In the field of medical injection molding, product deformation is a common problem that affects product quality and production efficiency. Medical injection-molded products, due to their high precision, high cleanliness, and special functional requirements, have deformation issues that not only may lead to product scrap but also impact the safety and effectiveness of medical operations. This article will delve into the reasons why medical injection-molded products are prone to deformation from four dimensions: material characteristics, process parameters, mold design, and product design, and propose corresponding preventive measures.

I. Reasons for the Deformation of Medical Injection-Molded Products

1. Differences in Material Characteristics
   Medical injection-molded products often use high-precision and high-performance engineering plastics, such as polycarbonate (PC), polyoxymethylene (POM), and polyether ether ketone (PEEK). These materials have different shrinkage rates. Crystalline materials (such as POM) have a shrinkage rate fluctuation range of 0.2% - 2.5%, and their shrinkage patterns differ significantly from those of non-crystalline materials. Additionally, hygroscopic materials (such as PA), if not fully dried before molding, can have water cause material degradation, increasing internal stress and leading to deformation.
2. Imbalanced Process Parameters
   Process parameters during the injection molding process, such as injection pressure, holding pressure, and cooling time, have a significant impact on product deformation. Insufficient injection pressure can result in incomplete filling and the formation of sink marks. Excessive holding pressure or a long holding time may over-compact the product, causing stress deformation. Inadequate cooling time means the product is not fully solidified and is prone to deformation during subsequent operations. On the other hand, an excessively long cooling time, although ensuring full product solidification, reduces production efficiency.
3. Defects in Mold Structure
   Unreasonable mold structure is an important factor leading to product deformation. Uneven distribution of cooling water channels, improper gate settings, and insufficient draft angles can exacerbate differences in product stress and cooling. For example, improper cooling water channel design can lead to uneven mold temperatures, resulting in inconsistent product shrinkage. Inappropriate gate positions may cause uneven filling of the molten material, generating internal stress.
4. Unreasonable Product Structure Design
   Unreasonable product structure design, such as uneven wall thickness and the presence of thin-walled cantilever structures, can cause different shrinkage rates in different parts, leading to deformation. When there is a large difference in wall thickness, the thick-walled part cools and shrinks by a larger volume than the thin-walled part, pulling the surrounding thin-walled part inward and causing deformation.

II. Preventive Measures for the Deformation of Medical Injection-Molded Products

1. Optimize Material Selection
   For the special requirements of medical injection-molded products, prioritize the use of low-shrinkage materials or add about 30% mineral or glass fiber fillers to control the shrinkage fluctuation within ±0.05mm. For hygroscopic materials, fully dry them before molding to avoid defects and increased internal stress caused by water. Also, test the properties of different batches of raw materials in advance to ensure consistency and reduce the risk of deformation from the source.
2. Precisely Control Process Parameters

• Injection Pressure and Speed: Adopt a segmented injection strategy. Initially, fill the mold cavity with 90% of the maximum injection pressure, and then gradually reduce it to 30% in subsequent stages to avoid excessive extrusion and internal stress generation. The injection speed should be adjusted according to the material characteristics to prevent excessive shear stress due to too fast a speed or premature solidification of the molten material due to too slow a speed.
• Holding Pressure and Time: It is recommended to maintain the holding pressure at more than 60% of the injection pressure and adopt a stepped holding pressure strategy. For example, the first stage of holding pressure is 80% of the injection pressure and lasts for 10 seconds; the second stage is reduced to 50% and lasts for 5 seconds. The holding time should be optimized according to the product wall thickness and material characteristics to ensure sufficient product replenishment.
• Cooling Time: Follow the "thickness square law," increasing the cooling time by 2 - 3 seconds for every 1mm increase in wall thickness. For thin-walled products, the cooling time can be appropriately shortened; for thick-walled products, it should be extended to ensure full product solidification.

3. Improve Mold Design

• Cooling System: Use 3D printing conformal cooling technology to manufacture irregular water channels, keeping the mold temperature gradient within ±3℃ and significantly improving cooling uniformity. For long, strip-shaped products, set 3 - 5 stepped gates with a spacing of 1/5 - 1/3 of the product length to ensure uniform filling of the molten material.
• Gate System: Optimize the gate position, form, and quantity according to the material characteristics. For example, for flat box-shaped plastic parts, using multiple gates can effectively prevent warping deformation.
• Ejection System: Ensure that the ejection system is reasonably designed, with balanced ejection pin arrangement and an appropriate cross-sectional area to avoid uneven ejection forces causing product deformation. For large, deep-cavity thin-walled plastic parts, a combination of multiple components or a combination of pneumatic/hydraulic and mechanical ejection can be used.

4. Optimize Product Design

• Wall Thickness Uniformity: Try to make the product wall thickness uniform and avoid areas that are too thick or too thin. If wall thickness variations are unavoidable, use a gradual transition method to reduce shrinkage differences.
• Rib Design: Appropriately setting ribs can improve the strength and rigidity of the product without increasing the wall thickness, while also helping to reduce shrinkage deformation. The thickness of the ribs is generally 0.5 - 0.7 times that of the product wall thickness, and the height should not be too high.
• Rounded Corners: Use rounded corners at corners instead of sharp internal corners to facilitate the flow of the molten plastic and uniform shrinkage.

5. Establish a Full-Process Quality Control System

• Mold Flow Analysis: Before production, use mold flow analysis software such as Moldflow to predict deformation trends and optimize process and mold designs in advance, which can reduce more than 80% of potential deformation problems.
• Process Monitoring: During production, use Statistical Process Control (SPC) to monitor key parameters such as injection pressure and mold temperature in real-time; conduct trial molding verification for each batch and adjust parameters in a timely manner.
• Regular Maintenance: Regularly clean scale from the mold cooling water channels and check for gate wear. Operators should strictly follow standardized procedures to reduce human-induced process fluctuations.

III. Conclusion

The deformation problem of medical injection-molded products involves multiple links such as materials, processes, molds, and product design, and requires source control and system optimization. By optimizing material selection, precisely controlling process parameters, improving mold design, optimizing product design, and establishing a full-process quality control system, the defect rate of deformation in medical injection-molded products can be effectively reduced, and product quality and production efficiency can be improved. In actual production, it is necessary to flexibly adjust the solutions according to product characteristics and production conditions to achieve precise matching between technology and production.