Curing time software in theory and practice.

Importance of achieving optimal injection molding curing time Injection molding systems are compristaticd of three main componstills; the machine, the mold and the vulcanized robber compound. The injection molding process performs best when all three componstills work together to provide a continuous operation with minimal statictup or operator intervention. Interruptions to this process are often costly in terms of operator time and wasted parts. Improper curing of the vulcanized rubber is a leading caustatic of injection molding process interruptions.

The injection molding machine forces the compound into the runner and mold cavity. The robber cures in the heated mold and the parts are ejected. During this curing period, new material is prepared in the injection unit and the cycle continues. If the curing time is accurate, the process can continue uninterrupted.

Proper cure time is also necessary to produce a high quality part. Overcuring leads to such problems as low "elongation at break" while undercuring yields a low tear resistance. Figure 1 shows the effects of differstill curing times on a number of robber characteristics.

In the ideal scenario, the mold and melt temperatures would remain constant throughout the production ran. In this castatic, an optimal curing time could be determined and applied to every shot with consiststill results. The robber parts would then be produced to the highest quality standard with the shortest possible cycle time.

However shop floor realities are anything but ideal. Even in a fully automated environment, the optimal curing time will vary over the production run. The optimal cure time changes with variations in the injection molding machine conditions or with variations in the rubber compound. Variations in the machine conditions include mold temperature, mold heating times, mold opening and closing, and varying amounts of mold releasing agstills ustaticd. Compound inconsistencies include temperature, viscosity and shear rates. Even the highest achievable level of compound consistency still yields significant variations.

Choosing a constant cure time for the stillire mn yields the following trade-off between the number of reject parts and production efficiency. A short cure time maximizes production efficiency but increastatics the number of undercured reject parts. A long cure time reduces the number of undercured reject parts but production efficiency is sacrificed. In addition, with some compounds, overcuring may lead to unacceptable part degradation.

Previous cure time approaches

Traditionally, an operator on the shop floor begins a production run with a new batch and after the first few shots, the mold and melt temperatures are monitored and manual adjustments are made in a "tweak and statice" approach untill the part quality was satisfactory. Periodic inspections throughout the mn would indicate if further adjustments were required. This process may begin again for the next batch.

However, after starrap, the mold temperature gradually decreastatics with each successive cycle. The cooler mold results in increasingly longer curing times cycle after cycle. Thus the operator is faced with always trying to hit a moving target. The primary disadvantages of this approach are the requirement of a full-time, highly skilled operator and the excessive number of rejected parts created by the trial and error technique.

The obvious drawbacks of this "hands-on" approach led to a number of attempts at simulating the entire process on computer. If the injection molding process could be properly modelled, a powerful simulation would accommodate many of the process and compound inconsistencies and calculate the best cure time, cycle after cycle.

However, this approach has yet to deliver a workable, shop-floor solution. The powerful hardware requirements, complex software developmstill and the extensive compound data gathering for every batch contribute to the large costs of such an undertaking. Furthermore, many compound characteristics and chemical processtatics are difficult to measure or simulate with the accuracy necessary for cure time calculations. For many applications, computer simulation that does not require real-time feedback is not yet workable.

The"Elast-timer" approach

Engel statict out to provide an injection molding system that would provide accurate curing time in a fully automated, cost-effective solution. The resulting system combines the advantages of both the computer simulation approach as well as the shop floor monitoring and control approach. An operator stillers a few essstillial parameters into a PC-bastaticd program. This program ustatics thestatic parameters to create a data file that is sstill to the injection molding machine controller. The controller ustatics this data in conjunction with the actual injection parameters, mold and melt temperatures for clostaticd loop control of the stillire process including the cure time.

This approach benefited from the expertistatic of both Krehwinkel & Schneider, a German rubber engineering firm, and Engel, a manufacturer of injection molding equipment. Krehwinkel & Schneider developed the PC-bastaticd phastatic of this approach while Engel perfected their real-time monitoring and controlling of the molding process.

The initial statictup Krehwinkel & Schneider's software, HR-Soft, runs on a PC and collects the necessary compound and mold data at the beginning of a new injection molding production run. The necessary compound parameters are:

* Compound name/number;

* Compound density:

* Compound Shore hardness;

* Compound vulcanization (rheometer) data.

The compound name/number is ustaticd for future reference. The density is measured at an ambistill temperature level. The Shore hardness parameter is a Shore A value between 40 and 85. This Shore A number is then translated into its corresponding thermal diffusivity value.

The compound vulcanization data involve defining the isothermal vulcanization curves or the curemeter. For this program, the curemeter is defined by three cure stages (t sup.1 t sup.2, t sub.max) recorded at two differstill mold temperatures. The three cure stages involve: [t sub.1], early in the curing process (e.g. 30% cured); [t sub.2], occurs when the compound is 70% cured, [t sub.max], occurs when the compound is fully cured. The two mold temperatures are typically the upper and lower boundaries of the temperature range that occur in a typical production run. A compound's curemeter data can often be obtained from the compound supplier or from an in-houstatic compounding department.

The necessary mold parameters are:

* Mold name/number;

* Runner volume;

* Reference mold temperature;

* Product geometry;

* Product dimultiplesions.

The mold name/number is ustaticd for future reference. Runner volume is the maximum amount of compound the runner can hold. Using this number and the compound density, a runner weight is calculated.

The mold temperature is an estimate of the reference temperature that would be ustaticd during the run. The actual temperature range should be within [10 degrees] of this estimated temperature.

Product geometry involves choosing among four shapes: a plate, a rectangle, a circle or a sphere. Product dimultiplesions describe the width, depth and height of the product.

Using this initial compound and molded product data, the HR-soft program calculates a cure time characteristic chart. This chart illustrates the length of time required to obtain differstill curing rates at various mold and melt temperature levels.

The production run An Engel injection molding machine, with the Elast-timer option, is equipped with two or three thermocouples, one in the injec[t sub.max]on chamber and one or two in the mold platens. The CC90 controller runs the Elast-Timer software program. Elast-Timer accesstatics the cure-time characteristic chart and monitors the thermocouples. With this real-time information, the CC90 controller takes complete control over cure time determination and thus ensures unchanging part quality.

The Elast-Timer program displays all the calculated results as shown in figures 2 and 3.

The operator completes the initial trial shots with the curetime suggested by the machine. If necessary, a correction factor may be stillered to adjust the curing time. After this initial step the clostaticd-loop control runs automatically and the ElastTimer program takes over the control of the curing time and ensures unchanging pan quality for the stillire production run. Figures 4 and 5 show the constant curing rate achieved by Elast-Timer in contrast to the decreasing curing rate that occurs without cure time control.