Difference Between Tg and Tm in Plastic Injection Molding

What is Transition Temperature (Tg)?

Glass Transition Temperature (Tg) refers to the point where a polymer shifts from a hard and glassy state to a soft and rubbery one.  Above the Tg, polymers become flexible and show some flow characteristics. Below this temperature, they are more rigid and less pliable.

Factors Affecting Tg

Several factors can influence Tg in polymers.

Molecular weight is a key factor; higher molecular weights often lead to higher Tg values. Intermolecular forces also play a role. Stronger forces make it harder for polymer chains to move, raising the Tg.

Plasticizers can be added to lower the Tg. They make it easier for the chains to slide past each other. On the other hand, adding fillers can increase Tg by restricting movement. Thermal history also matters. If a polymer has been heated and cooled many times, the Tg can shift.

What is Melting Temperature (Tm)?

When you heat a polymer, the crystalline regions turn from a solid state to a liquid state. This melting process happens at the melting temperature (Tm). This temperature differs among materials because of the crystal structure and bond strength within the polymer. In crystalline polymers, these patterns are more ordered, resulting in a sharp melting point.

Influence of Molecular Weight and Structure

The molecular weight of a polymer greatly impacts its melting temperature. Polymers with high molecular weight usually have a higher Tm. The reason is greater molecular weight increases chain entanglement, making it harder for the chains to move and melt.

The molecular structure also plays a role. Linear polymers might have a more distinct Tm than branched ones because of closely packed chains. Crystal perfection and chain alignment affect Tm too, with better alignment leading to higher temperatures.

The Difference Between Tg and Tm

Characteristic	Glass Transition Temperature (Tg)	Melting Temperature (Tm)
Type of Change	Second-order transition with no phase change; only physical property changes	First-order phase transition from solid to liquid
Material Type	Occurs in amorphous and semi-crystalline materials only	Occurs in crystalline materials
State Change	Glass state to rubber state without changing phase	Solid phase to liquid phase
Molecular Structure	No change in molecular arrangement; remains amorphous	Complete breakdown of crystalline structure
Reversibility	Fully reversible process	Reversible phase change
Temperature Range	Generally occurs at lower temperatures than Tm	Typically higher than Tg
Influencing Factors	– Chemical structure of polymer – Molecular weight – Plasticizer content – Flexibility	– Pressure – Chemical bonding – Shape and size of molecules – Molecular packing
Measurement Method	Typically measured using differential scanning calorimetry	Measured at specific pressure (usually standard pressure)
Industrial Significance	Critical in polymer processing and applications	Important for material selection and processing
Physical State	Material remains solid but becomes flexible/rubbery	Material completely transforms to liquid
Energy Change	Involves gradual softening	Involves complete phase transformation
Application Impact	Determines material’s flexibility and processing conditions	Determines material’s heat resistance and processing limits
Dependence on Heating Rate	More sensitive to heating rate changes	Less sensitive to heating rate changes

How Tg and Tm Define Polymer Categories

Thermoplastics vs. Thermosets: Tg and Tm help distinguish these categories. Thermoplastics have both Tg and Tm and can be reshaped multiple times upon heating. Thermosets, however, cross-link at high temperatures, setting the shape when they cool. They exhibit a Tg but not a typical Tm because they don’t melt.

Crystallinity and Properties: Polymers with high crystallinity have a clear, sharp Tm and generally higher mechanical strength. Those with higher amorphous content have noticeable Tg and greater flexibility but lower structural rigidity.

Tg and Tm in Mold Making

The glass transition temperature (Tg) and melting temperature (Tm) are critical parameters in the injection molding and mold die casting processes, as they directly influence processing conditions and the quality of the final product.  If the processing temperature is below Tg, the polymer remains rigid and brittle, leading to limited flexibility and a heightened risk of fracturing during the molding process. This can result in defects such as cracks or incomplete filling of the mold cavity, ultimately compromising the integrity of the molded parts.

In contrast, Tm sets the maximum temperature needed to fully melt the polymer, ensuring that the material can flow properly into the mold cavity. If the temperature exceeds Tm, the polymer may degrade or decompose, leading to poor material properties and defects in the final product.

When the processing temperature is near or exceeds Tg, the material properties can change significantly. The polymer becomes more pliable, allowing for better deformation without breaking, which enhances flow characteristics and facilitates the filling of intricate mold designs. However, elevated temperatures can also introduce new degradation mechanisms, such as thermal oxidation or chain scission, which can negatively affect the overall quality of the molded parts, including their mechanical strength and thermal stability.

The effects of Tg and Tm extend beyond processing temperatures to influence the cooling and cycle times of the molding operation. As the temperature approaches Tg during cooling, the material transitions from a rubbery state to a glassy state, which can affect the cooling rate and cycle time. If the cooling is too rapid, it may lead to internal stresses or warping in the final product. Conversely, if the cooling time is extended, it may allow for better crystallization in semi-crystalline polymers, enhancing their mechanical properties.

Additionally, Tg and Tm impact various material properties during processing, including thermal conductivity, optical clarity, and mechanical performance. For instance, polymers processed above Tg typically exhibit improved thermal and mechanical properties, as they can better accommodate the stresses imposed during molding. This is particularly important in applications where the final product is subject to mechanical loads or thermal cycling.

Critical processing parameters influenced by Tg and Tm include injection flow rate, mold wall temperature, packing pressure, and fiber orientation in composite materials. These parameters must be carefully controlled to ensure proper material flow, maintain the desired physical properties, and achieve consistent part quality. For example, an optimal injection flow rate is necessary to fill the mold effectively without causing defects such as short shots or excessive flash.

Reliability considerations also underscore the importance of temperature control. Exceeding Tg during processing can introduce new failure mechanisms, such as increased brittleness or reduced impact resistance, which can adversely affect both the electrical and mechanical properties of the molded parts. This, in turn, impacts the long-term reliability and performance of the final product, especially in demanding applications.

Tg of Common Plastic Injection Molding Materials

The following table lists the glass transition temperatures (Tg) in degrees Celsius for various plastic injection molding materials:

Cooperation With Moldie

From concept to production, moldie offers end-to-end services including part design, prototyping, mold design, and large-scale manufacturing.

We have:

– Advanced plastic injection mold making
– Precision die casting solutions
– Custom injection molding services
– Expert mold design and engineering
– Complete OEM/ODM services

Whether you need complex automotive components or precision industrial parts, Moldie delivers excellence with every project. Our state-of-the-art workshop and experienced team ensure your manufacturing needs are met with uncompromising quality and efficiency.