Eliminating Zinc Die Casting Defects

The Role of Alloys and Melting Processes in the Cause and Elimination of Zinc Die Casting Defects

John Titley, Director of The Brock Metal Company Ltd.

Zinc alloy pressure die castings are selected because the process-material combination will manufacture precise net shape parts which will accept a wide variety of finishes. Very high quality standards are achievable both consistently and cost effectively over large production quantities.

This, coupled with the range of physical and mechanical properties available and the highly productive manufacturing process, have made the Zinc alloy die cast part the first choice material for a huge range of mass produced components covering an extensive array of applications from electronics to locks, security and hardware for doors and windows, connectors and fittings for hydraulics and pneumatics to decorative parts for the automotive industry. All of these applications utilise more than one of the properties available from Zinc alloy pressure die-castings.

This diversity however, carries with it a penalty which the designer must consider when undertaking the manufacture of a new part.  The need for high quality decorative finishes will invariably mean that the finishing criteria will become more critical and this will affect the cost and the prospect of higher reject rates must be taken into account.

It therefore follows that the elimination of surface defects is a key requirement when manufacturing parts which require high quality surface finishes.  Even minor surface blemishes may cause rejection and can occur throughout the manufacturing process. Consequently the diecaster should consider any of the following as a possible cause of casting defects. In no particular order of importance:

  • Alloy chemistry:  the presence of excessive iron, lead, tin, cadmium, manganese and inter metallics/oxides are all possible sources of surface defects.
  • Metal temperature/metal handling:  machine pot temperature and temperature variations will change casting quality. Introduction of ingot or scrap to the machine furnace at the wrong time or in the wrong condition will affect casting quality.
  • Casting design: is a major controlling factor in the instigation of casting defects.  Section thickness changes, lack of fillet radii, surface textures and profiles may all promote surface defect problems if the casting design does not follow recognised design guidelines.
  • Die design:  attention to detail is critical, including the calculation of the runner and gate sizes, due account of machine performance, position of the gate relative to casting geometry, die temperature heating/cooling, use of over flow wells and venting.
  • Ejection: use of adequate ejection, size of ejector pins and their position, section thickness for ejection, removal from the die by robot, reduction of mechanical damage/impingements during ejection cycle or on conveyors etc.
  • Die lubrication:  control of quantity dispensed, type of lubrication, propensity for die surface deposits/coking/waxing.
  • Second operations: clipping/ break off, linishing and polishing, mass media finishing such as vibro polishing etc, the effects and requirements of these need to be considered at the casting/die design stage.

It is important for the diecaster to understand what defects can occur and to be able to identify the cause and the possible remedies available to resolve these issues. In order to implement some of these remedies the diecaster will need to have a thorough understanding of the machine performance and be conversant with the casting processes and cavity fill theories involved.

Die casting alloys:

The European die casting industry uses alloy ingots manufactured to the EN 1774 -1998 standard.  From its introduction in 1998 this ingot specification supercedes and renders all national specifications for countries within the European community obsolete. In most instances diecasters will be using either ZnAl400 (ZL3) or ZnAl410 (ZL5) which have similar casting characteristics and for the most part can be used in the same process and die.  Both alloys have 4% aluminium additions with 0.05% magnesium but ZL 5 has an additional 1% nominal copper addition offering slightly better mechanical properties than ZL 3.  However, there are other alloys listed in this EU norm which can be pressure die cast – some of which offer superior mechanical properties.

If we look at the metallurgical changes that can instigate surface defects in castings and what elements in the foundry may cause the metallurgy to change, it will become clear that this area is the least likely to generate serious casting problems or surface defects.

The surface finish of any zinc alloy pressure die casting is determined by several factors – die temperature, metal temperature at the end of the cavity fill, cavity fill time, cavity fill pattern and gate speed. Changes in the aluminium and magnesium content will influence some of these factors.  An aluminium content elow the minimum specified will reduce the fluidity of the alloy significantly, changing cavity conditions and increasing the fill time, both factors that have a major influence on the surface finish of cast parts. It is unlikely that the diecaster will increase the levels of aluminium in the alloy by accident.  In that rare event the mechanical properties of the alloys deteriorate markedly when aluminium content is 4.5% – 5.5%.  Improving again at around 8% aluminium, to allow the diecaster to use the more creep resistance ZnAl8Cu1 (ZL 8) in hot chamber pressure die cast machines.

