Injection Mold 設計 Guide: Principles, 材料s & CNC加工

A practical guide to 射出成形金型設計 — from part geometry rules to 金型材料の選定. Whether you are 設計ing a new plastic part or optimizing an existing mold for production, these principles reduce defects, speed up cycle times, and lower tooling costs. Written for 製品設計者、機械エンジニア、製造購買担当者 sourcing injection molding tooling.

重要ポイント

• 肉厚: Uniform 1.2-3.0 mm is the #1 rule — variations cause warping and sink marks
• 抜き勾配: Minimum 1° on untextured surfaces, 3°+ on textured — without draft, parts stick and molds wear faster
• ゲート位置: Always gate at the thickest section to ensure complete cavity fill before cooling
• 金型材料： P20 steel for 500K-1M shots, H13 for 1M+, アルミニウム 7075 for prototypes (5K-10K shots)
• CNC加工： 5-axis 精密 for complex mold cores — mirror polish down to SPI A-1 for optical parts
• DFMチェックリスト： 7-point verification before tooling starts — downloadable at the end of this guide

1. 肉厚 — 最も重要な設計ルール

Uniform wall 厚さ is the single most important 設計 principle in injection molding. 不均一な肉厚は冷却速度の差を引き起こす — thick sections stay molten while thin sections solidify, creating internal stresses that manifest as warping, sink marks, and voids.

• Recommended wall 厚さ range: 1.2 mm to 3.0 mm for most engineering thermoplastics (ABS, PC, PA6, POM)
• Thick-to-thin transitions: use 徐々にテーパーを付ける — maximum 3:1 厚さ ratio across any transition zone
• Rib 厚さ: 隣接する肉厚の40～60％ to prevent sink marks on the opposite face
• ボス外径：ねじ/インサート径の2倍。ボス周辺の肉厚は主肉厚の0.6倍。

最適な用途： Structural housings, 電子機器用エンクロージャ, consumer product shells.
避けるべき場合： Part has unavoidable thick bosses — consider gas-assist molding or structural foam instead.

2. 抜き勾配 — その重要性

Draft angles are the slight tapers added to vertical walls to allow the part to 引きずりや固着なく金型から離型する. Insufficient draft causes three problems: parts stick to the core side (increasing cycle time), ejector pins leave visible marks, and in extreme cases, the mold steel galls and requires repair.

• 無地肌面： Minimum 1° draft — but 1.5-2° is safer for production
• シボ面（VDI/SPI）： Add 1° per 0.025 mm of texture depth — typically 3-5° for medium textures
• 深いコアと高いリブ： 3-5° minimum — ribs taller than 10 mm need aggressive draft to avoid sticking
• シャットオフ面： 3-5° to prevent flash and mold damage at parting line interfaces

最適な用途： Any vertical wall parallel to mold opening direction.
避けるべき場合： Cosmetic outer surface with zero draft specification — use a 精密 CNC machined mold with mirror-polished cavity to reduce sticking at low draft angles.

3. ゲート配置とランナー設計

The gate is where molten plastic enters the cavity — its 位置 determines 充填パターン、ウェルドライン位置、および部品強度. Poor gate placement causes short shots, excessive warpage, and visible flow marks on cosmetic surfaces.

• 最も厚い部分にゲートを配置： Ensures the melt front reaches thin areas before solidifying
• 外観面付近へのゲート設定を避ける： Gate vestige (the small mark left after degating) should be on non-visible faces
• 大型部品にはマルチゲート： When flow length exceeds 200 mm, add secondary gates to reduce injection pressure
• サブマリン（トンネル）ゲート： Auto-degate during ejection — ideal for high-volume production without manual trimming

4. アンダーカットとサイドアクション

An undercut is any feature that prevents the part from being ejected straight out of the mold — holes perpendicular to the mold opening direction, snap-fit tabs, or recessed grooves. Undercuts require スライドコアまたはリフター, which add 20-40% to mold cost and increase cycle time by 2-5 seconds per shot.

• 除去するための再設計： Use pass-through cores (holes open on both sides) or split the part line to align with undercut features
• 内部アンダーカット用リフター： Angled pins that move during ejection — limited to 10-15 mm travel
• 外部アンダーカット用スライドコア： Hydraulic or cam-actuated — can handle larger travels (up to 100 mm)
• コラプシブルコア： For internal threads — expensive but eliminates unscrewing 機構s

最適な用途： Complex enclosures with snap-fits, connector housings with side ports.
避けるべき場合： Budget is under $5,000 — re設計 to eliminate undercuts or accept a 2-piece assembly.

金型材料の選定

The mold 材料 determines 工具寿命、表面仕上げ品質、および部品単価. より高い-grade steels cost more upfront but deliver 10-20× the shot life. For prototype and low-volume production, アルミニウム molds offer faster turnaround at lower cost. At BravoFabs, we CNC machine mold cores from all common 工具鋼 and アルミニウム合金s.

最適な用途： P20 for 80% of commercial 用途s. H13 when running glass-filled nylon or PPS. アルミニウム 7075 for prototypes and bridge tooling.
避けるべき場合： Using アルミニウム for 生産量s over 10,000 — tool wear becomes the dominant cost.

精密金型中子のCNC加工

Modern injection molds depend on 高精度CNC加工 for the cavity and core inserts. At BravoFabs, we machine mold components from hardened tool steel with 公差 as tight as ±0.005 mm. Our 出荷前に問題を発見します。 capabilities include:

• 5軸CNC加工： Complex curved surfaces and deep cavities in a single setup, eliminating alignment errors from multiple setups
• 高速加工（HSM）： 20,000+ RPM spindles for fine detail work on electrodes and direct-machined cavities
• ワイヤー放電加工： 鋭い内部コーナー, deep ribs, and features impossible to reach with rotating tools
• 研磨： Multi-stage diamond 研磨 from SPI B-3 (600 grit) to SPI A-1 (mirror, 3 μm diamond) for optical-grade surface finish
• テクスチャーエッチング： VDI 12 to VDI 45 textures applied post-machining for cosmetic surfaces

よくある設計のミスとその回避方法

製造性を考慮した設計（DFM）チェックリスト

Before 送信ing your part 設計 for tooling, verify these seven points. Each missed item can add 金型の手直しに500～2,000ドル and delay production by 2-4 weeks.

1. 部品全体の肉厚は均一ですか？ — Check with section analysis in CAD. Any section >3× the nominal wall is a sink risk.
2. すべての垂直面に最低1.5°の抜き勾配がありますか？ — Run a draft analysis with parting line defined. Red surfaces (negative draft) = stuck parts.
3. すべての内側コーナーのRは0.5mm以上ですか？ — Sharp corners concentrate stress and reduce mold life. Add fillets to all internal edges.
4. ゲートは最も厚い部分に配置されていますか？ — Mold flow simulation (Moldflow or similar) confirms fill pattern before cutting steel.
5. アンダーカットは排除または考慮されていますか？ — Each undercut requires a side action. Add cost estimate: $800-1,500 per side action.
6. パーティングラインは最も単純な形状に沿っていますか？ — Complex parting lines increase mold cost and create visible witness lines.
7. エジェクタピンの位置は外観面以外ですか？ — Avoid pin marks on Class A surfaces. Specify acceptable pin 位置s on the drawing.

For metal components that will interface with injection-molded parts — such as threaded inserts, mounting ブラケット, or reinforcement plates — see our 繰り返し注文の一貫性が保たれます。 and 出荷前に問題を発見します。 サービス for complementary manufacturing.