Improvement in Casting Quality & Productivity with Copper Titanium Plunger Tips

A. Introduction: With ever growing demand of aluminium die casting parts and growing competitiveness in the market, Aluminium Die Casters are constantly looking to optimize manufacturing operations and reduce costs to enhance their competitiveness and improve profitability. The biggest area of concern in the foundry industry is the rejection rate, which is impacted by any variation in one of the several process parameters that contribute to the casting process. It is estimated that most aluminium foundries across the world have rejection rates between 3% to 7% and sometimes even more. This directly affects the profitability. Amongst several other factors for high rejections rates, a major factor is poor plunger life and premature failure.

B. Reasons for Casting Rejections The Cause-and-Effect Diagram (Fig1) shows major reasons of porosity in a high pressure die casting part. There are 13 major causes that have been identified, which result in rejection of castings. Let us examine three major reasons for our discussion purpose since they are related to cooling.

1. Uneven Cooling The thermal conductivity of the die material i.e. tool steel is very poor and therefore it has limitation on the amount of heat it can extract from the molten aluminium and transfer it to the cooling water or air, that is in contact with the die external surfaces. In most castings, because of the intricate geometry of the part, the mass of the injected aluminium varies greatly across the casting. This leads to a widely varying cooling rate of the aluminium at thin and thick sections. The thinner sections will cool faster than thicker sections where bulk quantity of aluminium is injected. This leads to un-even cooling, resulting in shrinkages or porosity and thus a rejected casting.

2. Localized Hotspot The die cooling at locations like core-pins or inserts where surface area to volume ratio is low, is very poor because of the inability of such sections to extract sufficient heat. Such locations become the hottest part of the die. This allows molten metal to be in liquid stage longer and delay solidification leading to porosity or un-filled rejection.

3. Wear and Tear of Plunger Plunger is one of the most important part of the HPDC Machine and sometimes the most neglected part of the Die-Casting Machine. A large number of die-casting manufacturers use Cast Iron Plungers that are available locally in India at extremely low cost. Most Die-Casters do not spend much time studying the plunger dynamics and its contribution to productivity and rejections. As such very limited attention is paid to the cooling and other aspects of the plunger performance.

C. PLUNGER – Shot Sleeve Interaction The interaction between the plunger tip and shot sleeve is the most critical in the die casting process. It can be only effective if both components are operating at a balanced level of efficiency. The challenge is to maintain the least amount of gap without interference between the shot-sleeve and plunger even at elevated temperatures. This is important because the material of plunger tips and that of shot sleeve are different and therefore have different co-efficient of thermal expansion. The maintenance of the gap is also important to ensure minimal but optimum use of lubrication. Practical experience suggests that if gap between the plunger and shot sleeve increases beyond 0.1 mm (100 microns) during the casting process, then there will be back flash resulting in a rejected casting and a quickly worn-out plunger. Therefore it is important to maintain a gap of between 80 to 100 microns between the plunger OD and shot sleeve ID, during the casting process. It also therefore important that the plunger and shot sleeve temperature are maintained within a narrowband and control the expansion of the both plunger and the sleeve.

The following properties are critical for a Plunger Shot Sleeve interaction for an effective working:
  • Plunger Material
  • Enough Cooling inside plunger.
  • Alignment of Plunger and Shot Sleeve
  • Effective Lubrication
  • Clean Shot Sleeve
  • Shot Sleeve – Plunger Tip Clearance.
D. Plunger Materials/Solutions currently available

1. CAST IRON As mentioned above, majority of Die-Casters in India, use Cast Iron plungers because of its low initial cost. However, Cast Iron is a very poor thermal conductor and has low hardness. CI plungers wear out very quickly in about 800 ~ 1500 shots. In HPDC machines of 1000 MT and above, the plunger life has been found to be even less than 800, average being 500 shots.

WHY CAST IRON PLUNGERS FAIL?

Key to effective Plunger Shot Sleeve interaction is to maintain an “EFFECTIVE GAP” between Plunger and Shot sleeve, so that the plunger moves seamlessly with minimum friction. Cast Iron Plunger co-efficient of thermal expansion is very similar to Tool Steel Shot Sleeve. However, the Plunger is subjected to more heat and will expand more that the shot sleeve, leading to excessive wear and pre-mature failure. The thermal conductivity of cast iron is also poor which results in the plunger getting heated after a few cycles and failing due to reduced hardness at elevated temperatures.

