Guidelines for Hot Forming Design
1、 Overview of Hot Forming Process
Thermoforming is a manufacturing process that shapes thermoplastic sheets or films onto molds by applying heat and pressure. In this process, the sheet is first clamped and heated to the sagging point (above the glass transition temperature Tg of the polymer), usually achieved with the help of a radiation heating source. Subsequently, the sheet is pressed onto the surface of a mold made by FDM (fused deposition modeling, a 3D printing technology) or other methods using a vacuum source or air pressure to obtain the desired shape. According to different pressure methods, hot forming can be mainly divided into the following categories:
Vacuum forming: A vacuum is formed between the mold cavity and the sheet, and the sheet is tightly adhered to the mold surface using vacuum pressure (usually 14 psi). This method is suitable for forming relatively simple and thin-walled parts.
Pressure molding: Apply pressure (usually 50psi, up to 100psi) to the back of the sheet. This method is capable of processing thicker sheets and can form complex structures such as fine features, textures, and inverted buckles on the parts.
Mechanical pressure forming: Apply mechanical force directly to the sheet, and use a male mold plug that matches the appearance of the part to push the sheet to obtain the predetermined shape. Commonly used in situations where high precision and pressure control are required for molding.
Double sheet hot pressing molding: Using two sheets, double-sided hollow box parts can be produced, with features on both sides of the outside of the parts. This process expands the application scope of thermoforming and can replace rotary molding and blow molding in some cases to manufacture hollow box parts.
2、 Design points
（1） Part size design
Thermoforming is the process of stretching two-dimensional flat plastic sheets into complex three-dimensional geometric shapes. After forming, the wall thickness of the finished part will be less than the initial sheet thickness, and the stretching ratio and aspect ratio will vary in different parts of the part. Therefore, when designing the dimensions of hot formed parts, all dimensions should be marked on the side in contact with the mold. If using a convex mold, the dimensions are marked on the inside of the part; If using a concave mold, mark it on the outside of the part.
（2） Stretch ratio
The stretch ratio refers to the ratio of the total surface area of one side of the thermoformed finished part to the surface area of the initial plastic sheet. Generally speaking, the stretch ratio should not exceed 3:1. Excessive stretching ratio may cause the sheet to become excessively thin or even break during the stretching process, affecting the quality and performance of the parts.
（3） Aspect ratio
The aspect ratio is the ratio of the deepest depth of a thermoformed finished part to the minimum opening distance. Normally, the aspect ratio should not exceed 1:1. Excessive aspect ratio can cause local stretching of the sheet during the forming process, resulting in uneven wall thickness and even problems such as bottom cracking of the parts.
（4） Rounded corners
At the connection between walls, rounded or angled corners should be added to avoid designing sharp corners, which is particularly important at the intersection of the deepest three sides of the part. Larger rounded corners can make the wall thickness of each area of the part more uniform and improve the strength of the part. If the fillet is too small, it can easily lead to thin wall thickness, reduced strength, and stress concentration at the corner. Generally speaking, to avoid stress concentration, the fillet radius should be at least 75% of the wall thickness at that location. In addition, the deeper the part, the larger the minimum fillet required; Different plastic materials and types of thermoforming processes also have an impact on the minimum fillet size. For example, compared to PC and PE, ABS and PVC allow for smaller minimum rounded corners; Compared to vacuum forming, air pressure forming allows for smaller minimum rounded corners, which is one of the reasons why air pressure forming is commonly used for parts with complex shapes and structures. If conditions permit, the larger the fillet, the better.
（5） Inverted deduction
Inverted buckle is a practical feature on hot formed parts, which can be used to enhance part strength, achieve buckle function, fix features, and hide cutting marks. However, when designing the reverse buckle, attention should be paid to avoiding the buckle being too narrow or unable to be demolded, otherwise it will bring difficulties to the production of parts.
（6） Demoulding slope
To ensure that the parts can be smoothly removed from the mold, it is necessary to design a suitable demolding slope. The demolding slope of parts formed by convex molds is generally 4-6 degrees; Parts formed using concave molds have a demolding slope of 1.5-2 degrees. In some special cases, a 0-degree demolding slope is also feasible, but this requires the mold to have a complex motion structure. If there are bite marks or complex structures on the surface of the parts, it is necessary to increase the demolding slope appropriately. Compared to concave molds, convex molds require a larger demolding slope because the plastic shrinks and clamps the convex mold. If the demolding slope is too small, the first contact point of the plastic sheet will cool quickly when it extends in the mold, which will reduce the fluidity of the plastic and cause uneven material distribution, resulting in wrinkles on the surface of the part.
（7） Strengthening muscles
Strengthening ribs are used to enhance the strength of hot pressed molded parts. The design structure of reinforcing ribs in hot press molded parts is different from that in injection molded parts. The outer width of the reinforcing rib should be at least 1.75 times the depth of the reinforcing rib, and the thicker the wall thickness, the wider the requirement for the width of the reinforcing rib. For air pressure forming, the distance between the reinforcing ribs should be at least 1 times the depth of the reinforcing ribs, and the larger the air pressure, the wider the distance should be.
