Manufacturing Processes II Injection Mold Lab

Size: px

Start display at page:

Download "Manufacturing Processes II Injection Mold Lab"

Jerome Cross
7 years ago
Views:

1 Manufacturing Processes II Injection Mold Lab Prepared By Robert Dembzynski Course Instructor Professor Jack Zhou Mechanical Engineering and Mechanics Department Drexel University Spring 2002

2 Fundamentals of Injection Molding The Machine The principal components of an injection mold machine are the hopper, barrel, plasticating screw, heater bands, nozzle, coolant system, static and moving platens, ejection actuator, hydraulic system and control unit (see fig 1). The hopper is a container where the raw plastic material is stored prior to injecting. Other devices which perform pre-melt operations on the raw material may be built into the hopper or be a separate component. An example of such a device is called a dryer which simply removes moisture from the raw material. The barrel houses the plasticizing screw and contains the material as the screw melts the material and moves it toward the nozzle. The heater bands surround the outer surface of the barrel and mainly serve to limit heat loss through the barrel. It does help to melt the plastic but the majority of the melting is done through frictional heating generated by the plasticizing screw. The nozzle controls the flow of the polymer melt into the injection mold. The movable platen opens and closes the mold. Screw and Barrel Safety Gate Mold Material Hopper Hydraulic System Clamp Limit Switches Barrel &Nozzle Temp Control Injection Limit Switches Power Switch Control Panel

3 The Mold BackingPlate/ Pillar Coolant Line Plate A X Ejector plate Plate B Moving Platen Stationary Platen Coolant System Coolant Pump

4 The Screw There are various shapes and sizes of plasticizing screws. Normally there are three stages to a screw which are shown below. Selection of the size and type of screw is very important to successful molding.

5 The injection molding process The injection molding process can be divided into four main steps: 1) preparation of the material for injection: thermoplastic material will be melted and thermoset material mixed; 2) Filling: the molten polymer is injected inside the mold cavity; 3) holding: the polymer is compacted inside the mold and maintained under pressure; 3) cooling and ejection: the polymer is kept inside the mold (which can have an independent cooling system) for a time long enough to solidify it. During the cooling and ejection part of the process, the barrel is filled again with polymer by retraction of the screw. Filling and holding are two important steps of the process; they influence the final quality of the part. Moreover, the filling determines the orientation of the polymer in the cavity. As the filling depends on the part and on the mold designs, particular care should be taken during the part and mold conceptions.

6 Polymer Melt Rheology Rheology is the study of liquid flow and deformation properties in terms of stress, strain, and time. What does this mean relative to injection molding? The key to successful injection molding is to set and control the viscosity of the polymer melt. This is done by controlling flow the shear (deformation) rate and or the temperature. Shear rate is a function of pressure so really you are controlling the pressure. If you do not have the appropriate viscosity you will end up with defects in the molded part. The following are illustrations of typical defects. Sinks: Sink Flash: Flash

7 Jetting: Jetting Ejector Pin Marks: Ejector Pin Marks Important Notes: A. The pressure, temperature, and cycle time settings actually affect the mechanical properties of the final part. I.E. you can control impact resistance, tensile strength, etc. So if final product has to have a certain tensile strength keep your settings in mind. (Properties a function of Crystallinity) B. The raw material you get varies even from same manufacturer. So with a new shipment might have to do some trail runs (Molecular Weight Distribution)

8 Safety: Emergency Stop Button 1. Emergency Stop Button. Will shut down machine regardless of step in cycle. 2. Machine will not operate with Safety Gate in the open position as shown above. The gate must be pulled tight against stops. 3. Be careful handling parts immediately after ejection as the parts may still be very hot. 4. Be aware of your surroundings. Some of the surfaces on the machine are extremely hot and there are obviously moving components.

9 Operating Procedure: Step1: Check hopper to make sure there is a sufficient amount of the correct material is present. If not- Pour the plastic granule to the hoper (make sure there is no foreign material, such as metal, in it) and open the Gate at the bottom of the hopper Step 2: Turn on the power and supply cooling water to the injection machine and coolant pump. 1. Turn on the power to the injection Molding Machine by pushing up the Power Switch on the wall. 2. Turn on the Water Supply Main Valve. 3. Turn on the valves connected to the Mold Cooling Pipes, to the Barrel Entrance, to the Oil Pump. Injection Mold Power Switch Coolant Pump Power Switch Water Supply Main Valve Mold, Barrel, Oil Pump Coolant Supplies Step 3: Set Barrel Temperature and Coolant Temperature 1. Hit the start button on the coolant pump and set to desired temp 2. Set the two Barrel Temperature Controllers to the desired temperature for the material according to the material processing data sheet. 3. Set the Nozzle Tip Heating Power Controller to about 40 %.

10 On Switch Step 4: Turn Injection Mold Machine On (See fig above and to right) Step5: Set or Check other processing parameters Set or check other parameters such as Injection Speed, Injection Pressure, Holding Pressure, Injection Volume, and so on.

11 Process On Program Select Switch Manual Control Buttons Step 6. Make your first part 1. With the Program Select Switch in STOP mode, close the Mold Safety Gate. 2. Turn the Program Select Switch to SEMI-AUTOMATIC mode. 3. Press the Mold Close button. Step 7. The second part, and so on. 1. Check the injected part for burns, discolors, sinks, voids, warpage and other dimensional distortion. 2. Make necessary adjustments on barrel temperature, nozzle heating power, injection speed, injection pressure, holding pressure, injection volume, and so on. 3. Make another injection by pressing the Mold Close button or by closing the Mold Safety Hood. 4. Repeat step1 to step3, until you get satisfying part. Be Careful: In the SEMI-Automatic mode, closing Mold Safety Hood will execute an injection command. If you open the Mold Safety Hood and want to close it without injection, the Program Select Switch has to be in the MANUAL or STOP Mode.

12 Step 8. Make the parts automatically Put the program Select Switch to AUTOMATIC mode, and press the Mold close button, the machine will work automatically. Keep watching, problems happen sometime, you need to fix the problem and make the machine to continue. Step 9. Finishing the Job 1. Empty the barrel by injecting with Hopper Gate closed. 2. Turn the Program Select Switch to STOP position. 3. Turn off the oil pump by pressing the Stop button. 4. Turn off the Main Switch on the injection machine. 5. Turn off the Main Water Supply Switch. 6. Turn off Power Switch on the wall. 7. Clean the machine.

13 Effect of Mold Temperature On Crystallinity: Effect of Temperature on Melt Viscosity:

Plastic Injection Molding

Training Objective After watching this video and reviewing the printed material, the student/trainee will understand the principles and physical operations of the plastic injection molding process. An