On-Line Solutions To Injection Molding Problems
Installing And Setting Up The Mold
Sizing And Inspection:
Before installing a mold, a machine must be selected that is properly sized for the specific mold being installed. After the machine is selected it must be inspected to determine its status. This inspection includes items such as:
- Proper hydraulic oil level
- Heater bands in place and operating
- Mold temperature controllers operable
- Injection cylinder empty and screw forward
- Hopper shutoff closed and hopper wiped clean
- Proper material available and dried
- Granulator clean and available
- Safety gates and mechanisms operating and in good condition
- Vent hood(s) clean and operating
- Heat exchanger clean and operating
- Machine lubricated, or auto-lubrication working and filled
- Alarms and lights operable
After the machine inspection is completed, the mold can be installed. The following steps should be taken, but they are generic in nature and do not preclude the machine manufacturers instructions. Always follow those instructions first and foremost.
1 - Make sure that the mold has a connecting strap installed. This strap should connect the two halves of the mold and keep them from coming apart during transportation. Normally this is a metal strap mounted across the "A" and "B" plate parting line. It is not a safe or proper practice to install the mold as two separated halves.
2- Start the machine, make sure the injection sled is in the full back position, and set the barrel heaters to the proper temperatures. The profile should run from a cool rear setting to a progressively hotter front zone and nozzle as outlined on the setup sheet. Turn on the feed throat cooling water.
3 - Open the clamp wide enough to accept the mold. This is normally a dimension that equals a minimum of twice the height of the mold. This may require re-setting the mold open limit switches or control settings. Refer to the machine manual for instructions.
4 - Lower the mold from the top of the machine (or slide it in from the side) using a chain fall, and bring the mold up against the stationary platen, by hand. The mold should rest against the platen without assistance. This is accomplished by adjusting the location of the chain fall towards the platen. It is a good idea to have a thick metal plate placed across the lower tie bars at this point. The plate will act as a safety catch in case the chain fall breaks, or the connecting hook opens.
5 - At this point the mold must be raised and lowered slightly in an attempt to position it so the locating ring on the mold will slip into the locating hole on the platen. The chain fall should be connected such that the mold tilts slightly at the top.
6 - The tilted mold should be placed slightly above the locating hole of the stationary platen, and held against that platen as the mold is slowly lowered. The locating ring of the mold will automatically slip into the locating hole of the platen as the mold is gently lowered. A level can be placed across the top of the mold to assist in aligning the mold so it fits squarely on the platen. After leveling, the "A" half of the mold is ready to clamp in place.
7 - Position clamps, adjust, and bolt the "A" half of the mold to the stationary platen. The mold should be mounted with at least 1 clamp in each of the 4 corners. If the mold is very wide, additional clamps should be placed along the long dimension. If the mold is very small it may be possible to use only 2 clamps per mold half, although this is not recommended and smaller, specially built clamps may have to be utilized in order to use 4 clamps per mold half.
The best thing to do is purposely adjust the heel of the clamp away from the platen. This results in the clamping force being directed toward the platen through the toe of the clamp. The angle of direction should be minimal and can be such that the clamp heel is only 1/8" to 1/4" away from parallel.
The clamp is slotted for linear adjustment. The adjustment allows the clamping bolt to be located as close as possible to the mold because that, too, aids in creating maximum clamping pressure on the mold itself. The bolts should be tightened using a torque wrench, with 5/8" bolts requiring approximately 150 ft.lbs. torque and 3/4" bolts requiring approximately 200 ft.lbs. torque.
8 - If ejector rods are required, place them in the mold now. Then, slowly bring the clamp unit forward, under low pressure, to prepare for clamping the "B" half of the mold. This may require adjusting limit switches or settings. Check the machine manual for this information. Bring the moving platen up to within 1/4" to 1/2" of the mold base and set limits for the 'high pressure close" to activate at that point. Now, continue moving the clamp unit forward until it touches the mold base. Allow the press to build up clamp pressure to the desired setting. This ensures that the mold is fully closed.
9 - Shut off the machine. Locate clamps, adjust, and bolt the "B" half of the mold to the moving platen, making sure to follow the procedure mentioned above.
10 - Remove the chain fall hook, eyebolt, and connecting strap from the mold. To avoid losing the connecting strap, it may be desirable to keep it mounted but swung out of the way and tightened down so it will not come loose and cause mold damage.
11 - Recheck each and every clamp on both halves to make sure they are all tight. Start the machine and SLOWLY jog the clamp unit open under low pressure, watching for any indication of the mold halves seizing or binding together. Open the mold approximately 1/2" and stop. Shut off the machine and fully check the mold to make sure it is properly mounted.
