This article explains the warping and deformation of plastic parts most clearly! （1）

This article explains the warping and deformation of plastic parts most clearly! (1）

Background: Warpage refers to the deviation of the molded part's shape from the mold cavity shape, and it is one of the common defects in plastic products. There are many causes of warpage, and relying solely on process parameters is often insufficient. Based on relevant data and practical experience, this paper will briefly analyze the factors affecting warpage in injection molded parts.

Product Warpage Analysis: This section explores and studies the causes of product warpage and deformation, analyzes different causes, and proposes corresponding countermeasures. Based on the existing analysis and discussion, it provides a reference for the development of subsequent new models.

New Model Reference: Through understanding the warpage analysis and countermeasures, it provides a reference for the development of subsequent new models.

Analysis of the Causes of Warpage Deformation in Plastic Products
Factors Affecting Warpage Deformation in Plastic Products
Plastic Product Deformation
I. Mold Structure: 1. Gating System 2. Cooling System 3. Ejection System
II. Molding Stages: 1. Plasticizing Stage 2. Filling and Cooling Stage 3. Demolding Stage
III. Product Shrinkage
IV. Residual Stress
V. Metal Inserts

The plan is to analyze key factors 1-5 in the following order:

1. Factors 1, 2, and 3 in the mold structure

2. Factors 4 and 5 in the molding stage

Detailed Analysis of the Causes of Warpage and Deformation in Plastic Products

◆ Detailed Analysis of Factors Affecting Warpage and Deformation in Plastic Products

Gating System: The location, form, and number of gates in the injection mold affect the filling state of the plastic within the mold cavity, leading to deformation of the plastic part.

◆Cooling System: Uneven cooling rates during injection will result in uneven shrinkage of the plastic part. This difference in shrinkage leads to the generation of bending moments, causing warpage.

◆Ejection System: The design of the ejection system directly affects the deformation of the plastic part.

◆Filling and Cooling Stage: During this process, temperature, pressure, and speed interact, significantly impacting the quality of the plastic part and production efficiency.

◆Demolding Stage: Uneven demolding forces, unstable ejection mechanism movement, or improper ejection area can easily cause product deformation.

Detailed Analysis of the Causes of Warpage and Deformation in Plastic Products

Analyzing the causes of warpage and deformation, we inevitably ask:
Why do the location, type, and number of gates in the mold affect product deformation?
Why does the cooling rate of the plastic part lead to product deformation? How can we ensure that cooling meets our requirements?
Why does the design of the ejection system affect the degree of deformation? What design minimizes this effect?
Why do injection temperature, pressure, and speed affect the degree of deformation? How can we achieve a balance among these three factors?
Why do demolding force, ejection mechanism, and ejection area affect deformation? How can we obtain the desired result?
How do other factors influence product warpage and deformation?

Plastic Flow Analysis
Gating System
During injection molding, the flow of molten plastic within the mold cavity is influenced by the fact that the temperature of the mold cavity walls is generally lower than the melting point of the plastic. Therefore, the melt begins to cool from the moment it enters the mold cavity. A layer of melt in contact with the mold wall forms a stationary outer shell (frozen layer), while the interior remains warmer melt (flow layer).

The molding shrinkage rate of plastic varies depending on the flow direction; the shrinkage rate in the flow direction is much greater than that perpendicular to the flow direction (shrinkage rate anisotropy).

Red represents molten plastic, blue represents the solidified layer, and the red arrow indicates the direction of heat transfer.

The longer the flow distance, the greater the internal stress caused by the flow and shrinkage compensation between the frozen layer and the central flow layer; conversely, the shorter the flow distance, the shorter the flow time from the gate to the end of the part, resulting in a thinner frozen layer during mold filling, reduced internal stress, and significantly reduced warpage.

Plastic Flow Analysis

Taking B77 MID FRAME as an example, the first version of the design included the gate location and number, as shown in Figure A. Due to the long flow length and weak structure, excessive deformation on the long side was found after trial molding, failing to meet customer requirements. After modification, the number and location of gates were adjusted, as shown in Figure B, effectively improving the deformation problem.

Gates 1, 2, 3, and 4 are longer than the others.

Adding two more gates resulted in a more balanced flow length.

Injection Temperature, Pressure, and Speed ​​Analysis: Based on our previous analysis of plastic flow, we know that pressure has a significant impact on material filling, shrinkage, and stress deformation. So, what injection pressure is appropriate?

As shown in the diagram, higher pressure at the mold cavity inlet leads to a higher pressure gradient (pressure drop per unit flow length). This increases the melt flow length, necessitating an increase in inlet pressure to maintain the same pressure gradient and sustain the polymer melt velocity.

Diagram: Relationship between Pressure and Melt Delivery System and Mold Cavity