Fabricate and Repair Molds (Injection, Compression, and Blow Molds)

Manufacturing and Repairing Molds (Injection, Compression, and Blow Molding)

Chapter Introduction

This chapter covers the essential skills of the Tool and Die Maker for manufacturing, assembling, and repairing molds used in injection, compression, and blow molding processes. For the Red Seal exam, you must master not only machining techniques but also the understanding of physical phenomena (shrinkage, expansion, material flow) and applicable Canadian standards. This chapter is structured to follow the logic of the trade: design, machining, assembly, testing, and repair.

1. Fundamental Principles of Molds

1.1 Mold Classification

Mold Type	Process	Typical Materials	Typical Pressure	Typical Temperature
Injection	Thermoplastics and thermosets	P20 steel, H13, 420 stainless	50–200 MPa	40–120 °C (regulation)
Compression	Thermosets, composites	Tool steel, copper alloys	10–50 MPa	150–200 °C
Blow molding	Thermoplastics (bottles, tanks)	7075 aluminum, stainless steel	0.5–2 MPa (air)	180–230 °C (parison)

Key principle: The mold must withstand repeated thermal and mechanical cycles without deformation. Surface hardness (48–62 HRC) is achieved through heat treatment or nitriding.

1.2 Components of an Injection Mold

A typical injection mold includes:

Cavity: negative shape of the part.

Core (punch): positive shape, often movable.

Feed system: main sprue, secondary runners, gates.

Ejection system: ejector pins, blades, retention rings.

Cooling system: water circuits, plugs, O-rings.

Plates: fixed plate, moving plate, spacers.

Guides: guide pillars and bushings.

Shrinkage formula: Shrinkage is expressed in mm/mm or as a percentage. For a semi-crystalline polymer (e.g., PA66): shrinkage of 1.5–2.0%. For an amorphous polymer (e.g., PC): 0.5–0.7%.

Practical calculation: Cavity dimension = Nominal dimension × (1 + shrinkage/100). Example: 100 mm part in PA66 (1.8% shrinkage) → cavity = 100 × (1 + 0.018) = 101.8 mm.

2. Manufacturing Injection Molds

2.1 Material Selection

Material	Application	Treatment	Final Hardness
P20 (pre-hardened)	High-volume molds, non-abrasive plastics	None (28–32 HRC)	28–32 HRC
H13 (modified H13)	High-temperature molds, abrasive materials	Quench + temper	46–52 HRC
S7 (impact)	Cores subject to impact	Air quenching	48–56 HRC
420 stainless	PVC molds, corrosive applications	Quench + temper	48–52 HRC
Beryllium-copper (C17200)	Cores with conformal cooling	Aging	36–42 HRC

Canadian standard: Refer to ASTM A681 for tool steels, and CSA W59 for welded assemblies (if applicable).

2.2 Cavity Machining

Processes:

27.CNC milling: roughing and finishing with carbide end mills. Use cutting speeds of 150–250 m/min for P20 steel.

28.Wire EDM: for sharp internal corners and complex shapes. Typical tolerance ±0.005 mm.

29.Sinker EDM: for deep cavities with graphite or copper electrodes.

30.Grinding: for flat surfaces and precision fits (tolerance ±0.002 mm).

Golden rule: Always machine leaving a 0.3–0.5 mm allowance for finishing after heat treatment, because quenching causes deformation.

2.3 Feed and Ejection Systems

Runner types:

Hot runner: keeps the polymer in a molten state. Advantage: no sprue to remove. Disadvantage: high cost, risk of thermal degradation.

Cold runner: simpler, but generates waste.

Gate types:

Type	Diameter	Application
Direct sprue gate	3–6 mm	Thick parts
Edge gate	1–3 mm	Flat parts
Submarine gate	0.8–2 mm	Automatic ejection
Fan gate	Width 5–20 mm	Wide, thin parts

Ejectors: Standard diameter 1.5 to 25 mm. Required ejection force is calculated by: F = P × A × μ, where P is the holding pressure (MPa), A the projected area (mm²), μ the coefficient of friction (0.2–0.4 for steel/polymer).

