Most mold designers believe that surface shrinkage or internal shrinkage of injection molding parts can be improved by adjusting machine, which is related to injection molding pressure holding conditions, but not to mold structure. This is a completely wrong concept;
90% of plastic part surface shrinkage or internal shrinkage is related to design of mold gating system, most of which is caused by slender runners or small/thin gates, too early cooling and sealing, and insufficient "glue filling".
Following aspects must be optimized:
1. For thick-walled products, size of cold well of main and branch runners must be increased to prevent melt from cooling too early or too quickly in runner;
2. For thick-walled products, gate must be opened larger to ensure that need for long-term glue filling is met (mold matching process);
3. For plastic parts with uneven wall thickness and large wall thickness difference, gate position needs to be opened at the thickest part of plastic part, and gate needs to be enlarged a little to fully fill wall thickness. Ejector is set at rib position to reduce shrinkage;

 

4. Shrinkage of part far from gate can be improved by increasing number of gates or adding diversion grooves;
5. Concept of improving shrinkage of surface of plastic part or internal shrinkage hole should be "opening up glue filling channel", and do not rely on adjusting machine during injection molding to improve it;

 

Many people think that weld line (water line) on the surface of plastic parts cannot be improved, and in most cases, mold designers and injection molding personnel expect to improve it by adjusting machine. However, they do not know that weld line (water line) on the surface of plastic parts can only be thoroughly improved through mold design structure. Specific mold optimization measures are as follows:

 

1. Improve exhaust effect of mold at weld line;
2. Add electric heating rods to mold (where weld line is generated) to locally increase mold temperature;
3. Core matching accuracy is poor, resulting in wall of plastic part. Uneven thickness will produce weld lines;
4. Adjust position and number of gates to transfer weld lines to places that can be covered or not obvious after assembly;
5. Appropriately increase size of main and branch runners and cold material holes to prevent cold materials from entering mold and increasing obviousness of weld lines;
6. Increase number of mold water channels and reduce temperature gradient of mold to improve weld lines;
7. For annular products, change spoke gate to a central disc membrane gate;
8. Adjust gate position or number of gates to increase confluence angle of weld line (confluence angle is greater than 75 degrees), which can improve;
9. For weld lines caused by uneven wall thickness and uneven glue feeding, "flow limiting" and "flow guiding" design concepts can be used in mold design to ensure uniform flow of molten material, which can improve such weld lines;
10. For glass fiber reinforced plastics or color powders containing metal powder, when weld line cannot be improved, an overflow hole (cold material well) can be opened at weld line to move water pattern into overflow hole and then process it off (its thickness must be close to wall thickness to be effective;
11. Weld line can be improved by using a vacuum injection mold or a steam mold (high-temperature traceless mold);
12. For weld lines generated at molding hole or side concave hole, "core feeding" technology can be used to improve weld line;
13. Improve demoulding slope of mold and demoulding effect of plastic part to prevent weld line caused by release agent during injection process;

 

Since most of deformation of injection molded parts occurs after plastic parts are demolded, deformation of plastic parts has no obvious relationship with injection molding process conditions (except for stress deformation), but has a direct relationship with mold design. We must fundamentally improve problem of plastic part deformation by optimizing mold design. Deformation is divided into four types: structure, shrinkage, demolding and stress deformation.
Main mold optimization technologies are as follows:
1. Optimize demoulding effect (set ejector pin at rib/bone position) to improve changes caused by poor demoulding of plastic parts;

 

2. For deep cavity molds, add air intake (intake) devices to the front and rear molds to improve vacuum suction deformation;
3. Optimize ejection balance to improve deformation caused by unbalanced ejection of plastic parts;
4. For deformation of square plastic parts caused by structural reasons, adopt repair design from perspective of shrinkage rate differences at different positions to fundamentally solve deformation of such products;
5. For square plastic parts, strengthen cooling effect or insert beryllium copper at four corners of mold (heat accumulation at four corners) to improve its deformation;
6. For long strip plastic parts, set gate position at the end, and molten material flows along length direction, which can improve deformation caused by gate design in the middle;
7. For large plastic parts with complex structures, adding a "flow limiting pin" to mold can improve its deformation;
8. Increase number of cooling water channels to ensure uniform mold temperature and prevent deformation caused by uneven mold temperature and uneven shrinkage of plastic parts;
9. For thin-walled frame-shaped plastic parts, increase number of gates and shorten melt flow to improve deformation caused by molecular orientation;
10. Use reinforcing ribs to improve deformation of plastic parts (pull or support deformed parts);

