Thin Wall Injection Molding Technology
Advanced solutions for lightweight, high-precision plastic components in modern manufacturing
I. Introduction to Thin Wall Injection Molding Technology
Currently, China has become the world's industrial processing center (production base), especially in products such as color TVs, mobile phones, air conditioners, refrigerators, and washing machines, which account for 50% of global output. These products extensively use plastic parts, whose cost accounts for 50% to 80% of the total product cost. Therefore, while ensuring product quality, thin wall injection molding helps reduce product costs significantly. This is particularly relevant in low volume injection molding scenarios where cost efficiency is crucial.
In addition, mobile phones, audio players, digital cameras, handheld computers, and tablets increasingly require miniaturization and portability, necessitating thinner and lighter injection-molded parts to meet corresponding product requirements. Furthermore, the extensive use of disposable cups, bowls, spoons, lunch boxes, and other items has prompted relevant researchers at home and abroad to focus on thin-wall technology for injection-molded parts. Low volume injection molding has played a vital role in developing and testing these innovative thin-wall products.
The thinning of injection-molded part walls imposes special requirements on structural design, material performance, mold structure, injection molding processes, and more. Low volume injection molding allows manufacturers to experiment with different design approaches and material combinations before committing to large-scale production, making it an essential part of the development cycle for thin wall components.
II. Injection Molded Part Thickness and Structural Design
Injection molded parts not only have structural and appearance requirements but also need to accommodate internal components, requiring sufficient strength and rigidity while considering injection molding process requirements. Therefore, the thickness design of thin-wall injection molded parts is crucial, as illustrated in Figure 4-18. Low volume injection molding provides an excellent platform for testing different thickness configurations to find the optimal balance between structural integrity and material usage.
1. Thickness Design for Regular Injection Molded Parts
(1) Wall Thickness
If the wall thickness of an injection molded part is too thin, the flow resistance during molding becomes too great, making it difficult to fill large injection molded parts. Conversely, if the wall thickness is too thick, defects such as shrinkage marks and bubbles may occur. On the basis of ensuring rigidity and strength, the recommended wall thickness range for injection molded parts is 0.45~6.5mm, with the commonly used range being 1.5~3.0mm, requiring uniform wall thickness.
In low volume injection molding, precise control of wall thickness is even more critical due to the smaller production runs and the need for each part to meet exact specifications. According to usage requirements, when designing the overall wall thickness of injection molded parts, it is necessary to consider the impact of structures such as ribs, screw bosses, and other features on rigidity, strength, and appearance.
(2) Reinforcing Ribs
The installation of reinforcing ribs can improve the strength and rigidity of injection molded parts, prevent deformation of injection molded parts, and facilitate the flow of plastic melt. The structure and dimensions of ordinary reinforcing ribs are shown in Figure 4-19. From Figure 4-19, it can be seen that t = (0.40~0.75)T, L = (2.5~5.0)T, and α = 0.5°~1.5°.
In low volume injection molding, the design of reinforcing ribs must be carefully considered to ensure that parts can be produced efficiently even with smaller production quantities. The proper implementation of reinforcing ribs can often reduce the overall wall thickness required, saving material costs and reducing production time.
(3) Screw Bosses
Usually, self-tapping screws are needed inside injection molded parts to install other components, so screw bosses as shown in Figure 4-20 can be provided. Screw bosses are divided into two types: those with reinforcing ribs and those without. The length of the bottom of the reinforcing ribs c = (0.2~0.5) × the height of the screw boss.
Low volume injection molding allows for rapid testing of different screw boss designs to ensure they can withstand the required assembly forces without compromising the thin wall structure. Additionally, wall thickness also involves the design of structures such as bosses, corners, through holes, and blind holes. However, if a thin-wall structure is adopted, the structure and dimensions of the above-mentioned reinforcing ribs and screw bosses must be adjusted accordingly.
III. Material Selection for Thin Wall Injection Molded Parts
Due to the reduced thickness of thin-wall plastic parts, it is necessary to use materials with good fluidity. These materials must also have high impact strength, high heat distortion temperature, and good dimensional stability. In low volume injection molding, material selection can be more flexible as the cost implications of testing different materials are minimized compared to large-scale production runs.
1. Plastic Fluidity
When injection molding thin-wall plastic parts, the plastic must have good fluidity, with a flow length to thickness ratio (L/t) greater than 150. The fluidity of plastics can be characterized by their melt viscosity coefficient. The melt viscosity coefficients of commonly used plastics are shown in Table 4-5.
In actual production, the melt flow rate (MFR) of plastic resin is usually used as the basis for selecting its fluidity. Since the MFR of different grades and batches of plastic resin produced by different enterprises varies, it is necessary to conduct inspections before production. Low volume injection molding processes often include rigorous material testing to ensure that each batch meets the required specifications for thin wall applications.
| Plastic Material | Melt Viscosity Coefficient | Typical MFR Range (g/10min) | Suitable for Thin Wall? |
|---|---|---|---|
| Polypropylene (PP) | Low | 10-40 | Excellent |
| Acrylonitrile Butadiene Styrene (ABS) | Medium | 15-30 | Good |
| Polycarbonate (PC) | Medium-High | 5-20 | Limited |
| Polyethylene (PE) | Low | 2-25 | Good |
| Polyoxymethylene (POM) | Medium | 10-35 | Good |
Low volume injection molding is particularly valuable when working with specialized high-performance materials for thin wall applications, as it allows manufacturers to evaluate material performance without large capital investment. This is especially important when developing new thin wall products where material requirements may not be fully established.
