A short introduction to wax pattern production and troubleshooting
The goal of the investment casting foundry is to produce near net shape castings with minimal or no scrap and to achieve this aim strict quality and process control is required at each stage including wax pattern production and assembly. It falls to the wax manufacturer to provide highly repeatable materials to eliminate variation in this aspect allowing the foundry to focus on control of their process and producing high quality wax patterns and assemblies.
Process control and the overall quality of wax pattern production and assembly is a crucial step in the investment casting process as any defects at this early stage may either result in labour intensive rework or worse still, defects in the finished casting leading to costly scrap. The correct wax and process control is essential to avoid these issues.
Wax temperature control
This is possibly the single most important aspect to consider. Wax has a very poor thermal conductivity with any temperature change taking hours to achieve. As a rule of thumb allow 15 minutes per degree Celsius for a controlled increase or decrease in wax temperature. It is important that the liquid level in the holding tank does not fall too low and that the wax in the holding tank is fully conditioned and temperature controlled before injection. Temperature gauges give an indicative reading only and often do not reflect the wax temperature in the centre of the tank. Die temperature will change with use and this may give rise to changes in wax injection characteristics unless controlled.
Wax injection machine control
Key factors to consider here include wax tank temperature, wax injection cylinder temperature, wax hose temperature, injection nozzle temperature, platten temperature, injection flow, injection pressure, clamp force.
Advice on wax storage
Wax materials will age prematurely with sunlight, if there is no option but to store outside ensure they are covered, when stored inside ensure they are not located directly below a sky light. Wax pellets stored in cold conditions, for example just above freezing should not be affected unduly if in sealed bags. Remember wax takes a long time to rewarm. Pellets stored at elevated temperatures above 30 °C may soften and stick together so this should be avoided. Care should be taken to ensure that wax products are not in direct contact with solvents. Wax materials have a shelf life of typically 2 years under good storage conditions. With materials such as water soluble wax it is important to ensure they are protected from moisture in the atmosphere. Injected wax patterns are a very different story, if they subjected to cold environments then the wax may embrittle particularly as it contracts around inserts such as cores and this may lead to cracks. The ideal storage temperature for wax patterns and assemblies is 20 to 22 °C
Tips to minimise wax injection defects
Flow line defects appear as a negative surface indication often seen as a line from above. They are generally caused by a lack of energy within the wax and can cause weakening of the wax structure and be a source of ceramic inclusions on shelling. Flow lines are usually associated with
- Cold die
- Cold wax
- Incorrect injection pressure
- Incorrect flow rate settings
- Injecting a thick section through a thin section
Trapped air defects will either be evident on the wax surface or just below the surface. The cause is generally due to any air in the wax being unable to escape during injection and this can be a possible source of ceramic inclusions on shelling. Trapped air is usually associated with
- Wax too hot – causing turbulence during injection
- Flow rate too high – with wax flowing into the die faster than air escaping through joints or vents so becoming trapped
- Air entrapped in the wax in the injection machine – causing air bubbles to be injected with the wax
Surface pitting defects will leave a rough surface finish on the wax patterns which visually has an orange peel like appearance. The cause is generally due to lack of wax fluidity and can be a cause of ceramic inclusions on shelling. Surface pitting is usually associated with
- Die too cold
- Wax too cold
- Insufficient injection pressure
Cavitation defects also known as sink appear as a surface depression normally in the form of a bowl like shape. The cause is generally due to incorrect temperature or lack of feed to the area in question and results in dimensional issues. Cavitation is usually associated with
- Die temperature too high
- Wax temperature too high
- Insufficient injection pressure
- Insufficient Injection hold time
- Injection sprue too small
- Wax chill required on thick sections
- Injecting a thick section through a thin section
- Consider use of a filled wax if not already in place
Non fill defects also known as misrun or cold shut occurs when the cavity either does not fill completely or the pattern has rounded edges. The cause is mostly due to insufficient energy to fill the die cavity before the incoming wax feed freezes and results in incorrect wax pattern dimensions or definition. Non fill is usually associated with
- Cold wax
- Cold die
- Insufficient injection pressure
- Insufficient injection cycle time
- Wax flow restriction in the die
- Consider using a wax with increased fluidity
Summary
In summary the key process parameters that can be considered for most wax injection defects and fine tuning the process are
- Injection flow
- Injection pressure
- Injection hold time
- Die temperature
- Wax temperature to be considered as the final option