In the field of medical injection molding, product deformation and springback are one of the key issues affecting the quality of finished products. This not only relates to the appearance and dimensional accuracy of the products but also directly impacts their functionality and safety. This article will delve into how to effectively solve the problem of deformation and springback in medical injection-molded products by scientifically adjusting the holding time and pressure.

I. Understanding the Root Causes of Deformation and Springback

The deformation and springback of medical injection-molded products are mainly caused by uneven shrinkage during the cooling process of the plastic. When molten plastic is injected into the mold, as the temperature drops, the volume of the plastic shrinks. If the holding pressure is insufficient at this time, the pressure inside the mold cavity cannot be effectively maintained, leading to the formation of voids inside the product and subsequently causing deformation and springback.

II. Principles for Adjusting Holding Time and Pressure

1. Setting the Holding Time

The holding time refers to the period from the end of injection to the complete solidification of the gate, during which the pressure inside the mold cavity remains stable. Its setting should comprehensively consider factors such as material properties, product wall thickness, and mold design.

• Material Properties: Crystalline plastics (such as PP and PE) have a high shrinkage rate and require a longer holding time to ensure sufficient packing. Non-crystalline plastics (such as ABS and PC) have a lower shrinkage rate, and the holding time can be appropriately shortened.
• Product Wall Thickness: Thick-walled products require a longer holding time to compensate for volume shrinkage during cooling, while thin-walled products are the opposite.
• Mold Design: The size, location of the gate, and runner design also affect the holding time. Molds with small gates and long runners require a longer holding time to ensure that the molten material is fully filled.

2. Setting the Holding Pressure

The holding pressure is a key parameter for maintaining stable pressure inside the mold cavity. Its setting should be based on the injection pressure, typically ranging from 50% to 70% of the injection pressure.

• Thick-walled Products: Higher holding pressure is required to fully pack and prevent the formation of sink marks inside.
• Thin-walled Products: The holding pressure should not be too high to avoid defects such as flash or white marks on the ejector pins.
• High-precision Products: While ensuring the packing effect, the holding pressure should be minimized as much as possible to reduce internal stress and ensure dimensional stability.

III. Adjustment Strategies and Implementation Steps

1. Initial Setting

Based on the product characteristics and mold design, initially set the holding time and pressure. For example, for a medical injection-molded part with a wall thickness of 2-3 mm, the holding time can be set to 5-10 seconds, and the holding pressure can be set to 60% of the injection pressure.

2. Trial Molding and Observation

After completing the first injection-molded part, carefully observe its appearance and dimensions. If defects such as sink marks or depressions are found, it indicates insufficient holding pressure, and the holding time and pressure should be gradually increased. If defects such as flash or white marks on the ejector pins are found, it indicates excessive holding pressure, and the holding pressure should be appropriately reduced.

3. Gradual Fine-tuning

Follow the principle of "small-scale incremental/decremental" adjustments. Each adjustment should be controlled within a range of 5-10 MPa (holding pressure) or 1-2 seconds (holding time). After each adjustment, inject 1-2 trial parts and continue adjusting until the product has no sink marks or flash, and its dimensions and appearance meet the standards.

4. Final Verification

After determining the appropriate holding time and pressure, continuously inject 10-20 parts for batch verification. Check whether the product quality is stable and free from batch-related defects. Once confirmed, fix the parameters as the standard production values.

FAQ

Q1: What problems can be caused by an excessively long holding time?
A1: An excessively long holding time can increase the internal stress of the product, making it prone to deformation and cracking. At the same time, it extends the production cycle and reduces production efficiency.

Q2: What impact does an excessively high holding pressure have on the product?
A2: An excessively high holding pressure can easily cause defects such as flash and white marks on the ejector pins. It also increases the load on the mold and equipment, shortening their service life.

Q3: How can I determine whether the holding time and pressure are appropriate?
A3: Determine whether the holding time and pressure are appropriate by comprehensively observing the product's appearance (no sink marks or flash), dimensional stability (Cpk > 1.67), and weighing to confirm the material's compactness.