3. Manufacturing Compression Molds

3.1 Process Specifics

The compression mold is generally simpler: two mold halves (male and female) that close under pressure. The material (preform) is placed in the cavity, then the mold closes and heats.

Critical points:

Closing clearance: 0.05–0.15 mm to prevent flash.

Vents: grooves 0.05–0.15 mm deep to evacuate air and gases.

Heating: cartridge heaters or electric heating plates. Typical power: 2–4 W/cm² of surface area.

3.2 Manufacturing Closing Surfaces

The closing surfaces (land area) must be ground and lapped to ensure a proper seal. The width of the bearing surface is 3–10 mm depending on mold size.

Closing force calculation: F = P × A × 1.1 (10% safety factor). Example: projected area of 500 cm², compression pressure 30 MPa → F = 30 × 500 × 100 × 1.1 = 1,650,000 N = 165 tonnes.

4. Manufacturing Blow Molds

4.1 Types of Blow Molding

53.Extrusion blow molding: a parison (extruded tube) is pinched between two mold halves, then inflated with compressed air.

54.Injection blow molding: an injected preform is reheated then blown.

55.Stretch-blow molding: for PET (bottles). Mechanical stretching + blowing.

4.2 Blow Mold Design

Materials: 7075-T6 aluminum (lightweight, good thermal conductivity) or stainless steel for high-volume production.

Pinch-off points: The area where the two halves meet must have an angle of 15–30° and a land of 0.3–0.5 mm to cut off excess material.

Cooling: Critical for cycle time. Cooling circuits should be placed 8–12 mm from the cavity surface. Recommended water flow: 10–20 L/min per circuit.

Vents: Depth of 0.02–0.05 mm, width of 5–10 mm, placed at the highest points of the cavity to evacuate air during blowing.

5. Mold Repair and Maintenance

5.1 Defect Diagnosis

Observed Defect	Probable Cause	Solution
Flash	Excessive clearance, worn closing surfaces	Grind surfaces, reduce clearance
Part sticking	Insufficient draft, ejector pins too short	Increase draft, lengthen ejector pins
Flow marks	Poorly positioned gate, low temperature	Reposition gate, increase temperature
Burned part	Blocked vent, temperature too high	Clean vents, reduce temperature
Cavity wear	Abrasive material (glass, talc)	Nitride the surface, use harder steel

5.2 Repair Techniques

Welding: For cracks and chips. Use TIG with filler rod of the same composition as the mold steel. Preheat to 200–300 °C, weld in thin passes, then temper at 150–200 °C to avoid residual stresses.

Weld repair procedure: The welded area must be machined then ground to restore the exact geometry. Verify with a dial indicator or a coordinate measuring machine (CMM).

Nitriding: Surface treatment (gas or plasma) to increase surface hardness (up to 65–70 HRC) without deformation. Layer thickness: 0.1–0.4 mm.

Component replacement: Ejector pins, guide pillars, and bushings are wear parts. Fit tolerances:

Pillar/bushing: clearance of 0.01–0.03 mm.

Ejector/ bore: clearance of 0.02–0.05 mm.

5.3 Dimensional Inspection After Repair

Use:

Micrometer (outside): precision ±0.001 mm.

Inside micrometer: for bores.

CMM: for complex shapes, tolerance ±0.005 mm.

Optical comparator: for cutting profiles.

Standard: Refer to ISO 1101 for geometric dimensioning and tolerancing (GD&T) during verification.

6. Calculations and Thermal Considerations

6.1 Mold Thermal Balance

Heat to be removed by the cooling circuit: Q = m × Cp × ΔT, where:

Q = heat (J)

m = mass of polymer injected per cycle (kg)

Cp = specific heat capacity of the polymer (J/kg·K) — e.g., PP: 1900 J/kg·K

ΔT = difference between melting temperature and ejection temperature (°C)

Required water flow rate: V = Q / (ρ × Cp_water × ΔT_water), with ρ = 1000 kg/m³, Cp_water = 4186 J/kg·K, ΔT_water = 5–10 °C.

