Achieving Weight Reduction and Cycle Time Improvements
Sacrificing Surface Appearance
Michael H. Caropreso
Principal, Caropreso Associates
Methods to reduce part weight and improve cycle times continue to lure molders and OEMs to investigate these cost cutting technologies. Many of the processes however produce parts with splay or silver streaks on the surface, often making them unacceptable. The incorporation of a low cost process modification will significantly reduce and often eliminate the splayed surface. Counterpressure molding is proving to be very effective in applications with a wall thickness of 3mm and below. This paper will discuss counterpressure tool modifications necessary for achieving double-digit cycle reductions and moderate weight savings without sacrificing the surface appearance.
WHAT IS COUNTERPRESSURE?
Counterpressure is best described as an extension or modification of the standard injection molding process. The difference is how the melt front of the incoming resin is controlled during resin injection. Molten plastic injected into a typical mold flows in the path of least resistance. In other words, nominal walls fill first and then thinner walls or features are filled and packed nearer the end of the injection sequence. Often the top of a thin rib or boss is the last to fill and is evident in a short shot.
Counterpressure provides a controlled resistance to the flow front, forcing some of the melt to enter the thinner or harder to fill areas during the initial injection. When used with foaming agents or wet non-hygroscopic resins, splay is eliminated.
HOW DOES IT WORK?
The counterpressure process works like this: molten resin is injected into a pressurized mold. In order to maintain the proper pressure, the mold must be sealed. Sealing techniques are similar to the modifications necessary for vacuum molding. The modified mold requires a parting line seal. This seal is normally an O ring that surrounds the mold cavity. Depending on the tool complexity, slides, moving cores, lifters, inserts, ejector pins and any other areas of the mold that the counterpressure might leak, will require sealing. The gas pressure is dependent on resin type, mold design, blowing agent gas yield and sealing technique. The gas used is typically compressed air or nitrogen gas. Gas enters and exits the mold via strategically placed inlet and outlet locations.
The process sequence is as follows:
· The mold closes and clamps, compressing the parting line seal. Compressed air or nitrogen gas is introduced into the sealed mold. The injection of resin is delayed until the mold reaches a pressure of between 60 and 200 psi.
· Resin is injected into the pressurized mold.
· Compressed gas pressure created from the incoming resin is vented to atmosphere.
· When the resin injection is completed, all of the counterpressure is vented to atmosphere.
· After the resin cools and cures, the part is ejected.
Injecting against a known pressure and maintaining that pressure during injection, controls the resin flow. A typical scenario might be a counterpressure setting of 100 psi. During injection, the CP pushes against the incoming resin forcing it into all areas of the mold along its flow path. If a foaming process is used, the foaming gas is kept in solution not allowing gas bubbles to come to the flow front surface. The counterpressure gas in the mold compressed during resin injection is vented to atmosphere as the mold is being filled. At the end of injection, all of the pressure is vented. The skin has already been established so all foaming is done between the skins. If the pressure in the mold was high enough to stop the foaming gas from coming to the flow front, the part will have no visible splay.
Part appearance and or mold replication determines the optimum pressure and gas timing sequence. Splay will indicate too low of a pressure with parts using a foaming agent or molded with undried non-hygroscopic resin. In some cases, a sequencing problem may be the cause of the splay. With injection molding, unfilled features may be the indicator. Like all injection molding processes, each application will require its own set of optimal process parameters.
The gas used to pressurize the mold is introduced to the mold cavity via inlet and outlet ports. The size and location of the ports are critical to managing gas flow.
When a molder is considering the gas counterpressure process, the ability to properly seal the mold is often the determining factor in whether to move forward. The more complex the tool is the more difficult it will be to properly seal the mold. Stepped parting lines and complex moving cores are potential showstoppers.
Many techniques have been used to seal a mold. The use of sealed molds in negative pressure or vacuum molding is very common. The simplest mold to seal is one with a flat parting line, a single cavity, and a direct sprue gated. Sealing a mold becomes more challenging when stepped parting lines, moving cores, cavity inserts and numerous ejector pins are required.
The parting line seal is typically made from O ring gasket material, Neoprene being the most common substrate. The groove size is determined by the diameter of the O ring (Figures 1&2). A common practice is to make the groove slightly narrow and shallow. This technique requires the O ring to be stretched before installation. Stretching reduces the diameter of the gasket. The pressure produced by the gaskets memory to retain its original dimensions creates a friction fit.
