From what to how, 11 basic parameters of injection molding process, strength explanation!

Time:2021-06-11 16:42:27 / Popularity: / Source:

injection molding process 

1 Injection speed

01 What is injection speed
Usually injection speed we set refers to speed at which screw advances. But what is really important is speed at which melt advances in cavity, which is related to size of cross-sectional area in direction of flow.
02 How to determine injection speed
As a principle, injection speed should be as fast as possible.
Its determination depends on cooling rate and melt viscosity of melt: melt with fast cooling rate or high viscosity adopts high injection speed.
Note: Speed of cooling depends on properties of material itself, wall thickness and mold temperature.
injection molding process 
03 Injection speed is too fast/too slow
Injection speed is too fast: it is easy to appear focal spots, flash, internal bubbles or cause melt jetting
Injection speed is too slow: flow marks and weld marks are easy to appear, surface is rough and dull.

2 Turning pressure point

01 How to determine transfer pressure point
Generally speaking, pressure transfer point refers to switching point from injection to pressure holding when product is injected to 95% full under condition of zero holding pressure. For thin-walled products (such as cable ties): generally 98% of product is injection molded.
For non-equilibrium runners: generally 70%-80%, depending on specific circumstances. And it is recommended to use slow-fast-slow multi-stage injection.
02 Turning pressure point is too high/too low
Turning pressure point is too high: insufficient mold filling, weld marks, depressions, small size, etc.
Turning pressure point is too low: flash, difficult to demold, large size, etc.

3 Holding pressure

01 How to determine holding pressure
Optimized holding pressure is generally middle value of the lowest holding pressure and the highest holding pressure.
The lowest holding pressure: based on accurate transfer point, a certain holding pressure is given, and holding pressure is when product has just been insufficiently filled.
Maximum holding pressure: based on accurate transfer point, a certain holding pressure is given, holding pressure when product just appears burrs.
(From minimum and maximum holding pressure we can see safety range of product process fluctuations)
02 Generally speaking:
PA holding pressure=50% injection pressure
POM holding pressure = 80% injection pressure;
For products with high size requirements, 100% injection pressure can be achieved
PP/PE holding pressure=30-50% injection pressure

4 Holding time

01 How to determine holding time
Holding time is determined based on condensation of gate. It is determined by weighing product.
02 Holding pressure time is too long / too short
Holding pressure time is too long: affect cycle
Holding time is too short: insufficient weight, hollow inside of product, and small size.
Note: Holding pressure will affect length of holding time. The greater holding pressure, the longer holding time.

5 Screw speed

Goal of pre-plasticization is to obtain a uniform and stable melt (ie uniform plasticization, no cold material, no degradation, and no excessive gas)
01 How to determine screw speed
As a principle, screw speed must be determined so that pre-plastic time of screw, sum of suction time and retreat time of injection platform is slightly shorter than cooling time.
02 Screw speed is too fast/too slow
Screw speed is too fast: uneven plasticization (resulting in product cold material, insufficient filling and fracture, etc.), material decomposition (resulting in focal spots, chromatic aberration, fracture, etc.)
Screw speed is too slow: affect cycle

6 Cooling time

As a principle, cooling time should be set as short as possible. Basic requirements are that product does not deform, stick to mold, and have no excessively deep ejection marks.
And: screw speed:
POM <0.7 m/s;
PP/PE/PS <1.3m/s;
ABS/PC/PMMA <0.6m/s
injection molding process 

7 Back pressure

01 What is back pressure
Back pressure refers to force that hydraulic cylinder prevents screw from retreating when screw is pre-molded, its magnitude is equal to reaction force of melt at the front end of screw on screw.
02 How to determine back pressure
Determination of back pressure depends on properties of different materials and is usually provided by material supplier.
Generally speaking: PA: 20-80 Bar; POM: 50-100 Bar; PP/PE: 50-200 Bar
03 How to determine back pressure
Back pressure is too high: material decomposition; salivation; longer pre-molding time is required.
Back pressure is too low: uneven plasticization (especially for color masterbatch), improper plasticization (thus causing product bubbles, focal spots, etc.)

8 Suction volume

01 How to determine amount of suckback
Determination of suck back (combined with determination of back pressure) is based on principle of no salivation
02 Suction volume is too large/too small
Too much suck back: bubbles, focal spots, unstable material pad
Suction volume is too small: salivation, unstable material cushion (because check valve cannot be closed)

9 Clamping force

01 Determination of clamping force
Clamping force depends on projection area of cavity and injection pressure
02 Clamping force is too large/too small
Too much clamping force: poor exhaust (focal spots, insufficient mold filling), mold deformation
Clamping force is too small: flash

10 Melt temperature

01 How to determine melt temperature
Usually determination of melt temperature depends on properties of different materials and is provided by material supplier. (See attached table for melt temperature and mold temperature of materials used)
02 Setting of barrel temperature
injection molding process 
03 Melt temperature is too high/too low
Melt temperature is too high: material decomposes (thereby causing product bubbles, chromatic aberration, focal spots, fractures, etc.)
Melt temperature is too low: material is not uniformly plasticized, and melt contains cold material (thereby causing insufficient mold filling, cold material, product fracture, etc.)

11 Mold temperature

01 Why do we need mold temperature
No matter mold temperature is high or low, its role is always to keep mold at a certain temperature during stable production process and play a cooling role.
Really important mold temperature is temperature of mold cavity, not temperature displayed on mold temperature machine. Usually, cavity temperature will reach a stable dynamic equilibrium during stable production process, it will be about 10 degrees higher than display temperature.
(For large molds, mold must be fully heated before production, especially for thin-walled product molds with a large flow length ratio)
02 What will mold temperature affect
It will affect fluidity and cooling rate of melt.
Because it affects fluidity, it affects product appearance (surface quality, burrs) and injection pressure;
Because it affects cooling rate, it affects crystallinity of product, then affects shrinkage rate and mechanical strength of product.
03 Mold temperature is too high/too low
High mold temperature: good fluidity; high crystallinity; large shrinkage rate (thus resulting in smaller size); deformation; longer cooling time is required.
Low mold temperature: poor fluidity (resulting in flow lines, weld marks); low crystallinity; low shrinkage (resulting in larger size).

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