 

1. Multi-cavity molds improve flash (flashing) of gate position caused by optimizing balance of expansion force;
2. Add support columns (support heads) to mold to prevent flash caused by "punching" deformation of mold;
3. For plastic parts with thicker walls, appropriately increase size of main/dividing runners and gates to reduce holding pressure and expansion force to reduce flash of plastic parts;
4. Optimize design of mold exhaust system, improve mold exhaust effect, reduce flash of plastic parts by reducing holding pressure and expansion force;
5. Scientifically determine gap of sealing surface of exhaust system to prevent flash caused by gap of end of melt being larger than gap of overflow;
6. Optimize mold glue feeding balance to prevent flash caused by unbalanced glue feeding;

 

7. Add "anti-collision blocks" to mold. Prevent flash caused by collision or crushing of mold surface;
8. Install "wear-resistant blocks" on the side and bottom of mold slider to reduce flash caused by wear (large gap);
9. Ejector or sleeve is designed as an "exhaust type" structure to reduce flash caused by wear (large gap);
10. Select "good rigidity, high hardness, and wear resistance" for impact and moving parts of mold. 1. For glass fiber reinforced plastics, it is necessary to select steel with good wear resistance to reduce flash caused by wear; 2. For plastics with poor thermal stability and easy to produce abrasive decomposition products, it is necessary to select steel with good corrosion resistance to reduce flash caused by corrosion; 3. Optimize parting surface setting of mold, reduce the clamping force, and prevent flash caused by insufficient clamping force; 4. For molds where casting is set at the slider, positioning of slider needs to be reliable to prevent slider from generating gaps due to force, causing rubber parts to generate flash; 5. For molds that need to place hardware inserts, insert holes should be designed as insert blocks (for easy replacement), and hardened steel should be used to improve flash caused by wear of hardware insert holes;

 

Most traditional molds use guide pins and guide sleeves to position front/rear mold. After guide pins/guide sleeves are worn out after a period of use, their positioning accuracy will be reduced, resulting in uneven wall thickness of plastic parts, "grading" (misalignment) will occur when upper and lower covers are matched; for molds with center gates, uneven wall thickness caused by "eccentricity" of mold during injection molding causes trapped air, and plastic parts produce weld lines (difficult to improve);
For some molds with molding cores, plastic parts are "eccentric" or wall thickness of plastic parts is inconsistent, and many people mistakenly believe that it is caused by "eccentricity" of core.
"Three positioning" of mold refers to core positioning, guide column/guide sleeve positioning and mold pin positioning. Modern injection molding has high requirements for product size accuracy, so all molds must have "three positioning" devices to ensure stability of injection molding and accuracy of product size. It is important to attach great importance to improving reliability and durability of injection mold positioning.

In order to improve quality of mold design and prevent increase in number of mold trials, mold repairs and mold changes due to mold design errors, before mold design, relevant departments (injection molding department, engineering department, quality control department, mold company, etc.) must be organized to hold a "mold design requirements review". Only after confirmation, can subsequent work be carried out.
In order to prevent lack of molding knowledge and lack of consideration of problems, mold designers should listen more to suggestions of injection molding technicians, which is beneficial to optimizing mold structure, "getting it right from beginning" and taking fewer detours.
Holding a "mold design requirements review" before mold design is an important step to improve mold quality. Each new mold should insist on organizing relevant departments to hold a "mold design requirements review" before mold design, output corresponding process, equipment matching, quality control points and other requirements, and incorporate them into mold design considerations.

 

 

For later reading, please refer to "Talking about mold optimization from perspective of injection molding process 6, 7, 8".