Common defects in plastic injection molding process

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Common Injection Molding Defects & How to Fix Them

A: Blistering

Ever seen bubbles or raised bumps on a molded part? That’s blistering! It happens when gas or moisture gets trapped. The fix? Pre-dry the resin and improve mold venting to prevent those annoying bubbles.

B: Black Spots

Tiny dark specks on your part? That’s likely due to overheating or contamination in the material. To keep your parts spotless, maintain temperature control and ensure clean material handling.

C: Cold Slugs

These small, solid plastic bits create weak spots in your part. The solution? Use heated nozzles and sprue bushings to keep your material flowing smoothly.

D: Delamination

If your part is peeling or flaking, you’re dealing with delamination—usually caused by material contamination or incompatibility. Stick to pure materials and optimize processing for a strong, smooth finish.

E: Embedded Foreign Particles

Dust, dirt, or other unwanted fragments can get stuck in your part, ruining quality. Keep your workspace clean, and your parts will come out flawless.

F: Flash

That thin ridge of excess material on your part? That’s flash. It happens when molds don’t clamp properly or pressure is too high. Adjust mold alignment and pressure for a clean-cut finish.

G: Gloss Variation

Uneven shine across the surface? This comes from inconsistent cooling or mold defects. Solve it by fine-tuning temperature control and mold finishing.

H: Jetting

Noticing snake-like patterns? That’s jetting—caused by material entering the mold too quickly. Reduce injection speed and reposition the gate for a smoother look.

I: Short Shots

If the mold cavity isn’t fully filled, leaving an incomplete part, it’s a short shot! Boost injection pressure and shot size to ensure a full, well-formed part.

J: Sink Marks

Those little depressions on the surface? Sink marks happen when thick sections cool unevenly. Reduce wall thickness and optimize cooling to get a flawless finish.

K: Splay Marks (Silver Streaks)

Silvery or white streaks on your part? That’s moisture or gas escaping. Pre-drying your material helps eliminate this issue.

L: Surface Roughness

If your part feels rough or uneven, improper mold surface finishing or cooling is to blame. Polish the mold and control cooling for a smooth texture.

M: Tiger Stripes

Dark, wavy lines on large, complex parts? These tiger stripes come from inconsistent flow and shear stress. Adjust injection speed and gate location for a better result.

N: Uneven Surface Texture

If the surface feels inconsistent, it’s often due to irregular cooling or poor mold finishing. Control temperature and mold preparation for a smooth, uniform part.

O: Drag Marks

Scratches from the part sticking to the mold? That’s a drag—literally! Add draft angles and polish the mold to avoid these marks.

P: Burn Marks

Black or brown discoloration? That’s from trapped air overheating. Improve mold venting and slow the injection speed to prevent burns.

Q: Color Streaking

Streaky colors messing up your design? It’s likely poor pigment dispersion. Use high-quality masterbatches and better mixing techniques to fix it.

R: Flow Marks

Wavy lines on the surface? Uneven molten flow is the culprit. Adjust mold temperature and injection speed for a cleaner look.

S: Scratches

Scratches can ruin both looks and function. Careful handling and better ejection techniques will keep your parts pristine.

T: Weld Lines

Weak spots where two flow fronts meet but don’t bond well? Those are weld lines. Optimize temperature and pressure control to strengthen these areas.

U: Angular Deviation

If your part’s angles aren’t lining up as they should, mold misalignment or warpage could be to blame. Precision machining and controlled cooling help straighten things out.

V: Brittleness

Snap! If your part breaks too easily, excessive shear, overheating, or degraded materials could be the issue. Lower processing temperature and add impact modifiers to toughen it up.

W: Dimensional Instability

Parts changing size over time? That’s dimensional instability, often caused by residual stress or improper cooling. Post-molding treatments help keep your parts in shape.

X: Excessive Brittleness

Think brittleness, but worse. If your part is too fragile, adjust cooling profiles and use more flexible materials for better durability.

Y: Incomplete Filling

If a mold fills only halfway, you’ll see unformed sections and voids. Low injection pressure and insufficient material could be at fault. Increase injection speed and pressure to fill the mold completely.

Z: Insufficient Rigidity

Parts that bend or flex too much? That’s insufficient rigidity, often caused by poor material selection or weak design. Reinforce with ribs or fiber-filled materials for a stronger build.