It is important to note that magnesium is added to the alloy to improve hardness and because it controls the corrosion effects of certain impurities but excessive magnesium has only detrimental effects on surface finish, reducing fluidity and impeding cavity fill conditions. To overcome the problems associated with magnesium a low magnesium zinc alloy ZL 7 has been introduced. It is not included in the current EN standard but does offer the designer an alloy which is capable of being cast in thinner sections (down to 0.5mm) while retaining the high surface definition associated with the more traditional zinc alloys. (The effects of impurities in zinc alloys is covered in more detail in the ‘’Chemistry of Zinc foundry alloys’’ technical paper found on the Brock web site under Technical Resources.)

Iron is the most frequent contaminant arising within the die casting process. Most hot chamber die casting machines use cast iron or ferrous based melting/ holding furnaces. The Zinc alloy is in direct contact with the furnace and will attack the iron over time causing iron contamination in the alloy. The presence of aluminium in the alloy does prevent excessive iron pick up while close control of the melt temperature will prevent excessive erosion of the ferrous crucible.  However, if overheating does occur, the attack rate will increase dramatically at pot temperatures above 4500 C.

Excessive iron contamination depletes the aluminium content of the alloy exacerbating the iron pick up and causing the formation of intermetallic particles in the alloy. These are primarily AlFe3 or AlFe5 particles. In severe cases, the intermetallic will show after polishing as star shaped impressions on the polished surface. The intermetallic particle is diamond hard and this deflects the polishing mop forming a star of polishing ripples around the hard particle. The presence of this problem is often referred to as hard spots or inclusions and these are virtually impossible to remove by linishing or polishing.

It should also be noted that this phenomena will show through after plating or painting often giving a sharp grit like feature on the surface of the casting. Care should be exercised when remelting castings with this defect, after melting it is advisable to the remove the heat source and allow the melt to stand and acquiesce before removal of the dross and oxide on the surface. Continued application of heat moves the metal within the crucible causing the particles to circulate in the heat generated current and enlarges the intermetallic particles. The ‘fir tree’ like configuration allowing them to lock together with iron and aluminium in the melt, further depleting the aluminium and increasing the attack rate on the iron crucible.

Very small levels of some elements have a severe effect on casting quality and may affect the long term performance of the alloys.  Lead, tin and cadmium are typical of these; being controlled down to a maximum of 50 ppm or less in zinc alloys. The grain structure of a zinc alloy allows these elements to sit at the grain boundaries. If one or more of these elements is present above the maximum specified in the standard then certain conditions (warm with high humidity) will promote inter granular corrosion within the casting.  This is engendered by the formation of a minute galvanic cell stimulating the zinc to sacrifice itself at the grain boundaries. The effects in severe cases of this can be catastrophic evidenced by the total collapse of the casting structure – effectively converting the casting to oxide dust.

In less severe cases corrosion stress cracks are formed promoting premature part failure in service. The presence of these impurities is often indicated by hot shortness cracks during casting. These cracks can also be caused by other process based problems so it is always advisable to investigate all potential causes before taking action. In all instances, control of metal chemistry by use of a reputable zinc alloy supplier and implementing regular machine pot analysis procedures with good house keeping will limit the risk. The diecaster should also ensure that foundry personnel are adequately trained and understand the risks associated with the presence of certain impurities.

Melting practice:

Application of good house keeping and melt procedures will virtually eliminate any casting surface defects that can be attributed to the alloy chemistry. The zinc alloy pressure die casting process performs at the optimum when all process features are consistent. This particularly applies to melt temperatures. Low melt temperatures, large variations in melt temperatures and low metal levels in the machine pots are all potential sources of casting defects which should be eradicated by process control procedures. The diecaster should attempt to maintain a minimum pot temperature of 4150C when using ZL3 or 5 (ZL 2 will need 4350C).

