Positive and Negative Models in Thermal Molding
In hot press molding, molds provide molding reference for heated softened plastic sheets and determine the shape and size accuracy of the final product. The regular die is convex and the thin plate is formed by wrapping the outer surface of the die. The negative die is concave and the plate is drawn into the die cavity to be formed. Both molds directly affect the size accuracy, wall thickness distribution and surface quality of the product.
Positive Molds: Emphasize Internal Dimensional Accuracy
Positive die has a convex shape. The heated plastic sheet is a vacuum or pressure that forms on the outer surface of the mold. Since the inner surface of the product conforms directly to the requirements of the mold, this method ensures the accuracy of the internal dimensions of the product.
This high precision of internal dimensions makes positive moulds suitable for applications that require precise internal dimensions, such as specific containers or special packaging. However, the normal die usually results in uneven wall thickness distribution. The material at the top of the die is thicker and the edges and bottom corners are thinner, especially for deep cavity products. This thickness variation will affect the mechanical performance and service life of the product.
Negative Molds: Emphasis on External Accuracy and Wall Thickness uniformity
Negative die has a concave structure. Heated plastic sheets are pulled into mold cavity. The method prioritizes accuracy of external shapes and sizes for products such as food trays and electronic accessories that require precise external dimensions.
Negative pressure molding makes the wall thickness more even, which is very helpful for objects with complex geometries or deep cavities. Negative mold may also make crisper surface textures and details since the exterior surface of the plastic touches the inside wall of the mold directly. This makes it better for uses that need greater aesthetic quality.
Comparison of Comparing Dimensional Accuracy and Wall Thickness Distribution
Positive and negative modes have different applications in size accuracy, wall thickness distribution, and use cases.
Dimensional Accuracy
-Positive molds: The internal dimensions are highly precise, as the plastic interior surfaces come into direct contact with the mold. This precision favors products that require precise internal structures.
-Negative mould: the external dimensions are highly precise and the the product's outer surface matches the mould. Suitable for products with strict external dimensions.
Wall Thickness Distribution
-Positive molds: The top and bottom of the product are usually thicker and the sides and bottom are thinner. This thickness distribution can cause problems, especially for products with deeper depths.
-Negative die: It allows for more uniform wall thickness, which is essential for complex or deep design.
Application Positive and Negative Molds
The selection of positive and negative modes depends mainly on the expected application and product requirements.
Format: Suitable for applications where internal accuracy is critical, such as specialized containers and packaging solutions.
-Negative mould: more suitable for items that require precise external dimensions, such as food trays and aesthetically pleasing shells.
Advantages and disadvantages of positive and negative modes
According to the process requirements and product specifications, mold type has its specific advantages and disadvantages.
Advantages of Positive Molds
1. High surface finish: Positive mold produces smoother surfaces, leading to high quality surface aesthetics, ideal for appearance of key products.
2. Lower initial investment: Positive die mold manufacturing requires less initial investment and a longer service life.
Defects of Positive Molds
1. Loss of patterning detail: In some cases, details may become shallow or disappear due to stretching during molding.
2. Limit to simple geometric shapes: regular models are best suited to simple shapes; complex patterns and tight radii are challenging.
3. Size limitations: Due to difficulty in obtaining uniform thickness and precise dimensions, regular die is not suitable for larger parts.
Advantages of Negative Molds
1. Lower mold manufacturing costs: Typically lower mold production and setup costs.
2. High-quality surface designs: They offer intricate surface designs, ideal for complex designs that require polished aesthetics.
3. Generality of large complex parts: Negative die can accommodate large products and complex parts design. Weaknesses of (negative mold) technology:
1. Low material utilization: When using inner threaded molds, more plastic material is required to achieve even wall thickness, resulting in higher material waste.
2. Surface quality limitations: Products manufactured with inner threaded molds may have surface quality problems due to variations in wall thickness or uneven cooling.
3. Higher investment cost: due to the complexity of the inner threaded die, more specialized equipment and higher investment costs are usually required.
Notes on Mold Manufacturing
The design and manufacture of the die require attention to several key aspects to ensure the quality and functionality of the final product.
Pointing and Streaming
Sharp angles in the die can limit the flow of materials, creating an "assembly line" that affects the appearance and function of the product. Using rounded corners or smooth transitions can improve material flow and uniformity.
Edge Cooling Marks
Uneven cooling of mold corners will cause cooling marks and affect the appearance of the product. Optimize cooling system to ensure uniform heat dissipation and improve surface quality.
Uneven Wall Thickness
Uneven thickness of the wall, especially the uneven edges, will reduce the mechanical properties of the product. Precise mold design and strict control of wall thickness distribution are essential.
Difficult Demolding
Die wall Insufficient draft angle, can lead difficult demolding and product damage mold walls. Properly designed draft angles facilitates smooth disassembly and maintains the shape and surface quality of the product.
Considerations for Negative Mold Manufacturing
The production of negative mold also requires careful control to ensure consistency in product quality.
Edge Thickness Uniformity
Maintaining uniform edge thickness is critical to the structural integrity of the product. Thickness variations can lead to stress concentration and decrease product strength. High precision manufacturing process and regular mold inspection are necessary.
Monitoring Corner Thinning
In the process of hot molding, the angle can excessive thinning or even perforated. In the process of mold design and process control, it is necessary to strengthen the angular structure.
Mold Type Selection Principles
The mold type should be selected according to specific production requirements and workpiece characteristics:
Negative die is suitable for complex workpieces or large product, with high precision and durability.
Positive die is more suitable for simple geometry or application which requires high surface quality, with high production efficiency and smooth surface.
Thermal molding Mold Process Requirements
Draft angle
For easy demolding, a draft angle is required on the side of the die. Larger angles can improve demolition efficiency, and a minimum of 1 degree is generally recommended.
Aspect Ratio
Due to prestretching, the aspect ratio of positive molds may be low; negative molds usually require an aspect ratio of 1.4 or higher to form effectively.
Mold Leakage Test
leakage test is the key to ensure uniform wall thickness and product durability. The test method is to test potential defects by simulating production conditions by gradually injecting water and pressurizing the mold with a designated injection port seal. This test is particularly important for the thermoforming of materials such as HIPS because it effectively prevents wall thickness imbalance and structural failure.
Mold selection should be evaluated according to actual production requirements and product characteristics. Choosing the right mold type can improve thermoforming quality, production efficiency and economic efficiency.
