Thinwall Molding Plastic

Molding parts with thinner wall sections has been prevalent in the packaging industry for many years, but the know how developed in packaging items is limited to the polyolefins and styrenics used in that product area.The benefits of thinwall molding are now sought in technical applications such as cellular phones and notebook computer housings.

When I was a worker at PT Nagai Plastic Indonesia, we produce printer part for PT Epson Indonesia. We are specialize in big part such as housing, coverprinter, paper support and stacker paper. Our facility has 80 Ton till 960 Ton injection machine and as a Maintenance Supervisor, I have to keep that facility work well everytime.
These parts has thinner wall section almost every section. Especially on the rib. Many rib for wall part support. Off course many problem appear. But many merit too. Lighter, smaller parts molded with less resin and fastercycle times are being produced which translate into lower unit costs for themolder. However, these products require higher viscosity engineering polymers such as PC, ABS, and PC/ABS blends, which raises new challenges. Part design,resins, processing, and molding equipment are all impactedand there is a need for boosting performance requirements to meet the high speeds and pressuresdemanded. New hot runner designs play a key role.
Thinwall molding can be defined as either wall sections of less than 1.5 mmor flow length over wall thickness ratios (L/T) of greater than 100. By eithermeasure, thinwall molding pushes conventional molding equipment to the limitby its effect on the flow area as shown in the following example.

Flow path for thinwall molding is 1/10th that of conventional molding

The nominal wall thickness of conventional vs. a thinnwall design are 3.0 mm and 0.75 mm, respectively.In both cases as the molten resin travels throughthe mold, a skin layer is formed approximately 0.25 mm thick immediately atthe mold wall. The resulting flow paths are cut to 2.5 mm for the conventionalmolding and 0.25 mm for thinwall. Reducing the thinwall flow area to just one-tenth that of conventional molding can push standard molding equipment beyond its limits.
The high injection speeds and pressures employed in thin wall molding to overcome this reduced flow path narrow the process window, but higher performance equipment widens the window sufficiantly to produce precision parts. In order to produce thinwall parts, an injection molding must be able to meet these three requirements :

• High injection speeds and pressures;
• Rugged large clamp/small injection unit combination;
• High performance controls and hydraulics.
  

Fill times of less than 0.5 s are typical in thinwall molding in order to fillthe mold before the skin layer builds up and blocks the flow channel.An injection unit equipped with an accumulator can achieve such rapid fill times. The accumulator allows for near instantaneous delivery of a largevolume of oil to the injection cylinder during fill. The accumulator shouldbe located as close as possible to the injection cylinder in order to minimize pressure drop and maximize responsiveness. This also reduces the amount of fittings and hoses which are a potential source of leakage.

Filled resins are used in some thinwall applications to impart necessary performance characteristics. Glass and graphite fillers of up to 20% by weight are used which results in excessive wear to the runner system, particularlyat the gate area where the resin experiences high shear rates and often makes a 90 deg. turn to enter the cavity. Standard beryllium copper tips should be replaced with more wear resistant materials that also provide high conductivity, easy access, and replaceability.Thinwall molding with engineering resins places new demands on injection molding equipment. High pressure, precision molding requires higher performance machines and hot runners that are specially suited to meet these rquirements.