To avoid excessive variations the diecaster should use the appropriate furnace technology, avoid charging ingots directly into the machine melt and ensure that the maximum level of metal is always present when making castings. This latter point prevents dross entry into the goose neck and helps maintain temperature stability.

The diecaster should endeavour to maintain a stable level of alloy in the machine furnace crucible. Low levels allow the pot temperature to increase and necessitate large additions of possibly cooler material which obviously affect the melt temperature adversely. This set of circumstances extends the time required to recover the optimum working conditions and promotes instability and inconsistency in the process. The use of two cell machine pots does eliminate this problem completely but the diecaster should always be aware of the problems associated with machine furnace operation.

Where single furnace crucibles are in use, the diecaster should avoid feeding the scrap, sprues and runners directly back into the machine furnace melt. The addition of hot sprues may appear attractive from remelt energy savings but there is a downside in terms of casting quality. Sprues and runners may have a surface residue of die lubricant which will increase dross levels and cause some fume. More importantly, adding sprues to the machine pot will, by virtue of the surface area to alloy volume ratio, increase the level of oxide present in the melt. This will increase dross on the melt as the sprues are melted.

Some of the oxide released by the melted sprues will become entrained in the melt causing casting defects and increasing the risk of casting failure/fracture etc. This entrained oxide will also show on the surface of parts as minute oxide folds – these are impossible to remove by polishing and will show through a plated surface. The interface between these folds and the cast alloy structure may absorb plating solutions during the plating process. These are subsequently released after plating, breaking down the adhesion of the copper substrate and causing the plated surface to blister and lift off. Peeling off these blisters will often show a clean zinc casting surface with the under side of the blister showing a copper surface.

Poor Cavity Flow – Magnified 20xPhotograph 1 Poor Cavity Flow – Magnified 20x

Zinc alloy casting surface showing typical effects of cold flow and poor cavity fill at the edge of a casting, magnified at 20 x 1.

This defect will may be caused by one or several of the following (from most to least likely).

  • Die temperature too low – causing the alloy to partially freeze prior to cavity filling

Recommended actions:

Reduce die cooling, increase die heater cooling range, optimize die temperature at 160 – 1800 C. Reduce die cycle time if casting is thin wall. Remember this is a common fault at die start up!

  • Metal temperature too low – causing premature freeze of the alloy prior to filling

Recommended actions:

Increase metal temperature in the machine pot to maximum 4250 C (ZL3 and 5) advisable to check thermo couple is measuring accurately before increasing temperature of metal.

  • Application of excessive die lubricant which has chilled the die face

Recommended actions:

Reduce delivered amount of die spray, check that ratio of water to oil is correct, change delivery to minimal spray mist application

  • Cavity fill time too long – may be the result of low plunger injection speed, insufficient gate, runner or nozzle area reducing delivery of metal to the cavity

Recommended actions:

Check machine plunger injection speeds to ensure that 1st and 2nd phase are set correctly, increase plunger speed on second phase. Finally, check that die runner and gate calculations are correct, check casting weight against estimate, verify that die maker has cut runner to the size specified in calculations. Modify runner only as last option!

  • Machine plunger too small to deliver adequate metal to fill the part.

Recommended actions:

Increase plunger size or machine size if the plunger is at maximum available. Increasing the injection speed may not improve the situation if the plunger is too small.

  • Casting weight or flow distance within the cavity was underestimated and is extending the cavity fill time.

Recommended actions:

Increase gate area to accommodate, remembering to ensure gate area does not exceed runner areas.  May necessitate change in plunger speed.

Photograph 2 Blistering – Magnified 5x

Blistering - Magnified 5x

Surface of Black powder coated automotive part showing a blister (circled) due to entrapped gas under the surface of the casting.  (Damage to the surface on right of circled area is mechanical damage to the powder paint before curing was complete).

