What are five elements of injection molding?
Pressure (Mpa)
Speed (%)
Stroke (mm)
Time (S)
Temperature (℃)

 

Injection molding process interface

Pressure is used to overcome flow resistance and reflect speed; setting value of injection pressure is limit value of pressure in filling stage. Actual pressure can be seen from display of injection molding machine "Pressure Curve" (yellow curve is actual pressure, and chart is read from right to left)

 

Injection molding pressure curve
Practical application of curve:
When product has a local micro-short shot (lack of material), how to determine whether to increase pressure, speed, or both pressure and speed?
When actual pressure has not reached set value, but actual speed has reached set value, speed setting value should be increased;
When actual pressure has reached set value, but actual speed has not reached set value, pressure setting value should be increased;
When actual pressure has reached set value, and actual speed has also reached set value, pressure and speed setting values need to be increased.

 

Injection speed curve

Transition point:
How to determine transition point? Generally speaking, transition point refers to switching point from injection to holding pressure when product is filled to 95% under zero holding pressure. For thin-walled products: generally 98% of product is filled, and for unbalanced flow channels: generally 70%-80%, depending on specific situation. It is recommended to use slow-fast-slow multi-stage injection.
When transition point is set by position, set transition time must be 3-5S longer than actual injection time.
Transition point is too high: insufficient mold filling, weld marks, depressions, small size, etc. Transition point is too low: flash, demolding difficulties, large size, etc.

How to determine press holding pressure, optimized press holding pressure is generally middle value between minimum press holding pressure and maximum press holding pressure, and press holding pressure is generally set from low to high. Minimum press holding pressure: Based on accurate transition point, a certain press holding pressure is given, and press holding pressure is when product just has insufficient mold filling. Maximum press holding pressure: Based on accurate transition point, a certain press holding pressure is given, and press holding pressure is when product just has burrs. (From minimum and maximum holding pressures, we can see safe range of product process fluctuations) Generally speaking:
PA holding pressure = 50% injection pressure
POM holding pressure = 80% injection pressure; for products with high dimensional requirements, 100% injection pressure can be achieved
PP/PE holding pressure = 30-50% injection pressure

During melting and plasticizing process of plastic, molten material continuously moves to the front end of barrel (inside metering chamber), more and more, gradually forming a pressure to push screw backward. In order to prevent screw from retreating too quickly and ensure that molten material is evenly compacted, it is necessary to provide a reverse pressure to screw. This reverse pressure that prevents screw from retreating is called back pressure. Back pressure is also called plasticizing pressure, and its control is achieved by adjusting return oil throttle valve of injection cylinder.
Back pressure valve is installed at the rear of injection cylinder of pre-plasticizing screw injection molding machine to adjust speed of injection cylinder oil leakage when screw rotates backward, so that cylinder maintains a certain pressure. Screw backward speed (resistance) of full motor is controlled by AC servo valve.

1. It can compact molten material in barrel, increase density, and improve stability of injection volume, product weight and size
2. It can "squeeze out" gas in molten material, reduce gas marks and internal bubbles on the surface of product, and improve uniformity of gloss
3. Slow down screw backward speed to fully plasticize molten material in barrel, increase mixing uniformity of color powder, masterbatch and molten material, and avoid phenomenon of color mixing in product
4. Properly increasing back pressure can improve shrinkage of product surface and glue running around product
5. It can increase temperature of molten material, improve plasticization quality of molten material, and improve fluidity of molten material when filling mold.

1. When back pressure is too low, screw retreats too fast, density of melt flowing into the front end of barrel is small (relatively loose), and more air is trapped
2. It will lead to poor plasticization quality, unstable injection volume, large changes in product weight and product size
3. Surface of product will have shrinkage, air marks, cold material marks, uneven gloss and other undesirable phenomena
4. Bubbles are prone to appear inside product, periphery and bone position of product are prone to insufficient glue

1. Melt pressure at the front end of barrel is too high, material temperature is high, viscosity decreases, melt backflow in screw groove and leakage flow between barrel and screw increase, which will reduce plasticization efficiency (amount of material plasticized per unit time)
2. For plastics with poor thermal stability (such as PVC, POM, etc.) or colorants, melt temperature increases and heating time in barrel increases, causing thermal decomposition, or colorant discoloration increases, and surface color/gloss of product deteriorates
3. Back pressure is too high, screw retreats slowly, and pre-plasticization return time is long, which will increase cycle time and reduce production efficiency
4. Back pressure is high, melt pressure is high, and nozzle is prone to melt drooling after injection. Next time glue is injected, cold material in water inlet flow channel will block water inlet or cold material spots will appear in product
5. In plastic injection process, nozzle often leaks glue due to excessive back pressure, which wastes raw materials and causes heating ring near nozzle to burn out
6. Mechanical wear of pre-plastic mechanism and screw barrel increases

