There are numerous reasons you don't want to cut open the screen to upgrade internal components. More on this below. Best is to upgrade these externally which is entirely possible to make it completely useful
External SSD Upgrades
#1 upgrades to make these iMacs much faster is an SSD. External SSD will in most use cases be completely sufficient.
USB 3.0 speeds via USB-A (5Gbps) sufficiently matches an internal SATA 3 SSD (6Gbps) before any protocol overhead differences. You will not notice any difference other than an SSD hanging out the back in an enclosure. Doing this will speed up any HDD or most Fusion Drive iMacs to be way faster and completely useful even running newer patches macOS like OCLP macOS's.
Fusion drives are terrible designs. They wear out the SSD at a rapid pace. Best practice is to split the Fusion Drive into 2 separate drives (SSD and HDD) 24GB SSD is unfortunately too small to be useful for macOS system install but 128GB is good enough to run macOS and basic usage.
Note while external SSD upgrade can be very useful. It's not nVME speeds so probably limits video editing workstation type of work. But it is certainly fast enough for most common usage such like browsing etc.
I've also often encountered older Samsung SSDs that require more power than USB-A supplies so would need a powered enclosure. Haven't encountered this with other SSD brands (WD, Sandisk etc)
Why Don't Want to Cut Open the Screen
There are many reasons
If you really have to cut open the screen. Here are some tips
Pizza wheel encounter hard to cut portion
It is likely the wheel is cutting between the liner and the glass. Try from the other direction. It is still possible to eventually cut open the tape but liner will be folder in wards a little reducing adhesive surface.
On 27", I avoid tilting up the screen aggressively to remove the bottom tape
The bottom glass of the screen is wedged against the metal body. I worry about amount of pressure can crack the glass if tiling at aggressive angles. I use a ultra thin plastic card with a bent L shaped tip to slowly and gradually cut from both edge while tilting it ~30 degrees periodically to loosen the tape.
Peel off tape residue vertically
Don't do at an angle, vertically pulling it up is the cleanest removal
Clean off residue and adhesive primer
I use ISO to remove old sticky residue. Be careful of the microphone holes on the bottom edge if you have one. ISO will ruin those microphones.
Bought adhesive primer (3M 94) from aliexpress. I'm guessing its not the real thing but seems to have worked well so far
Reapplying tape
The top 2 corners have an air breathing channel. Easy to close this channel trying to align the tape (and stretching it slightly) If channel is closed, cut it slightly so remains open. I'm guessing hot air escapes through here rather than expanding inside and pushes the screen away from the resealed tape.
Alignment
It's really hard to align the bottom edge of the LCD against the metal chassis. No robotic laser alignment tools are available. If the LCD doesn't reset against the bottom of metal chassis, then likely the weight will tug on the resealing tape potentially loosening it.
After positioning the screen and before peeling the tape. Masking tape on top edge and corner of the screen can help alignment as LCD won't move around too much. Also some some spacers (popsicle sticks) can help maintain a gap freeing both hands. Another pair of hands can probably be very helpful.
I start by peeling off and exposing the sticky surface on the bottom edge of the screen. You only get 1 shot at putting this down as it doesn't want to move as soon as it's stuck onto the tape. NOTE for 27" screen, the center will sag from weight and stick onto the tape even if both sides have a tiny spaced gap. Once this happens, the position is stuck.
Once bottom edge is sealed to the tape, raise the screen very slightly just to access the tape to remove the protection to expose sticky surface. Start from the 2 sides and finally the top. Remove all the 3 remaining edge protection before dropping down the LCD for final seal. If drop down the screen after removing just the sides (and not the top) you will not be able to access the top anymore. Again, 2 pair hands helpful here and/or use spacer tools on the top edge to prevent sides from sticking after sticky surface protection removal.
A roller with gentle pressure can help to get a good seal along the edges.
I am able to generally get the bottom edge to sit against the chassis and left and right side maybe < 1/4 mm off using this method.
Extra Protection
Some people have added sticky assemblies + fishing lines to help save the screen if it falls off. Not easy to do while handling all the exposed sticky tapes along the edges. Furthermore, if put up too high, the tapered top edge of screen will not close.
External SSD Upgrades
#1 upgrades to make these iMacs much faster is an SSD. External SSD will in most use cases be completely sufficient.
USB 3.0 speeds via USB-A (5Gbps) sufficiently matches an internal SATA 3 SSD (6Gbps) before any protocol overhead differences. You will not notice any difference other than an SSD hanging out the back in an enclosure. Doing this will speed up any HDD or most Fusion Drive iMacs to be way faster and completely useful even running newer patches macOS like OCLP macOS's.
Fusion drives are terrible designs. They wear out the SSD at a rapid pace. Best practice is to split the Fusion Drive into 2 separate drives (SSD and HDD) 24GB SSD is unfortunately too small to be useful for macOS system install but 128GB is good enough to run macOS and basic usage.
