The Importance of Setting the Appropriate Mold Temperature

In the injection molding industry, it’s common for newcomers to inquire: why does a higher mold temperature result in a shinier finish for plastic products?

This article will introduce you to the importance of mold temperature and how to set it properly.

Mold temperature-1

1. The Impact of Mold Temperature on Appearance

Firstly, when the mold temperature is too low, it reduces the melt’s flowability, potentially leading to incomplete filling.

Mold temperature also affects the crystallinity of plastics.

For ABS, if the mold temperature is too low, the product’s surface finish will be inferior. On the other hand, with higher mold temperatures, plastics are more prone to migrate to the surface.

Therefore, when the injection mold temperature is elevated, the plastic components come closer to the mold’s surface, resulting in better filling, increased brightness, and a shinier appearance.

Mold temperature-2

However, the injection mold temperature should not be too high either, as excessive heat can cause mold sticking and result in noticeable bright spots on the plastic parts.

On the other hand, if the injection mold temperature is too low, it can lead to the parts adhering too tightly to the mold, making them prone to surface damage, especially on textured areas.

For addressing issues related to part quality, multi-stage injection molding can be employed. For instance, if there are visible weld lines on the product during the filling stage, stage-injection molding can be used as a solution.

In the injection molding industry, for high-gloss products, higher mold temperatures yield a shinier surface finish, while conversely, lower temperatures result in a less glossy appearance.

However, for products made from UV-resistant PP material, the higher the temperature, the lower the surface gloss of the product. Lower gloss levels correspond to higher color variation, meaning that gloss and color variation are inversely proportional.

Therefore, the most common issue resulting from mold temperature is the rough surface finish of molded parts, which is often caused by excessively low mold surface temperatures.

The mold shrinkage and post-mold shrinkage of semi-crystalline polymers primarily depend on the mold temperature and product wall thickness. Non-uniform temperature distribution within the mold leads to different shrinkage rates, making it challenging to ensure that the parts meet specified tolerances.

In the worst-case scenario, whether processing unreinforced resin or reinforced resin, the shrinkage exceeds the correctable value.

Mold temperature-3

2. Impact on Product Dimensions

Excessive mold temperature can lead to thermal decomposition of the melt, resulting in increased shrinkage when the product is exposed to the air, causing a reduction in product dimensions.

In cases where product dimensions increase under low-temperature mold conditions, it is generally due to the mold surface temperature being too low. This is because when the mold surface temperature is too low, the product experiences lower shrinkage in the air, resulting in larger dimensions.

Low mold temperatures accelerate the “frozen orientation” of molecules, leading to an increased thickness of the frozen layer within the mold cavity. Additionally, low mold temperatures hinder crystal growth, thus reducing the product’s molding shrinkage.

Conversely, higher mold temperatures result in slower melt cooling, longer relaxation times, lower orientation levels, and facilitate crystallization, leading to a larger actual shrinkage of the product.

If the warm-up process takes too long before the dimensions stabilize, it indicates poor mold temperature control. This is because the mold takes a longer time to reach thermal equilibrium.

Non-uniform heat distribution in certain parts of the mold can significantly extend the production cycle, leading to increased molding costs.

Maintaining a consistent mold temperature can reduce fluctuations in molding shrinkage and enhance dimensional stability.

For crystalline plastics, higher mold temperatures facilitate the crystallization process. Fully crystallized plastic parts will not undergo dimensional changes during storage or use. But higher crystallinity results in greater shrinkage.

For softer plastics, it is advisable to use lower mold temperatures during the molding process to promote dimensional stability.

Regardless of the material, maintaining a consistent mold temperature and uniform shrinkage is beneficial for improving dimensional accuracy.

Mold temperature-4

3. The Impact of Mold Temperature on Warping

If the mold’s cooling system is designed improperly or the mold temperature control is inadequate, insufficient cooling of the plastic parts can lead to warping and deformation.

When it comes to mold temperature control, it should be determined based on the structural characteristics of the product:

This includes temperature differentials between the front mold and the rear mold, the core and the mold wall, and the mold wall and insert components. By controlling the cooling and shrinkage rates of different mold parts, one can counteract the tendency of the part to bend in the direction of the higher temperature side after demolding. This helps to offset orientation-related shrinkage differences and prevent warping deformation in accordance with orientation patterns.

For plastic parts with completely symmetrical shapes, the mold temperature should be consistent throughout to ensure even cooling of all parts of the part. Stable mold temperatures and balanced cooling can reduce part deformation.

Significant differences in mold temperature can result in uneven cooling and inconsistent shrinkage in the plastic parts. This can lead to stress-induced warping and deformation, particularly in parts with uneven wall thicknesses and complex shapes.

On the side of the mold with the higher temperature, after cooling, the deformation will inevitably occur towards the side with the higher mold temperature. It is recommended to choose the front and rear mold temperatures reasonably according to the specific requirements.

Mold temperature-5

4. The Impact of Mold Temperature on Mechanical Properties (Internal Stress)

Low mold temperatures result in pronounced weld lines on plastic parts, leading to reduced product strength.

