(Imagine this: Molten plastic screaming inside the mold, "I can't hold it anymore! Let me out!" The holding pressure system replies coldly, "Wait 5 more seconds. This is your KPI!")
In the wild world of injection molding, hold time is the most "uncomfortable" phase. After the molten plastic is injected into the mold, the holding stage is like a cruel "pee - holding competition":
Too short: The plastic "bursts out" instantly, and the part shrinks like a deflated balloon (shrinkage rate can reach 20%)!
Too long: The plastic "develops internal injuries," and surface stress cracks appear like spider webs (stress exceeding 10 MPa spells disaster).
Worst of all: If holding pressure and time are mismatched, plastic will squirt out of the mold like an out - of - control fountain (a sight too terrible to behold).
The "Shrunken Water Cup" Incident: A factory shortened hold time by 3 seconds. The produced water cups were 1 cm shorter. Customers thought they got kids' versions (actual volume shrank by 15%).
The "Spider - Web Phone Case" Tragedy: Excessive hold time + high pressure caused the phone case surface to crack like it was crawled by 10,000 spiders (stress test showed 12 MPa).
The "Flash Fountain" Accident: Sudden pressure release during holding made plastic squirt out of the mold. The machine panel was covered in "plastic magma" (clean - up took 3 hours).
| Plastic Type | Optimal Hold Time | Consequences of Over/Under Holding |
|---|
| ABS | 10 - 15 seconds | Under = surface sinks (18% shrinkage); Over = internal voids |
| PP | 8 - 12 seconds | Under = dimension shrinkage (2.5% linear shrinkage); Over = warping |
| PC | 15 - 20 seconds | Under = reduced light transmittance (10% haze increase); Over = cracking |
| POM | 5 - 8 seconds | Under = unstable dimensions (tolerance > ±0.1 mm); Over = decomposition |
Data Source: Institute of Plastic "Pee - Holding" (made up, but the data is real)
Holding pressure = Injection pressure × (30% - 50%)
(Example: If injection pressure is 100 MPa, set holding pressure at 30 - 50 MPa. Otherwise, plastic will "wet itself.")
Hold time = Part wall thickness × (3 - 5 seconds/mm)
(Example: For 2 mm wall thickness, hold time is 6 - 10 seconds. Thicker walls need longer holding.)
First stage: 70% injection pressure × 5 seconds (Hold most of the melt first).
Second stage: 50% injection pressure × 3 seconds (Relax a bit but don't release completely).
Third stage: 30% injection pressure × 2 seconds (Release slowly to prevent "incontinence").
Result: A car parts factory reduced shrinkage from 12% to 3% with this method. The mold technician proudly stuck a "Water - Saving Star" certificate on the machine.
Gate diameter = Part wall thickness × (0.8 - 1.2)
(Too small = "Urethral blockage" during holding; too large = "Dripping" after holding).
Cooling channel distance from cavity = 3 - 5 times wall thickness
(Too close = Plastic "freezes and can't pee"; too far = "Bladder inflammation" from over - holding).
Add 15% glass fiber: Hold time can be reduced by 20% (Glass fibers act like straws to speed up "peeing").
Use low - shrinkage PP: Cut hold time by 10 seconds (Material has a natural "anti - pee - holding constitution").
Case Study: A toy factory switched to low - shrinkage material, cutting hold time from 12 to 8 seconds. Production capacity soared 33%. The boss gave the machine a "Efficiency King" red envelope.
"Hold time isn't about holding longer;
Match pressure and time like a song.
ABS holds for 15 seconds max,
PP shouldn't go past 12, relax.
Step - by - step holding is the key,
First hard, then soft, then set it free.
Mold gates shouldn't be too tight,
Cooling shouldn't give plastic a fright.
Choose the right material, less toil,
Boost efficiency and production with style!"
(Last reminder: Mess up hold time, and the consequences are dire—either parts turn into "dwarfs" or molds become "fountains." Remember: Let plastic "pee scientifically" to produce perfect parts that are "shrink - free, crack - free, and flash - free"!)
