Nowadays there are mechanical engineering softwares that help the mold makers and molding professionals to understand to evaluate the part design for plastic injection molding and also check the tool design before cutting the steel. Although the simulation software’s results are not 100% reliable, it saves time and cost by predicting the week area of the design.
Although the results are not reflecting 100% accurate results, it gives an idea to the designers to observe the tool functionality and evaluate their design before cutting the tool. Also, it will reduce development time and save money for companies. Moldflow and Moldex are two examples of software that are very useful to simulate injection molding of plastic.
Below you can see two graphs produced by Mold Flow analysis.
There are over 200 different parameters that must be established and controlled to achieve proper action molding of a plastic part. These parameters fall within four major areas :
4 parameter areas [1] Reference : A guide for injection molders , by D.M. Bryce
To the troubleshooter, all the 4 areas are important, but the pressure and temperature areas are the ones most commonly considered during the troubleshooting process. Based on the requirements of any particular plastic material, the must be sufficient to inject the plastic material and to hold the mold closed. In addition, the temperature of the injected plastic and mold must be correctly maintained.
1 . PRESSURE
Pressure is found primarily in the injection area, but there is also pressure found in the clamp unit of the molding machine.
1.1 Back Pressure
The first pressure to consider is back pressure. This is pressure that is created during t he return action of the screw after injecting material. The screw turns (augers) to bring fresh material into the heating cylinder. This material is placed in front of the screw and nudges the screw backwards. A buildup of pressure is created at the front end of screw. This pressure is used for better mixing of the plastic (especially if colors are added to the press), removing small amounts of trapped air, and controlling the weight of the shot by maintaining an accurate density of a given volume of melt. The back pressure setting should start at 50 psi and be increased in 10 psi increments as needed, with a maximum setting of 300 psi. The maximum setting is needed because anything over that will cause too much shearing of tthe plastic and result in thermally degraded plastic.
The first pressure to consider is back pressure. This is pressure that is created during the returns action of the screw after injecting material. The screw turns (augers) to bring fresh material into the heating cylinder. This material is placed in front of the screw and nudges the screw backwards. A buildup of pressure is created at the front end of screw. This pressure is used for better mixing of the plastic (especially if colors are added to the press), removing small amounts of trapped air, and controlling the weight of the shot by maintaining an accurate density of a given volume of melt. The back pressure setting should start at 50 psi and be increased in 10 psi increments as needed, with a maximum setting of 300 psi. The maximum setting is needed because anything over that will cause too much shearing of the plastic and result in thermally degraded plastic.
1.2 Injection Pressure
The next type of pressure to consider is injection pressure. This is the primary pressure for injecting 95% of the molten plastic into the closed mold. Normally, the highest pressure and the fastest fill rate are the best conditions. However, high pressure will increase molded-in stress. And the stress will be released at some time. There is no question as to its being released, only as to when it will be released. And remember, the hotter the plastic, the more fluid it becomes and the lower the pressure can be to fill the mold.
The next type of pressure to consider is injection pressure. This is the primary pressure for injecting 95% of the molten plastic into the closed mold. Normally, the highest pressure and the fastest fill rate are the best conditions. However, high pressure will increase molded-in stress. And the stress will be released at some time. There is no question as to its being released, only as to when it will be released. And remember, the hotter the plastic, the more fluid it becomes and the lower the pressure can be to fill the mold.
1- Setup: This includes manual operations. In this mode, every press action requires manual pushing of an appropriate button to bring about the desired action.
2- Semi-Auto or Full-Auto: In automatic operation timers, relays, and limit switches are electrically coordinated to produce the proper sequence of operations, so that each cycle is repeated from shot to shot, and the end result is a finished product with consistent characteristics.
No buttons should be actuated if the machine is in a shut-down condition. In that case, the first move should be to open the water lines to all connections, such as the heat exchanger, the hopper throat, and any other component requiring water coolant. Next one should make sure that all pump suction valves are open.
One shoud check the setup record, which indicated not only the set of parameters for the job but also the accessoroes needed.
Having assembled all the items needed for mold mounting and setup, one can proceed to manipulate the machine.
Before any model is selected, power has to be available in the control
Yellowness Index: A measure of the color on the yellow scale.
Yield Point: In tensile testing, the yield point is the first point on the stress-strain curve at which an increase in strain occurs without an increase in stress.
Yield Strength: The stress at which a material exhibits a specified limiting deviation from the proportionality of stress to strain.
Young’s Modulus: The ratio of tensile stress to tensile strain below the proportional limit
Warpage: Distortion caused by nonuniform internal stresses.
Water Absorption: The amount of water absorbed by a plastic article when immersed in water for a stipulated period of time. All plastics will absorb moisture to some extent.
Weld Line: Where melted material flows together during molding to form a visible line or lines on a finished part that may cause weakening or breaking of the component.
Wisps: Similar to stringing but smaller in size. These also may occur as slight flashing when the mold is over packed or forced open slightly. Mold-parting-line wear or misalignment can also cause wisps.
