理想的な射出成形の壁厚とは？

肉厚^[1] is arguably the single most important design parameter in injection molding. Get it right, and your part molds cleanly, functions reliably, and costs less. Get it wrong, and you’re dealing with sink marks, warpage, voids, and cycle times that eat your margin.

This guide covers everything engineers need to know about 射出成形 wall thickness: how to choose the right value, what happens when walls aren’t uniform, material-specific guidelines, and the most common mistakes from thousands of DFM reviews.

What Is Wall Thickness in Injection Molding?

Wall thickness is the distance between the outer and inner surface of a molded part at any cross-section. It determines how plastic flows through the injection mold cavity, how quickly the part cools, and whether final dimensions hold to specification.

Thinner walls save material and reduce cycle time, but increase injection pressure requirements and risk short shots. Thicker walls flow more easily but cool slowly, extending cycle time and increasing the risk of voids and sink marks. The sweet spot for most engineering thermoplastics is 1.5–3.0 mm. Always verify your chosen thickness against the material supplier’s data sheet and flow simulation results before finalizing the design.

Why Is Uniform Wall Thickness So Important?

Non-uniform wall thickness is the root cause of more molding defects than any other single design error. When walls vary significantly, thick sections cool and shrink at a different rate than thin sections. This differential shrinkage射出成形^[2] creates internal stresses that manifest as warpage, sink marks, and dimensional instability.

The guideline is straightforward: maintain wall thickness within ±25% of the nominal value throughout the entire part. If your nominal wall is 2.5 mm, every section should fall between 1.9 mm and 3.1 mm.

What Is the Recommended Wall Thickness by Material?

Different materials have different flow characteristics and shrinkage rates. Here’s a practical reference table based on extensive production data.

素材	Min Wall (mm)	Ideal Range (mm)	Max Practical (mm)
ABS	0.8	1.5–3.0	4.5
PC（ポリカーボネート）	0.8	1.5–3.0	4.5
PP（ポリプロピレン）	0.6	1.2–2.5	5.0
PA (Nylon 6/66)	0.6	1.0–3.0	4.0
POM (Acetal)	0.8	1.0–3.0	4.0
PMMA (Acrylic)	0.8	1.5–3.5	5.0
PBT	0.8	1.0–3.0	4.0
PE（ポリエチレン）	0.6	1.0–2.5	5.0
PS（ポリスチレン）	0.8	1.0–3.0	4.5
TPE/TPU	0.5	1.0–3.0	5.0

The minimum wall values represent what’s technically possible with optimized processing, not what’s recommended for production. For reliable manufacturing, stay within the ideal range.

How Do You Transition Between Different Wall Thicknesses?

Sometimes wall thickness variation is unavoidable. When it happens, the transition between thick and thin sections is critical. The standard guideline is a 3:1 taper ratio: for every 1 mm of thickness change, provide at least 3 mm of gradual transition.

Abrupt thickness changes cause flow hesitation, stress concentrations, and visible sink marks on the opposite surface. In severe cases, parts crack at thickness transitions during assembly because residual stress exceeds the material’s yield strength.

What Happens When Walls Are Too Thick?

Thick walls create three problems: excessive cycle time, internal voids, and sink marks.

Cycle Time Penalty

Cooling time scales approximately with the square of wall thickness. A part with 2 mm walls might cool in 15 seconds; the same geometry with 4 mm walls could take 50–60 seconds. Across a production run of 100,000 parts, that’s thousands of additional machine hours.

Internal Voids

When thick sections cool, the outer skin solidifies first while the interior is still molten. As the interior shrinks, it pulls away from the solidified skin, creating internal voids that reduce structural integrity — particularly problematic in load-bearing applications.

シンクマーク

Sink marks are the surface manifestation of the same phenomenon. When material at a thick section shrinks, it pulls the surface inward, creating a visible depression especially noticeable on glossy surfaces. Rib-to-wall ratios directly control sink severity: ribs thicker than 50–60% of nominal wall almost always produce visible sink marks.

What Happens When Walls Are Too Thin?

Thin walls carry their own risks. The most immediate is short shots — the plastic melt freezes before completely filling the cavity. This is especially problematic with high-viscosity materials like polycarbonate and long flow paths, where the melt viscosity is already high.

Thin walls also increase injection pressure requirements. If required pressure exceeds machine capability, you get incomplete fills and high residual stress.

Structural integrity is another concern — always include a safety margin for thin-wall parts in consumer products subject to drop testing. A part that survives static loads may crack on impact if walls are too thin.

