Investment casting uses five main types of wax: pattern wax (filled and unfilled), runner wax, assembly wax, and water-soluble wax. These waxes aren’t actually pure wax anymore. They’re complex blends of natural and synthetic materials engineered to meet precise manufacturing requirements.
Modern investment casting waxes combine multiple ingredients to achieve specific melting points, flow characteristics, and dimensional stability. Here’s what goes into these specialized materials:
Most pattern waxes blend paraffin and microcrystalline waxes with polymers and resins. This combination flows well at injection temperatures (70-90°C) while staying solid during normal handling.
Unfilled pattern wax contains no solid fillers – it’s a pure blend of waxes, resins, and polymers that melts completely when heated. This virgin wax excels at capturing intricate details and producing smooth surface finishes.
The wax flows easily into complex dies at low injection pressures, filling even the tiniest features. Its low viscosity when melted means it needs minimal pressure to inject, reducing wear on equipment.
Unfilled wax melts out completely during dewaxing, leaving zero residue. You can reclaim and reuse nearly 100% of the material, cutting costs significantly.
However, unfilled wax shrinks considerably as it cools. Thick sections often develop internal voids (cavitation) that require metal chills in the die to prevent.
Large or thin-walled patterns may warp or distort because unfilled wax lacks structural rigidity. Its thermal expansion coefficient of about 50×10⁻⁶/°C can stress ceramic shells during dewaxing if you heat too quickly.
Filled pattern wax contains 10-50% organic filler particles that dramatically reduce shrinkage and improve dimensional stability. The filler occupies volume that would otherwise change during cooling, preventing cavitation even in thick sections. These waxes produce stronger, more rigid patterns that hold their shape better.
The thermal expansion drops by 20-50% compared to unfilled wax. This reduced expansion means less stress on ceramic shells during dewaxing, improving yield rates.
Filled wax costs more and requires special handling. The melted wax needs constant stirring to keep fillers suspended – if they settle, the wax properties change.
You can’t easily recycle filled wax because the fillers complicate filtration. Many foundries limit reuse or discard it entirely to maintain quality.
Runner wax forms the channels that feed molten metal into your patterns. It’s designed to melt at a lower temperature than pattern wax – typically below 60°C – so it drains out first during dewaxing.
This early melting prevents pressure buildup that could crack the ceramic shell. The wax flows out quickly and completely, creating clear pathways for the metal.
Runner wax uses higher paraffin content for excellent fluidity. It fills simple sprue shapes easily without voids or defects.
The formulation includes tackifiers that help it bond securely to pattern waxes during assembly. Yet it maintains enough strength to support multiple patterns on a casting tree.
Runner wax isn’t suitable for detailed features – it’s purely functional. The soft consistency means wax trees may need careful handling or water quenching to set properly.
Sticky wax acts like a specialized adhesive for joining wax components and fixing minor defects. It contains high levels of rosin or resin that make it extremely tacky when heated but hard and brittle when cool.
A small flame or wax pen melts the sticky wax into a syrupy liquid that bonds instantly to other waxes. It solidifies within seconds, creating strong joints for assembling complex trees.
Repair wax has a softer, putty-like consistency for filling scratches or voids. You can warm it, apply it to defects, then carve or smooth it after cooling.
These waxes only work for small joints and patches – never for creating actual patterns. The brittle nature means joints can crack if stressed, especially in cold conditions.
Excess sticky wax must be trimmed away to avoid creating defects in the ceramic coating. The application requires skill with heating tools to avoid accidentally melting the main pattern.
Water-soluble wax creates dissolvable cores for casting parts with internal cavities or undercuts. It’s based on polyethylene glycol (PEG) rather than traditional wax, with up to 50% inorganic filler for stability.
You inject the soluble wax first to form the core shape. Then you inject regular pattern wax around it to create the outer form.
After the pattern cools, you dissolve the core in warm water. This leaves a hollow cavity inside the wax pattern – something impossible to achieve with conventional molding.
The high filler content provides excellent dimensional stability with minimal shrinkage. Cores maintain precise tolerances for critical internal features like cooling channels in turbine blades.
Soluble wax cores are fragile and require careful handling. The high viscosity needs increased injection pressure and constant agitation to prevent air entrapment.
| Wax Type | Composition | Melting Point | Thermal Expansion | Best Use Case | Main Advantage | Main Disadvantage |
|---|---|---|---|---|---|---|
| Unfilled Pattern | Paraffin/microcrystalline blend, no fillers | 60-70°C | High (50×10⁻⁶/°C) | Small/medium detailed parts | Excellent detail capture, 100% recyclable | High shrinkage, prone to cavitation |
| Filled Pattern | Same base + 10-50% organic filler | 70-85°C | Low (20-40% less) | Large/complex parts | Minimal shrinkage, high stability | Higher cost, difficult recycling |
| Runner/Sprue | High paraffin content, tackifiers | <60°C | Low-Medium | Gating systems | Melts first, protects shell | Only for simple shapes |
| Assembly/Sticky | Rosin-infused wax | >70°C | N/A (tiny amounts) | Joining and repairs | Fast bonding, quick setting | Brittle joints, manual application |
| Water-Soluble | PEG polymer + inorganic fillers | 55-65°C | Very Low | Internal cavities | Creates complex hollows | Single-use, fragile cores |
You shouldn’t mix different wax types because they have different melting points, expansion rates, and chemical compositions. Mixing them can cause unpredictable shrinkage, poor bonding, or dewaxing problems that ruin your castings.
Unfilled pattern wax can be recycled almost indefinitely with proper filtering, often using 30-50% reclaimed wax in production. Filled wax typically can’t be recycled effectively because the fillers complicate processing and alter the properties when remelted.
Natural waxes now serve mainly as minor additives (usually under 5%) to improve specific properties like surface finish or flexibility. Pure beeswax alone shrinks too much and costs too much for modern industrial casting, but small amounts can enhance synthetic wax blends.