Figure 2 illustrates the suggested groove dimensions for a .210 diameter gasket. The location of the O ring should be approximately 1.5 from the cavity.
Modifying an existing tool that has a stepped parting line becomes challenging. O ring material does not like to bend more than 30 degrees. Anything further can result in a tear and eventually a leak. New molds should consider incorporating a flat parting line around the perimeter of the complex shaped part.
INLET & OUTLET LOCATIONS
In order to manage the gas flow into and out of the mold, strategically placed inlet and outlet ports need to be incorporated into the tool. Inlet locations should be placed somewhere near the resin gate. The size and number of inlets will determine how long it will take to pressurize the cavity. Outlets are usually located near the end of fill and provide the required venting of the mold.
Most inlets and outlets are produced by drilling a hole in the side of the mold to a depth that just goes beyond the O ring groove (Figure 3). A second hole is drilled in the face of the mold to meet the hole drilled from the side. The hole drilled from the face should be located in the relief area of a vent.
The inlet locations will allow the gas to flow through the drilled holes, into the relief area and over the vent to pressurize the cavity. The outlets are produced the same way. Gas exiting the mold will pass over a vent, into the relief area and through the drilled holes.
To ensure good gas flow to and from the mold, .250 diameter drilled holes are commonly used. Often, more than one inlet or outlet is required. In this event, create manifolds by connecting all of the inlets together and all of the outlets together using short pieces of tubing or hose. In some cases, parting line inlets or outlets may not be sufficient for proper gas management. Ejector pins can be used for either inlets or outlets if modified with a flat connected to a groove. The bottom of the pin must be sealed to ensure the gas flows into or out of the cavity and not back to the ejector plate (Figure 4).
Ejector pins typically have a .001 clearance around the diameter. This clearance is required for proper pin movement. Although O rings have been used as a sealing method, cup seals or lip seals are more often utilized. These types of seals have a rubber like material in a metal U shaped holder. They are commonly used with hydraulic cylinders. A counter bore is required to insert the seal either in the cavity or in the pin plate. The seal must be held in place by some means to avoid the seal from common dislodged.
With very small parts, the ejector pins may be too small or too close together to seal individually. This condition may require sealing the entire ejector system. Depending on the complexity of this type of sealing method, the molder may choose to disregard sealing the ejectors and compensate for minor leakage by supplying more gas pressure to the mold.
CAVITY & CORE BLOCKS
Cavity and core blocks are common in multi-cavity tooling. These blocks contain the ejectors as well as the cooling lines. Counterpressure can find its way between the block and the mold frame. Counterboreing the ejector pin clearance hole in the mold frame will seal the ejector pins. The seals will be held in place when the cavity block it bolted in. An additional O ring around the perimeter of the cavity and core insert is required to stop pressure from finding its way to the hold down bolts.
Most cavity/core inserts have cooling lines. These lines are built with o ring seals to eliminate water leakage. These seals should be designed so that the counterpressure cannot push the O ring into the water line. This is best done by fitting the O ring in a groove instead of a counter bore. (Figure 5)
SLIDES & MOVING CORES
Sealing moving mold components is the biggest challenge in designing a tool for the counterpressure process. As mentioned before, the complexity of the mold is often the determining factor in whether or not counterpressure is a real option for a particular application.
Each application will be unique when designing a seal. If possible, the moving steel should reach a positive stop that can be modified to include a flexible seal. The pressure in the cavity will want to leak past and space created when the slide is in the forward position. When ever possible, a flat parting line should be designed to encapsulate the motion portions of the tool.
Current economic conditions warrant faster cycle times and reduced resin usage. The counterpressure process is a proven method for achieving both.
The use of endothermic foaming agents enhances resin flow while reducing required melt temperatures. The foaming provides both weight reduction and cycle reduction. The splayed surface finish that results from the standard foam process is an unavoidable by product. Paying attention to the details of proper mold sealing will enhance the overall surface finish of the molded part as well as ensure the maximum cycle time reduction.
Michael H. Caropreso is founder of
Caropreso Associates, a
Before starting his consulting business, he spent twenty-eight years with GE where his primary responsibility was the development of low-pressure injection molding processes.
He is a active member of the SPE and serves on the conference committee for the Structural Plastics Division of the SPI.