Why Does Plastic Shrink More in Thick Sections in Injection Molding

 

1. Why Does Plastic Shrink More in Thick Sections—and How Do You Compensate for It?

All thermoplastics shrink as they cool—but thick sections shrink more, and unevenly. The outer skin solidifies first, while the core remains molten longer. As that core cools, it pulls inward, creating differential shrinkage that leads to sink marks, internal voids, or dimensional drift.

At HOPO, we don’t guess shrinkage—we engineer around it. Using material-specific data (e.g., 1.8–2.2% for PP, 0.6–0.8% for glass-filled PA66), we adjust cavity dimensions in CNC programming. For complex geometries, we run mold flow simulations to map predicted shrinkage and pre-distort the mould steel—ensuring your final part lands within ±0.1 mm tolerance, even in 5 mm-thick zones.

Fun fact: A 6 mm wall in unfilled PP can shrink nearly twice as much as a 2 mm wall in the same part—causing warpage no amount of clamping force can fix.

 

2. How Does Wall Thickness Affect Gate Design and Filling Patterns?

Thick walls require more material volume and slower packing, which changes everything about how you fill the cavity.

• Too small a gate? You’ll get high shear heating, degrading the polymer.
• Too fast injection? Jetting or air entrapment in thick cores.
• Poor gate placement? Weld lines form in high-stress areas—critical in load-bearing parts like chair legs or crate corners.

For thick-walled parts (e.g., bucket bases, washing machine drums), we often use fan gates, diaphragm gates, or multiple sequential gates with valve control. This ensures uniform packing pressure and minimizes knit lines. At HOPO, gate design isn’t an afterthought—it’s co-developed with your wall strategy during DFM.

 

3. Can You Use Simulation Software to Predict Sink and Warp in Thick-Walled Parts?

Yes—and you absolutely should.

Manual rules of thumb (“keep walls under 4 mm”) are helpful, but real parts have bosses, ribs, and transitions. That’s why HOPO runs Moldex3D or Autodesk Moldflow on every complex project. These tools let us:

• Visualize cooling time gradients across thick sections
• Predict sink depth on visible surfaces
• Optimize cooling channel layout to balance solidification
• Simulate fiber orientation in glass-filled resins

We once helped a client redesign a thick-walled dustbin lid that kept failing drop tests. Simulation revealed a hidden void near a thick boss. By adding a local cooling insert and reducing rib thickness, we eliminated the defect—before cutting steel.

Simulation isn’t magic—it’s risk reduction. And for high-volume products, it pays for itself in the first production run.

 

4. What’s the Relationship Between Wall Thickness and Part Cost?

Let’s talk numbers.

• A 3 mm wall in a PP bucket might use 220g of material.
• Increase it to 5 mm, and you’re at ~360g—a 64% material cost increase.
• Worse: cooling time jumps from 28 seconds to 52 seconds, slashing output by ~46% on the same machine.

Over 100,000 parts, that’s tons of extra resin and hundreds of extra machine hours. At HOPO, we help clients right-size thickness—not to cut corners, but to maximize value. Sometimes, adding ribs or draft gives you 3× the stiffness for 10% more material.

Strong design isn’t about mass—it’s about efficiency.

 

5. Are There Industry Standards or Guidelines for Wall Thickness by Application?

Yes—and they exist for good reason.

• Home appliance housings (e.g., AC units): 1.8–2.5 mm (balance of stiffness, surface finish, and cycle time)
• Automotive interior trim: 2.0–3.0 mm (must pass OEM drop/impact tests)
• Industrial crates/dustbins: 3.0–5.0 mm (structural, but with controlled ribbing)
• Pipe fittings: 2.5–4.0 mm (pressure-rated, with uniform wall to avoid burst points)

These aren’t arbitrary. They’re backed by decades of field performance and standards from SPI, ISO, and major OEMs (like Midea, Haier, or GE Appliances). At HOPO, we cross-reference these benchmarks during DFM—so your mould meets not just your specs, but industry expectations.

 

6. How Do Material Suppliers Recommend Wall Thickness for Their Resins?

Check any technical datasheet from SABIC, BASF, DuPont, or LG Chem—and you’ll find a section titled “Recommended Wall Thickness” or “Design Guidelines.”

For example:

• SABIC LNP™ Stat-Kon (conductive PC): max 3.2 mm
• BASF Ultramid® A3WG6 (30% GF PA6): optimal 1.5–4.0 mm
• Dow INNATE™ TF (thinfilm PE): not for thick sections—designed for <1 mm

Why does this matter? Because material performance is validated only within recommended ranges. Go outside them, and you void real-world guarantees on impact, creep, or warpage.

At HOPO, we never assume—we consult resin guides and collaborate with compounders to ensure your material-thickness combo is certified for your use case.

 

7. When Should You Consider Alternative Processes Like Rotomolding or Blow Molding Instead?

Honest advice: injection molding isn’t always the answer.

If your part is:

• Hollow and large (e.g., 200L water tank)
• Walls >6 mm with no internal features
• Low volume (<5,000 pcs/year)

…then rotomolding or extrusion blow molding may be more cost-effective. Injection molding thick, solid parts wastes material and time.

At HOPO, we’re injection mould specialists—but we’re also solution partners. If your design leans toward rotomolding, we’ll say so—and even refer you to trusted partners. Our goal isn’t just to win a mould order—it’s to help you succeed.

 

Final Thought: Thickness Is a System—Not a Spec

At HOPO Mould, we know that great parts aren’t born from thick walls—they’re born from smart design, precise tooling, and material-process harmony.

Located in Huangyan, Taizhou—with 40-minute access to Taizhou Airport and 15 minutes to the railway station—we combine deep local manufacturing expertise with global standards to serve clients worldwide.

Have a thick-walled part in development? Send us your 3D model. We’ll review it not just for “can we mould it?”—but for “how can we make it better, faster, and more reliable?”

Because in injection molding, the right thickness isn’t the biggest number—it’s the wisest one.