6 steps mold qualification process

1. Viscosity Curve

Viscosity of the resin and a process with stable viscosity is one of the most critical parameter in part quality and the stability of the process. In injection molded parts the injection speed is the important variable that determines the viscosity of the resin. To confirm the optimum speeds to be used is one of the most important steps for a stable molding process.

Procedure to confirm the optimum injection speed

· Set the melt temperature at the mean value of the given range

· Set holding pressure and time to zero

· Set transfer point so that only 95% of the part is filled

· Start from 95% of the injection speed and record the fill time

· Reduce 10 percentage point and record the fill times as shown

· Keep reducing the injection speed until the injection stalls.

· Record the data and plot the graph as shown

2. Cavity Balance test

In multi cavity molds the filling pattern between each cavity can vary depending on the distance from the sprue, mold temperatures, dimensions of the runner, gate size and venting of the mold. To understand the differences between the cavities the weight will give a good indication of the balance between cavities. In case there is imbalance then the above-mentioned mold characteristics to be studied and corrective measures to be done before next trial.

Procedure to confirm the optimum Cavity Balance

• Set the melt temperature at the mean value of the given range
• Set holding pressure and time to zero
• Set the screw recovery delay time about the same as estimated hold time
• Set the injection speed as confirmed by viscosity curve study
• Only by adjusting the transfer position mold parts that are just short of a full part
• If there is cavity imbalance, then the biggest part should be just short of a full part
• Make three such shots and take average weight of each cavity and plot a graph as shown

3. Pressure drop test

The pressure drop test shows if the process pressure is capable of meeting the required target.
It gives a good map if there is any abnormal pressure drop that is restricting the process by the time resin fills the cavity completely. Consider the plastic flows through the following sections: The nozzle of the machine, the sprue, the primary runner, the secondary runner, the gate and the end of fill. 

The procedure to determine the pressure drop is as follows.

1. Set the machine to the maximum available pressure.

2. Build a shot on the molding machine and take an air shot. Note down the peak pressure required to do so.

3. Next start molding but mold only the sprue and note the peak pressure.

4. Mold the primary runner only and note the peak pressure.

5. Mold the secondary runner only and note the peak pressure.

6. Mold a shot such that it just enters the gate and note the peak pressure.

7. Mold a shot such that it just reaches the end of fill and note the peak pressure.

8. Generate a graph as shown and look for the sections that have a high pressure drop.

4. Gate seal test

The gate seal time shows at what time the gate freezes. Beyond which the holding pressure is ineffective.

Procedure to confirm gate seal time

· Set the hold time to 0 and mold 5- 8 shots to stabilize

· Then increase the hold time to 1 sec and weigh the part after  3 shots.

· Next continue increasing the hold time by I sec and weigh the part after 3 shots

· Continue increasing by 1 sec until no change in part weight occurs

· Stop the process and plot the chart

5. Process window study

The process window study generally gives a good idea of the process ,after all the before mentioned tests how big is our operating window

Procedure to confirm Process window study

· Set the Barrel temp on the lower range of the recommended temperature

· Set the injection speed as per viscosity curve

· Set all holding pressures and time to zero

· Start molding and make parts which is about 98 % full

· Set the hold time based on gate seal study

· Increase hold pressure in small increments by about 3 % points

· Note down this pressure as Low temp-Low pressure corner ( Point A )

· Increase the hold pressure until the part flashes out and note down as low temp –High pressure corner ( Point B )

· Repeat the same steps at higher range of the melt temperature and note down High temp-Low pressure and High temp-High Pressure corner ( Point C & D )

· The whole graph shows the process window how wide it is or shows the risk in processing window.

· Set the process to the center of this window

6. Dimensional/ cooling study

This helps determine the part accuracy at different temperatures and determine at what temperate the dimensions are going out of spec.

Procedure to confirm cooling study

· Mold three shots at various cooling times.

· Measure the critical dimensions.

· Plot a graph of dimension versus the cooling time.

· Analyze the data to see how the critical dimensions are influenced with the cooling time.

· Decide on the cooling time that best fits the data.

· Run 30 shots at this cooling time and perform a statistical analysis to determine the process capability at this cooling time