thin wall injection molding

A wide range of thin-wall applications can be handled by standard molding machinery. Compared to machines a decade ago, new standard machines offer much more capability. The use of new materials, gating technology, and design techniques has further expanded the filling capabilities of standard machines.

Nevertheless, as wall thickness continues to shrink, a more specialized press with higher speed and pressure capabilities may become necessary. It is not uncommon to have fill times of less than 0.5 seconds and injection pressures greater than 30,000 psi with a portable electronics part less than 1 mm thick. Injection and clamping cycles are generally driven by accumulators in hydraulic machines designed for thin-wall molding. Additionally, high-speed and high-pressure electric and hybrid hydraulic models have started to appear.

To withstand high pressures of projected area, clamp forces should be at least 5-7 tons/square inch. In addition, heavy platens reduce flexure due to a drop-off in wall thickness and an increase in injection pressure. In thin-wall machines, the tiebar distance is typically 2:1 or less than the platen thickness. Further, thin walls can also affect filling and packing at high speeds and pressures when closed-loop control is applied to injection speed, transfer pressure, and other process variables.

Large barrels have a tendency to have too many shots. The shot size should range between 40% and 70% of the barrel’s capacity. If the parts are thoroughly tested for property loss due to possible material degradation, reducing the minimum shot size to 20%-30% of barrel capacity may be possible due to the greatly reduced total cycle time typical of thin-wall applications. It is important for users to be aware that smaller shot sizes can lead to longer barrel residence times, which will degrade the material’s properties.

Make your molds more durable

Thin-wall molding is characterized by speed as one of its key attributes. In order to prevent freeze-off in thinner cavities, faster filling rates and higher pressures are needed. For a standard part filled in 2 seconds, a 25% reduction in thickness could result in a 50% drop in fill time down to 1 second.

Using thin-wall molding has the advantage that it requires less cooling as the wall sections drop. By reducing wall thickness aggressively, cycle times can be cut in half. It is possible to prevent runners and sprues from decreasing that cycle-time advantage with careful management of the melt-delivery system. To minimize cycle time, thin-wall molding sometimes uses hot runners and heated sprue bushings.

It is important to review mold material as well. In conventional applications, P20 steel is widely used, however, due to the higher pressures of thin-wall molding, molds must be more robust. Thin-wall tools are more safe when they are made of steels like H-13. In addition, you should choose a molding material that will not accelerate mold wear when injected at high speed into the cavity.

Nevertheless, robust tools are expensive – perhaps 30% to 40% more expensive than a standard mold. However, increased productivity often helps offset the cost. Thin-wall approaches are often used to decrease tooling costs. Increasing productivity by 100% can eliminate the need for building molds, saving money over the course of a project.

More tips on thin wall tool design:

You should use steel that is harder than P20 for aggressive thin-wall applications, especially when corrosion and wear are expected to be high. H-13 and D-2 steels are suitable for gate inserts.

• A molded interlock can sometimes prevent misalignment and flexing.
• By telescopeing into the cavity, cores can be less likely to shift and break.
• Under the cavities and sprue use thicker support plates (often two to three inches thick) with pillars (typically pre-loaded 0.005 inches).
• Using larger and more ejector pins than with conventional molds will reduce pin pushing.
• Think strategically about where to place sleeve and blade knockouts.
• By polishing cores and ribs with Diamond No. 2, sticking problems can be eliminated. Also, nickel-PTFE can improve part release when it is applied to mold surfaces.
• The part can be vented along up to 30% of the parting line around the core pins and ejector pins, as well as using vents on the core pins and ejector pins. Vents usually measure between 0.0008 and 0.0012 inches. They are between 0.200 and 0.0400 inches width. Occasionally, the parting line can be sealed with an O-ring so that a vacuum can be pulled on the cavity to quickly evacuate the gas.
• As injection speeds increase, gates with larger walls help reduce material shear and wear, as well as delay freezing.
• Rockwell (Rc) hardnesses of 55 or more are usually found in gate inserts used to withstand high injection pressures.
• To reduce stress at the gate, aid filling, and reduce part damage when degating, use gate wells when gating directly onto a thin wall with a sprue, pinpoint, or hot-drop.
• Runner systems can be improved with hot manifolds, but they require at least 0.5-in.-diameter pipes. There should be no sharp corners or dead zones in the passages. Manifolds should be heated externally rather than internally. If valve gates are used, they must be built to handle high pressure and must be non-restrictive.
• Furthermore, thin-wall applications present a greater challenge to cooling of the cores and cavities. A couple of recommendations:
• It is generally a good idea to locate non-looping cooling lines directly in the mold core and cavity block to maintain as consistent a mold surface temperature as possible.

It is generally better to increase coolant flow through the tool instead of decreasing coolant temperature to maintain the desired steel temperature. As a rule of thumb, there shouldn’t be a temperature difference greater than 5° to 10° F between delivery coolant and return coolant.