Definition: During molding, uneven shrinkage of plastic or insufficient plastic compensation during holding pressure stage leads to an uneven surface of molded part. This defect is called shrinkage.
Location of Occurrence: Shrinkage generally occurs in areas of greater wall thickness in plastic part.
Causes:

 

A. Insufficient Cross-sectional Area of Gate and Runner: If cross-sectional area of mold's gate and runner is too small, filling resistance will increase, making shrinkage more likely. Therefore, cross-sectional area at corresponding locations should be enlarged.

 

B. Poor Mold Sealing: If mold seal is poor, pressure inside cavity will be uneven and low, leading to shrinkage. Therefore, sealing performance of mold should be carefully checked. Many factors affect sealing performance of mold, such as mold processing, etc. Excessive wear, poor guiding, and foreign objects on parting surface should be addressed individually after inspection.

 

C. Poor Mold Venting: Poor mold venting affects melt filling, leading to shrinkage and under-expansion. Therefore, it is essential to ensure venting system functions properly; this can be checked through design and usage.

 

D. Asymmetrical Gate Positions: Asymmetrical gate positions result in uneven melt entry speeds into different cavities, causing uneven cooling of plastic parts and resulting in depressions. Therefore, gates should be placed as symmetrically as possible.

 

E. Uneven or Insufficient Mold Cooling: Uneven or insufficient mold cooling easily leads to depressions. Therefore, design and manufacture of cooling system must be carefully considered. For areas prone to dents, cooling should be enhanced.

 

F. Unstable feeding system: An unstable feeding system in injection molding machine leads to inconsistent material supply, easily causing dents. Feeding system should be inspected to ensure adequate material supply.
G. Nozzle orifice too small or partially blocked: If nozzle orifice is too small or partially blocked, it will cause excessive localized pressure loss. A larger diameter nozzle should be used, or nozzle should be cleaned and unblocked.
H. Excessive moisture or volatile matter in raw material: If raw material contains excessive moisture or volatile matter, gas generated after heating will accumulate in mold cavity, making it difficult for melt to fill mold smoothly. Raw material should be cleaned and ventilated.
I. Insufficient Lubricant in Raw Materials: Insufficient lubricant in raw materials results in poor melt flow and potential sink marks. Amount of lubricant should be increased appropriately.
J. High Resin Shrinkage: A higher resin shrinkage rate leads to greater potential shrinkage. Therefore, resins with low shrinkage rates should be selected as raw materials whenever possible.

K. High Melt Temperature or Mold Temperature: Excessively high melt or mold temperatures can cause insufficient cooling and shrinkage in molded parts. Melt and mold temperatures should be appropriately reduced.
L. Injection Time and Holding Time Too Short: If injection and holding times are too short, melt will not fill mold sufficiently, easily leading to sink marks. Injection and holding times should be appropriately extended.
M. Injection and holding pressures are too low. Insufficient injection pressure reduces filling speed; insufficient holding pressure prevents melt from fully filling mold. To address this, injection and holding pressures should be appropriately increased.
N. Plastic parts with inserts. Plastic parts with inserts often have inserts made of metal, particularly steel. Significant difference in thermal expansion coefficients between steel and plastic makes area around insert prone to shrinkage. Using a non-ferrous metal with a smaller difference in thermal expansion coefficient and thickening plastic layer around insert will greatly reduce shrinkage.
O. Uneven wall thickness of plastic part. If wall thickness of a plastic part varies too much, shrinkage is likely to occur. Therefore, when designing plastic parts, wall thickness should be made as equal as possible or difference should not be too large.
EXAMPLE: As shown in image below, when molding back cover of M5209 machine tool, a three-point injection system was used to ensure good filling in all areas.

 

However, situation is not ideal. (See image below)

 

Improving molding conditions here is quite difficult because it is far from gate. Applied holding pressure cannot fill depressions in this area. Furthermore, considering product's structural characteristics, due to areas with thinner sections, incomplete filling is likely, making low-speed injection impossible. Therefore, surface shrinkage in this type of product can only be reduced, not eliminated.