Shenzhen Alu Rapid Prototype Precision Co., Ltd.

Shrinkage in injection molding occurs as the molten plastic cools and solidifies, causing a volumetric reduction (typically 0.3%–3%, depending on the material) due to thermal contraction, crystallization, and molecular reorientation. This can lead to dimensional inaccuracies, warping, sink marks, or voids. Reducing it involves optimizing design, material choices, and process controls to ensure uniform cooling, better packing, and minimal anisotropy (directional differences in shrinkage).

1. Optimize Mold and Part Design

Design choices influence how evenly the material flows, cools, and shrinks. Focus on uniformity to minimize differentials.

a.Ensure uniform wall thickness: Vary thicknesses as little as possible (ideally 1.5–2 mm for most parts) to promote even cooling rates and reduce warping from uneven shrinkage. Thicker walls cool slower, increasing crystallization and shrinkage in semi-crystalline materials like polypropylene. 

b..Strategic gate placement: Position gates near thick sections or use multiple gates for large/complex parts to enable symmetrical filling, ensuring uniform material flow and pressure distribution. This reduces localized shrinkage and voids. 

c.Incorporate ribs, gussets, and radiused corners: Add ribs (thinner than adjacent walls) for reinforcement without excess thickness, and round sharp corners to distribute stress evenly and prevent sink marks. 

d.Design for shrinkage compensation: Oversize mold cavities by the expected shrinkage rate (use material datasheets for values) and ensure proper venting to release trapped air/gases, promoting even filling. 

e.Uniform cooling channels: Place channels strategically for consistent mold temperatures, avoiding hot spots that cause differential shrinkage. Verify cooling lines are efficient and unobstructed.

2. Select Appropriate Materials and Additives

Material properties dictate baseline shrinkage—amorphous polymers shrink less uniformly than semi-crystalline ones.

a.Choose low-shrinkage materials: Opt for amorphous thermoplastics like ABS, polycarbonate, or polystyrene (0.3%–0.7% shrinkage) over semi-crystalline options like nylon or polypropylene (1%–3%) for better dimensional stability. 

b.Incorporate fillers and reinforcements: Add mineral fillers (e.g., talc, calcium carbonate) to reduce overall shrinkage by providing non-shrinking volume and improving isotropy. Use fiber reinforcements (e.g., glass fibers) to lower shrinkage in the flow direction (by 50%–80%), but balance to avoid warpage from anisotropy. 

c.Consider polymer modifiers: Additives like hyperbranched polymers can enhance toughness and further minimize shrinkage in high-shrink materials.

3. Fine-Tune Process Parameters

Adjust machine settings to improve packing and control cooling dynamics.

a.Control temperatures:

Melt temperature: Use moderate levels to ensure good flow and packing without excessive expansion/contraction—higher temps can increase shrinkage if packing is inadequate. 

Mold/cavity temperature: Maintain uniform, moderate temps (e.g., 40°C–80°C depending on material) to slow cooling evenly; hotter molds allow better molecular packing before solidification, reducing shrinkage, while colder ones solidify outer layers first (risking unevenness). 

b.Optimize pressures and speeds:

Injection pressure and speed: Increase pressure for denser packing; reduce speed to allow gradual filling and minimize shear-induced orientation that amplifies anisotropy. 

Holding/packing pressure and time: Apply higher pressure (e.g., 50%–80% of injection pressure) for longer durations to force additional material into the mold as it shrinks, compensating for up to 0.5% volume loss—especially effective with large gates. 

c.Extend cooling time: 

Allow sufficient time (based on wall thickness) for full solidification before ejection to minimize post-mold shrinkage and internal stresses; for semi-crystalline materials, longer cooling increases mold restraint. Optional: Immerse parts in cold water post-ejection to rapidly solidify the core, but monitor for induced stresses