230 Mold Design Experiences (Part 2)

Stay tuned for Part 2 and Part 3 for more in-depth, industry-specific guidance. Here continue Part 2.

This technical article is the first installment of a three-part series dedicated to sharing practical, field-tested mold design experience. Spanning Part 1, Part 2, and Part 3, the full content compiles 230 actionable tips refined from years of hands-on practice in mold manufacturing, covering core aspects such as mold structure optimization, material selection, precision control, and cost reduction strategies. Targeted at mold designers, process engineers, and manufacturing teams, the tips in this series are tailored to common scenarios in injection mold and stamping mold production. Whether you are refining the design of complex lifters, optimizing mold latch structures, or seeking solutions to 3D printing deformation issues for mold components, you can find targeted insights here. Part 1 will focus on basic design principles and common structural pitfalls, laying a solid foundation for the advanced topics to be covered in the subsequent installments. Stay tuned for Part 2 and Part 3 for more in-depth, industry-specific guidance. Let's continue to Part 2.

This article contains technical specifications and best practices for high-precision plastic injection mold design and manufacturing, particularly for optical and electronic components (like camera bodies and lenses).

Technical Specifications for Mold Design and Manufacturing

1. Venting and Clearances

79. Venting Grooves: On standard molds, venting grooves are typically 0.5mm deep at the outer edge and 0.02mm deep near the cavity. For precision molds (e.g., camera housings), the outer edge is 0.07–0.1mm deep, while the cavity side is strictly 0.007–0.01mm.

80. Parting Surface (PL): To ensure a tight seal between the moving (B-plate) and fixed (A-plate) sides, the PL usually stands 0.02mm higher than the mold base. C10–20 chamfers (0.5–1mm deep) are often milled at the four corners of the B-plate (#103) to prevent interference with the A-plate (#102).

2. Material Shrinkage and Machining

81. Precision Tolerances: Polyacetal (POM) parts typically have a dimensional tolerance of ±0.2%. Adding a cavity increases this tolerance by roughly 5.8%; for four cavities, the tolerance increases by 1.4x (reaching ±0.28%).

82. Milling NAK80: Using a Kennametal ¢16 blade (KCM25) on NAK80 steel, optimal settings are: 0.4mm depth of cut, width at 2/3 tool diameter, 55m/min linear speed, and 0.5mm/rev feed rate with air cooling.

83. Grinding: Grooves as narrow as 0.5mm can be achieved via grinding.

84. Return Pins: Return pins usually have a hardened surface layer of ~0.5mm, while the core remains soft for toughness.

85. Surface Finishing: For precision flat surfaces, use a step-over of 2/3 to 4/5 of the tool diameter and a slow feed rate.

86. Slider Grooves: The tolerance for slider (carriage) slots is typically ±0.01mm.

3. Design Philosophy and Client Communication

87. Design Review (DAFM): Before designing, it is mandatory to confirm the parting line, ejector pin locations, undercut treatments, gate types, wall thickness vs. shrinkage, and tolerances with the client. Designers must not make arbitrary decisions.

88. Hot Runners: These are generally suitable for high-volume production exceeding 240,000 cycles.

4. Ejection Systems

89. Submarine Gates: For long gates, a draft angle of 0.5°–1° must be added to the straight portion of the gate to prevent ejector pin breakage or ejection failure.

90. High Pressure: High-precision PC parts (e.g., Olympus camera back covers) may require injection pressures up to 200 MPa.

91. Cold Slug Wells: For molds with large runners, the cold slug well must be elongated (e.g., extended by 14mm after the first T1 trial).

92. Pre-designed Venting: Venting should be planned during the design phase, not added after trials. Typically, a shallow groove (less than the flash limit) is machined around the perimeter.

5. Precision Inserts and Alignment

93. Chamfered Inserts: If an insert's C-angle meets the mold core, the length tolerance from the base to the C-angle should be +0.05mm to prevent flashing.

94. Surface Roughness (Ra): * Standard: 7μm

Precision: 4μm

High-aesthetic/Optical: 2μm

95. Material Sourcing: Order mold steel 3–5mm larger than the required final dimensions.

96. Sprue Pullers: Avoid fixing puller pins with rear-locking screws as the resulting stress causes breakage. Floating designs are preferred.

