What Are The Factors Affecting The Shrinkage Rate in Injection Molding?

1. Influence of Material Properties on Shrinkage

(1) Plastic Type:

Different resin materials exhibit distinct shrinkage rates. Shrinkage can even vary between batches of the same resin grade from the same manufacturer, or between different manufacturers. Furthermore, inherent resin characteristics dictate that some materials have a wide shrinkage range, while others are narrower.

(2) Glass Fiber (GF) Content:

Shrinkage behavior for the same plastic type changes with varying GF content. As glass fiber content increases, shrinkage generally decreases. Typically, adding 20% to 40% by weight GF to a thermoplastic resin can reduce shrinkage by one-quarter to one-half.

However, molding practice reveals that in the flow direction, this reduction is largely unaffected by part wall thickness. Conversely, in the direction perpendicular to flow:

• At constant wall thickness, shrinkage decreases as GF content increases.

• In thin-walled sections, shrinkage is almost unaffected by GF content.

2. Influence of Mold Design Features on Shrinkage

(1) Parting Line and Gate:

The mold parting line, gate type, and gate dimensions directly influence melt flow direction, density distribution, the effectiveness of pressure holding/compensation, and cycle time.

• Using a direct gate or a large cross-section gate reduces shrinkage but increases anisotropy: shrinkage is lower along the flow direction and higher perpendicular to it.

• Conversely, a small gate thickness can cause premature gate freeze-off. Plastic shrinking within the cavity then lacks timely compensation, resulting in higher shrinkage.

• Pin-point gates seal quickly. Where part design allows, using multiple gates can effectively extend holding pressure time and increase cavity pressure, thereby reducing shrinkage.

(2) Part Geometry:

Part shape, size, wall thickness, the presence of inserts, the number of inserts, and their distribution significantly impact shrinkage. Generally, parts that are complex in shape, small in size, thin-walled, contain inserts, or have multiple symmetrically distributed inserts exhibit lower shrinkage.

(3) Insert Design:

While metal inserts in injection molded parts serve functional requirements locally, they hinder free shrinkage. The part remains constrained within the mold until ejection (an in-mold constraint effect). Inserts disrupt melt flow, density distribution, and shrinkage in their vicinity, and the insert itself acts as a heat sink. Consequently, parts with inserts generally exhibit lower shrinkage than parts without.

• Furthermore, overly complex or large inserts can cause differential shrinkage between different sections of the part due to mutual constraints. Complex geometries generally show lower shrinkage than simple ones due to these internal constraints.

(4) Cooling System:

The layout of the mold cooling channels affects cavity surface temperature, which in turn influences the cooling rate and shrinkage process at different points on the part.

• Areas of the cavity closer to cooling channels cool faster. This shortens the time for thermal changes, increasing the gap between the polymer's actual specific volume and its equilibrium value. Additionally, by the time in-mold shrinkage begins, the surface temperature here is already low, limiting further shrinkage.

• Cooling channel layout and size design directly impact mold temperature distribution and part cooling. Poor design can lead to shrinkage variation: faster-cooled areas tend to exhibit higher shrinkage. Complex part geometries, varying wall thicknesses, and sequential filling often cause non-uniform cooling, resulting in significant shrinkage variation.

• To mitigate this, cooling water can be directed first to warmer zones; even using warm water in fast-cooling areas and cold water in slow-cooling areas. This minimizes shrinkage variation and prevents part warpage or cracking.

3. Influence of Processing Conditions on Shrinkage

(1) Pressure:

Injection molding pressures (Injection Pressure, Holding Pressure, Cavity Pressure) significantly influence part shrinkage behavior.

• Higher Injection Pressure generally reduces shrinkage. Increased pressure raises injection speed, accelerating filling. This generates shear heating (raising melt temperature and reducing flow resistance) and allows earlier transition to the holding/packing phase while the melt is still hot and less viscous. This is particularly critical for thin-walled parts and parts with small gates due to their rapid cooling.

• Higher Holding Pressure and Cavity Pressure promote cavity packing, reducing shrinkage. Holding pressure has a particularly pronounced effect. This is explained by the compression of molten resin under pressure: higher pressure = greater compression = greater elastic recovery after pressure release = part dimensions closer to cavity dimensions = less shrinkage.

• Crucially, cavity pressure is not uniform throughout the part; pressure varies between hard-to-fill and easy-to-fill areas. Furthermore, pressure should be balanced across cavities in multi-cavity molds to prevent differential shrinkage between cavities.

(2) Temperature:

  Melt Temperature: Temperature critically affects polymer melt viscosity. Above the flow temperature, viscosity decreases exponentially with increasing temperature (increased free volume, reduced intermolecular forces), enhancing flowability. Temperature is the primary means to control viscosity for mold filling.
 While principles of crystallization, orientation, and thermal expansion suggest shrinkage should increase with higher melt temperature during packing/cooling, some experiments show the opposite.
  This is because higher melt temperature reduces viscosity. If Injection and Holding Pressure are constant, gate freeze-off is delayed, extending holding time, improving compensation, increasing density, and thus reducing shrinkage.
  Therefore, the net effect of melt temperature on shrinkage results from the interplay of thermal shrinkage, crystallization shrinkage, orientation shrinkage, and packing compensation. If the first three dominate, shrinkage increases with temperature. If packing compensation dominates, shrinkage decreases with temperature.
 Mold Temperature: After injection, the melt solidifies by releasing heat. Different plastics require specific optimal mold temperatures for efficient molding, minimal stress, and minimal warpage.
  Mold temperature primarily controls cooling/solidification and affects shrinkage mainly after gate freeze-off until ejection.
 Before gate freeze-off, higher mold temperature increases thermal contraction tendency but also delays gate freeze-off, allowing Injection/Holding Pressure to act longer, enhancing compensation and increasing negative shrinkage (packing).
 Overall shrinkage is the net result of these opposing effects and does not necessarily increase with mold temperature. After gate freeze-off, pressure effects cease. Higher mold temperature then prolongs cooling time, and shrinkage upon ejection generally increases.

(3) Time:

 Injection Time (Filling Time): The duration the screw advances to inject and fill the cavity under pressure.
  Shorter injection times (before gate seal) lead to higher shrinkage and greater shrinkage variation. Once injection time meets or exceeds the gate freeze time, further extension has no effect on part weight or shrinkage. Injection time control is closely tied to gate thickness, which largely determines gate freeze time.
  Extending injection time after gate freeze is ineffective, reduces productivity, and can cause defects (e.g., cracks near the gate). For a given part, injection time is determined by injection speed.
 Holding/Packing Time: Longer holding times allow better melt packing and compensation, increasing part density and reducing shrinkage. Once the gate seals, holding pressure becomes ineffective. Excessive holding time unnecessarily extends cycle time.
 Cooling Time (In-Mold): The time the part cools within the closed mold before ejection. Its effect on shrinkage depends on resin type, part thickness, melt temperature, mold temperature, and crystallization behavior.
  Generally, longer cooling times allow more uniform solidification, resulting in parts closer to cavity dimensions and thus lower shrinkage.
  For amorphous resins, cooling time has minimal impact on final shrinkage.

  For semi-crystalline resins, prolonged cooling can allow more complete crystallization, potentially increasing shrinkage due to higher crystallinity. However, the dominant effect of adequate cooling time is usually reduced shrinkage through better dimensional stability.