12 - Start the machine and continue to open the mold slowly until the "B" half disengages fully from the "A" half. Then stop the mold at the point described for fully open. This would normally be a minimum of approximately 2 times the depth of the part being molded, to make sure the part will fall free after ejection. It is acceptable to open the mold farther than that rule-of-thumb, but it should not open farther than necessary because of the additional time required to do so. If there are slides, or other actions, in the mold, make sure they are still properly engaged upon full opening if at all possible. This will minimize the potential for breakage. Check for broken springs or other obvious damage.
13 - Adjust settings for proper ejection. Ejection should NOT pulsate. One stroke should be adequate for part removal. If this is not enough there is something wrong and it should be corrected before continuing production. The amount of ejection stroke should not exceed 2-1 /2 times the depth of the part in the "B" half of the mold (assuming ejection is located on the "B" half). Ejection stroke should be kept to a minimum and is only required to get that part of the plastic that is molded in the "B" half freely out of the mold. More than that only adds to the overall cycle time.
14 - Lubricate all moving components such as ejector guides, leader pins, and slides. Wipe off all excess. Gently clean the cavity surfaces. Close the mold and turn off the machine.
15 - Attach the hose lines from the mold temperature control unit. Blow air through the cooling lines of the mold to make sure they are not obstructed and to observe the proper path for connecting hoses. Do not loop the "A" half and "B" half together on a single line, but attach separate "In" and "Out" lines for each half, and use separate control units for each half. Make sure there are no kinks in the hoses and that they will not be crushed or stretched when the mold closes or opens. After inspecting for proper attachment, activate the temperature control units and adjust for the proper temperature setting.
16 - Recheck all clamps.
17 - Check to determine if the barrel is up to heat. This normally takes 45 minutes to an hour to properly come to preset temperatures and soak. Make sure all heater bands are operable and properly connected. Once the barrel is at proper temperature it is a good time to check that the machine nozzle is properly aligned with the mold's sprue bushing and has the correct radius and sprue opening as well. (The nozzle may be adjusted so that it is exactly centered with the sprue opening). This can be checked by carefully inserting a very thin piece of cardboard or heavy paper between the nozzle and the sprue and then closing the nozzle against the bushing. Pull the barrel sled back to view the paper image and adjust the nozzle if required. Another way to check is simply mold a few parts and closely inspect the runner system sprue to make sure the nozzle opening is centered in the sprue opening. Some molders use bluing ink and place it on the nozzle tip to see where it leaves a smudge when pushed against the sprue bushing. Once the nozzle is correctly centered you may move to the next step.
18 - Ensure that the hopper feed gate is closed and the hopper magnet is in position. Place fresh, dry material in the hopper. A purging compound may be required first, depending on what material was in the barrel last.
19 - After the barrel is up to heat, and allowed to soak for 10 to 15 minutes, open the feed gate on the hopper and allow material to drop through the feed throat and into the cylinder.
20 - Purge the machine as follows:
21 - The injection screw should have been left in the forward position of the barrel when the last job was shut down. It should stay in that position while preparing material for air shots. Activate the screw rotation until fresh material is brought to the front of the barrel. This will be obvious because the screw will spin freely to begin with but slow down considerably as the fresh material is brought forward.
22 - Set the screw return limits to the desired point and allow the screw to return to that point. The screw rotation will stop once the screw returns to the set point. Allow the material that was brought forward enough time to absorb heat from the cylinder This will normally be only a minute or two.
23 - With the sled still in the back position, take 3 air shots. An air shot consists of injecting a full shot of material into the air, under molding pressure, and allowing it to accumulate on a special plate designed to catch purgings. Make sure that proper time is allowed between these air shots to allow the upcoming material to come to proper heat. This time usually amounts to the total cycle time of the job that will be running in production. Using a fast acting pyrometer with a probe, measure the melt temperature of the material injected during the air shots. This is the temperature that must be controlled for proper molding. Adjust settings as necessary. If a different material or color is being used it may require 15 or 20 air shots to clear the old material out.
24 - Set all limits for injection and cycle. These include injection forward speed and pressure, holding pressure, cushion distance, cooling time, mold open and close settings, and others as outlined in the following sections.
25 - Prepare a full charge of material for the first shot. Bring the injection sled forward until the nozzle seats against the sprue bushing of the closed mold. The mold must be closed to absorb the force of the injection sled against the sprue bushing. If the mold is open at this point the "A" half will be pushed off the platen. Lock the sled control in place.
26 - Open the mold and bring the clamp unit to a full open position.
27 - Set the cycle indicator to "Manual", "Semi-automatic", or "Automatic" depending on requirements.
28 - Close the safety gate to initiate the first cycle.
29 - Observe the injection process. The pressures and feeds should be set such that a short shot will be taken first. Then pressure and feed settings can be adjusted until a properly molded part is produced. This should be done over a long period of time ( 15 to 20 shots) and not hurried.
30 - Inspect part dimensions (after 8 hours) and appearance and continue if acceptable. If not acceptable, determine remedy and reset.
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