Blistering - Magnified 20xPhotograph 3 Blistering – Magnified 20x

The picture shows a section through the blister. The cavity under the surface of the casting is clearly visible.  The thickness of the zinc layer directly beneath the paint is approximately 0.45 mm. The areas circled show the separation cracks between the inner surface of the laminated blister and the basic casting substrate.

Blisters can originate for many reasons, for instance in the curing of lacquer or paint, and it is a misconception that this problem occurs only when parts are subjected to post casting heat. Although it must be stressed that heat applied to castings does exaggerate the number or severity of the problem.

Blisters may be caused by one or more of the following (from most to least likely)

  • Too much die lubrication applied to a moderately hot die

Recommendations: Reduce die spray to the minimum required for consistent die operation. Check the die for areas where the part may be sticking, lubrication traps, and cold spots in the cavity. Remember die lubrication which is not volatilised or evaporated will form pockets of fume within the casting which is the most common form of large isolated blisters as illustrated in the photograph. Blisters of this type often appear in the same place on the casting demonstrating that the fume is being trapped by the cavity fill pattern. As shown by the photograph it is possible that the rest of the casting is relatively porosity free.

  • Air trapped in the metal caused by the cavity fill pattern and /or casting geometry

Observations;  Entrapped air pockets are normally spherical in appearance and will often appear in the same place consecutively if related to cavity fill pattern or casting geometry. Once a casting fill pattern has been determined by the gate /runner size and position; each shot will give the same pattern unless a dramatic change is made to the die or casting geometry. Most common causes are the differences in casting sections being too large. Blisters show in the thin sections (on inside and outside of casting), isolated blind bosses in thin sections, oval or round parts filled using taper tangential runners (fill the outer edge before the centre effectively sealing the cavity), box shape parts filled from on edge using taper tangential runners (fill around the edge on the part line –seal cavity), badly designed runner gate fills part but by passes one area sealing air in one section of the part.

  • Gate speed too slow (less than 40 M/sec.).

Recommendations: Check that machine settings agree with those used to establish runner and gate size.  Check also plunger and nozzle size for same reason. It may seem irrational, but ensure machine used is that specified in gate runner calculations. Reduce the first phase injection speed, increase second phase injection speed – change ratio of 1st to 2nd phase to extend slow filling stage. Increase freeze/stress time to ensure optimum packing is achieved. Look for venting opportunities remembering comments related to edge fill / cavity sealing risks. Change fill pattern by either extending gate length, thickening gate, adding over flow pockets before metal flow reaches areas that isolated blisters occur.  The extra volume will change metal flow rate and direction, moving the trapped air. Care should be taken to avoid introducing more air into the casting from the over flow well.

  • Paint curing temperature too high or finishing contractor’s process is out of control limits or varies due to intermittent running, operator error or faulty control equipment etc.

Recommendations: Ensure paint specified by the end user meets temperature range suitable for Zinc alloys suggest 2100 C maximum but 1800 C is preferable. Remember that at these temperatures Zinc is within its creep boundaries and will be soft enough to allow the thin lamination to distort forming a blister – see preceding photograph of a formed blister. It should also be noted that blisters form on “as cast faces” of some castings illustrating what can be expected after finishing is completed. Quenching will eliminate this if no heat treatment is expected during the subsequent manufacturing process.

Whole books have been written about casting design for plating and the effects that casting geometry has on the finishing and finish of painted and plated castings. There are well established and documented guidelines which will assist the diecaster to avoid the obvious pit falls. Some of these, though well documented, are ignored, sometimes through lack of knowledge but more often because the aesthetic consideration and/or the mechanical function parameters of the part override good design practice.

