A very core and critical issue in injection molding process. Plasticization quality is indeed cornerstone for determining final product performance, appearance qualification rate and production stability. As a process technician, if you cannot accurately judge quality of plasticization effect, it is like "driving with your eyes blindfolded". You cannot effectively analyze root cause of defect, let alone optimize process.

 

Common injection molding defects such as gas marks, silver wires, trapped air, scorching and other gas-related defects, source of 、gas is indeed closely related to plasticization effect. Poor plasticization may lead to:
1. Incompletely melted particles: Flow obstacles or hot spots are formed in subsequent filling, resulting in local degradation and gas production.
2. Overheating or degradation: Plastic stays in barrel for too long or local temperature is too high, thermal degradation occurs, and a large amount of small molecule gases (such as CO, CO2, monomers, etc.) are produced.
3. Uneven mixing: Masterbatch, additives, recycled materials, etc. are not mixed sufficiently, and local concentration is too high, which may lead to decomposition or cavitation.
4. Entrained air/water vapor: Improper screw design, back pressure, and speed during plasticization fail to effectively discharge air brought in by raw materials or incompletely dried water (water vapor turns into steam at high temperature).
Therefore, in actual production, to accurately determine whether plasticization quality has reached a "good" state, a comprehensive evaluation from multiple dimensions is required. Let's talk about this issue now
I. Visual observation (most direct and most commonly used)
1. Injection action and melt flow state:
Injection smoothness: observe state of melt flowing out of nozzle during injection.
Good state: Melt flows out in a uniform, continuous, and smooth "rat tail" shape, without interruption, spraying, or dripping. Flow rate is stable and there is no obvious fluctuation.
Bad state: Melt is sprayed in particles, flows out intermittently, drips severely, or flow rate is fast and slow. This indicates that melt has poor uniformity, unstable viscosity, or contains unmelted particles/gas.
Melt appearance:
Good state: Melt surface is smooth and glossy (specific glossiness is related to material itself), color is uniform and consistent (consistent with set color sample), and there are no visible unmelted particles, impurities, bubbles, moire or smoke.
Bad state:
Graininess/Moire: Rough surface, with lines or visible tiny particles, indicating uneven mixing or incomplete melting.
Bubbles: Bubbles emerge from inside or surface of melt, indicating that gas (air, water vapor, degradation gas) is not fully removed.
Discoloration/burnt particles: Uneven color, yellowing, blackening or black/brown spots, indicating local overheating and degradation.
Smoke: Obvious smoke (especially abnormal color smoke, such as yellow smoke and black smoke) during injection is a sign of severe overheating and degradation.
Stratification/Flower: Melt color or texture stratification, usually caused by different batches of raw materials, too high proportion of recycled materials or uneven mixing.

 

2. Empty shot strips (the most important and intuitive evaluation method):
Operation: Under premise of ensuring safety, close mold and perform empty shot (shoot melt into air or a special collection container), and take a few cooled strips for observation.
Key points for judging: Appearance:
Good condition: Surface of material strip is smooth, dense, with uniform luster, consistent color, no spots, no bubbles, no unmelted particles, no moire or stripes. Cross section is dense and uniform, without pores or voids.
Bad condition:
Surface roughness/graininess: It feels obviously rough, and tiny unmelted particles or crystal points (especially transparent materials) can be seen. Indicates that plasticizing temperature is insufficient or screw shear is insufficient.
Bubbles/voids: There are bubbles or holes on the surface or inside. Indicates that gas (moisture, air, degradation gas) has not been completely removed. Fine and uniform bubbles may be caused by excessive moisture; large bubbles or voids may be caused by low back pressure, insufficient screw compression ratio or severe degradation.
Moire/stripes/stratification: There are cloud-like, wavy or different colored stripes on the surface. Indicates uneven mixing of raw materials (masterbatch, additives, recycled materials), uneven plasticization or laminar flow of different melt viscosities.
Discoloration/burnt particles: Material strip changes color as a whole or partially (yellow, gray, black), or there are black/brown impurity particles. Indicates overheating degradation.
Dull/dim: Surface of material strip is dull and gray, which may be due to material degradation or lubricant precipitation (sometimes also related to material itself).
Physical properties (judged by hand feel/visual inspection):
Good state: Material strip has moderate flexibility (depending on material), is not easy to break brittle, and has a certain elasticity when bent. Cross section is dense.
Bad state:
Too hard/brittle: It is easy to break brittle when bent (especially high-temperature materials should have a certain toughness at normal plasticizing temperature), which may be insufficient plasticization (not fully melted) or severe degradation leading to molecular chain breakage.
Too soft/sticky: Material strip is abnormally soft or even sticky, which may be due to excessively high plasticizing temperature leading to degradation or precipitation of low molecular weight substances (such as plasticizers, lubricants)..