6.2 Thermal Expansion

Steel expands at 11 × 10⁻⁶ /°C (linear coefficient). For a 300 mm long mold, a 50 °C variation causes expansion of: 300 × 11 × 10⁻⁶ × 50 = 0.165 mm.

Important: Account for expansion when machining at room temperature if the mold operates hot. Machine the dimension cold by subtracting the expected expansion.

7. Standards and Safety

7.1 Applicable Canadian Standards

CSA B149.1 (Natural Gas and Propane Installation Code): applicable if the mold is gas-heated (rare, but possible for compression molds).

Canadian Electrical Code, Part I (CE Code): for cartridge heaters and electrical connections. Rule 8-200: protection of conductors against overcurrent.

CSA Z432: Safeguarding of Machinery — applicable to injection and compression presses.

CSA B51: Boiler, Pressure Vessel, and Pressure Piping Code — if the mold uses pressurized fluids (steam, thermal oil).

7.2 Handling Safety

Always lock out the press (lockout procedure) before working on the mold.

Use thermal gloves when handling hot molds (temperature > 60 °C).

Check the condition of hydraulic circuits (maximum pressure, hoses) before each trial.

Wear safety glasses during grinding and polishing.

8. Testing and Commissioning

8.1 Initial Mold Trial

106.Cold check: manual closing, clearance inspection, guide alignment.

107.Dry run: cycle without material, verify ejection movements.

108.Trial with material: first parts, adjust parameters (temperature, pressure, injection speed).

109.Part inspection: dimensions, appearance, weight. Compare to specifications.

8.2 Common Adjustments

Flash: increase clamping force or reduce clearance.

Incomplete fill: increase mold temperature, injection pressure, or enlarge the gate.

Excessive shrinkage: increase hold time or reduce mold temperature.

Pitfalls to Avoid

116.Forgetting shrinkage: Never machine a cavity to the nominal part dimension. Always apply the shrinkage factor, otherwise the part will be too small.

117.Confusing expansion and shrinkage: The mold's thermal expansion adds to the polymer's shrinkage. Calculate both separately.

118.Neglecting vents: A mold without vents produces burned or incomplete parts. Always provide vents of 0.02–0.05 mm.

119.Welding without preheating: Welding tool steel without preheating causes cracks. Always preheat to 200–300 °C.

120.Ignoring guide tolerances: Excessive clearance in guide pillars causes misalignment and flash. Check regularly.

121.Using the wrong steel: P20 steel for a high-temperature mold (where H13 is required) leads to premature wear. Always verify the service temperature.

122.Not documenting modifications: Any mold modification must be recorded (sketches, dimensions, dates) for future repairs.

123.Forgetting lockout: Working on a mold without locking out the press is a serious violation and dangerous. Always follow the procedure.

Summary

Injection, compression, and blow molds share common principles: cavity, core, feed system, ejection, cooling.

Polymer shrinkage is the most critical parameter for cavity dimensioning. Formula: cavity = nominal dimension × (1 + shrinkage/100).

Common materials: P20 (pre-hardened), H13 (high temperature), S7 (impact), 420 stainless (corrosion).

Machining tolerances: ±0.005 mm for EDM, ±0.002 mm for grinding.

Weld repair requires preheating (200–300 °C) and post-weld tempering.

Canadian standards: CSA B149.1 (gas), Canadian Electrical Code, Part I (Rule 8-200), CSA Z432 (machinery), CSA B51 (pressure).

Mold trials follow a logical sequence: cold check, dry run, trial with material, part inspection.

Safety is paramount: lockout, thermal gloves, safety glasses.

For the exam: Memorize the shrinkage and clamping force formulas, steel hardness ranges, and fit tolerances. Questions often involve simple calculations and defect diagnostics. Practice reading mold drawings and identifying components.