Combining Resin Conversion Technologies for Achieving Optimal Part Performance.
A Gas Assist Molding Case Study
Michael H Caropreso
The replacement of sheet metal with plastic has become somewhat of a common practice. Often an application that appears to be quite simple actually creates a bigger challenge to the plastic part designer than he or she anticipated.
Reasons for converting to plastic typically revolve around reduced metal assembly times, the need for tighter tolerance requirements, because of softer design possibilities or in this case significant shipping cost reductions.
Like all plastic designs, criteria pertinent to the part performance, appearance, environment and total cost will determine not only the resin to use but also the conversion technology used to manufacture the part.
This case study will review the Hewlett Packard “Rosebowl” 8U and 9U side panels. A project that replaced a large sheet metal door with smaller plastic modules. This paper describes the development of the process combination of gas assist molding with sequential injection molding.
The HP “Rosebowl” project replaces a large sheet metal side panel used to protect the electronic equipment mounted inside a peripheral computer rack system. The metal door. referred to as the 2 meter door, requires a service person to remove the entire panel for equipment servicing. The metal panel is heavy, hard to handle and vulnerable to bending damage.
The concept of redesigning the sheet metal door with smaller modular panels reduces overall cost significantly. The $40 metal door cost $150 to ship because of its size. Often, the door is delivered damaged. A smaller sheet metal door has nearly the manufacturing cost as the larger door. Plastic panels are less expensive, can be shipped individually for much less money and are far more durable. The 8U and 9U door panels allow field service person to access specific areas inside the rack. In addition, a snap fit design would make the removal and re-installation of the panel much faster and easier. In order to adequately cover and protect the internal components, the side panels must not have any gaps between each panel. When fully installed, each panel must snap into place and maintain a constant style line creating a more elegant and contemporary appearance.
Most large flat panels molded in plastic utilize either standard injection molding, structural foam, blow molding, RIM orvacuum forming. Most recently gas assist molding has entered this arena. Process selection required careful review of the features and benefits that each process offered.
The gas assist process was chosen as the manufacturing method for this high volume, highly aesthetic application. This process allows for this relatively large part to be molded on a reasonable clamp tonnage machine. More importantly, the gas process will greatly reduce the stress in the part which in turn reduces the tendency for warpage. Also the gas channels can increase part stiffness and eliminate sink marks from heavy wall features.
One item that truly affects both process and design is the wall thickness. Obviously, the thinner the wall the lighter the part and the faster the cycle. The panels are approximately 35 inches long by 16 inches wide. Nominal wall thickness is .140”. Injection molded, the projected area of the part dictates a molding machine in the 1200 to 1500 ton range. Also, thin walls become more dependent on resins with long flow length capabilities.
The intent is to mold a part with texture and then to evaluate the need for paint in Order to meet the appearance requirements.
Gas Assist Design Criteria
Gas assist molding is often able to eliminate the need for hot manifold systems. A direct sprue gate is used to feed flow runners is commonly seen as a method to produce large parts. One of the draw backs to this approach is the creation of what is known as “read through” . This change in gloss over the gas channels is often undesirable and may result in the need for a mist coat of paint. The gloss change is more noticeable with darker colors.
Even though the 8U and 9U panels would be molded using a light beige resin, it was decided that this was a chance that we did not want to take. Instead, a two drop hot manifold system with hydraulic valve gates was chosen as the primary resin gating method.
The use of valve gates offers this application unique advantages. First two gate locations will significantly reduce the clamp tonnage requirements if sequentially operated. Opening one gate at a time decreases the overall projected filling area during resin injection. Another advantage is the control of the resin flow fronts. This feature makes it possible to move or eliminate weld lines. The technology combination of sequential injection molding with gas assist molding offered HP the opportunity to achieve the desired part performance the panels demanded.
The part design included gas channels that were placed on the edge of each long side of the panel providing stiffness and creating a channel for straightening purposes. The channels were intentionally ended at the far corners of the panel. They would be lengthened after the first shots were made in order to fine tune the packing of some of the molded features.
Prior to building prototype molds, a mold filling analysis was performed. The analysis provided valuable information needed to properly locate the valve gates, determine gas channel sizes and the gas pin locations. It will also verify the required clamp tonnage. Once complete, the analysis was interpreted. Several iterations involving gate location and channel size eventually produced a design that every felt confident with. A key factor to the successful results was the incorporation of sequential injection. The part was filled by opening one valve gate long enough to allow the flow front to go beyond the second gate. The second gate was then opened to complete the filling. Gate placement was slightly off center to ensure the panel filled from one end to the other. Predicted clamp tonnage required was only 650 tons.
The concept was to allow the part to fill from the bottom of the part to the top. In other words, the short or unfilled area of the mold would be at the end of the gas channels. This desired filling technique ensures optimal gas channel coring. Because of the valve gate location, the nominal wall was filled during resin injection. When the resin reached the gas channels along the side, the thicker cross section Allowed the plastic to flow faster or “race track”. Before this created a flow problem, the first gate was shut off and the second gate opened. A new flow front was created into the nominal wall. Eventually this new front reached the gas channels resulting in a desirable filling pattern.
