Deep Hole Drilling for the Confectionery Food Processing Industry: Chocolate Temperer Scraper Shafts, Candy Mould Cooling Channels, and Licorice Extruder Dies

A manufacturer of chocolate tempering machines (scraper shaft 80 mm x 1.5 m, requiring 40 mm x 1.5 m axial bore for thermal fluid) used a carbide gun drill (40 mm, Vc = 80 m/min, f = 0.05 mm/rev, oil at 40 bar). Bore Ra 0.4 microns. Tested at 50 C dark chocolate tempering temperature: surface temperature uniformity +/-0.3 C along the full 1.5 m length.

Confectionery Component Comparison ​

Comparison of Confectionery Equipment Components Requiring Deep Hole Drilling ​

Drilling Parameters for Confectionery Processing Materials ​

FAQ ​

Why is the chocolate tempering scraper shaft bore temperature uniformity so critical for cocoa butter crystallisation? ​

The scraper shaft bore temperature uniformity is critical because the tempering of chocolate — the controlled crystallisation of cocoa butter into the stable Form V crystal structure — requires the chocolate to be cooled from approximately 45°C (the fully melted state) to 27–28°C (for milk chocolate) or 28–29°C (for dark chocolate), then warmed to 30–31°C (for milk chocolate) or 31–32°C (for dark chocolate) to melt out the unstable Form IV crystals. If the scraper shaft surface temperature varies by more than ±0.5°C along its length, the chocolate in contact with the hotter sections of the shaft will not crystallise correctly (it will remain in the unstable Form IV state), while the chocolate in contact with the colder sections will over-crystallise (forming Form VI crystals that give the chocolate a gritty texture). The thermal fluid flowing through the gun-drilled bore must therefore maintain the shaft surface temperature at a uniformity of better than ±0.5°C along the full heat exchanger length (typically 1–3 m). This is achieved by: maintaining a high thermal fluid flow velocity (2–4 m/s in the bore) to achieve turbulent flow (Reynolds number > 4000), which provides uniform heat transfer along the bore length; using a temperature-controlled fluid supply with a PID controller that maintains the fluid inlet temperature within ±0.1°C; and ensuring that the bore surface is smooth (Ra < 0.4 µm) to prevent localised flow disturbances that create temperature variations. The gun-drilled bore provides the straight, smooth passage required for turbulent flow — a rough or non-straight bore would create laminar flow pockets that cause localised temperature variations. The temperature uniformity is verified by mounting 5–10 thermocouples on the shaft surface at intervals along the length and measuring the temperature distribution during operation. If any thermocouple deviates by more than ±0.5°C from the set point, the thermal fluid flow rate is increased or the bore is re-honed to improve the surface finish.

How are licorice extruder die multiple parallel bores drilled, and what determines the bore L/D ratio? ​

Licorice extruder dies are steel plates (P20 or H13 tool steel, 50–100 mm thick, 100–300 mm diameter) with multiple parallel axial bores (2–8 mm diameter) through which the licorice paste is extruded to form the characteristic multi-strand licorice ropes. The bores are drilled on a multi-spindle gun drilling machine with 10–50 spindles (one spindle per bore). The spindle pitch equals the desired strand spacing (typically 4–10 mm). The dies must be replaced frequently (every 2–8 weeks of production) because the abrasive sugar and flour particles in the licorice paste gradually enlarge the bore diameter, increasing the strand diameter and changing the product weight. The L/D ratio of each bore (bore length divided by bore diameter) determines the back pressure and the extrusion rate, which affects the final product texture and density. A higher L/D ratio (> 10:1) creates higher back pressure, which forces the licorice paste to flow more slowly and uniformly through the die, producing a denser, chewier product with a smoother surface texture. A lower L/D ratio (< 5:1) reduces the back pressure, resulting in a softer, more flowing product with a less defined shape. The bore drilling parameters for tool steel dies: Vc = 60–100 m/min, feed f = 0.04–0.08 mm/rev, carbide gun drill with TiAlN coating. After drilling, the bores are hand-polished to Ra < 0.4 µm (using diamond paste on a wooden lap) to reduce the friction between the licorice paste and the bore wall — a rough bore surface causes the licorice to tear or shred as it exits the die. The bore diameter tolerance is ±0.03 mm (to maintain consistent strand weight), and the L/D ratio tolerance is ±0.05 (to maintain consistent back pressure from one die to the next). The completed die is flow-tested with a standard licorice paste at production temperature and pressure, and the extrusion rate per bore is measured.

