25.0 DIE CASTING DEFECTS
25.1 Introduction:
The various faults commonly encountered in die casting can be classified into three groups.
Cold type defects:
Lack of fill.
Cold shut.
Severe chill
Chill.
Flow lines
Hot type defects:
Soldering.
Cracks.
Broken part.
Bent part.
Heat marks or shrinkage pits.
Miscellaneous defects:
Scale.
Blisters.
Porosity.
Excessive flash.
Mechanical defects.
Ejector pin marks.
When correcting faults during a casting run, it is important that the right corrective action is taken as soon as possible. A fault correcting procedure, intended for shop floor use is
given below.
25.2 Coldtypedefects
a) Lack of fill
This condition has three basic causes. The first cause is inadequate metal in the gooseneck or cold chamber. The metal level in the holding furnace of a hot chamber must be maintained above the goose neck inlet ports. For cold chamber machines, the correct size ladle is used, and care must be taken to ensure that the ladle is full for each shot. When an automatic ladle is used, it must be proper1y adjusted to ladle exactly the correct amount of metal into the cold chamber. Secondly this defect may be caused by cold metal, cold die, or both. The temperatures should be checked and adjusted as necessary. Finally a lack of fill may be the result of slow shot speeds. The shot control hydraulic valve should be opened the proper amount.
b) Cold shut:
Cold shut, like lack of fill is caused by cold metal, slow shot or low die temperatures. If air vents and /or over flows are clogged with flash they may also contribute to the problem. All these factors must be checked and corrected as necessary.
c) Severe chill:
Severe chill is similar to cold shut but it will cover a large surface of casting instead of being a single line. Excessive release material as well as cold metal, slow shot, low die temperature or clogged air vents may be the cause. Severe chill usually appears when shots are made into a cold die, but will rarely occur during normal operation.
d) Chill:
This defect has the same appearance and is caused by the same conditions as severe chill but is less noticeable. Slight or faint chill lines on the surface cause for the rejection of certain types of castings. However castings used for ornamental parts will usually require a chill free surface. Low die temperature, low metal temperature, slow shot speed, or excessive die release material are all causes of chill.
e) Flow lines:
Flow line defects are similar to chill and cold shut. Flow lines can usually be reduced or eliminated by increasing the die temperature, metal temperature or both. Like chill, the defect may be corrected by higher shot speed or less die release material.
25.3 Hot type defects
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a) Soldering:
This condition is the result of the cast metal bonding to the die surface. Upon ejection the casting tears away leaving layer that has bonded to the die. When soldering occurs, it may create additional problems such as cracking or bending on the casting, out of tolerance dimensions, depressed ejector pin marks in the casting and/or porosity within the casting.
Soldering may be caused by excessive metal temperature, incorrect die temperature (too hot or too cold) or insufficient die release material. If the condition is severe or when other methods fail to remove the solder, it must be cleaned from the die. Caustic solutions provided for this purpose may be used or the material may be polished out of the die. It is recommended that the die maker or other person with special training polish the die cavity. If the polishing is not done properly, a rough die surface may be created that will increase the soldering condition. Care must also be exercised when increasing the amount of die release material applied to the soldering area. Excessive amounts of this material may create other defects such as porosity, chill or blisters. When adjustments to the above conditions do not eliminate the soldering condition, it is likely that the problem may have been caused by the metal alloy being cast or it may result from the die construction.
b) Cracks:
Castings may crack from internal stress or from abnormal pressure during ejection. The first cause, internal stress is created from excessive metal or die temperatures. If the condition persists after several temperature adjustments have been made, it may be necessary to increase the shot or machine timer settings. The timer should be adjusted only after everything else has failed.
Cracks from abnormal ejection pressure may be indirectly the result of soldering. The operator should care fully inspect the casting for signs of soldering. If soldering exists, the appropriate corrective measure should be taken. Insufficient drafts or a rough cavity finish in the die can also cause abnormal ejection pressure. Additional die release material may help, but it will not correct this situation. In severe instances die repair may be necessary.
c) Broken part:
A portion of the casting may stick in the die during ejection and the rest of the casting break away and eject normally. The cause is the same conditions as described for cracks - excessive metal or die temperature, insufficient die release material or soldering.
d) Bent parts:
Castings may bend instead of breaking when part of the casting sticks in the die. This is a different result from the same conditions that cause a broken part.
e) Heat marks:
Have the appearance of surface pits and are caused by excessive die temperature or excessive metal temperature. Depressed areas in the castings and sharp inside comers are the most susceptible to this defect. Some times die temperature balance can be adjusted to eliminate heat marks. For example the
Flow of cooling water to the area affected may be increased and decreased to the die half in the same area. Clogged vents or excessive die release may cause large volumes of gas to become entrapped in the die.
Such gases will increase the size of heat marks and may cause the pits to become rounded and smooth in the inside.
f) Wave or lake:
Irregular lines or slight steps on otherwise smooth casting surfaces are usually caused by excessive die temperature in the area of the defect. Increasing the flow of die cooling water or increasing the water (or solvent) dilutions of the die release material will reduce such defects.
g) Sink marks:
Shallow smooth depressions on the casting surface are called sink marks. Such marks usually appear on the casting surface opposite any heavy section such as a rib or boss, and are caused by uneven shrinkage of the casting. Reduced die temperature in the area of the sink mark. Reduced metal temperature and some times increasing the temperature of the other die half will minimize these defects. Some times increase injection (shot) pressure coupled with higher die temperature between the defects and the gate will help reduce sink marks.
25.4 Miscellaneous defects
a) Scale:
Build - up of die release materials (or their oxides) on the die results in an irregular rough surface on the casting. The material must be removed with caustic solutions or by polishing the die. The same care is needed as described for soldering. After cleaning the die, the amount of release material applied for each shot must be reduced. Sometimes it may be desirable to change the type of release material and /or the ratio of the release material to solvent (or water.)
b) Blisters:
Bubble like bumps on the casting surface are caused by air or other gases trapped inside the casting. Slower shot speed, clean vents, reduced die temperature, or less die release material will usually eliminate blisters.
c) Porosity:
Large holes in the castings are called porosity. Low die temperatures (particularly in the runner and gate areas), low shot pressure, clogged vents, or excessive release material can cause porosity. Porosity is also often related to lack of fill, cold shut, heat marks, and blisters. When such a relationship exists the correction for the related defect will often improve or eliminate the porosity.
d) Excessive flash:
Excessive flash results from material such as flash sticking to the die faces and holding the die open, excessive injection pressure or speed, or insufficient clamping force. The first problem is corrected by cleaning the die faces. Flash that has become embedded into the die face must be scraped off corrections to the injection speed and pressure must be made by adjusting the appropriate hydraulic valves. The clamping force is increased by adjusting the tie bar nuts. Very slight adjustments to these nuts are usually all that is required. Flash indicates that an extra thick casting is being made. Extra thickness causes extra heat input to the die, and may result in additional problems.
e) Mechanical defects:
The operator must be aware of all moving and fragile parts of the die cavity, which are subject to wear, breakage or other failures that could cause defective castings. Small cores or thin blades of the die forming deep narrow holes or slots in the casting can be easily broken or bent. Ejector pins, and moving cores can wear, break or not seat properly. In any of this situation the die will not make the part of the correct shape.
f) Ejector pin marks:
High and low ejector pin marks: ejector pins may push into the casting when solder and / or a rough cavity surface result in the casting sticking in the cavity. High casting temperature at the time of ejection also can let the pins push into the part. Increased flow of cooling water or more liberal application of release material will sometimes reduce low ejector pin marks.
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