2. COPPER BERYLLIUM

Some Die-Casters use Solid Copper Beryllium tips, which have a simple design like cast iron. Copper Beryllium co-efficient of thermal expansion is about 50% more than Cast Iron, therefore it is important to modify the tolerances on the plunger OD, to control the effective gap between the plunger and shot sleeve at elevated temperatures.

CuBe has reasonably good hardness and much better thermal conductivity and thus enhanced heat transfer properties. It transfers heat to the cooling water quite efficiently, and hence the temperature of the plunger can be maintained in a narrow band, thereby maintaining “Effective Gap” between the plunger and shot sleeve. This significantly reduces wear due to friction, and the plungers last longer.

However, Copper Beryllium is expensive and very hazardous to handle, due to the presence of Beryllium in these tips. Globally, users of Copper Beryllium have become aware of the serious health hazards that Beryllium poses to the users and therefore many countries have severely restricted use of Copper Beryllium or effectively banned its manufacture and use due to the serious health hazard they pose to the users.

Disposing of used Copper Beryllium plungers has to be done with care. It causes serious environment pollution, as a used or corroded Copper Beryllium Plunger is 135 times more toxic ! than a new unused plunger.

CBD Diseases

The disease caused by exposure to Copper Beryllium is known as Chronic Beryllium Disease (CBD). This primarily affects the lungs of humans and results in difficulty in breathing and severe impairment of the pulmonary function. CBD may occur among people who are exposed to the dust or fumes from beryllium metal, metal oxides, alloys, ceramics or salts. CBD usually has a very slow onset, and even very small amounts of exposure to beryllium can cause the disease in some people. Some latest research suggests that skin can be a route of exposure and sensitization in chronic beryllium disease. As stated above, corroded Copper Beryllium could be as much as 135 times more toxic and harmful than Non-Corroded Copper Beryllium.

3. HYBRID

To overcome the challenges posed by the cost and hazards of Copper Beryllium, some Die-Casters use hybrid tip assemblies of a complex design with steel and copper beryllium alloy parts. These tips are very expensive and difficult to use & maintain.

Hybrid Plungers are complex integration of Steel Body and Copper Beryllium rings or sleeves. Some designs even have rings of steel in the front portion of the plunger. The goal of such designs is apparently to find the optimum solution of hardness, lower cost and higher thermal conductivity. However, most of the designs available in the market, have not achieved this goal completely. High performing plungers tend to have a higher Copper Beryllium content and therefore higher cost and risk. Those having lower CuBe tend to give a relatively poor life and therefore beat the goal. In additional special tools or fixtures are required to clamp and unclamp rings. Hybrid Plungers cannot be machined down to lower diameter, which is a common practice with solid plungers, thereby limiting its use to only one machine/size.

E. Copper Titanium Plunger Tips Copper Titanium is the strongest / hardest known Copper Alloy after Copper Beryllium. It has good thermal conductivity, strength and hardness and a perfect material for Plunger Tips. Solid Copper Titanium plunger tips offer exceptional performance and eliminate all the dis-advantages of solid Copper Beryllium or Hybrid Plunger Tips. Copper Titanium is completely safe material at all stages of its manufacture, use and end-of-life disposal. To further enhance the life of plungers we have adopted a process of PVD coating on our Copper Titanium Plungers which increases the surface hardness of the plunger without compromising its thermal conductivity, and hence significantly increasing the life of Plungers.

Copper Titanium (QTi®) Plunger Tips, eliminate all the above stated disadvantages of (hazardous) Cu-Be or (low life) Cast Iron. QTi® Plunger Tips deliver 25,000 to 40,000 shots, depending on the quality of interacting shot sleeve and machine alignment, and offer a superior and stronger material which non-hazardous.

Thus Copper Titanium alloy is a breakthrough innovation, which has been long awaited by the industry in their bid to improve productivity, reduce rejections and enhance profitability of HPDC.

F. Copper Titanium Core Pins/Inserts To balance the heat transfer in various parts of the die and improve solidification patterns, many Die-Casters use pure copper pins, inserts and Die Parts to provide for extra cooling to reduce rejection and porosity. While this succeeds in achieving the fast cooling objective, the Copper pins/inserts wear out very quickly, as copper is a soft material. As these pins/inserts wear out, the castings start getting rejected due to wear or scoring marks on the casting.

Core-Pins-inserts

Copper Titanium has a unique advantage of high hardness (300 BHN) and high conductivity making it ideal material for inserts and Core Pins. The life of these pins can increased further by coating them with an appropriate PVD coating.