Cavity Pressure Transducers Enhance Mold's Process Development

Mold cavity pressure transducers are frequently used at Tech Group. Such usage provides increased data about what takes place inside the mold's cavity. The benefit is not limited to what happens in the cavity, but extends to monitoring injection pressure and melt viscosity. For example, if melt viscosity is decreased, the resulting cavity pressure is increased. This is caused by a lesser Delta P -- pressure drop -- in the mold.

In the example above, a new mold qualification was in progress. The dotted line is a template or master trace documenting what the process should look like if the process is normal. The solid line indicates the current cycle is different. The difference is higher EOF (end of fill) cavity pressure by 249.9 psi. The post gate cavity pressure is also greater by 194.9 psi and the melt viscosity is reduced - value not listed in this screen, but other screens tell the engineer that the viscosity has dropped by 13 percent. This alone sets off alarms as does the cavity pressure cycle integrals and peak values. Such alarms result in product that is automatically rejected by the robot. This immediately receives attention to perform root cause analysis to see what has caused the change.

A quick check indicates the resin lot is the same, the melt temperature is correct, the molded parts look good, but something has changed. Further examination yields that the resin is not fully dried. After additional drying, the process is restarted with process conditions in proper conformance to the template curve.

This type data collection is done via RJG's ePak system and Insight software. This technology is used for most new mold qualifications at Tech Group.

Benefits include the following:

	Verify minimum cavity pressure at EOF is at least 3000 psi
	Establish curves for future reference
	View cavity pressure decay
	View gate seal performance
	ID resin effects on peak pressure and pressure decay - cooling rate

Once in production, these cycle integrals are more comprehensive than just looking at peak pressures. The cycle integral looks at area under the pressure curves (cycle integral reflects indicated pressure for full duration of the molding cycle). This data is more comprehensive because the EOF cavity pressure area can change based on many sources of variation: melt viscosity changes, cooling rate changes, peak pressure differences, resin lot differences, melt temperature changes. A water flow blockage or reduction can cause enough of a cooling rate change to alter the area under curve or the slope of pressure decay creating an alarm. Such alarms indicate the process has changed which might adversely affect the molded product. Root cause analysis is then performed to correct the problem.

Tech Group molds many close tolerance parts for medical and electronic applications which require this level of control.

The red cross hatched area is the injection fill viscosity - area under filling phase of injection pressure curve. The dotted line is a master trace or template. The red cross hatched area indicates resin viscosity is reduced. Injection fill viscosity has gone down because the resin is not fully dried. This type process variation can be detected with good process monitoring equipment and knowledge based molding.

The area under blue and green curves are also monitored as well (blue equals EOF cycle integral and green equals post gate cycle integral).

In this case the packing pressure was maintained, but the reduced viscosity has resulted in higher cavity pressures which may cause defects ... the root cause is identified, corrected and quality maintained. Process monitoring equipment used: RJG instrumentation: ePak, Insight software, Lynx style cavity pressure transducers ... molding press and mold at Tech Group.

written by:
Jay W. Carender
Principal Engineer
Tech Group Scottsdale

In this display of injection pressure and cavity pressure consistency, the curves are overlaid for two hours run time. This equals approximately 280 cycles or shots molded. As can be seen the data indicates excellent cavity pressure repeatability. This press happens to be an all electric press that yields reduced energy consumption and process consistency that is seldom realized. Such control requires mold temperature control units (MTCUs) with increased pump capacity for high water flow rates, excellent drying of the resin, well tuned barrel temperature controllers, molds that are well built with proper balance and venting characteristics and a machine that is repeatable.

In short, all high quality components to yield a molding system that can perform. Excellent equipment and mold PMs are also needed to keep such a system running at optimum levels.

Even when a system is created that performs at this level, process monitoring equipment is needed to catch inevitable hiccups to injection molding processes. Such hiccups (process variation) can result from the following:

	Equipment failures or malfunctions
	Utility supply interruptions or changes (pneumatic or electric)
	Dryers which might run low corrupting residence time equilibrium or other resin handling issues
	Ambient condition changes
	Mold vents becoming clogged affecting vent performance consistency
	Screw, barrel or check ring wear
	Electrical spikes or noise affecting equipment controls
	Heater band failure
	MTCU dump valve failures or coolant flow blockages including water supply turned off
	Random gate blockages from dirt debris or metal

There are process techniques which are used to help minimize the affects of some of these sources of variation, but in the end an effective process monitoring system is useful to reject discrepant product at the press immediately after molding.