IV. Structural Design of Thin Wall Injection Molded Part Molds
1. Overall Mold Structure
Thin-wall plastic parts use thin-wall technology during the injection molding process, so the materials used have poor fluidity in the mold, thus requiring higher injection pressure. Therefore, the rigidity and strength of the mold used must be correspondingly increased. Therefore, when designing the mold's moving template, fixed template, and their support plates, their thickness is usually 30% to 50% thicker than that of traditional mold templates, and support columns must be added.
The mold clamping surface (fixed template and moving template) must be provided with conical positioning (integral conical surface positioning or conical positioning blocks) to ensure precise positioning and good lateral support, preventing bending and offset. In low volume injection molding, where molds may be used for multiple prototype iterations, this robust construction helps maintain precision over multiple production runs.
In addition, thin-wall plastic parts require high-speed injection from the injection molding machine, which increases mold wear. Therefore, the mold cavity, core, gate, and other components must be made of materials with high hardness, strength, rigidity, and wear resistance. Usually, mold steels such as S136, 2344, SKD61, and PMS are used, and are pre-hardened or heat-treated to achieve a surface hardness of 48~52 HRC.
2. Gating System
For plastics with excellent fluidity such as PP, a pinpoint gate form can be used. For plastics with medium fluidity (such as ABS, polyoxymethylene, etc.), the gate should be designed as much as possible in the thicker part of the plastic part, and the injection process should transition from thicker to thinner to reduce depression and warpage.
Multiple gate forms can be used (see Figure 4-24), making it easier for the plastic melt to fill the cavity and reducing pressure drop. Hot runner technology can also be used to reduce the viscosity of the plastic melt and achieve rapid injection into the mold cavity. Low volume injection molding often employs hot runner systems to minimize material waste and ensure consistent part quality, even with smaller production quantities.
Sprue Design Considerations
- Short length to minimize pressure loss
- Proper taper (2-5°) for easy ejection
- Smooth surface finish (Ra 0.8 μm or better)
- Adequate diameter to accommodate flow rate
Runner System Optimization
- Balanced runner layout for multi-cavity molds
- Circular cross-section preferred for minimal pressure loss
- Proper sizing relative to part thickness
- Thermal insulation for hot runner systems
In low volume injection molding, the gating system design becomes even more critical as each part must meet strict quality standards without the benefit of long production runs to optimize the process. Careful consideration of gate location, type, and size helps ensure proper filling and packing of thin wall sections, reducing the need for post-production rework.
V. Thin Wall Injection Molding Equipment
Standard injection molding machines can be used to produce many thin-wall products. Currently, the performance of new injection molding machines has greatly exceeded that of machines from 10 years ago. Advances in materials, gate technology, and design have further expanded the capabilities of standard injection molding machines for filling thin-wall parts. However, as wall thickness continues to decrease, thin-wall products are best produced using injection molding machines with high-speed and high-pressure capabilities.
Hydraulic injection molding machines used for thin-wall injection molding are designed with accumulators that can frequently drive injection and clamping. All-electric injection molding machines and electric-hydraulic injection molding machines with high-speed and high-pressure performance are also used for thin-wall injection molding. These specialized machines are equally valuable for low volume injection molding applications, where precision and repeatability are essential even with smaller production quantities.
When producing thin-wall products, as wall thickness decreases and injection pressure increases, large templates help reduce bending. Closed-loop control of injection speed, pressure, and other processing parameters helps control filling and packing under high pressure and speed conditions. Low volume injection molding benefits significantly from this precise control, as it allows for consistent part quality even when producing small batches of complex thin wall components.
Equipment Performance Requirements for Thin Wall Injection Molding
High Injection Speed
Minimum 300 mm/s, preferably 500-800 mm/s to fill thin sections before material cools.
High Injection Pressure
Capable of 2000-2500 bar to overcome flow resistance in thin wall sections.
Rapid Response
Fast valve gate control and precise pressure/flow transitions for consistent filling.
Closed-Loop Control
Real-time adjustment of process parameters to maintain dimensional stability in thin walls.
High Clamping Force
Adequate clamping force (4-8 tons per cm² of projected area) to prevent flash.
Precise Temperature Control
Accurate barrel and mold temperature control (±1°C) for consistent material flow.
For low volume injection molding of thin wall components, the equipment must offer quick changeover capabilities to accommodate frequent material or design changes. This flexibility allows manufacturers to efficiently produce small batches of specialized thin wall parts without sacrificing quality or incurring excessive setup costs. The combination of advanced machine capabilities and low volume injection molding expertise enables the production of high-quality thin wall components that meet the demanding requirements of modern products.