Solutions of Excessive Wall Thickness in Injection Molding

1. Introduction: The Neglected "Invisible Killer"

In the daily production of injection molding, excessive local wall thickness seems like a minor issue, but it is actually an "invisible killer" that can trigger a series of troubles. If designers fail to control the local wall thickness of plastic parts, resulting in a thicker section than the surrounding area, the finished products often have problems such as pitted surfaces and overall bending deformation after demolding. This not only affects the appearance and functional performance of the product but may also directly lead to mass scrapping and a significant increase in production costs. This article will detail the hazards of excessive local wall thickness and corresponding countermeasures.

2. Main Defects Caused by Excessive Local Wall Thickness

2.1 Sink Marks (Dents)

Sink marks are the most common defect caused by excessive local wall thickness. During injection molding production, the cooling rate of molten plastic in the thick-walled area is much slower than that in the thin-walled area. When the internal plastic shrinks sharply due to cooling, it exerts a pulling force on the externally cooled and shaped surface, causing the outer surface to be "drawn in" and form sunken sink marks.

2.3 Warpage Deformation

Uneven wall thickness leads to significant differences in shrinkage rates across various areas of the part: the thick-walled area cools and shrinks slowly, while the thin-walled area cools and shrinks rapidly. This difference in shrinkage speed creates unbalanced internal stress in the part, which is like being pulled in two directions, eventually causing bending or twisting deformation. This problem is particularly prominent in long strip and plate-shaped parts. A real production case once occurred: a corner of an automobile instrument panel part had a wall thickness only 1mm exceeding the standard, but after cooling, the entire part warped by 2mm, directly leading to assembly misalignment. The root cause is that the internal stress accumulated in the thick-walled area exerts a "dragging" effect on the part during the release process, causing its shape to deviate.

2.3 Bubbles and Voids

Bubbles and voids are also common defects in parts with local thick walls. Molten plastic flows slowly in the thick-walled area, and it is easy to entrap air or volatile substances of the material itself during the flow process. When these gases cannot be discharged in time, they will form air holes inside the part; if the gas accumulates near the surface, it may also cause the outer surface to bulge. In severe cases, these voids will damage the structural integrity of the part, reduce its mechanical strength, and make the part prone to cracking when stressed.

2.4 Other Hidden Risks

• Reduced production efficiency: Thick-walled areas require longer cooling time, which directly prolongs the entire injection molding cycle and reduces the output per unit time. For factories, every additional few seconds in the production cycle will significantly reduce the total daily output, indirectly increasing production costs.

• Serious material waste: The thick-walled design itself consumes more raw materials, and the high scrap rate caused by defects further aggravates material waste, making the production cost "worse".

• Long-term use risks: Stress concentration is prone to occur in thick-walled areas. During long-term use, the part may break due to stress fatigue; this risk will be more obvious especially in harsh environments such as high temperature and chemical corrosion.

• Stress-related defects: The thick-walled area may also have problems such as stress whitening and surface scorch marks, affecting the product appearance and performance stability.

3. Solutions to Excessive Local Wall Thickness

3.1 Design Optimization: Control from the Source

• Ensure uniform wall thickness: Control the overall wall thickness of the part within the golden range of 1-3mm to avoid sudden local thickening; if the structural requirements cannot avoid thick walls, use arc transitions instead of sharp corners to reduce sudden changes between thick and thin areas and achieve smooth transitions.

• Reasonable stiffener design: Add stiffeners in the thick-walled area to disperse stress and assist heat dissipation; it should be noted that the thickness of the stiffener must not exceed 1.5 times the wall thickness of the main body, and a smooth transition should be adopted between the stiffener and the main body to avoid new stress concentration points.

• Advance simulation and prediction: Use injection molding simulation software in the design stage to simulate and analyze the shrinkage and deformation of the part, and detect and optimize hidden dangers in the wall thickness design in advance.

3.2 Process Adjustment: Optimize the Molding Process

• Adjust holding pressure parameters: Appropriately increase the holding pressure and extend the holding time to ensure that the plastic in the thick-walled area is fully filled and compacted, reducing sink marks and voids caused by shrinkage.

• Balance mold temperature: Appropriately increase the mold temperature for the thick-walled area and keep the normal temperature for the thin-walled area. By balancing the cooling speed of each area, the shrinkage difference is reduced; at the same time, optimize the cooling water circuit and add cooling channels in the thick-walled area to accelerate the cooling of the thick area.

• Control injection speed: Adopt a segmented injection speed of "slow-fast-slow" to avoid gas entrapment caused by too fast      injection, and ensure that the material can fill the thick-walled area smoothly.

• Auxiliary post-processing: Perform annealing treatment on the molded parts. By heating, heat preservation and then slow cooling, the internal stress accumulated in the thick-walled area is released, and warpage deformation is reduced.

3.3 Mold Improvement: Strengthen Ventilation and Molding Assistance

• Add ventilation structure: Add vent grooves in thick-walled areas and positions where weld lines are prone to occur to ensure that the gas entrapped in the molten plastic during the flow process can be discharged in time, reducing bubbles and voids; the size of the vent grooves should be reasonably set according to the material characteristics to avoid flash.

• Adopt gas-assisted injection molding technology: For parts with large thick walls, gas-assisted injection molding technology can be used. By injecting inert gas into the thick-walled area to form a hollow structure, he shrinkage pressure in the thick-walled area is reduced, and sink marks and deformation are fundamentally reduced.

3.4 Material Control: Reduce Molding Hidden Risks

For materials with strong hygroscopicity such as TPE, sufficient drying treatment must be carried out before molding. The moisture in the material is removed through professional drying equipment to prevent the moisture from vaporizing due to heat during injection molding to form bubbles; at the same time, materials with appropriate fluidity are selected to ensure that they can flow smoothly and fill fully in the thick-walled area.

4. Summary

The problem of excessive local wall thickness in injection molding is easy to be ignored, but it is directly related to product quality, production efficiency and cost control. To solve this problem, we must adhere to the principle of "design as the core, process as the auxiliary, and mold as the guarantee", optimize the wall thickness design from the source, combine process adjustment and mold improvement, and do a good job in material control. It is recommended to carry out small-batch test molding in actual production, and find the best solution through repeated testing and parameter adjustment. In the injection molding industry, "details determine success or failure". Properly handling the wall thickness problem can significantly improve product competitiveness and improve production quality and efficiency.