Among these are the following:

  • Failure to use adequate fillet radii and soft external edges:  Internal and external radii improve flow during cavity fill, reduce turbulence, improve polishing and mass media finishing effectiveness. They also  increase tool life and reduce die maintenance, increase casting strength by removing notches and stress raisers and reduce the effect of dramatic section changes.
  • Failure to control section changes, and adopt the accepted guidance:  Use the three circle rule to control ‘section to section’ changes. Reduce the thinnest section to minimum 1.5 mm and adjoining sections should be limited to 4.5mm max, increasing progressively to the desired or necessary section.
  • Failure to adopt curved surfaces and other design aids which disguise plating and polishing blemishes: Large flat surfaces exaggerate the edge build up associated with plating and powder coating. Minute curvature is easier to polish, improves material flow during cavity fill, reducing turbulence and flow defects.
  • Deep blind pockets or holes: Plating will not ‘throw’ into these areas. They also tend to block with polishing debris and plating solutions, and invariably need cleaning out after finishing. As a casting feature, holes and slots are useful for plating jigging points. Bear in mind that jigging leaves scars and these scars are sources of corrosion defects as well as being unsightly.
  • Gate scars and part line defects: These areas are a source of porosity and casting defects.  Clipping and break off, machining, polishing often reveal porosity and fracture defects at the gate scar or along the part line. These defects are process generated or caused by temperature imbalance at the casting stage. The part designer should specify ‘show’ faces on parts which should not be used for gating or positioning of over flow wells. Defects will occur at these points even with good die design and therefore it is wise to control where these problems are acceptable or cause the least inconvenience.
  • Vertical part line changes: The casting designer should always specify the maximum draft allowable to prevent the casting from sticking in the die. Generous draft promotes easy ejection, low die face wear and fast efficient die cycling and should be adopted as a first principle. This policy should also be implemented across the part line where zero draft has more additional severe consequences. Normally, zinc die castings need a minimum draft angle of 10/side but this is insufficient if used on die face part lines particularly if these are greater than a few millimetres. It is advisable to use angles in excess of 50 on die part line features. This is a casting defect issue and affects die maintenance and, in the longer term, die life. In short, vertical faces formed in the line of draw cannot be adjusted for wear. When wear occurs it will allow flash to develop at the part line which cannot be removed by clipping, necessitating expensive, time consuming second operation work. Using increased draft allows the die to be adjusted by ’bedding in’. This is a simple, standard procedure which will eradicate the flash and extend the die life without expensive die face welding and re-machining.
  • Paint curing temperatures: Specify finishes with low cure temperatures to diminish the risk of blisters, cissing and exaggeration of minute casting imperfections. High gloss paints will also amplify the evidence of minute surface defects and the use of matt or semi matt finishes will assist the diecaster. In any event, make the surface finish criteria very clear at the enquiry stage to avoid expensive surprises at later stages in the project.

Reference to zinc alloy casting guidelines is both worthwhile and cost effective if the end user is to get the best from the die casting process and the benefits offered by the range of zinc alloys available.

Good quality die castings are manufactured from good quality dies. There are no short cuts to achieving this objective and maintaining the desired quality long term. Having established the part design required, the design of the die is critical to ensure casting quality, cost effectiveness and long term manufacturing viability/stability. The number of impressions, die cycle times and minimal scrap all feature in the die design process and will be influenced by the decisions made at the die design stage. It is important to establish the quantity of parts required per annum or batch as this will affect machine size and manufacturing costs will be used to establish the die design.

As mentioned previously, casting quality criteria, finish, accuracy etc will influence the design of the die. It is possible for the die designer to calculate the feed system and gate size required to achieve the finish, cycle time, size of machine required and to predict the flow path of the molten material in the cavity. This can be achieved at the quotation stage and before the final casting and tool design is formulated.

The size of ejector pins and position of fixed and sliding cores will be critical to the performance of the die and these will be influenced by the casting profile, section thicknesses, aesthetic features and limitations. Positioning ejectors pins on areas of the casting thick enough to support the ejection force when the casting alloy is still hot and ductile; will reduce surface bumps where the ejector distorts the casting surface. Placing ejectors on bosses within the casting is used to avoid this problem; as is the use of vertically gated ejector bosses outside the casting edge. However, these measures add additional weight to the casting and or shot which may not be effective where casting cost is a prime objective. It may also be necessary, where zero draft cores are employed, to introduce sleeve ejectors which apply force directly around a problem core or casting feature.

