1. Uniform Wall Thickness

Sink marks show up where a thick section meets a thin one. The thick area keeps shrinking after the thin area has frozen, and the surface caves in. Buyers often blame the material. The fault sits in the part design.

Keep nominal wall as even as possible. Where thickness must change, step it gradually instead of jumping. A 3:1 transition over a short distance will sink on almost any resin.

Ribs deserve the same discipline. A rib thicker than 0.6 times the wall will sink the surface behind it, just like a thick section does. Keep ribs at 0.5 to 0.6 times the wall, no taller than three times the wall, and you get stiffness without sink.

Benefits

Even walls cut sink, hold shape, and let the melt fill without racing ahead in thick spots. That habit follows the same part design rules we apply to every new tool.

Recommended Wall Thickness

As a rule of thumb, hold thickness variation within about 10% across the part.

2. Draft Angles in Plastic Injection Mold Design

Start with 1° per side. That is the minimum that releases reliably on smooth, polished surfaces. As texture depth grows, add more; 2° to 5° covers most textured finishes. Deep ribs and tall bosses need extra.

No draft means the part drags on the steel during ejection. You get scuffs, or worse, a part stuck hard enough to need a press-side fix. I have seen a zero-draft boss stop a line for half a shift.

For textured finishes, a working rule is 1° of draft per 0.025 mm of texture depth, over a 1.5° base. Ribs and bosses need at least 0.5° per side, or they will drag.

3. Gate and Runner System Design

Gate location is a cost decision, not just a flow decision. Put the gate in the wrong spot and you add weld lines on a visible face. Or you pay for a hot runner where a cold runner would do. Ask where the gate mark will sit before you approve the mold layout.

Common Gate Types

• Edge gate
• Pin gate
• Submarine gate
• Fan gate
• Hot runner gate

Design Rules

Balance the runners so every cavity fills at the same pace. Keep flow paths short. Gate into thick sections and keep gate marks off cosmetic faces. Good mold design shows in the parts, not in the CAD file.

Match the gate to the part. Edge gates suit simple shapes and are easy to maintain. Submarine gates hide the mark on cosmetic parts but need higher pressure. Hot runner gates leave almost no waste, which pays off at high volume.

4. Cooling System Design

Cooling eats 60–70% of the cycle. That single fact makes cooling layout the most valuable place to spend design effort. Channels should run close to the cavity wall with even spacing, so the steel temperature stays uniform shot after shot.

Cooling Design Guidelines

Where the geometry allows, conformal cooling follows the part shape instead of drilled straight lines. It costs more upfront. It pays back on thick or complex parts through shorter cycles and less warpage. For published test data on conformal cooling, see Plastics Technology. Our own notes on cooling channel layout go deeper on spacing and flow rates.

Cooling time grows with the square of wall thickness. Double the wall and the cooling time roughly quadruples. That is why uniform thin walls beat thick sections every time.

5. Venting Design

Burn marks and short shots often trace back to trapped air, not to the press settings. Air caught in the cavity compresses, heats up, and scorches the plastic. Vents give it a way out.

Benefits of Proper Venting

Cut vents 0.02–0.05 mm deep at the parting line and at the last places to fill. Go deeper and the vents will flash. Go too shallow and they might as well not exist. Venting is the most skipped step in mold design, and it is also the cheapest to fix. Put vents where the melt arrives last, usually the far end from the gate.

6. Ejection System Design

Ejection fails quietly. The part looks fine until it warps a day later, because the pins pushed on a soft, still-warm section. Spread the ejection force across stiff areas of the part, such as ribs, bosses, and thick walls.

Common Ejection Methods

• Ejector pins
• Sleeve ejectors
• Stripper plates
• Air ejection

On deep thin-wall parts, add air assist before you add more pins. Extra pins leave marks; air does not.

7. Mold Steel Selection

Match the steel to the job, not to the budget alone. P20 handles most general molds. H13 takes the heat of high-temperature resins. S136 polishes to a mirror for optical or cosmetic parts.

Common mold steels:

Cheap steel in a high-volume tool is the most expensive choice you can make. A re-cut costs more than the upgrade ever did. We wrote up our own approach to choosing mold steel separately. For an outside reference on steel grades, see MoldMaking Technology.