How Do Ribs and Bosses Affect Wall Thickness?

Ribs and bosses are the most common features that interact with wall thickness. For ribs: base thickness should be 50–60% of nominal wall, height should not exceed 3× nominal wall, and multiple thin ribs are better than one tall, thick rib.

For bosses, coring is the solution — hollow out the center with a core pin to maintain uniform wall thickness. Boss outside diameter should be no more than 2–3 times the hole diameter. Parts that follow these proportinjection moldingional 射出成形金型^[3] mold cleanly; those that deviate generate ongoing quality issues.

How Does Wall Thickness Affect Cycle Time?

で 射出成形, 冷却時間^[4] typically represents 50–70% of total cycle time, governed by the thickest cross-section.

Nominal Wall	Typical Cooling Time	Relative Cycle Cost
1.5 mm	8–12 seconds	1.0× (baseline)
2.0 mm	12–18 seconds	1.3×
2.5 mm	18–25 seconds	1.6×
3.0 mm	25–35 seconds	2.0×
4.0 mm	40–60 seconds	3.0×

Going from 2.0 mm to 3.0 mm walls roughly doubles per-part manufacturing cost through cycle time alone. Thin-wall design, when structurally feasible, is one of the highest-ROI optimizations in injection molding.

What Are the Most Common Wall Thickness Mistakes?

1. Neglecting uniform thickness. Parts designed without thickness analysis have areas 3× thicker than nominal alongside sections at half nominal wall, causing sink marks, warpage, and extended cycle times.
2. Over-thickening for strength. Engineers add material when a rib would be lighter, faster to produce, and more dimensionally stable.
3. Ignoring rib proportions. Ribs at 80–100% of nominal wall cause deep sink marks. The 50–60% rule applies to every material.
4. Abrupt thickness transitions. Sharp changes without taper create stress risers and cosmetic defects.
5. Not running flow simulation. Modern tools predict fill patterns, pressure, and cooling with high accuracy. Skipping simulation on complex parts usually loses.

What Should You Check Before Submitting Your Design?

Before submitting your design for tooling, run through this checklist. Each item takes seconds to verify and can prevent costly tooling revisions.

Check Item	Pass Criteria
Nominal wall within material ideal range	✓
Wall variation within ±25% of nominal	✓ or noted
All ribs ≤60% of nominal wall	✓
Thickness transitions use 3:1 taper	✓
Boss OD ≤3× hole diameter	✓
Thickest section identified and reviewed	✓
Flow simulation completed	✓

Optimizing wall thickness before the mold is built is essential — the cheapest place to fix a thickness problem is in CAD, not in steel.

Frequently Asked Questions About Wall Thickness

射出成形における最小肉厚は何ですか？

For most engineering thermoplastics (ABS, PC, Nylon), 0.8 mm is the practical minimum for short flow paths. For high-flow materials like PP and PE, you can go as thin as 0.5 mm. These minimums require high injection pressure and carry risk of short shots.

部品全体で肉厚は変化しますか？

はい、ただし変動は公称壁厚の±25%以内に収め、異なる厚さ間の遷移は3:1のテーパー比を用いて段階的に行う必要があります。

壁厚は収縮とどのように関連していますか？

厚い部分はより多くの材料が冷却収縮するため、より大きく収縮します。この不均一な収縮が射出成形部品の反りの主な原因となります。

壁厚は部品の強度に影響しますか？

Yes, but not linearly. Doubling wall thickness more than doubles bending stiffness (it scales with thickness cubed). However, thickening walls also increases residual stress and void risk. Properly proportioned ribs often achieve better strength-to-weight performance.

肉厚はどのように測定しますか？

CADソフトウェア（SolidWorks、Creo、およびほとんどのMCADパッケージに組み込まれています）の肉厚分析ツールを使用します。物理部品では、超音波肉厚計による非破壊測定、または切断した断面をノギスで直接測定します。生産中は、超音波測定が継続的な品質監視の標準方法です。

薄肉成形とは何ですか？

薄肉成形とは、肉厚が1.0 mm未満（電子機器筐体では0.3 mm程度まで）の部品を指します。非常に高い圧力（200 MPa以上）に対応できる高速成形機と専用の金型設計が必要です。

Bottom line: 肉厚は1.5～3.0 mmに保ち、±25%の均一性を維持し、3:1のテーパ遷移を使用し、リブは基準肉厚の50～60%にします。これら4つの規則により、肉厚関連の欠陥の90%を防止できます。

---