The designer should introduce die features which allow adjustments of die faces to remove flash such as wear pads and over flow wells to change fill patterns and remove some trapped air/fume in the cavity, or include vacuum technology if casting performance requires minimal levels of porosity. The use of robotics for automation and efficiency will have design implications which may lower the ‘casting to shot weight’ yield ratio. This is a critical part of the costing process that may be offset by the process automation benefits if applicable.

The surface finish of any zinc alloy die casting is reliant on die surface temperature which is controlled by the use of water or oil pipes designed into the die. The size and position of these is determined at the die design stage and will be related to the weight/volume of the castings, number of cavities, the cycle time and finish required. This latter point is also influenced by the cavity fill time and gate velocity which can be correlated to the die casting machine performance; optimising the size of the gate and runner for efficient running and yield.

Die face temperature is one of the process parameters requiring consistency and control. To achieve the defect free matt finish required for electro plating with minimal polishing, the die temperature needs to be controlled at an optimum 1800 C. Higher temperatures may be required if longer flow distances are present in the cavity or where the casting has thin sections less than 1.25 mm. To achieve the necessary surface finish in these instances will require good die and metal injection temperature control, and a cavity fill time of less than 20 milliseconds. These three parameters should give an end of fill metal temperature greater than 4050 C in the cavity and prevent the formation of oxide surface defects and cold shuts within the casting.

The elimination of this type of defect will also require a smooth cavity fill pattern designed to reduce oxide generated by turbulence within the die cavity. To maintain these conditions the diecaster will need to create consistent process conditions, cycle times and interruption free running – this can only be achieved if the design criteria covered earlier in this paper is put in place.

Realisation of these aims, once the design criteria have been implemented, relies on process control, one part of which is die lubrication. In zinc alloy die casting, die spray as it is sometimes referred to, is a lubricant not a coolant. It is normally water based soluble oil for health and safety and environmental reasons and is sprayed in mist form onto the die faces in sufficient volume to allow the die temperature to be maintained while evaporating the water content of the spray.

Under ideal conditions, evaporation of the water leaves the die face covered in a microscopic coating of die lubricating oil. Control of the volume of spray and use of the correct type of lubricant can be critical to the maintenance of good casting surface finish.

Surface defects can arise from oil deposits burning onto the die cavity face forming fine coking which shows on the casting surface. This is usually the result of exceeding the die temperature guidelines (over 2000 C), oil rich die spray, incorrect selection of lubricant, or excessive application of die lubricant. It must be stressed that the application of die lubricant to a die which is too cold (less than 1600C), will result in severe cold flow defects, excessive porosity from fume and steam in the casting and interrupted cycling which further exacerbates the problem.

Post Casting Defects

At the outset it was mentioned that the Zinc alloy pressure die casting process produces parts of precise finish and accuracy which will accept a wide variety of high quality finishes. The process however, stops at the production of the casting and there are several stages to consider after the casting is complete. Most of these stages involve post casting operations which identify, create and/or generate reject defective parts after finishing.

These are invariably known as second operations and this term covers clipping, de-gating, linishing and polishing, vibro or mass media finishing, machining and plating/painting etc or other forms of finishing.  Rejects can arise from each of these operations but from a finishing point of view some forms of reject can be ignored for example machining scrap due to dimensional inaccuracy.

The movement of parts, ie. the act of pick and place, is responsible for the same number of rejects or surface defective parts as the casting process. Multiple handling elsewhere means that impingement damage is common unless care is taken when parts are transferred from die to storage and transit container, from locations for machining and clipping in jigs and tools, transit packaging before/after plating and assembly fixtures and tools. Often the level of polishing or mass media finishing is insufficient to remove the edge cuts, nicks and impingements introduced by handling after the casting leaves die.

After the casting cycle is complete the part is ejected and, in some cases, this involves dropping the total shot (casting plus runner) onto a conveyor or slide tray and then into a stillage or small steel or plastic storage pan.  During this free fall process many minor impacts with steel edges and other castings can occur – causing impingement or nicks on susceptible faces or edges. The die designer should consider this factor when positioning the casting in the die, particularly at the die open stage.  Ejector and face pins, core slide edges on both die sides need to be accommodated when the shot falls forward or off the ejection. The same discipline should also apply during the clipping stage.