Adjustment of injection molding back pressure should be determined by performance of raw materials, drying conditions, product structure and quality conditions. Back pressure is generally adjusted to 3-20kg/cm3. When there is a little gas flower, color mixing, shrinkage on product surface, product size and weight change greatly, back pressure can be appropriately increased. When nozzle leaks glue, drools, melt decomposes overheated, product changes color, and return is too slow, you can consider reducing back pressure appropriately.
Back pressure is one of important parameters for controlling quality of melt and product in injection molding process. Appropriate back pressure plays an important role in improving product quality

 

a Under premise of being able to close mold, low pressure should be set as low as possible, as long as it can reliably offset friction resistance of mold (friction resistance between mold plate and guide column, friction resistance of guide column and guide sleeve after moving mold contacts, friction resistance of inclined guide column slider, friction resistance between moving mold plate and supporting parts, etc.)
b To set a sufficient low pressure protection stroke (should be greater than size of product, and should be greater than size of fixture of robot when there is a robot). Too large a low pressure protection stroke will affect accurate setting of low pressure protection pressure and low pressure protection time, so it should be set reasonably and appropriately
c To measure actual time from start to the end of low-pressure protection, set low-pressure time to be slightly longer than actual time (with a margin of less than 0.1 seconds to ensure that output of mold clamping action signal is stopped in time when there is foreign matter). Principle of low-pressure protection is that within a certain low-pressure time, computer cannot sample high-pressure signal, and computer determines that there is foreign matter in mold cavity. At this time, computer should stop output of mold clamping signal in principle, alarm and output mold opening signal at the same time.

According to theoretical clamping force calculation method, calculate theoretical required clamping force, and set actual clamping force according to size of theoretical clamping force. Actual high-pressure pressure is generally slightly larger than theoretical clamping force.
Set high-pressure stroke position to be larger first, then gradually reduce high-pressure stroke position according to experience until high pressure cannot be raised or injection molding machine arm cannot be straightened, and slightly increase high-pressure stroke so that clamping force can be reliably achieved.
Take following figure as an example: 1.3 position cannot generate high pressure, 1.7 position can generate high pressure, then we can increase 0.1-0.3 on the basis of 1.7. If setting is too small, there will be more abnormal alarms for not closing mold, and if setting is too large, protection factor will be reduced.

 

 

Definition: Speed at which molten plastic is injected from nozzle into mold under a certain pressure
Function:
Increasing injection speed will increase filling pressure
Increasing injection speed can increase flow length and make product quality uniform
High viscosity and fast cooling at high speed injection, suitable for long process products
Flow balance at low speed, stable product size
Setting principle:
Prevent mold expansion and flash
Prevent burning due to excessive speed
Try to choose high-speed filling to shorten molding cycle while ensuring product quality. Too fast injection speed is prone to scorch spots, flash, internal bubbles or melt injection. Too slow injection speed: easy to appear flow marks, welding marks, cause rough and dull surface

Definition: speed of mold opening and closing
Function: reasonable mold opening and closing speed is a necessary condition to ensure normal operation of machine and production
Setting principle:
Ensure smooth operation of machine with minimum vibration
Set mold opening and closing speed should make time required for mold opening and closing as short as possible

Maximum mold opening and closing distance should be completed by maximum mold opening and closing speed
Follow principle of slow-fast-slow

Definition: speed of ejector ejection and retreat after mold opening
Function: Ensure that product is demolded smoothly without deformation, distortion or top cracking
Setting principle:
Front section ejects at low pressure and slow speed of 10-20MM to prevent product deformation or top cracking (effectively protect mold)
Rear ejection is fast, but ejection must be smooth
Action stroke
Stroke should include injection/metering/relaxation/opening and closing molds/ejection
Metering stroke refers to length of material storage chamber in front of screw
When injection molded product is 95% in state, it switches to holding pressure, which is switching of injection stroke
When injection molding program is terminated, screw is at the front position of barrel. When pre-plasticization program starts, screw starts to rotate. Screw retreats under counter pressure of material until screw retreats to set position
When metering is completed, plastic will usually flow. At this time, setting release stroke to achieve glue extraction can solve flow phenomenon;
Mold opening stroke depends on size of mold. As long as it can be easily and quickly taken out For products, the smaller mold opening stroke is, the better to shorten molding cycle.
Ejection stroke also depends on size of mold. As long as product can be taken out quickly and easily, the smaller ejection stroke is, the better to shorten molding cycle.

