. 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 0R 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 and knit at converging flow fronts 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.the plastic, and excessive back pressure will thermally degrade the plastic.

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 welding at knit line locations. If the mold is too cold, the molecules solidify before they are packed and the knit lines will be more evident and much weaker.Solution: Increase the mold temperature to the point that the material has the proper flow and packs out the mold with maximum knit line strength. 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 an unpacked condition and knit lines will be weaker and more evident.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 because of frictional drag and cool off before the cavity is properly filled. The flow front breaks into many streams and they have difficulty welding back together before they solidify.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 GATE LAND LENGTH
Explanation: The area that surrounds the gate itself is called its land. It determines the distance a material must travel in a restricted state immediately before it enters the cavity. The length of this travel (land) should be no more than 1/8''. The land acts like a tunnel when the mold is closed and if the tunnel is too long the material begins to cool off before it can get to the cavity. This causes the material to split into streams that can create knit line conditions.Solution: Decrease the gate land length. It is best to construct the mold so that the gates are located in replaceable inserts. That way they can be replaced easily at times when adjustments are needed. The insert should include the land area. This land length should be no less than 0.030'' and no greater than 0.125''.

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 breakup of the flow front as the material enters the gate. This may cause streams of flow that cannot weld back together properly and they will form weak knit lines.Solution: Utilize a material that has the stiffest flow possible without causing knit lines. 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 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 fill the mold before they fully solidify. The flow front may break up into streams that cannot weld back together properly and weak knit lines may 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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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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