The use of robot/pick and place units does reduce the risk of impingement damage in many instances assuming that the same precautions are put in place to prevent it.

Brock Metal offers a free design advisory service for customers and will help the customer to resolve casting problems as part of its technical support to the industry.

Photographs 1, 2 and 3 Cracking – Magnification 50:1

Cracking – Magnification 50:1 Cracking – Magnification 50:1 Cracking – Magnification 50:1

The picture shows a section through a 900 corner.  There is evidence of the first stages of a hot tear but the casting exhibits minimal porosity in the enlarged etched section below. This casting failed consistently when subjected to long term fatigue tests. This fracture is typical of the minute cracks which form when fillet radii are omitted and micro cracks are a common cause of in service failure because their size makes them undetectable under normal visual examinations.

It is interesting to note that the fracture is following grain boundaries in the structure, application of stress will propagate or develop this crack following the established trend.

Alternatively, the photograph below shows a fracture instigated by a cold lap formed at a corner with no fillet radii.  This fracture developed at 10% of the expected test life cycle and prompted premature failure of the part at less than 20% of the anticipated life cycle.  It should be noticed that the path of the fracture has been influenced by the presence of minute porosity adjacent to the cold flow defect. (Circled.)

Gas PorosityPhotograph 4 Gas Porosity

In this example there are numerous large pores evident which are approximately 1mm in diameter. This sample is included to show a case of excessive gas porosity. At these levels of porosity the mechanical properties of the alloy will be significantly reduced and it will be impossible to polish or apply a surface finish which involves heat or plating solutions. The surface of the part was relatively defect free giving no indication of the level of porosity present in the casting. Routine weight checks highlighted this problem; allowing corrective action before any castings were dispatched. The part weighed less than 80% of the normal calibrated weight for the casting.

Micro PorosityPhotograph 5 Micro Porosity

Enlarged view of micro porosity – directly under the CuNiCr plated surface.  This porosity was present on the majority of the outer (fixed die half) casting surface, forming a permeable, absorbent surface which retain small deposits of the plating solutions. It proved impossible to establish any permanent adhesion of the copper to the Zinc alloy substrate. The problem was wide spread across several thousand parts and was traced to incorrect die lubrication additions.

ImpingementPhotograph 6 Impingement

This shows an enlarged view of an impingement defect on the surface of a casting. The copper and nickel layers of the plated surface can be clearly identified and these have partially filled the indentation.  It is clear that there are defects present in the copper layer at the root of the ‘nick’.  It should be noted that although the copper and nickel have penetrated the fault there is still visible evidence of the defect on the surface after plating.  There were numerous examples of this defect on one face of this particular casting: the photograph shows a section through an isolated example.

Photograph 7 Sub Surface Porosity on Fused Powder Paint Finishes

Sub Surface Porosity on Fused Powder Paint Finishes Sub Surface Porosity on Fused Powder Paint Finishes

Illustrating the effects of minute sub-surface porosity on fused powder paint finishes. This common defect known colloquially as ‘cissing’ indicates the presence of minor gas, air or steam pockets below the surface of the casting. When the surface of the casting is heated to fuse the powder the air etc is forced through the surface of the casting and the soft paint, leaving a small volcano like mound on the surface as seen above.  The photograph shows defects which were between 0.5 and 2.0 mm in diameter. Removal of a mound by light abrasion of the surface normally reveals a ring of zinc substrate with fused paint inside the ring itself.  Very fine hair like cissing sometimes occurs when minute oxide or cold flow is present on the surface of the casting. The cold flow defects will absorb water from cleaning solutions which result in cissing when the casting is heated during the curing part of the paint process. It is very important to control curing temperatures in this process and also to ensure that the paint specified has the lowest cure temperature available to achieve the finish, wear and corrosion resistance specified.

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