Relationship of cooling time in plastics injection molding with HDT/VSP
The importance of Heat Deflection Temperature (HDT) and Vicat Softening Point (VSP) as crucial thermomechanical properties can hardly be disregarded in the complex realm of plastic processing. Unravelling their effect on the cooling characteristics during plastics processing is key to achieving optimal performance and maintaining dimensional stability. In this blog, we'll explore how HDT and VSP relate with the cooling process, and help in shaping plastics from the moment they are melted to the final, solid state during the injection molding process.
HDT is a crucial parameter that measures the ability of a plastic material to withstand a specified load at elevated temperatures without undergoing deformation. It is often a decisive factor in applications where dimensional stability under heat is paramount. HDT is determined by subjecting a plastic specimen to a load while gradually raising the temperature until the material deforms in a specified quantity. HDT is the temperature at which a given polymer test bar will be bent by 0.25 mm under a given load. It can be measured with either method A (1.80 MPa), B (0.45 MPa), or C (8.00 MPa) as per ISO 75. Furthermore, VSP is another critical temperature-related property that characterizes a plastic material's resistance to deformation under a specific load. Unlike HDT, VSP is determined by observing the penetration of a needle into the plastic specimen at an increasing temperature. VST is defined as a temperature in °C at which a circular indenter of 1 sq.mm flat cross section area penetrates in a sample of minimum 3 mm thickness immersed in oil bath heated at 50 +/-5°C per hour heating rate at 5 kg load, by 1 mm. If any of these parameters are changed, VST results will not be reliable.
Let us move on to the process of injection molding, a widely used manufacturing process in which plastic materials are melted, injected into a mold, and then cooled and solidified to produce a wide range of plastic parts and products. This process is commonly employed for mass production of identical items, making it an efficient and cost-effective method for creating various plastic components. An injection molding cycle consists of melting the material in the screw barrel assembly, injecting it into the mold, holding the pressure, cooling/solidification and ejection of the article while the machine gets itself ready for the next cycle to begin. The cooling time in the cycle is often the highest of all the steps and may well go up to 80-85% (depending of a lot of factors) of the injection molding cycle. 
HDT/VSP Testing Machine and Setup
The cooling time can be mathematically calculated using the following formula:
where, h represents the thickness of the part, α is the thermal diffusivity which can be calculated by multiplying density of the plastic with its specific heat and dividing this value with thermal conductivity of the material, and T is the surface temperature
Image showing marks on the plastic part due to ejector pins
(Credits: https://waykenrm.com/blogs/ejector-pins-injection-molding/)
Let's revisit the topic of the relevance of HDT and VSP for cooling during injection molding. The variable "T" in the equation above represents the surface temperatures of the melt, mold, and ejected part, denoted by the alphabet. It is essential to acknowledge that the melt temperature is the point at which the material transitions from a solid to a liquid state. This distinction can be challenging to articulate for amorphous polymers, while it tends to be more straightforward for semi-crystalline ones. Moreover, the mold is maintained at a temperature where the melt temperatures are high. This temperature regulation contributes to achieving the best surface finish replication of the cavity surface. The third value corresponds to the temperature during ejection, which aligns with the Heat Deflection Temperature (HDT) of the material. From a manufacturing perspective, it is crucial for the ejection temperature to be below the HDT. Therefore, a 20% buffer in cooling time is incorporated into the calculated value. This addition is necessary to ensure that the part doesn't distort under the action of the ejector assembly, wherein the pin might exert force on the part in a balanced or unbalanced manner, potentially lacking support at one end. In cases where consumers observe deep pin marks or experience pins tearing components during the manufacturing process, it is advisable to reassess the values and engage with the raw material provider, especially if certain parameters are missing. This proactive approach ensures a more accurate understanding of the molding conditions and aids in addressing potential issues related to cooling, ejection, and part distortion.
The understanding of these thermomechanical properties intricately shape the plastic behavior from molten to solid states, exerting a profound influence on the cooling process during injection molding. Both HDT and VSP, help understand the resistance of plastics to deformation under a specific load and provide insights into its behavior during cooling and solidification. As we get into the injection molding process, cooling time emerges as pivotal, with calculations influenced by HDT and VSP ensuring distortion-free solidification during ejection. This meticulous approach, incorporating a buffer, optimizes the cooling phase, enhancing the overall efficiency of injection molding. The comprehensive mathematical formula considers parameters such as part thickness, thermal diffusivity, and surface temperatures, empowering manufacturers to produce high-quality plastic components with precision. This informed understanding enables proactive mitigation of potential issues related to cooling, ejection, and part distortion, fostering excellence in injection molding practices within the dynamic landscape of plastic manufacturing.
If you have any other questions or would like to suggest topics for us to write about, please feel free to contact us at prashant.gupta@polymerupdateacademy.com
Author
Dr. Prashant Gupta
Faculty, Polymerupdate Academy
