Процесс литья под давлением: Полное пошаговое руководство

Injection molding is the dominant manufacturing process for plastic parts at scale. It converts raw polymer pellets into finished components in seconds, producing everything from bottle caps to automotive dashboards with high precision and repeatability. Understanding how the process works helps engineers design better parts, avoid common defects, and communicate effectively with molders.

How Injection Molding Works: The Basic Cycle

The injection molding cycle consists of five distinct phases, each with specific parameter controls that determine part quality.

1. Clamping

Before injection begins, the mold must be securely clamped shut. The clamping force must exceed the injection pressure (typically 60-150 MPa) to prevent flash—thin layers of material escaping the mold cavity at the parting line. Clamp tonnage is typically calculated at 3-8 tons per square inch of projected part area, depending on material and part geometry.

2. Injection

Plastic pellets are fed from a hopper into a heated barrel where a rotating screw melts and homogenizes the material. The molten plastic is then injected into the mold cavity at high speed and pressure. Injection speed and pressure profiles are critical—they must fill the cavity completely before the material begins to solidify.

3. Packing and Holding

After the cavity is filled, additional pressure (holding pressure) is applied to compensate for material shrinkage as the part cools. This phase typically lasts 5-20 seconds depending on wall thickness. Inadequate holding pressure results in sink marks and dimensional shortfalls.

4. Cooling

The part solidifies as heat transfers to the mold walls. Cooling time is the longest phase of the cycle, typically 10-60 seconds. Cooling channel design in the mold is critical—well-designed cooling reduces cycle time and improves part quality.

5. Ejection

Once the part has solidified sufficiently, the mold opens and an ejection system (typically spring-loaded pins or a stripper plate) pushes the part out of the cavity. Draft angles on part walls facilitate ejection. Ejection too early causes warpage; too late wastes cycle time.

Основные параметры технологического процесса

Параметр	Typical Range	Effect on Part
Температура плавления	220-300 degrees C (material dependent)	Higher = better fill, more degradation risk
Температура пресс-формы	20-120 degrees C	Higher = better surface, longer cycle
Давление впрыска	60-150 MPa	Higher = better fill, flash risk
Поддержание давления	40-80% of injection pressure	Compensates shrinkage, affects density
Cooling Time	10-60 seconds	Longer = less warpage, slower cycle
id #ddd;”>Total Cycle Time	15-120 seconds (typical)	Drives production cost per part

Common Defects and Their Causes

Sink Marks

Dimensional voids or depressions on the surface, typically over thick sections or ribs. Caused by insufficient holding pressure, short holding time, or excessive melt temperature. Fix: increase packing pressure and time, reduce wall thickness transitions.

Flash

Thin excess material at the parting line or around cores. Caused by insufficient clamp force, excessive injection pressure, or worn mold. Fix: increase clamp tonnage, reduce injection pressure, inspect mold for wear.

Warping

Non-planar distortion of the finished part. Caused by uneven cooling, excessive shear from high injection speed, or residual stress from insufficient annealing. Fix: balance cooling channels, adjust gate location, use模具temperature control.

Short Shot

Incomplete filling—the cavity is only partially filled. Caused by insufficient injection pressure/speed, material that is too cold, or clogged gates. Fix: increase injection parameters, verify material drying, check gate clearance.

Weld Lines

Visible lines where two flow fronts meet in the cavity. Caused by multiple gate locations, complex geometry, or low

melt temperature. Weld lines are weak points and stress concentrators. Fix: raise melt temperature, redesign gate locations, add venting at weld line locations.

Burning Marks

Dark spots or streaks near the end of fill, typically from trapped air that ignites. Caused by excessive injection speed, poor venting, or excessive moisture in material. Fix: reduce injection speed, add vacuum vents, verify material dryness.

Injection Molding vs. Other Processes

Фактор	Литье под давлением	Обработка с ЧПУ	3D-печать
Best for Quantity	Более 1 000 деталей	1-500 parts	1-100 parts
Толерантность	+/-0.05 mm typical	+/-0.01 mm achievable	+/-0.1-0.5 mm typical
Отделка поверхности	Mold finish dependent	Tool mark dependent	Layer lines visible
Стоимость оснастки	High ($10K-$500K+)	Low-medium	Нет
Расточительство материалов	Sprue/runner system	Chip evacuation	Support structures

Our Injection Molding Capabilities

We offer injection molding services for prototyping and low-to-mid-volume production:

• Multi-cavity tooling for competitive unit pricing at moderate volumes
• Overmolding and insert molding for multi-material assemblies
• Hot runner and cold runner systems optimized for part requirements
• Full DFM analysis before mold construction to minimize defects

Related Reading

• 3D Printing vs Injection Molding: Which Manufacturing Process Is Right for Your Project?
• Калибровочный куб для 3D-принтера: полное пошаговое руководство
• От прототипа к производству: Масштабирование производственного процесса