In the process of medical injection molding production, material shortage is a relatively common defect that seriously affects product quality and production efficiency. Medical products have extremely high requirements for the precision, performance, and safety of injection-molded parts. Material shortage not only may lead to product function failure but also trigger medical safety hazards. Thoroughly exploring the reasons for material shortage in medical injection molding is of utmost importance for ensuring the quality and production stability of medical products.

Raw Material Factors

Raw Material Characteristics

The raw materials used in medical injection molding have unique physical and chemical properties. For example, some medical plastics have poor fluidity and are difficult to fully fill the mold cavity during the injection molding process. Take polycarbonate (PC) as an example. Its melt viscosity is high, and its fluidity is poor at low temperatures. If the injection molding process parameters are not set reasonably, material shortage is likely to occur. In addition, the shrinkage rate of raw materials is also a key factor. Different medical plastics have significant differences in shrinkage rates. Materials with high shrinkage rates experience obvious volume shrinkage during the cooling process. If the mold design or injection molding process fails to fully consider this characteristic, material shortage may occur in some areas of the cavity due to material shrinkage.

Raw Material Quality

Unstable raw material quality is an important cause of material shortage. Medical injection molding has extremely high requirements for the purity and quality of raw materials. Any impurities or composition deviations can affect the properties of the materials. If other plastics or additives are mixed into the raw materials, it will change the melt fluidity and viscosity of the materials, leading to uneven filling during the injection molding process. Moreover, uneven particle size distribution of raw materials can also affect the injection molding effect. Raw materials with large particle sizes may not be fully plasticized during heating and melting, while those with small particle sizes may form bridging during conveying, hindering the normal flow of materials and thus causing material shortage.

Mold Factors

Mold Design

Unreasonable mold design is a common cause of material shortage in medical injection molding. The design of the mold's runner system is crucial. If the runner size is too small, the length is too long, or the layout is unreasonable, it will increase the flow resistance of the melt in the runner, causing the melt pressure to drop when it reaches the end of the cavity and preventing it from fully filling the cavity. For example, in molds for medical injection-molded parts with complex shapes, if the design of the diverting runners does not fully consider the flow distribution of each cavity, material shortage may occur in some cavities. In addition, the design of the mold's venting system cannot be ignored. If the venting is not smooth, the air in the mold cavity cannot be discharged in a timely manner, forming high-pressure air resistance during the melt filling process and hindering the flow of the melt, resulting in material shortage.

Mold Wear

With the increase in usage time, molds will experience varying degrees of wear. Wear on the cavity surface of the mold will change its dimensions. Gaps will appear between the originally tightly fitted cavity and core, causing the melt to leak during the filling process and preventing it from reaching the predetermined filling amount. Moreover, the gate area of the mold is also prone to wear. When the gate size becomes larger, the flow rate of the melt passing through the gate increases, and the pressure loss also increases, which may affect the filling effect of the melt in the cavity and lead to material shortage. In addition, wear on the guiding components of the mold will cause the mold to shift during opening and closing, affecting the accuracy and sealing of the cavity and thus triggering material shortage problems.

Injection Molding Process Factors

Injection Molding Parameter Settings

The setting of injection molding parameters directly affects the filling process of the melt. Injection pressure is one of the key parameters to ensure the full filling of the melt in the cavity. If the injection pressure is too low, the melt cannot overcome the resistance in the mold cavity and is difficult to reach all corners of the cavity, resulting in material shortage. Injection speed also affects the filling effect. A too-slow injection speed will make the melt cool for too long in the cavity and solidify prematurely, preventing it from completing the filling; while a too-fast injection speed may cause turbulence in the melt in the cavity, forming vortices and bubbles and affecting the filling quality. In addition, holding pressure and holding time are also important parameters. Insufficient holding pressure or too short holding time will prevent the already filled melt from being fully compacted, leaving voids inside the product, which appears as material shortage.

Temperature Control

Temperature control includes barrel temperature and mold temperature. Excessively high barrel temperature will cause the plastic to decompose and generate gases, affecting the fluidity and filling performance of the melt; while excessively low barrel temperature will result in insufficient plasticization of the plastic, increasing the melt viscosity and making it difficult to flow, which can easily cause material shortage. Mold temperature also has an important impact on the cooling and filling of the melt. If the mold temperature is too low, the melt cools too quickly in the cavity, reducing its fluidity and making it difficult to fully fill; if the mold temperature is too high, although it is beneficial for the flow of the melt, it will prolong the cooling time of the product, which may cause difficulties in product demolding and also affect the dimensional accuracy and performance of the product.

Environmental Factors

Workshop Environmental Humidity

The humidity of the medical injection molding workshop environment has a certain impact on the properties of raw materials. If the workshop humidity is too high, some medical plastics with strong hygroscopicity will absorb moisture from the air. During the injection molding process, this moisture will vaporize at high temperatures and form bubbles, hindering the flow of the melt and causing material shortage. Moreover, moisture will also affect the molecular structure of the plastic, reducing its mechanical properties and thermal stability and further affecting the quality of the product.

Workshop Temperature Fluctuations

Temperature fluctuations in the workshop will also have an adverse impact on the injection molding process. Temperature changes will cause the temperature of the mold and raw materials to be unstable, affecting the fluidity and filling effect of the melt. For example, in a low-temperature environment, the mold temperature drops quickly, and the melt cools rapidly in the cavity, making it prone to material shortage; in a high-temperature environment, the raw materials may soften or decompose due to heating, affecting the injection molding quality.

Material shortage in medical injection molding is the result of the combined effects of multiple factors such as raw materials, molds, injection molding processes, and the environment. Only by thoroughly understanding these reasons can we better prevent and control material shortage problems in the medical injection molding production process and ensure the quality and safety of medical products.