Understanding Mold Temperature: Its Impact and Optimization in Injection Molding

Mold temperature plays a pivotal role in the injection molding process, influencing not only the quality but also the appearance and dimensions of the final product. Achieving a uniform and optimal mold temperature is crucial to ensure the molded parts meet the desired specifications and quality standards. This article delves into the intricacies of mold temperature, and its effects on various aspects of the molding process and offers insights into setting the right temperature based on the material used. Join us as we explore the significance of mold temperature and its profound impact on the world of injection molding.

Defining Mold Temperature and Its Significance in Molding

Mold temperature, often referred to as cavity temperature or tooling temperature, is the temperature of the mold cavity’s surface during the molding process. In the realm of mold design and the setting of molding conditions, it’s paramount not only to maintain an appropriate temperature but also to ensure its uniform distribution across the mold.

An uneven distribution of the mold temperature can lead to inconsistent shrinkage and internal stresses in the molded part. This inconsistency can, in turn, result in the molded part becoming prone to deformations and warping. The molding temperature directly impacts the molding cycle and the quality of the molded product. In practical operations, the mold temperature is initially set based on the minimum appropriate temperature for the material being used. Adjustments are then made depending on the quality of the output.

Impact of Mold Temperature on Injection Molding

In the injection molding industry, newcomers often ask: Why does a higher mold temperature result in a shinier finish on the molded parts? Let’s delve into how to optimally set the mold temperature and its effects on the molding process.

Effect on Appearance

When the mold temperature, also known as tooling or cavity temperature, is too low, the flowability of the molten plastic decreases, potentially leading to short shots. Mold temperature influences the crystallinity of plastics. For instance, with ABS, a low mold temperature results in a less glossy product finish. At higher mold temperatures, the plastic tends to migrate more easily to the surface. This means that when the mold temperature is high, the plastic composition is closer to the mold surface, leading to better filling and a shinier, more lustrous finish. However, an excessively high mold temperature can cause the plastic to stick to the mold and produce noticeable bright spots on the part. Conversely, a too-low mold temperature can cause the plastic to grip the mold too tightly, risking damage during ejection, especially if the part has intricate surface patterns.

Influence on Product Dimensions

A high mold temperature can lead to thermal decomposition of the melt, resulting in a higher shrinkage rate when the product is exposed to air, thus reducing its size. On the other hand, if the part size increases when the mold is operated at low temperatures, it’s typically due to the mold surface temperature being too low. This is because a lower mold temperature results in reduced shrinkage in air, leading to a larger product size. A low mold temperature accelerates the “freeze orientation” of the molecules, increasing the thickness of the frozen layer inside the mold cavity. Conversely, a high mold temperature slows down the cooling of the melt, leading to a higher actual shrinkage rate.

Impact on Deformation

Improper design of the mold’s cooling system or inadequate temperature control can result in insufficient cooling of the plastic part, leading to warping or deformation. For symmetrical parts, it’s essential to maintain a consistent mold temperature to ensure uniform cooling. A significant temperature difference within the mold can cause uneven cooling, leading to inconsistent shrinkage and, consequently, warping or deformation.

Effect on Mechanical Properties (Internal Stress)

A low mold temperature can result in visible weld lines on the plastic part, reducing its strength. For crystalline plastics, a higher degree of crystallinity increases the tendency for stress cracking. For non-crystalline plastics with high viscosity, like PC, the tendency for stress cracking is related to the internal stress in the part. Raising the mold temperature can help reduce this internal stress.

Influence on Heat Deformation Temperature

Especially for crystalline plastics, if a product is molded at a lower mold temperature, the molecular orientation and crystallization are instantly frozen. Under a higher temperature environment or secondary processing conditions, the molecular chains might partially rearrange and crystallize, causing the product to deform even at temperatures much lower than the material’s heat deformation temperature (HDT). The best practice is to produce at a mold temperature close to its crystallization temperature, ensuring sufficient crystallization during the injection molding stage.

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

Recommendations for Determining the Right Mold Temperature

As molds become increasingly intricate, creating the right conditions to effectively manage mold temperature becomes more challenging. Beyond simple parts, mold temperature control systems often involve a compromise. The following suggestions serve as a general guideline.

Suggested Temperatures for Different Plastics

Knowing the right temperature for different plastics is key to getting the best results in the finished product. Here’s a detailed table that provides recommended mold temperatures for different plastics, along with some insights:

Additional Mold Temperature Recommendations

1. Temperature Control in Mold Design:

During the mold design phase, consider temperature control for the external shape of the parts being processed.

2. Heat Transfer Considerations:

When designing molds for low injection volumes and large mold sizes, prioritize good heat transfer properties.

3. Fluid Flow Design:

Design fluid channels in the mold and sprue with some margin. Avoid using joints, as they can severely obstruct the flow of temperature-controlling fluids.

4. Use of Pressurized Water:

If possible, use pressurized water as the temperature control medium. Employ high-pressure, high-temperature-resistant hoses and manifolds.

5. Temperature Equipment Specifications:

Provide detailed specifications for temperature control equipment that matches the mold. Datasheets from mold manufacturers should offer essential figures regarding flow rates.

6. Insulation:

Use insulating plates at the junction between the mold and the machine platen.

7. Separate Temperature Control Systems:

Use different temperature control systems for the moving and fixed molds.

8. Isolated Temperature Control Systems:

Use isolated temperature control systems on any side and center, allowing different startup temperatures during the molding process.

9. Series vs. Parallel Circuits:

Temperature control system circuits should be connected in series, not in parallel. Parallel circuits can lead to uneven flow rates due to resistance differences, resulting in more significant temperature variations.

10. Temperature Display:

It’s beneficial to have displays for both supply and return temperatures on mold temperature control equipment.

11. Process Control:

The goal of process control is to incorporate a temperature sensor into the mold, allowing for temperature variation detection during actual production.

12. Establishing Thermal Equilibrium:

Establish thermal equilibrium in the mold through multiple injections during the entire production cycle, typically at least ten injections. Many factors influence the actual temperature in thermal equilibrium. The actual temperature of the mold surface in contact with the plastic can be measured using a thermocouple inside the mold (reading at 2mm from the surface). A more common method involves using a handheld pyrometer with a fast-response probe.

13. Adjusting Mold Temperature:

Adjust the mold temperature to the appropriate value. Recommended mold temperatures for different materials are usually provided in material datasheets, considering factors like surface finish, mechanical properties, shrinkage, and processing cycle.

14. Considerations for Precision Components:

For mold processing precision components or parts with strict appearance or safety standards, a higher mold temperature is typically used. For parts with lower technical requirements and where production costs need to be minimized, a lower processing temperature can be used. However, manufacturers should be aware of the drawbacks of this choice and inspect parts thoroughly to ensure they meet customer requirements.

Finding an injection molding supplier who is proficient in mold temperature

Because of the importance of mold temperature to injection molding, it is especially important to find an injection molding supplier who is very good at what they do. At Prototool, we don’t just understand the intricacies of mold temperature; we master them. Specializing in plastic products manufacturing, our expertise in the injection molding process is second to none. Our in-depth knowledge ensures that every product we produce is of the highest quality, consistently meeting and often surpassing client expectations. As the industry continues to evolve, Prototool remains at the forefront, dedicated to integrating the latest insights and technologies and ensuring unparalleled excellence in every plastic part we manufacture.