Injection Molding Basic & Systematic Mold Proving ③

4. Systematic Mold Test

1) Information

The relevant information and testing purposes for molds, plastic materials, and molding products should be available in the early stage of all mold testing processes.

2) Procedure

This mold test should be performed in accordance with the flow chart as shown in the previous page.

3) Partial Filling

It is very important to proceed with partial filling for each test and analysis of the defect causes. In this case, it is also essential that the molten resin doesn’t flow into the mold cavity during the supplementary pressure stage. The supplementary pressure switchover point predefined should be visually inspected and selecting a very small value of supplementary pressure volume is important.

It is necessary to secure the sufficiently long delay time in order to prevent the molten resin from entering the mold cavity during measurement. 

The partial filling stage should be performed in a step-wise manner at the places having diameter change or surface flaws for clear shaping of a flow process. It also should record the corresponding switchover point or the measurement volume so that the flow shear can be traced. Sufficient prefilling is required in the production with a partial filling in order to secure the corresponding ejector positions.

4) Injection Section

The initial velocity of injection, which is referred to as the distance covered during the beginning of injection in the injection section, must be relatively constant to achieve consistent surface quality.

At the injection rate constantly predefined, the molten resin is injected at a constant amount. Figure 2.1 illustrates the diameter of a small resin shear that is close or far from the gate and the resin shear rate is higher in these areas than in the middle.

Partial filling of the parts on flat surface

In order to obtain a more constant resin shear rate, the injection profile having a low-high-low speed must be set. After setting the size and position of the substantial flow shear diameter of the molding product, the screw forward speed should be adopted to obtain an approximately equal injection shear rate.

5) Supplementary Pressure Switchover Point

The supplementary pressure switchover point determines the transition from injection to the supplementary pressure stage. To prevent the overfilling of molding products and the possible mold damage, the supplementary pressure switchover should be conducted when the process reaches approximately 98% of the total capacity of molding products.

The supplementary pressure switchover point should be checked whenever the amount of melting being measured is affected by the variable changes.

• Pressure: Compression of Melting Resin

• Suck Back: Changes of resin amount in the screw shear area

• Initial Injection Stage: Closing Behavior of Screw Check Ring

6) Effective Supplementary Pressure Time

Effective supplementary pressure time can be determined by measuring the weight of the molding product according to the increasing supplementary pressure time.

Determination of effective supplementary time

The amount of supplementary pressure must be maintained constant. As soon as the considerable weight increase stops, the effective supplementary pressure time has been reached. It is to ensure that the molten resin does not leak backwards from the molding products to the gate system or into the space of the screw shear by an unclear method. This may be the result of the weight or dimensions change in a molding product.

7) Cooling Time

Cooling time should be as short as possible for economic reasons. The values by formula or experience can be used as a fixed value for the corresponding resin. The deterministic factors are product thickness, mold temperature and dimorphic temperature.

With regard to the effect of mold temperature on the cooling time, it should be kept in mind that the mold temperature is presumably very high in production processes. A change in temperature of 10℃ causes a cooling time change by approximately 20%. Furthermore, the dimorphic or mold temperature also affects shrinkage. Consequently, a modification in mold dimension should be done with the molding products produced at regular cycles.

8) Process Area

The quality of injection molding products can only be guaranteed within a certain range of quality-related setting values. This range is sometimes referred to as the process area. In the middle of the process area, only the machine setting value can secure a low defect rate caused by errors in materials, machines, and peripherals, etc. It is possible to define the allowable error value of the change in machine settings.

If any quality issue occurs during production, it is mandatory to inspect all composition items in advance for perfect functioning. It is necessary, in general, to measure the process temperature, to inspect the resin drying, and to compare both the configured value and actual value of the variables.

The root cause can be identified only when the configured value of the machine is systematically optimized.

9) Variable Change Procedure

If the optimization is proceeded with the following measures, only one variable should be changed and record at a time. In particular, if there are changes in melting temperature and mold temperature, the molding products should be evaluated when the thermal balance is reached. 

10) Correlation between Variables

If the correlation among several variables and their influences on the quality of the molding products cannot be evaluated, a systematic mold test should be performed. To this end, each quality-related variable such as injection ratio and supplementation pressure should be included in the test plan so that they can be systematically changed. The number of mold tests is 2n.

The test should be conducted according to the change in the configured temperature because the time required for the test should be spent as short as possible.

In case of the table above, the tests shall be conducted in the order of 2, 3, 4, 7, 1, 5, 6, and 8.

* Source: www.lsinjection.com