Definition: Position change of screw during injection process (including metering stroke)
Function: Combine speed and pressure to control flow state of plastic
Setting principle:
Metering stroke is determined by filling amount of finished product. Usually, a 10-20MM buffer is added to this value to determine final setting
First stroke is usually set based on low pressure and slow speed to fill hot runner and material handle
Second stroke is usually set based on high pressure and fast speed to fill 90% of product
Third stroke is usually set based on low pressure and slow speed to fill 95% of product
(Setting principle: the simpler the better, process fluctuation is small, and product size is relatively stable)

Definition: Position of male mold after male and female molds are separated:
Function: To ensure that all actions can be performed smoothly when demolding and taking out parts
Setting principles:
Spacing between each switching position shall not be less than 30MM
Maximum distance should be completed at maximum speed
Maximum mold opening position should be based on principle of convenient removal of parts (including manipulators) and not touching female mold surface when taking out parts
Maximum mold opening position should be based on the shortest molding cycle

Definition: Limit position of ejector ejection and retraction
Function: Used to limit distance of ejector forward and backward to ensure smooth demolding of products and accurate reset of ejector
For some molds with spring reset, ejection stroke also has function of ejection limit protection
Setting principles:
Ejection distance should follow principle of smooth demoulding from small to large
Spring reset molds without ejection limit pins should ensure that spring is not crushed during ejection
Ejector pin should not be higher than male mold surface when it is retracted
Ejector pin of mold with a slider must be retracted to avoid interference between mold clamping slider and ejector pin
Ejection backstroke is generally not set to 0, but 0.5-1.0mm to avoid excessive alarms
Injection molding machine ejector action setting interface

 

 

Key points: When injection time just meets 95% of product state; setting principle is to set it as small as possible to shorten molding cycle

Determination of pressure holding time is based on gate condensation and is determined by product weighing.
Holding time is too long: overweight, prone to white tip, tear, flash, difficult demoulding, oversized size. Holding time is too short: underweight, internal voids, shrinkage, undersized size

Reduce cooling time until following conditions are met;
No white tip, tear, or crack will occur when finished product is ejected.
Finished product size will not decrease with decrease of cooling time
Actual temperature of mold will not increase with decrease of cooling time

 

Filling time
Holding time
Cooling time (including metering time)
Opening and closing mold Demolding Neutron action Installing inserts Pick-up time
Measuring time is: storage time, cooling time must be greater than storage time (less than is meaningless); cooling time is generally about 1/2 of the total cycle;

 

Quality statistics interface
Whether product is stable, whether molding cycle is stable: whether position of pressure transfer + end position of pressure holding is stable;

For hydraulic presses, it is heat energy generated by friction of hydraulic oil movement due to continuous operation of machine. It is controlled by cold water. When starting machine, make sure that oil temperature is around 45℃. If oil temperature is too high or too low, it will affect transmission of pressure
Melt temperature: (adjusted by barrel stability setting: actual barrel temperature is ±5℃ of barrel setting temperature)
How to determine melt temperature Usually determination of melt temperature depends on performance of different materials and is provided by material supplier. (Melt temperature and mold temperature of material used are shown in material property table)
Barrel temperature setting. Melt temperature is too high: material decomposition (causing bubbles, color difference, burnt spots, fractures, etc. in product) Melt temperature is too low: uneven plasticization of material, cold material in melt (causing insufficient mold filling, cold material, product fractures, etc.)

Why mold temperature is needed? Regardless of mold temperature, its function is always to keep mold at a certain temperature during stable production and to cool it. Mold temperature that really matters refers to temperature of mold cavity, not temperature displayed on mold temperature meter. Usually, cavity temperature will reach a stable dynamic balance during stable production process. (For large molds, mold must be fully heated before production, especially for thin-walled and large-flow-length-ratio product molds)
Mold temperature affects 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 product crystallinity, which in turn affects product shrinkage and mechanical strength properties.
High mold temperature: good fluidity; high crystallinity; large shrinkage (resulting in smaller size); deformation; requires longer cooling time
Low mold temperature: poor fluidity (resulting in flow lines and weld marks); low crystallinity; small shrinkage (resulting in larger size)

 

Injection molding machine temperature setting
Insulation setting: For materials such as POM, PC, and ABS, long-term stay in barrel will cause material decomposition. After setting insulation temperature, barrel temperature can be temporarily maintained at insulation setting value to prevent material decomposition.