In order to prove out the design, it was determined that two prototype mold be built, one as a single cavity mold for the 8U panel and one single cavity for the 9U. Due to the possibility that these molds may be pressed into service for initial pre-production parts, they were constructed with steel mold bases. Only the core side inserts were made of aluminum. The tools included the hot manifold and valve gates with collapsible core inserts for the snap fit features.
Initial Tool Trials
The initial molding trials proved to be very successful. The parts filled almost exactly as predicted from the analysis. The gate sequencing provided not only the reduced clamping pressure but produced parts that showed no visible knit or unfilled area of the mold would be at the end of the lines. Gate location would be further optimized in the production molds.
Some of the features on the back side of the panel did exhibit slight sink marks. As you recall, this was expected and would require the extension of the gas channel to these areas for better packing. The gas channels did prove to be the correct size and the gas pins were in the correct location.
The panels were straight and rigid. Most importantly, they snapped onto the rack perfectly with no space between panels. Other than some minor engineering changes, the prototype parts were everything that was expected.
Getting Ready For Production
Moving from the prototype phase to the production phase involved several key factors. First of all, this was the molders first gas assist application. With a program of this magnitude, everything needed to be in place and working in order to meet production schedules.
For the molder this was a monumental task. The primary production site was in Ireland. Plastic molding would be a new addition to the current sheet metal fabrication plant. Literally everything needed for the manufacturing of the doors would be new. Molding machines, auxiliary equipment such as dryers and mold heaters, robotics, nitrogen generation, gas assist equipment and most important, trained molding technicians needed to be ready and waiting for the production tooling. Project management and good communications were critical. Using the data and experience from the prototype molding trials, the new “Rosebowl” molding plant was born.
A second critical milestone was to finalize the production tool design. The manifold was shifted to one side approximately 2 inches to better optimize the resin filling pattern. The gas channels were extended around the far corners of the door to permit gas pressure to pack out the snap fit details. The base of the stand off ribs were reduced to 60% of the adjacent wall to eliminate sink marks. These ribs were over designed and did not require the thicker walls.
The last detail was to establish a robust molding process and a troubleshooting technique for the new molder in Ireland.
Establishing The Molding Process
The 8U and 9U panels require a surface finish that has no visible sink marks, invisible weld lines and required no painting. They also need to be straight and have snap fits with no gas penetration. The process must be repeatable, produce consistent parts and have a reasonable processing window.
In order to achieve these goals, the gas assist process needs to be fully understood. The three most important phases of gas assist molding is resin filling, gas filling and gas packing. Using these phases as a guideline, the process and troubleshooting information was established
- Resin filling information starts with the resin manufacturers recommended temperature and pressure settings. Once optimized the mold was filled using only one valve gate. A series of short shots are made until the flow front just reached the second valve gate. Noting the ram position for the resin to reach this point, valve gate number two is set to open just beyond that position. The final shot size is determined by the amount of unfilled mold cavity.
- Gas filling begins after the valve gates are both closed. A delay time is often required to allow the molten resin to set up slightly. This is resin and application dependent. Gas is introduced via two gas pins, one in each gas channel. Each channel is adjusted individually for gas pressure and timing to obtain optimum coring. A gas bubble that extends to the end of the gas channel insures the shortest overall cycle time. A bubble that extends out of the gas channel and into the nominal can create weak areas at the base of the snap fits.
- Gas packing starts as soon as the initial bubble is formed. The gas pressure continues to pack the part as it cools, The bubble will continue to elongate slightly during this time. It is important that the gas pressure remains on until the part is completely cured.
The combination of conversion technologies is often overlooked when considering manufacturing options. Taking advantage of the features and benefits of sequential injection molding with gas assist molding has created a viable system for producing large plastic applications.
The reduction in clamp tonnage provides a significant cost advantage. Reducing the resin injection projected area by opening one valve gate at a time combined with the traditional gas assist short shot process reduces clamping forces to one ton per square inch or less. In most cases, press sizing is determined by shot capacity and tie bar spacing.
Aesthetics are enhanced by the ability to move or eliminate weld lines. Valve gate control allows for the manipulation of the flow fronts to achieve a desired surface finish. Gas assist molding ensures that textured surfaces as well as internal features are true and crisp because of the more uniform packing pressures the gas supplies.
The ability to design parts with gas channels that enhance stiffness and strength while providing a path for packing selected external features, increases the part performance without sacrificing aesthetics.
The HP 8U and 9U door panels are excellent examples of how process technology combination can be utilized to achieve optimal part performance.
The author would like to acknowledge the HP Roseville CA Industrial Design Team of Brian Tsuyuki, Eric Jensen, Horst Zittlau and Brad Mousa, Max Limon of LB Molds, GE Plastics and the APW enclosure team from Ervine and Dublin, Ireland.