（8） The angle between walls
On the vertical section of the component, the larger the angle between the walls, the better. A larger angle is beneficial for the uniform stretching of the sheet during the forming process, reducing stress concentration and improving part quality.
（9） Tolerance
Hot pressing has its specific universal tolerance standards, for example: in terms of standard hole to hole tolerance, the tolerance is ± 0.015 inches for dimensions within 12 inches, and ± 0.001 inches for every additional inch of tolerance above 12 inches; In terms of standard forming and trimming tolerances, the tolerance is ± 0.030 inches for sizes within 12 inches, ± 0.060 inches for sizes between 12 inches and 60 inches, and ± 0.001 inches for every additional inch above 60 inches.
3、 Materials suitable for hot forming
Common plastics that can be used for thermoforming include PS (polystyrene), HIPS (high impact polystyrene), PE (polyethylene), PP (polypropylene), ABS (acrylonitrile butadiene styrene copolymer), PVC (polyvinyl chloride), PVC/ABS (alloy of polyvinyl chloride and acrylonitrile butadiene styrene copolymer), PMMA (polymethyl methacrylate, also known as organic glass), PVC/PMMA (alloy of polyvinyl chloride and polymethyl methacrylate), PC (polycarbonate), PC/ABS (alloy of polycarbonate and acrylonitrile butadiene styrene copolymer), TPO (thermoplastic polyolefin), and PETG (ethylene glycol modified polyethylene terephthalate). These materials have different characteristics, such as high transparency and easy processing of PS; ABS has good comprehensive performance, high strength and toughness; PC has excellent impact resistance and heat resistance. When selecting materials, it is necessary to comprehensively consider factors such as the usage environment, performance requirements, and cost of the parts.
4、 Application of hot forming
Thermoforming technology has become an ideal choice for many industrial fields due to its multifunctionality and cost-effectiveness. Hot formed parts are commonly used to replace metal sheet metal parts and have unique advantages compared to fiberglass (fiber-reinforced plastic) parts and parts manufactured through resin transfer molding (RTM) processes. It has a wide range of applications, covering the following aspects:
Medical field: diagnostic and imaging equipment enclosures, bed and furniture components, auxiliary equipment, as well as wall and ceiling panels, etc. The hot forming process can manufacture parts that meet the appearance, dimensional accuracy, and hygiene requirements of medical equipment, and the cost is relatively low.
Automotive industry: dashboard, seat components, interior panels, bumpers, and air ducts, etc. Hot formed parts can achieve complex shape design while possessing certain strength and lightweight characteristics, which helps to improve the overall performance of automobiles and reduce production costs.
Aerospace field: air ducts, seat components, interior panels, kitchen equipment, curtains, etc. In the aerospace field, there are extremely high requirements for the quality and performance of parts. Hot forming technology can achieve lightweight design, reduce the weight of aircraft, and improve fuel efficiency while ensuring the quality of parts.
Office equipment: fax machines, printers, computer and copier enclosures, power boards, wall panels and ceilings, as well as furniture, etc. The hot forming process can quickly produce various shapes of office equipment casings and components, meeting the diverse needs of the market for product appearance and functionality.
5、 Advantages and disadvantages of hot forming
（1） Advantages
Wide range of parts that can be produced: Thermoforming can produce extremely small parts, such as tablet packaging materials or watch batteries, as well as large parts, such as ship hulls 3-5 meters long. The thickness range of the molding material is 0.05-15mm, and for foam materials, the thickness can reach 60mm.
Low mold cost: Due to the use of lower pressure, there is no need for complex mold structures and expensive mold materials. In the production of large part samples and small batch production, the cost of parts is highly competitive, making it an ideal process.
Easy design modification: The low cost of molds and short delivery cycles make part design modifications simple and can quickly respond to changes in market demand.
High material quality and durability: Hot forming uses sheet plastic, resulting in better product quality and durability.
Widely applicable materials: Almost all thermoplastic materials can be used for thermoforming, providing rich material choices for product design.
Simplified product structure: capable of manufacturing large parts and simplifying products originally composed of multiple parts into one part, saving material and assembly costs.
Excellent performance of parts: Hot formed parts have a strength not lower than that of steel and fiberglass, and also have the characteristics of light weight, corrosion resistance, and better ductility than fiberglass.
（2） Disadvantages
Difficulty in controlling wall thickness: It is difficult to accurately control the wall thickness of parts during the hot forming process, and it is not suitable for processing parts with significant differences in wall thickness.
Depth limitation of parts: The depth of thermoformed parts is limited to a certain extent. The aspect ratio of a general container is 0.5-2, and the ideal aspect ratio is less than 1.
Poor consistency and uniformity: It is difficult to ensure the consistency of structure or size between different parts using hot forming method, and it is also difficult to ensure the uniformity of wall thickness in different parts of the same part. In addition, some details of the mold during the vacuum forming process are difficult to fully reflect in the parts.
Increased raw material costs: The sheets used in thermoforming are semi-finished products made from pellets or powders, and compared to other plastic molding processes such as injection molding, raw materials will increase additional costs.
Increased process complexity: After hot forming, the sheet material needs to be cut, which increases the process complexity, and the cut scraps cannot be directly recycled.
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