Requirements for Thin-wall Molding
The demand for small and lightweight components has made thin-wall injection molding a critical performance enhancement for injection machines.
“Thin-wall” is typically defined as light weight electronic plastic parts with wall thicknesses of less than 1mm. For larger automotive plastic components, “thin-wall” can be 2mm. Generally, thin-wall products require changes in processing techniques, including higher pressures and speeds, shorter cooling times, and modifications to ejection and gate arrangements.
The following are the hardware requirements for thin-wall injection molding in terms of injection machines and Thin wall molds.
Injection Machines Requirement
Standard injection machines can be used to produce various thin-wall products. The performance of modern injection machines significantly exceeds those from 10 years ago. Advances in materials, gating technology, and design have further broadened the capabilities of standard injection machines to fill thin-wall components.
However, as wall thickness decreases, a more specialized injection machine with high-speed and high-pressure capabilities is required. For example, it is common to have an injection time of less than 0.5 seconds and injection pressures exceeding 210MPa for electronic components with thicknesses less than 1mm.
Hydraulic injection machines designed for thin-wall injection molding feature accumulators for frequent injection and mold closing. High-speed and high-pressure capabilities are also available in fully electric injection machines and electric/hydraulic hybrid injection machines. To withstand the high pressures of modern injection machines, the minimum clamping force required is 5-7 tons per square inch of projected area.
Furthermore, as injection pressures increase with decreased wall thickness, larger molds help reduce deflection. The ratio between the pull rod diameter and mold thickness for thin-wall products injection machines is typically 2:1 or lower. Controlling injection speed, pressure, and other processing parameters in a closed-loop manner contributes to controlling filling and holding pressure under high pressures and speeds.
Regarding shot size, the recommended injection amount is often around 40% to 70% of the barrel capacity since the large barrel diameter may not be necessary. With significantly shortened total molding cycles for thin-wall products, it is possible to reduce the minimum shot size to 20% to 30% of the barrel capacity. Care must be taken by the user during injection as a smaller shot size can result in increased residence time of the material in the barrel, potentially impacting the performance of the product.
Mold Requirement
Speed is one of the key factors for successful thin-wall injection molding. Rapid filling and high pressure enable molten thermoplastic materials to be injected into the mold cavity quickly, preventing gate freezing. If a standard part can be filled within two seconds, reducing mold thickness by 25% can potentially reduce filling time by 50%, resulting in one second.
One advantage of thin-wall injection molding is that as the thickness decreases, less material needs to be cooled. With thinner walls, the molding cycle can be halved. Proper arrangement of melt delivery systems prevents hot runners and sprue bushings from hindering the reduction of molding cycles. The use of hot runners and sprue bushing inserts helps minimize the molding cycle time. Additionally, consideration should be given to the mold material.
While 1.2312 steel is widely used for traditional molded products, the higher pressures in thin-wall injection molding require molds to be extremely robust. Therefore, internal molds typically adopt 1.2344-ESR, while special injection molding materials use S136H or 420 ESR, as well as other hard steels, to provide additional safety factors for thin-wall molds. However, the cost of robust molds may be 30% to 40% higher than standard molds. Nevertheless, the increased cost is usually offset by the improved production performance.