Cold slug is cold slug generated during melt injection molding process that enters mold cavity, causing molded plastic part to be mixed with these cold slugs, resulting in obvious unfused small micro-blocks on the surface of product. Color and performance of cold slug are different from those of main plastic. Following is a brief analysis of causes and improvements of cold slugs.

 

(1) Influence of cold slug well in pouring system
If there is no cold slug well in pouring system, cold slug generated during injection molding will enter mold cavity and affect quality of product. Therefore, a suitable cold slug well is opened in runner.
(2) Influence of cold slug well capacity, size and position
If cold slug well capacity in pouring system is too small or position is incorrect, cold slug cannot be accommodated by cold slug well and enters mold cavity to form cold slug. In this regard, cold well should be opened correctly. Cold well is usually set at the end of main runner. When length of branch runner is long, a cold well should also be opened at the end and have a suitable capacity.

(3) Consider cold material problem when designing diving angle and gate size of submerged gate. Size of diving angle is conducive to demoulding, gate size is conducive to division of labor of same product and control of generation of cold material.

 

Diving angle diagram of runner

 

Gate size diagram

(4) Add an R angle between submerged gate and branch runner to reduce friction between submerged gate and sharp corner of mold during demoulding, and prevent sharp corner of mold from scraping gate out of material.

 

R angle at connection between submerged gate and branch runner

(1) Generation of cold material at nozzle. Head of nozzle is prone to cold material, especially when using a long nozzle. Number of nozzle heating rings must be ensured, and nozzle head can also get sufficient heat.
(2) Nozzle and main runner bushing must be aligned, and centering must be adjusted to prevent leakage.
(3) Matching dimensions of nozzle R angle and mold runner R angle should be well designed. Nozzle R angle of injection molding machine is 1-2 degrees smaller than mold runner R angle, which can achieve a better match.
(4) Check damage of mold and nozzle matching position frequently to prevent collision or deformation causing leakage or cold material.
3. Injection molding process
(1) If melt temperature is too low, melt will be poorly plasticized. It is also easy to cool and solidify prematurely during injection process to produce cold material. For this, melt temperature should be increased.
(2) If mold temperature is too low, melt will cool quickly after entering runner and mold, which is easy to produce more cold material. If cold material cannot be accommodated by cold material well in time, it will enter cavity and produce cold material. For this, mold temperature should be appropriately increased.
(3) If injection pressure is too low, it will affect injection speed and shear heat of melt. When melt enters cavity, temperature is low and speed is slow, which is easy to produce more cold material. For this, injection pressure should be appropriately increased to ensure that melt can reach set injection speed.