1. Pressure distribution during injection
2. Factors affecting injection pressure
3. Relationship between injection pressure and position, number of injection ports
4. Relationship between injection pressure and product thickness
5. Relationship between injection pressure and material
6. Relationship between injection pressure and injection time
7. Relationship between injection pressure and melt and mold wall temperature
8. Relationship between injection pressure and injection speed
Pressure distribution during injection molding
Injection molding pressure: is to overcome resistance of melt flow process. Conversely, resistance in flow process needs to be offset by pressure of injection molding machine. Melt must be compacted and compensated at a certain speed to ensure a smooth filling process.
As flow length increases, resistance that needs to be overcome along way also increases, and injection molding pressure increases accordingly.

 

Distribution of injection pressure along melt flow path
Factors affecting injection pressure
There are many factors affecting melt injection pressure, mainly three categories:
A-type material factors: plastic type, viscosity.
B-type structural factors: such as injection system type, number and position, mold cavity shape, product thickness, etc.
C-type is process factor of molding.
Injection pressure is proportional to melt viscosity, flow length, melt flow speed, and inversely proportional to cross-sectional area of nozzle or runner. (Figure)

 

Relationship between injection pressure and gate position and number
The longer flow length, the greater pressure lost by melt during flow process. Therefore, pressure required to reach end of product is also higher. The shorter flow length, opposite is true.
The only way to shorten flow length is to increase number of gates and adjust gate position:
Influence of gate position on injection pressure and melt flow. (Figure)
Effect of gate position on injection pressure and melt flow

Relationship between injection pressure and product thickness
The smaller thickness of finished product, the easier it is for melt to cool when it flows, and flow is restricted, so the higher pressure required for melt filling
The longer flow length of melt, the thicker product must be (L=30-40t2), flow length ratio (flow length/minimum thickness of product).
Relationship between injection pressure and material
Different materials show different flow speeds and require different pressures
Viscosity of melt is the most significant factor affecting injection pressure due to flow properties.
Injection pressure selection range reference data (chart)
Reference data for injection pressure selection range

Relationship between injection pressure and injection time
Generally speaking, the shorter filling time, the higher volume flow rate of melt and the higher required injection pressure.
Filling time also depends on cooling effect of mold wall.
Relationship between injection pressure, melt temperature and mold temperature
Temperature of melt and mold will affect: injection pressure, surface quality of products, shrinkage/deformation of finished products, molding cycle, internal stress, etc.
Within molding temperature range of a specific material, every 10℃ increase in melt temperature will lead to a decrease in melt viscosity and cause injection pressure to decrease by about 10%
Excessive increase in barrel temperature will degrade plastic and affect surface quality and strength of product. According to experience, for every 1℃ increase in barrel temperature, injection pressure tends to decrease by about 1.5Ma:
Relationship between injection pressure and melt/mold wall temperature (example)

Relationship between injection pressure and injection speed
Optimal injection speed distribution allows melt to pass through gate area at a slower rate to avoid jet flow and excessive shear force
Injection pressure is to overcome flow resistance, which is basically constant, and speed change affects flow of melt.
Increasing flow rate allows melt to fill most of mold cavity.
Injection pressure is divided into two stages: stage of injecting molten material into mold at high speed, pressure at this time is called primary injection pressure. Pressure applied after material fills mold is called secondary injection pressure. (Pressure holding).
Generally, secondary injection pressure is about (80-120MPa) 800-1200Kg/cm2
In normal process debugging, it should start from low pressure and gradually increase

1. Definition
2. Relationship and position (switching point) between holding pressure and injection pressure
3. Control of holding pressure process
4. Control of holding pressure time
2. Holding pressure
When injection process is about to end, injection pressure switches to holding pressure, and holding pressure stage will begin. During holding pressure process, injection molding machine continuously feeds cavity from nozzle to fill volume vacated by shrinkage of part; if cavity is filled without holding pressure, part will shrink by about 25%, especially ribs will shrink too much and form shrinkage marks. Holding pressure is generally about 85% of maximum filling pressure, which should be determined according to actual situation. As shown in following figure, Figure a indicates start of filling, Figure b indicates that cavity is filled to about 90%, Figure c indicates start of holding pressure, and screw slowly moves forward, and Figure d indicates end of holding pressure, and cavity is completely filled.

 

Holding pressure control is very important for reducing flash and preventing plastic parts from sticking to mold. Good holding pressure control methods can help reduce product shrinkage and improve product appearance quality. Holding pressure is generally 75%~85% of injection pressure. Using following holding pressure control curve can help reduce injection pressure and clamping force and maintain good product quality.
Too long or too short holding time is not good for molding. Too long will make holding uneven, increase internal stress of plastic part, and plastic part will be easily deformed. In severe cases, stress cracking will occur; too short will make holding insufficient, part volume shrinks seriously, and surface quality is poor.
Holding curve is divided into two parts. One part is holding of constant pressure, which takes about 2~3s, called constant holding curve; the other part is holding pressure gradually reduced and released, which takes about 1s, called delayed holding curve. Delayed holding curve has a very obvious effect on molded parts. If constant holding curve becomes longer, part volume shrinkage will decrease, otherwise it will increase; if slope of delayed holding curve becomes larger and delayed holding time becomes shorter, part volume shrinkage will increase, otherwise it will decrease; if delayed holding curve is segmented and extended, part volume shrinkage will decrease, otherwise it will increase.

