First of all, you can add a styling agent specifically for polypropylene plastic, ZH-DX-2 styling agent, which can significantly improve the shrinkage and deformation problem. Secondly, it can be solved through the following points:

1. Solutions in mold design

1.1 Waterway design

Reasonable waterway design makes the mold temperature on the cavity surface as consistent as possible. If necessary, strengthen cooling in areas with larger wall thickness or poor heat dissipation. Strengthen cooling on the mold surface corresponding to the ribs, allowing the surface solidified layer to form faster. When the surface solidified layer is thicker, the rigidity is greater and shrinkage marks are less likely to occur.

When the corresponding surfaces of the movable and fixed molds forming the ribs are made of steel, shrinkage marks are likely to occur. If the ribs are replaced with ceramic or plastic inserts, the solidified layer above will form faster and be more rigid. Large, the finally cured high-gloss PP plastic is sucked inward, so that the top surface will not collapse and sink marks can be prevented.

1.2 Gate design

The gate of the part should be designed in an area with large wall thickness, or close to the location where shrinkage marks and shrinkage holes appear, to facilitate pressure maintenance and shrinkage. The size of the gate should be large enough to slow down the cooling of the gate so that more melt can enter the cavity for feeding during the pressure holding stage. Generally speaking, the gate thickness should not be less than 50% of the wall thickness, and it is best to reach 80% of the wall thickness.

1.3 Flow channel design

Circular flow channels are preferred because they have the largest effective cross-sectional area, followed by trapezoidal flow channels. It is best not to use semicircular flow channels. road. The larger the effective cross-section of the flow channel, the stronger the ability to maintain pressure and shrinkage, and the less likely it is for parts to have shrinkage marks or shrinkage holes. In addition, the size of the runner should be large enough to reduce the mold filling resistance and provide sufficient holding pressure for the cavity.

1.4 Pull rod design

Pull rods are often used in three-plate molds. The design of the pull rods should avoid extending into the flow channel, causing the effective cross-section of the flow channel to become smaller. The filling resistance increases, which is not conducive to the pressure maintaining and shrinkage of the parts. For materials with poor fluidity such as polycarbonate (PC), special attention needs to be paid to the design of the pull rod to avoid excessive pressure loss in the flow channel causing insufficient actual pressure holding, resulting in sink marks or shrinkage holes in the product.

1.5 Exhaust design

The exhaust of the mold is smooth, higher pressure and speed can be used during injection molding, the effect of pressure maintaining and shrinkage is better, and shrinkage marks or shrinkage holes are reduced possibility. Typical exhaust groove design, depending on the material, the depth of the exhaust groove will also be different, but the same is that the length of the exhaust groove should not be too long, preferably around 2mm.

2. Solutions in the molding process

2.1 Mold temperature

The influence of mold temperature on shrinkage marks or shrinkage cavities is relative. When the mold temperature is too low, the surface layer of the part will easily condense and become thicker, the thickness of the core layer will be relatively reduced, the pressure-maintaining and shrinkage channel will become narrower, and the far end of the part will not receive enough feeding, forming sink marks or shrinkage cavities. ; In addition, the low mold temperature makes the pouring system, especially the gate, easy to freeze, and the parts cannot maintain sufficient pressure and shrinkage, and it is also easy to form shrinkage holes or shrinkage marks. When the mold temperature is too high, the cooling efficiency of the mold is low and the cooling is slow. Because the cooling time is too long, the shrinkage becomes larger. If there is not enough pressure maintaining and shrinkage, it will easily lead to sink marks or shrinkage holes. But relatively speaking, shrinkage cavities are prone to occur when the mold temperature is low, and shrinkage marks are prone to occur when the mold temperature is high.

2.2 Effective holding pressure

The effective holding pressure is low, causing the resin filling to be less than the shrinkage of the part. When the mold temperature is high, dents are easily formed, and when the mold temperature is high, dents are easily formed. When it is low, it is easy to form holes. The main reasons for low holding pressure are as follows: low holding pressure setting, short holding time, small gate size, and thin runner.

2.3 Other process parameters that have a greater impact

Other process parameters that have a greater impact on sink marks and shrinkage cavities include melt temperature, injection speed, V/P conversion position, Back pressure and residual glue amount, etc. The higher the melt temperature, the lower the viscosity of the material, which is more conducive to mold filling and pressure-maintaining shrinkage, and is beneficial to preventing sink marks and shrinkage cavities. However, the higher the melt temperature, the corresponding cooling time is also higher; reasonable injection molding speed , can effectively perform pressure-holding and feeding before the gate freezes; the V/P switching position is generally selected when the part is filled to about 95% to 98%. Switching too early can easily cause sink marks or shrinkage holes; appropriate back pressure can Increasing the density of the melt will help prevent sink marks or shrinkage cavities; the amount of residual glue is generally controlled at 5 to 10mm, and an appropriate amount of residual glue can ensure the effect of maintaining pressure.

2.4 Post-cooling treatment

For some parts that require no shrinkage marks in appearance but allow internal shrinkage cavities, they can be quickly immersed in freezing water after being released from the mold to make the parts shorter. It solidifies and cools within a period of time to prevent the occurrence of shrinkage marks. This method is more effective for products with larger wall thickness.

3. Solutions for PP materials

3.1 Crystalline and amorphous materials

The shrinkage of crystalline materials is greater than that of amorphous materials. Because during the cooling of crystalline materials from the molten state to room temperature, the molecular chains are arranged in an orderly manner to form crystals, so the volume shrinkage of crystalline materials is greater than that of amorphous materials. Therefore, crystalline materials are relatively more prone to sink marks or shrinkage cavities.

The shredder shell of a certain project uses reinforced PP instead of ABS. Although the shrinkage rate of the reinforced PP material is similar and there is no problem with the size of the product, the shrinkage marks at the ribs are more obvious than those of ABS, and the thickness or base surface of the ribs needs to be adjusted. Thickness, or adjust the size of the runner and gate to strengthen pressure-holding and shrinkage

3.2 Viscosity

The higher the viscosity of the PP material, the greater the filling resistance and the more difficult the filling. The worse the pressure-holding and shrinkage effect, the easier it is to produce sink marks or shrinkage cavities. Therefore, to improve the sink marks and shrinkage cavities of parts, it is a feasible solution to improve the fluidity of modified PP materials

3.3 Fillers

The addition of fillers is conducive to increasing the The strength of the surface layer of the part resists the shrinkage stress of the core layer. The part is not prone to shrinkage marks but tends to produce shrinkage cavities. It should be noted that there is a large difference in shrinkage of fiber-reinforced materials in parallel and vertical flow directions. Since the glass fiber is oriented parallel to the flow direction and plays a supporting role, it shrinks less in this direction and shrinks more perpendicular to the flow direction. It is very important to choose good modified PP raw materials.