Variación Dimensional
Causas, Soluciones y Prevención en Moldeo por Inyección
La inestabilidad dimensional proviene de contracción inconsistente, inestabilidad de proceso o compensación incorrecta del molde.
A continuación encontrará información general sobre este defecto. Para obtener una solución precisa adaptada a su máquina, material y parámetros de proceso utilizando la base de datos de resolución de problemas de moldeo por inyección más completa del mundo — regístrese y acceda a diagnóstico asistido por IA, 7 calculadoras de ingeniería y seguimiento detallado de proyectos.
🔍Síntomas e Identificación Visual
⚠️Causas Raíz por Categoría
⚙️ Proceso
- Shot weight variation inconsistent pack dimensional scatter
- Presión de mantenimiento variation
- Cooling water temperature fluctuation
- Temperatura del molde drift
🔲 Molde
- Shrinkage calculated incorrectly averaged min max should test with flat plate mold
- Cavity imbalance in multi-cavity mold
- Cooling system imbalance differential shrinkage
🧪 Material
- MFI drift between lots shrinkage drift
- Additive change talc in PP changes shrinkage
- Proporción de remolido demasiado alta lower MW more shrinkage
- Moisture hydrolysis lower MW more shrinkage
🗂 Diseño Molde
- Shrinkage calculation in mold is incorrect
- Runner and cold slug well problems in mold
- Gate location / flow length ratio is incorrect
- Sprue and runner cross-section area and shape are incorrect.
- Cooling system faulty
- Dimensional differences between cavities in multi-cavity mold
🔧 Estado del Molde
- Mold parting line shift
📐 Diseño Pieza
- Tolerancing is incorrect
- Corner designs at 90° (no radius)
- Mold-dependent and independent dimensional tolerances not specified in drawing
📋 General
- Wrong tolerance system DIN 7168 metal used instead of DIN 16742 plastic
- Sharp corners in part shear stress dimensional change
- Screw position repeatability shot weight variation
✅Soluciones y Acciones Correctivas
⚙️ Proceso
- Stabilize shot weight fix check valve barrel heaters thermocouple contacts
- Monitor holding pressure and cooling water temperature
- Use SPC on shot weight and key dimensions
🔲 Molde
- Test shrinkage on flat plate mold for exact value not average
- Balance multi-cavity runner system
- Fix cooling circuit imbalance
- Polish parting line by procedure only 11-step protocol
🧪 Material
- Materials with high MFI have greater shrinkage. Lower MFI grades can be tried.
- If additive use is decided after mold construction, part dimensions tend to come out larger. For example, adding talc to PP makes it behave like PS — shrinkage decreases and parts come out longer.
🗂 Diseño Molde
- In a flat plate mold, first measure the mold dimension. Then inject the material and measure the part. The difference precisely determines the shrinkage rate.
- Incorrect runner diameter and cross-section design can cause dimensional variation. Runner cross-section must be full round. Missing cold slug wells can also reduce polymer flow into the cavity. Runner cross-section and cold slug well design must be done correctly.
- The maximum flow length at minimum wall thickness must be known and design should be made accordingly
- Runner cross-section size must be calculated correctly based on part weight and must be full round. In hot runner systems, correct sizing must be selected based on material type and part weight.
- As a principle, mold surface temperature must be uniform at every point. If not achieved, differential shrinkage will cause dimensional errors.
- If cavity balance is incorrect, part dimensions vary depending on injection speed.
🔧 Estado del Molde
- Incorrect pressure during mold polishing causes mold dimensions to grow after certain use. Polishing should be done on a bench in the mold shop, not on the machine, to apply correct pressure.
📐 Diseño Pieza
- DIN 16742 tolerance system is used for plastics. If tolerances are not given according to mold-dependent and independent shrinkage values per material type, different dimensions result. Designers sometimes use DIN 7168 which is a metalworking tolerance.
- Sharp-cornered plastic part designs create shear stress and internal stress. Minimum 0.2 mm radius should be given in plastic part design.
- If the part designer does not account for these tolerances in the drawing, the mold maker machines directly on CNC. These dimensions are given in DIN 16901.
📋 General
- Apply DIN 16742 tolerances not DIN 7168
- Get MFI certificate for each incoming lot
- Reducir proporción de remolido
🛠Solución Paso a Paso
Verificar parámetros de proceso
Stabilize shot weight fix check valve barrel heaters thermocouple contacts
Inspeccionar el molde
Test shrinkage on flat plate mold for exact value not average
Verificar estado del material
Materials with high MFI have greater shrinkage. Lower MFI grades can be tried.
❓Preguntas Frecuentes
¿Qué causa la Variación dimensional en moldeo por inyección?
Variación dimensional es causado por varios factores que incluyen: Shot weight variation inconsistent pack dimensional scatter; Presión de mantenimiento variation; Shrinkage calculated incorrectly averaged min max should test with flat plate mold; Cavity imbalance in multi-cavity mold; MFI drift between lots shrinkage drift; Additive change talc in PP changes shrinkage.
¿Cómo se corrige la Variación dimensional en moldeo por inyección?
Comenzar verificando los parámetros clave: Peso de inyectada (g), Tiempo de enfriamiento, Temperatura del molde, Compensación de contracción. Stabilize shot weight fix check valve barrel heaters thermocouple contacts
¿Qué plásticos son más afectados por la Variación dimensional?
Materiales comunes: PA6, PA66, POM, PP, PBT. Preste especial atención al secado, contracción y temperatura del molde para cada grado.
¿Cómo prevenir la Variación dimensional en futuras producciones?
La prevención se basa en tres pilares: (1) optimización de proceso — verificar Peso de inyectada (g), Tiempo de enfriamiento; (2) mantenimiento del molde — verificar entradas, venteos y canales de enfriamiento; (3) control de material — asegurar secado correcto y proporción virgen/remolido.
¿Cuál es la diferencia entre la Variación dimensional y defectos similares?
Part dimensions vary batch to batch or within a batch. Part does not fit assembly. Dimensions shift over time. Esto lo distingue de defectos de superficie similares durante la inspección visual.
📐Calculadoras Relacionadas
Verifique sus parámetros de proceso y prevenga este defecto con nuestras calculadoras.
🔗Defectos Relacionados
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