The Defects of Plastic Mold Shrinkage and Improvement Methods

Fundamental Phenomenon of Plastic Mold Shrinkage

During the injection molding process, due to insufficient pressure in some parts of the mold cavity, when the melt begins to cool. The volume of the thicker wall on the plastic mold shrinkage slowly and forms tensile stress. If the surface hardness of the product is not enough, and there is no when the melt is replenished, the surface of the product is stretched by stress, this phenomenon is called Plastic Mold shrinkage.

The phenomenon of Plastic Mold Shrinkage mostly occurs in the part where the melt gathers in the mold cavity and the thick-walled area of the product, such as the junction of the reinforcing ribs, support columns, etc. With the surface of the product.

Plastic Mold Shrinkage: Defect Causes and Repair Methods

Surface shrinkage occurs on injection molded components. It not only alters the way plastic parts look, but it also makes them less durable. The type of plastic used, the injection molding procedure, the plastic parts and mold structure, etc. are all directly related to the plastic mold shrinkage problems.

• Raw Materials for Plastic

Different plastics shrink at varying speeds. Crystalline plastics are the main raw ingredients that are simple to shrink (such as nylon, polypropylene, etc). The molecules are ordered at random when the crystalline form is heated during the injection molding process and turns into a fluid state.

Injecting something into a somewhat the plastic molecules will progressively align to create crystals when the mold cavity is cool. As a result, the volume shrinks more than the prescribed amount, meaning that plastic mold shrinkage has taken place.

• Injection Molding technique of Plastic Mold Shrinkage

A few factors that contribute to shrinkage during the injection molding process include inadequate holding pressure, a slow injection speed, a low mold or material temperature, an inadequate pressure holding time, etc.

To prevent shrinkage issues, it is crucial to examine the molding conditions and ensure that there is sufficient pressure while configuring the injection molding process’s settings. Increasing the stay time generally guarantees that the product has enough time to cool and replenish the solution.

• Mold Structure and Plastic Parts

The root cause of shrinkage is the uneven thickness of plastic products. A typical example is that plastic parts are very easy to shrink on the surface of reinforcing ribs and support columns. In addition, the runner design of the mold, the size of the gate, and the cooling effect also have a great influence on the product. Due to the low heat transfer capacity of the plastic, the farther it is from the mold cavity wall, the slower the cooling will be.

Therefore, there should be sufficient melt fills the mold cavity, which requires that the melt will not reduce the pressure due to backflow when the screw of the injection molding machine is injecting or maintaining pressure. In addition, if the runner of the mold is thin, too long or the gate is too small and the cooling is too fast, the semi-solidified solution will block the runner or gate and cause the cavity pressure to drop, causing the product to shrink.

The reasons For The Plastic Mold Shrinkage are as follows

1. Insufficient plastic material in the mold.

2. Improper material temperature (too low or too high).

3. Mold temperature is too low or too high.

4. Insufficient cooling time (bones, tendons, molding, and sinking).

5. Poor exhaust of the mold at the shrinkage (air trapped).

6. The plastic part’s bone position and the column’s glue wall are too thick.

7. Wear of screw check ring (large reverse flow).

8. The gate position is improper or the process is too long.

9. The runner is too thin or too long.

*Corresponding Improvement Method of Mold Shrinkage

1. Enhanced sol injection volume.

2. Adjust to the appropriate material temperature.

3. Increase the mold temperature or appropriately reduce the mold temperature.

4. Extend the cooling time as appropriate.

5. Open an exhausted groove at the plastic mold shrinkage.

6. Make the glue thickness as uniform as possible (change to gas-assisted injection molding).

7. Removing and replacing the check ring (over-ring glue).

8. The gate is opened at the wall thickness or increases the number of gates.

9. Widen the main and shunt channels and shorten the length of the runners.

Inadequate mold feed material typically shows up where?

1. Insufficient melt amount.

2. Insufficient injection pressure.

3. Insufficient pressure holding or pressure holding switching position too early.

4. Injection speed is too fast or too slow.

5. Injection time is too short.

6. The gate size is too small or the glue feeding is unbalanced (multi-cavity).

7. The nozzle is blocked or the heating ring is burned out.

8. Mouth leaking glue.

*The Improvement Method Of Insufficient Rubber Material In the Mold

1. Increase sol metering stroke.

2. Increase injection pressure.

3. Increase holding pressure or delay switching holding pressure.

4. Extended injection time (with pre-ejection action).

5. Speed up injection or slow down injection.

6. Increase the gate size or balance the injection of the mold.

7. Remove and clean the foreign body in the nozzle or replace the heating ring.

8. Re-align the nozzle, tighten the nozzle, or reduce the back pressure.