For crystalline plastics, higher crystallinity increases the tendency for stress cracking in the parts. To reduce stress, mold temperatures should not be excessively high (e.g., for PP and PE).

In the case of high-viscosity, amorphous plastics like PC, stress cracking is related to internal stresses in the part. Elevating mold temperatures is advantageous for reducing internal stresses and mitigating the tendency for stress cracking.

The manifestation of internal stress is the presence of visible stress marks.

This occurs primarily because the formation of molding internal stress is mainly due to the varying thermal shrinkage rates during cooling. After the product is formed, its cooling progresses from the surface towards the interior. Initially, the surface undergoes rapid cooling and hardening, followed by the gradual cooling of the interior. During this process, differences in shrinkage rates between layers give rise to internal stress.

When the residual internal stress within a plastic part exceeds the resin’s elastic limit or when exposed to certain chemical environments, cracks can form on the surface of the part.

Research on transparent resins like PC and PMMA has shown that residual internal stress in the surface layer is in a compressive state. While in the inner layers, it is in a tensile state. Surface compressive stress depends on the surface cooling conditions, with colder molds causing the molten resin to cool rapidly, resulting in higher residual internal stress in the molded product.

Mold temperature is the most fundamental factor in controlling internal stress, and even slight variations in mold temperature can have a significant impact on residual internal stress.

In general, each product and resin has its own minimum mold temperature limit for acceptable internal stress. When molding thin-walled parts or parts with longer flow distances, the mold temperature should be higher than the minimum limit for typical molding scenarios.

5. Impact on the Heat Deflection Temperature (HDT) of the Product

Especially for crystalline plastics, if a product is molded at lower mold temperatures, the molecular orientation and crystallization are instantly frozen.

When exposed to a high temperature environment during end-use or secondary processing conditions, the molecular chains partially rearrange and crystallize, causing the product to deform even at temperatures well below the material’s heat deflection temperature (HDT).

The correct approach is to produce the product at mold temperatures close to the recommended crystallization temperature, ensuring that the product undergoes sufficient crystallization during the injection molding stage. This prevents post-crystallization and post-shrinkage when exposed to high-temperature environments.

In summary, mold temperature is one of the most fundamental control parameters in the injection molding process and is a primary consideration in mold design.

Mold temperature-6

6. Recommendations for Establishing the Right Mold Temperature

Today, molds are becoming increasingly complex, making it more challenging to create ideal conditions for effective mold temperature control.

Except for straightforward parts, mold temperature control systems are often a compromise solution.

Therefore, the following recommendations serve as a rough guideline only:

(1) Temperature control for the outer shape of the productmust be considered during the mold design phase.

(2) Leave a margin when designing the mold runner.Avoid the use of connectors, as this can severely hinder fluid flow controlled by mold temperature.

(3) If possible, use pressurized water as the temperature control medium. Utilize resilient pipes and manifolds that can withstand high pressure and temperature.

(4) Provide detailed specifications for temperature control equipment that matches the mold. Data tables from the mold manufacturer should include necessary figures regarding flow rates.

(5) Use insulating boards at the junction between the mold and machine platen.

(6) Employ separate temperature control systems for the moving and stationary molds.

(7) Use separate temperature control systems on each side and at the center to enable different starting temperatures during the molding process.

(8) Different temperature control system circuits should be connected in series, not in parallel.

If circuits are connected in parallel, differences in resistance can lead to varying flow rates of the temperature control medium, resulting in greater temperature variations compared to when circuits are in series.

The operation works effectively only when series circuits are connected to the mold, and the temperature difference between the inlet and outlet is less than 5°C.

(9) It is advantageous to have displays for supply and return temperatures on the mold temperature control equipment.

(10) The purpose of process control is to incorporate a temperature sensor into the mold so that temperature changes can be monitored during actual production.

(11) Establishing thermal equilibrium within the mold by multiple injections, for example 10 injections.

The actual temperature at which thermal equilibrium is reached is influenced by numerous factors.

The actual temperature of the mold surface in contact with the plastic can be measured using a thermocouple inside the mold (reading 2mm from the surface).

A more commonly used method is to use a handheld high-temperature thermometer with a fast-responding probe.

Determining mold temperature requires measuring multiple points, not just a single point or one side’s temperature.

Subsequently, adjustments can be made based on the set temperature control standards. Mold temperatures should be adjusted to the appropriate values.

These recommendations are typically provided with the consideration of achieving the best balance between factors such as high surface finish, mechanical performance, shrinkage, and processing cycle.

(12) Molds for processing precision productwith strict aesthetic requirements or specific safety standards typically require higher mold temperatures. This helps reduce post-molding shrinkage, achieve a shinier surface finish, and maintain consistent performance.

For parts with lower technical requirements and a need for cost-effectiveness in production, lower processing temperatures can be employed during molding.

However, manufacturers should be aware of the drawbacks of this choice and thoroughly inspect the parts to ensure they still meet customer requirements.

Leave a Comment

Contact Us