97. Wire EDM Corner Radius: Wire EDM naturally leaves a 0.2mm radius (R) in sharp corners. Designs for insert holes or square pin holes must account for this to avoid fitment issues or burrs.

98. Slider Draft: Use a 2–3° draft on the contact surface between sliders and cores to reduce wear and provide pre-load.

6. Polishing and Fitting

99. Coating & Polishing: Coating thickness is ~0.02–0.03mm; mold polishing also removes ~0.02–0.03mm. Account for these in the final fit dimensions.

100. Insert Fitting: An insert is perfectly fitted if it slides slowly and feels firm (no wobble) when inserted 1/4 of the way into the pocket.

101. High Rigidity: For lenses/gears, heat treat auxiliary mold plates (S45C/S55C) to 45 HRC to maintain rigidity.

102. Lens Steel: Use YAG-250 powder metallurgy steel (Daido Steel) heat-treated to 56±1 HRC for ultra-pure optical surfaces.

103. Micro-Polishing: For small recessed dots, use a toothpick in a high-speed drill (6,000–10,000 RPM) with diamond paste under a microscope.

7. Maintenance and Quality Control

104. Ejector Clearance: Standard relief depth is 0.1mm (+0/0.02). For precision molding, it is 0.03mm (+0/0.01), requiring extremely strict control over all plate thicknesses and pin lengths.

105. Inspection Checklist: Check for burns, flow marks, scratches, short shots, flash at PL, sink marks, ejector marks, and relief depths.

106. Stripper Plates: For multi-cavity stripper plate molds, use integrated (one-piece) cores to ensure balanced ejection.

107. Diamond Paste: #5000 to #8000 grit is sufficient for a mirror finish.

108. Reaming: The runout for reamed holes is typically 0.05mm.

8. Specialized Components and Structures

136. Ejector Pin (EP) Relief: Ejector pins should be relieved within the core to reduce friction.

137. Springs: Use "Blue" (Light Load, TL type) springs for small pull rods to ensure sufficient tension.

138. Sprue Puller Pins: Use RLRT types. The clearance hole in the 101 plate should be 0.2mm larger (bilateral) without tolerance. Ensure the 101 and 101A plates are Wire EDM'd together for perfect concentricity.

139. Gas Traps: Use specific design structures to avoid trapped air during high-speed processing.

140. Fiber-Reinforced Materials: For materials like Glass Fiber (GF), use inserts for high-wear areas as the material is highly abrasive.

145. Point Gates: Relief depth (B) is usually 0.5–0.6mm, slightly larger for ABS or PC.

147. Three-Plate Mold Opening: * PL1 (Runner strip) = Waste length + 10mm (for robot) or Runner-to-gate length + 15mm (for gravity drop).

PL2 (Part ejection) = Part height (A) + 2–3mm.

150. Metrology: Human error in measurement is typically ~2μm. When using a micrometer, three "clicks" of the ratchet indicate proper contact. Measurement should occur at 22°C ± 0.5°C.

151. Mirror Molds: Often use "insert ejection." For large parts, use guided ejection with guide bushes and pillars to prevent tilting or scratching the cavity.

154. Machining Sequence for Bosses/Studs:

Mill the flat surface (leave bosses unmachined) and apply texture (etching).

Check flatness.

Machine the bosses (CNC milling only—avoid EDM to ensure uniformity).

Leave 0.03mm for polishing; polish under 30x–50x magnification.

Material-Specific Settings & Constraints

Material	Key Specification	Notes / Applications
POM (Polyacetal)	±0.2% Base Tolerance	Multi-cavity molds increase tolerance significantly (up to ±0.28%).
PC (Polycarbonate)	200 MPa Injection Pressure	Used for precision camera back covers (e.g., Olympus).
PPE	60°C Mold Temperature	Ideal for stable thermal performance.
LCP	0.14mm Wall Thickness	Excellent flow properties for ultra-thin precision parts.
PSF	Anti-static properties	2-3x the cost of PC; used in high-end electronics.
NAK80	55m/min Linear Speed	Machining parameters for ¢16 cutters (KCM25).
YAG-250	56±1 HRC	Powder metallurgy steel used specifically for lens molds.

Next article we will continue Part 3, the last Part of Mold Design Experiences. We hope these valuable experiences will be helpful to our actual design work. If you have other design experience, you are welcome to share it with us or contact us at info@jstmould.com. Then we can keep improving all these methods and share to others to achieve good result for our works.