Professional Analysis of Injection Molding Defects in Plastic Products (Part 1)

1. Unstable Plasticization What is Unstable Plasticization? Unstable plasticization refers to the phenomenon where resin cannot be consistently fed into the barrel or the feed volume fluctuates. Thi

1. Unstable Plasticization



What is Unstable Plasticization? Unstable plasticization refers to the phenomenon where resin cannot be consistently fed into the barrel or the feed volume fluctuates. This typically manifests in the following patterns:

• Failure to pre-plasticize entirely.

• Inconsistent pre-plasticization (dosing) time.

• Intermittent short shots (insufficient filling). Essentially, the volume of molten resin delivered to the front of the barrel varies shot-by-shot.







The Principle of Plasticization The key to efficient plasticization lies in the friction differential: the resin pellets should have high friction against the barrel wall (to be pulled forward) and low friction against the screw surface (to slide along the flights).







Root Causes of Instability

• Improper Screw Speed: Higher speeds generally increase conveying power. However, if the speed is too low, the feeding force weakens, leading to dosing errors. Conversely, if the speed is too high, the pellets may rotate with the screw rather than moving forward.

• Incorrect Back Pressure: While back pressure helps eliminate gas and stabilizes melt density, excessive back pressure resists the backward movement of the screw, destabilizing the dosing process.

• Inconsistent Temperature Profiles: The temperature near the hopper (feed zone) is critical. If it is too high, pellets melt prematurely and stick to the screw, causing "bridging" and reducing conveying efficiency.

• Regrind Material: Reground material often has irregular shapes and dust, which increases inter-pellet friction and can lead to inconsistent feeding compared to virgin pellets.





Solutions

1. Optimize Screw Speed: Start by measuring the dosing time over 50–100 cycles to identify fluctuations.

2. Adjust Feed Zone Temperature: Gradually lower the temperature under the hopper (in increments of 10°C) to prevent premature melting.

3. Manage Regrind: Ensure regrind size is uniform and minimize dust/fines before mixing with virgin material.



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2. Gate Blush (Gate Vestige/Splay)

What is Gate Blush? Gate blush appears as a dull or discolored "cloudy" patch or small flow marks near the gate area, significantly impacting the aesthetic quality of the part.

Causes: Flow Instability Gate blush is primarily caused by unstable flow patterns as the melt enters the cavity:

• Process: Low mold temperatures or excessive injection speeds.

• Tooling: Gate sizes that are too small or gates that cause "jetting" (where the melt shoots into the cavity without contacting the walls).

• Material: Low-viscosity resins or materials with poor flow characteristics.







Solutions

• Multi-Stage Injection: Use a profiled injection speed—start slow as the melt passes through the gate, then increase speed to fill the cavity.

• Increase Mold Temperature: This prevents the "skin" from freezing too quickly and improves surface finish.

• Optimize Tooling: Enlarge the gate cross-section or change the gate location to ensure the melt hits a wall (impingement) upon entry.



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3. Bubbles and Voids

What are Bubbles? Bubbles manifest as either surface bulges or internal voids trapped within the part's cross-section.

Root Causes

1. Entrapped Air: Caused by high screw speeds, low back pressure, or excessive decompression (suck-back). In the cavity, it can be caused by excessive injection speeds or "race-tracking" effects.

2. Material Degradation: If the melt stays in the barrel too long or at too high a temperature, the polymer chains break down, releasing gas.

3. Moisture: Inadequately dried hygroscopic materials (like PA or PC) will create steam bubbles.





Solutions

• Air Entrapment: Increase back pressure to compress the melt and reduce decompression distance. Perform a "filling study" to observe the flow pattern.

• Prevent Degradation: Reduce barrel temperatures and ensure the machine shot size is appropriately matched to the part weight (avoiding long residence times).

• Venting: Improve mold venting at the last point of fill to allow air to escape.



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