Common Misconceptions in Injection Molding Melt Temperature Control
Melt temperature is a critical parameter directly influencing plasticizing quality, flow behavior, dimensional stability, and mechanical properties. However, many operators rely on empiricism rather than scientific principles, leading to defects, material waste, and reduced productivity. This section outlines the five most common misconceptions and the correct technical approaches.
Misconception 1: Blindly Increasing Temperature to Improve Flowability
A common reaction to short shots or weld lines is to raise the melt temperature, assuming higher temperatures always improve flow. In reality, every material has a defined processing window. Exceeding this range causes thermal degradation: PE may discolor and become brittle above 300°C; ABS yellows and develops silver streaks above 270°C; PC degrades rapidly above 320°C, releasing harmful gases and reducing mechanical strength. Even below degradation temperatures, excessive heat increases shrinkage, leading to warpage and nozzle drooling. The correct approach is to first optimize screw speed and back pressure, then increase temperature in small increments (≤5°C) only when necessary.
Misconception 2: Ignoring the Need for a Rational Temperature Profile
A proper temperature gradient (Rear Low → Middle → Front High → Nozzle Slightly Lower) ensures gradual, uniform melting. Setting all zones to the same temperature or overheating the rear zone causes early melting, excessive shear heat, and localized degradation. An underheated front zone results in incomplete melting, causing bubbles, specks, and poor surface quality. Industry standards recommend: Rear zone 10–20°C below the melting point, middle zone increasing by 5–10°C, front zone near the upper processing limit, and nozzle 5–10°C lower than the front zone.
Misconception 3: Using a Fixed Temperature for All Part Geometries
Wall thickness and mold temperature significantly influence melt temperature requirements. Using the same temperature for thin- and thick-walled parts is a mistake. Thin parts require higher flowability, while thick parts need lower temperatures to avoid excessive shrinkage, sink marks, and internal stresses. Mold temperature must also be considered: a cold mold with high melt temperature causes rapid surface cooling, creating internal stresses and potential cracking; a hot mold allows lower melt temperatures to improve cycle time. For example, PP parts with wall thickness >5mm typically require 10–15°C lower melt temperature and a mold temperature of 50–60°C.
Misconception 4: Over-Reliance on Temperature to Solve Defects
Many defects are incorrectly attributed to temperature. Flash is often caused by insufficient clamping force or excessive injection pressure, not high temperature. Silver streaks and bubbles usually stem from poor drying. Weld lines can be improved by optimizing gate location or increasing mold temperature, not just by raising melt temperature. A systematic approach—checking melt appearance, drying conditions, and process parameters—is essential.
Misconception 5: Frequent, Large Temperature Adjustments
Temperature stability is critical for consistent part quality. Large, frequent adjustments cause severe fluctuations in melt viscosity and plasticizing quality, leading to unstable dimensions and mechanical properties. Industry standards recommend maintaining a temperature fluctuation within ±3°C. Adjustments should be made in small steps (5°C), followed by a stabilization period of 3–5 cycles before evaluating part quality. Regular calibration of the temperature control system is also necessary to prevent thermocouple errors.
Summary
The core principles of melt temperature control are precise matching, rational profiling, dynamic adjustment, and stability. Operators must move beyond empiricism and adopt a systematic approach based on material characteristics, part geometry, and process conditions to ensure high-quality production and cost efficiency.