What cooling channel layout is used in candy mould plates, and how does it affect cycle time? ​

Candy mould plates (for hard candy, lollipops, and fondant) use a grid of gun-drilled cooling channels (6–12 mm diameter) arranged in a serpentine pattern that covers the full mould plate area (typically 200–500 mm × 300–600 mm). The channels are drilled in two directions: primary channels (parallel to the mould length, 6–12 mm diameter, 200–500 mm length) are gun-drilled from one side of the plate, and secondary channels (parallel to the mould width, 6–12 mm diameter, 300–600 mm length) are drilled from the adjacent side. The primary and secondary channels intersect at 90° junctions, forming a continuous serpentine flow path. The ends of the secondary channels are sealed by welded or threaded plugs. The channel spacing (pitch) is typically 2–3× the channel diameter (12–36 mm for a 12 mm channel), and the distance from the channel to the mould cavity surface is 5–10 mm. The uniform cooling provided by the grid pattern reduces the candy moulding cycle time by 30–50% compared to straight-drilled channels (which cannot maintain a constant distance from the curved cavity surfaces). The drilling parameters for P20 tool steel: Vc = 80–120 m/min, feed f = 0.05–0.08 mm/rev, carbide gun drill with TiAlN coating, emulsified oil coolant at 40–60 bar. The channel network is pressure-tested after drilling (water at 5 bar for 5 minutes, zero leakage) and flow-tested (the coolant flow rate at the operating pressure must be within ±10% of the calculated value). If the flow rate is below the calculated value, one or more channels is partially blocked by a drill chip or a burr at an intersection, and the intersection must be inspected by borescope and cleared by flushing.

How is the gum base kneader shaft drilling different from other food processing shaft drilling? ​

The gum base kneader shaft (for chewing gum manufacturing) operates at 100–200 bar kneading pressure and must resist both torsional and thermal loads while maintaining food-grade surface finish. The shaft is made from 316L stainless steel (for corrosion resistance against the acidic gum base ingredients, pH 4–6) with a gun-drilled axial bore (20–50 mm diameter, 1500–3000 mm length) that circulates heating or cooling water. The key difference from other food shaft drilling is the requirement for the bore diameter to be stepped: the shaft has a larger bore diameter at the drive end (to accommodate the drive shaft connection) and a smaller bore diameter along the kneading section (to maintain the wall thickness required for the 100–200 bar kneading pressure). The stepped bore is produced by gun drilling with a stepped gun drill (a single drill with two diameters, similar to the step drills used in coaxial RF connector manufacturing). The hot-melt adhesive applicator roll bore (for folder-gluers) is typically a single diameter; the gum base kneader shaft requires a stepped bore because the kneading pressure is transmitted as a torsional load through the shaft wall, and the drive end must have a thicker wall (smaller bore) to transmit the torque, while the kneading section can have a thinner wall (larger bore) to reduce the shaft weight. The gun drilling parameters for 316L: Vc = 50–70 m/min, feed f = 0.03–0.05 mm/rev, PCD-tipped gun drill (preferred for production) or carbide gun drill with TiAlN coating, NSF H1 oil coolant at 40–60 bar. The bore is honed after drilling to Ra < 0.4 µm and is verified by borescope inspection at 50×.

What are the waffle cone mandrel drilling requirements, and how is the stepped bore produced? ​

Waffle cone mandrels (for rotating waffle cone baking machines) are cast iron or aluminium bronze components with a gun-drilled axial bore (20–50 mm diameter, 300–600 mm length) that houses a gas burner or an electric cartridge heater for the 160–200°C baking temperature. The mandrel has a tapered outer shape (the cone shape of the finished waffle) and the bore must follow the taper: the bore diameter is larger at the base of the cone and smaller at the tip. The tapered bore is produced by a two-step gun drilling process: first, a straight bore is gun-drilled to the smaller diameter at the tip, drilling from the tip end to the base end. Second, a stepped reamer is inserted from the base end and removes additional material in the larger-diameter section, creating the tapered profile. The waffle cone mandrel bore must maintain the wall thickness between the bore and the outer taper within ±0.5 mm to ensure uniform heating. The drilling parameters for cast iron: Vc = 80–120 m/min, feed f = 0.06–0.12 mm/rev, uncoated carbide gun drill, emulsified oil coolant at 30–50 bar. The wall thickness is verified by ultrasonic measurement at 10 positions along the mandrel.

The information provided in this article is for general informational purposes only and does not constitute professional engineering advice. Always consult qualified food processing engineers, confectionery manufacturing specialists, and equipment manufacturers for specific applications. Data and parameter recommendations are based on published research and industry experience as of 2026.