Formation
Benefits of adjusting back pressure
Problems that may occur if back pressure is too high or too low
Formation of back pressure
Back pressure (also called plasticizing pressure) is a pressure gradually formed during melting and plasticizing process of plastics as molten material continuously moves toward front end of barrel, pushing screw backward. In order to prevent screw from retreating too quickly and ensure that molten material is evenly compacted, a reverse pressure is applied to screw. This pressure that prevents screw from retreating is called back pressure.
Benefits of adjusting back pressure
Benefits of properly adjusting back pressure:
Compact molten material in screw, increase density, increase product weight and dimensional stability:
Vent (expel gas in molten material), improve gloss uniformity
Improve plasticization, increase mixing uniformity of color powder, masterbatch and molten material
Can improve surface shrinkage and flash of products
Can increase molten material temperature, improve plasticization quality, improve molten material fluidity during mold filling, no cold glue lines on the surface of product.
Problems that may occur when back pressure is too high or too low
Low density of melt, easy to trap air (low)
Large changes in product weight and product size (low)
Slow retraction of screw melt, increased cycle time (high)
Salting of melt, cold material blocking glue port, and producing cold material spots (high)
Nozzle leakage (high)
High mechanical friction between pre-plasticizing mechanism and screw (high)
Back pressure is adjusted to 3-15Kg/cm2

Definition
How to calculate clamping force
Relationship between injection pressure and clamping force
Definition
Clamping force: It is set to resist injection pressure of molten material. Generally, injection pressure at the end of mold cavity is a fraction of initial pressure. Therefore, average pressure of mold cavity in injection molding is generally around 35-50MPa:
How to calculate clamping force
·Calculation formula: F>AXPX10-3
·F-----Clamping force (t)
·A-----Total projected area (cm2)
·P-----Average pressure in cavity (35-50MPa)
Another formula F=projected area x system pressure x2x10/screw cross-sectional area

 

Relationship between injection pressure and clamping force
Average pressure in mold cavity must be less than minimum clamping force.
Clamping force can be reflected by clamping high pressure, but it is not equivalent to it.
Clamping force is constant force maintained when injection pressure acts on mold cavity. High pressure is an instantaneous force.

Range of processing temperatures for commonly used materials
How to detect actual temperature of melt
How to set barrel temperature
Relationship between melt temperature and injection pressure
Range of processing temperatures of commonly used materials
Melt temperature: It is an important condition in injection molding and an important factor affecting injection pressure.
Current molding temperature of company's materials:

How to detect actual temperature of melt
Temperature of material pipe is controlled by heating coil, rise and fall of heating temperature is detected by temperature sensing line. Therefore, when setting temperature, difference between set value, detection temperature, and melt temperature should be considered.
Detection of actual temperature of melt: Use method of air injection. After molten material is ejected, insert temperature measuring instrument into melt and rub it repeatedly. Detected temperature is actual temperature of melt (including friction heat and compression heat of plastic in screw)
How to set barrel temperature
Principle: Generally, it is set by maintaining a certain gradient
Temperature setting from rear feed port to front nozzle is gradually increased
Feeding section temperature is for preheating plastic.
Temperature setting of the first half of compression section should be slightly lower than melting point of material.
Temperature setting of second half of compression section and metering section should be slightly higher than melting point of material

 

Relationship between mold temperature and product quality, time
Relationship between mold temperature setting and materials
Introduction to mold temperature of commonly used materials
Relationship between mold temperature and product quality, time
Mold temperature has an important relationship with molding operation efficiency (molding cycle) and product quality
The lower mold temperature, the faster cooling speed, the shorter molding cycle and the higher efficiency. If mold temperature is too low, it is easy to cause poor appearance of product, such as: flow marks, weld lines and shrinkage marks.
The higher mold temperature, the better surface gloss and surface transferability of product, but the longer cooling time, the longer molding cycle and the lower efficiency.
Normal mold dynamic and fixed mold temperature difference is best controlled at around 10℃.

 

Relationship between mold temperature setting and materials
For crystalline resins, crystallization rate is governed by cooling rate. If mold temperature is increased, cooling rate is slow, which can increase crystallinity, which is beneficial to improve dimensional accuracy and mechanical properties of product. For example, crystalline resins such as nylon resin, polyoxymethylene resin, and PBT resin all require higher mold temperatures for this reason.