HPDC casting defects

Types of defects in HPDC (High Pressure Die casting)

In this paper we will discuss about aluminum Die casting Casting . Casting defects may be defined as characteristics which create a deficiency or imperfection to quality specification imposed by design and service requirement. Casting defects are caused by non optimized process, failure of material,casting equipment. So the defects can be tolerated and repaired. In aluminum die casting the molten aluminum is poured or injected into metallic die. There are many sources to cast defects and have many names. In this paper an attempt has been made to provide aluminum casting defects with their causes and remedies.

1. visual defect.

    a.  non filling

    b.  cold flows

    c.  Discolored

    d.  lamination

2. Porosity.

    a. shrink

    b. flow

    c. gas

3. leakage.
4. blisters
5. sinks
6. drags
7. soldering
8. bending. 
9. distortion
10. trim damage

VISUAL DEFECTS

This kind of defect occur because it is always a race between the time the molten metal arrives at a location in the die and the rapid solidification taking place.

if the metal is partially solidify when two flow come together ,they form wrinkle or laps and lamination that are characteristics of surface defect.

A. NON FILLING:- 

Non filling is the defect when metal not reached at that place due to some unfulfilled conditions, this defect can be occur due to technical parameter setting and die condition.
Some common factor that cause of non filling.
1.Flow pattern
2.Casting Shape
3.Die temp.
4.Metal Temp.
5.venting
                               
1. Flow Pattern: flow pattern of metal depend upon the factors
The geometry of the part; mostly the flow distance, the number of reflections before the end of flow, and whether the flow can directly reach critical areas.
       1.Filling Speed.
       2. Fill time
       3.Wall thickness of casting.
Filling speed :-  higher the filling speed higher will be good surface quality. A plunger speed variation that causes a fill time variation of 30% may not be significant with a wall thickness of  3.175 mm or greater, but a plunger speed variation of 10% can be extremely significant if the wall thickness is 2 mm or less.

Fill time :-   lower the fill time higher will be good surface quality.Fill time is the time start from when metal reached at gate to the end cavity fill and if overflow weight is much less than casting the time to fill overflow also considered. The important relationships between fill time, die temperature, metal temperature, and wall thickness can be approximated with the formula referred to earlier, and is the one used in the NADCA gating course. This is:

Where:

K = a constant
T = average casting wall thickness
Ti = metal injection temperature
Tf = metal flow temp (solidus)
Td = die temperature
S = percent solids at cavity full
Z = conversion for latent heat

Wall Thickness:- for thin wall speed of plunger (filling speed) require higher than the thick wall.
      For example if the wall thickness is fairly thick ( >3.175 mm) then fill time factor is much less important factor,because the metal has less restriction to flow in the cavity, but if the wall thickness is thin (<1.525 mm ) than fill time is by far a most important variable.The wall thickness becomes very critical when it is below about 2.28 mm in aluminum.The minimum wall thickness consideration will depend on this calculation,but it will be about 1.5 mm for aluminum.

2.Casting Shape: The geometry of the part; mostly the flow distance, the number of reflections before the end of flow, gate design, gate area, and whether the flow can directly reach critical areas. Turbulence can be avoid by gate design, it should be study in die design.
Part shape is a very important factor when troubleshooting surface defects and often can be the most important; but unfortunately, it is the most difficult to change.
Important factors in the shape are:
• Flow distance (distance as the metal has to travel from the gate to the
furthest point to fill)
• Complexity of the metal flow path (how many reflections are required
for the metal to reach it’s final destination)
• Blind fill areas (cores, fins, etc.)
• Shaded areas (areas that are directly behind an object that divides the
metal flow)
• Draft and radii allowed
• Allowable gate locations
• The shape also causes hot or cold spots in the die, which in turn affects
surface defects

3. Die temp. In aluminum Die casting die temp is a important factor that affect the surface quality of casting. 

   Alloy                             Good finishing                                                  Average finishing
                                   temp.(degree Celsius)                                         temp.(degree Celsius)

Aluminum                            240-310                                                               190-310
Zinc                                      230-290                                                               190-290
Magnesium                          230-290                                                                200-290

To maintain Die temp. following factors are to be standardized.
A. Die Spray.
B.Water/ Oil flow rate.
C.cycle Time.
           
4. Metal temp. Metal temp of alloy is also a great affect on cavity filling, a low temp i.e less than specify temp. cause of non filing. low temp of alloy does not able to fill cavity completely with less filling time, and the thin section of casting remains unfilled. High metal temp also cause of flash in the die which cause of biscuit variation and generate non filling in the casting.
The Best practical range in HPDC of aluminium alloy is 660-690 degree Celsius.
  
5. Venting and overflow. Overflow and chillvent are provided in a die for air removal of die cavity and to removal air impurities at end of shot. If the chillvent or overflow stuck in a die it block the Passage of air flow, means it block the metal to fill the cavity.
a stuck over flow in Die can be cause of breakout on the gate area of overflow, and also cause of porosity in the casting.