Note while external SSD upgrade can be very useful. It's not nVME speeds so probably limits video editing workstation type of work. But it is certainly fast enough for most common usage such like browsing etc.
I've also often encountered older Samsung SSDs that require more power than USB-A supplies so would need a powered enclosure. Haven't encountered this with other SSD brands (WD, Sandisk etc)
Why Don't Want to Cut Open the Screen
There are many reasons
- Delicate glass screen can easily crack even if using plastic pizza wheel cutter.
- There is an ultra thin liner attached to the glass which double sided tape sticks on to. Pizza wheel can get under this liner making it very difficult to cut open. Furthermore, compromised liner can weaken the new double sided tape's stickiness
- Most aftermarket reseal double sided tape are junk. I've compared ebay purchases with the higher quality tape I use (from mobilesentrix which is a pro repair shop supplier, need business license to purchase parts) ebay one was much thinner and didn't have the proper microphone location cutouts. Anyway, many have reported screens falling off after reassembly
- Proper reassembly require proper cleaning of old tape residue, use an adhesion primer, proper alignment (very difficult, see below), avoid sealing off breathing channel on upper 2 corners, and finally high quality double sided tape.
- ifixit directions and YouTube videos are far from sufficient in outlining all the tricky points and challenges. Just skim through the comments and you will see plenty of people that had cracked or fallen LCDs.
- Upgrade is the worst reason to this this. SSD can be attached externally and used effectively. > 8GB RAM on 21.5" is generally unnecessary.
- Failed hardware. Even if failed hardware is internal HDD or Fusion Drive (The smaller sized 24GB SSD often gets trashed by heavy use) can just avoid using them and boot/run off external SSD.
- The pivoting hinge is a failure spot. External support can be used.
- Dust in the lower corners of the screen. This unfortunately occurs on these screens. 27" more than 21.5". Dust enters and deposits onto back of the LCD which require removing the screen and disassembling the LCD slightly to clean. As noted, cutting open the screen has major risks. Perhaps just live with this and generally not so noticeable.
If you really have to cut open the screen. Here are some tips
Pizza wheel encounter hard to cut portion
It is likely the wheel is cutting between the liner and the glass. Try from the other direction. It is still possible to eventually cut open the tape but liner will be folder in wards a little reducing adhesive surface.
On 27", I avoid tilting up the screen aggressively to remove the bottom tape
The bottom glass of the screen is wedged against the metal body. I worry about amount of pressure can crack the glass if tiling at aggressive angles. I use a ultra thin plastic card with a bent L shaped tip to slowly and gradually cut from both edge while tilting it ~30 degrees periodically to loosen the tape.
Peel off tape residue vertically
Don't do at an angle, vertically pulling it up is the cleanest removal
Clean off residue and adhesive primer
I use ISO to remove old sticky residue. Be careful of the microphone holes on the bottom edge if you have one. ISO will ruin those microphones.
Bought adhesive primer (3M 94) from aliexpress. I'm guessing its not the real thing but seems to have worked well so far
Reapplying tape
The top 2 corners have an air breathing channel. Easy to close this channel trying to align the tape (and stretching it slightly) If channel is closed, cut it slightly so remains open. I'm guessing hot air escapes through here rather than expanding inside and pushes the screen away from the resealed tape.
Alignment
It's really hard to align the bottom edge of the LCD against the metal chassis. No robotic laser alignment tools are available. If the LCD doesn't reset against the bottom of metal chassis, then likely the weight will tug on the resealing tape potentially loosening it.
After positioning the screen and before peeling the tape. Masking tape on top edge and corner of the screen can help alignment as LCD won't move around too much. Also some some spacers (popsicle sticks) can help maintain a gap freeing both hands. Another pair of hands can probably be very helpful.
I start by peeling off and exposing the sticky surface on the bottom edge of the screen. You only get 1 shot at putting this down as it doesn't want to move as soon as it's stuck onto the tape. NOTE for 27" screen, the center will sag from weight and stick onto the tape even if both sides have a tiny spaced gap. Once this happens, the position is stuck.
Once bottom edge is sealed to the tape, raise the screen very slightly just to access the tape to remove the protection to expose sticky surface. Start from the 2 sides and finally the top. Remove all the 3 remaining edge protection before dropping down the LCD for final seal. If drop down the screen after removing just the sides (and not the top) you will not be able to access the top anymore. Again, 2 pair hands helpful here and/or use spacer tools on the top edge to prevent sides from sticking after sticky surface protection removal.
A roller with gentle pressure can help to get a good seal along the edges.
I am able to generally get the bottom edge to sit against the chassis and left and right side maybe < 1/4 mm off using this method.
Extra Protection
Some people have added sticky assemblies + fishing lines to help save the screen if it falls off. Not easy to do while handling all the exposed sticky tapes along the edges. Furthermore, if put up too high, the tapered top edge of screen will not close.
