How to set reasonable injection process parameters?

Time:2022-12-30 09:06:37 / Popularity: / Source:

Debugging of injection molding machine refers to continuous adjustment of various parameters of injection machine for specific mold until qualified plastic parts are produced. Various parameters of plastic injection machine can be roughly classified as follows:

1. Preliminary comprehensive parameters:

For a specific set of molds, following three parameters need to be considered first before mold start work:

1.1 Allowable die size:

It is Moho*Move* (Mothmi~Mothma) of injection molding machine. Its items must be larger than corresponding items of mold: Mwid*Mlen*Mthi (width*height*thickness)

1.2 Maximum injection volume:

It is weight SHWT (g) of the largest plastic that can be injected by injection molding machine. The total weight of each injection must be less than (or equal to) 85% SHWT and greater than (or equal to) 15% SHWT. (When the total weight of each injection>85%SHWT will reduce injection efficiency)

1.3 Clamping force:

That is, maximum separation force that mold can bear after mold is closed. Its size is approximately proportional to projected area of plastic part, and rough calculation formula is as follows:
Clamping force (tons) = projection surface of the cavity (inch 2) * material pressure coefficient
Among them, material pressure coefficient of PS, PE, PP is 1.7; ABS, AS, PMMA is 2; PC, POM, NYLON is 3. For a specific mold, actual clamping force of injection machine is less than or equal to rated clamping force of injection machine * 90%. Excessive clamping force is not beneficial to injection machine and will cause mold to deform.

2. Temperature parameter (T):

Temperature in production process of injection is different according to different rubber materials. It can be divided into following types:

2.1 Material temperature:

In production of injection, moisture content in raw materials needs to be dried to below a certain percentage. Because moisture content of raw material is higher than a certain proportion, it will cause defects such as air blooming and delamination.

2.2 Barrel temperature:

Barrel from hopper to nozzle can be divided into: conveying section, compression section, metering section, heating temperature of each section is collectively referred to as barrel temperature. Barrel temperature is from low to high. In addition, nozzle temperature is usually slightly lower than temperature of metering end.

2.3 Mold temperature:

Refers to cavity surface temperature. Setting temperature is different according to shape of each part of mold cavity. Generally, mold temperature of parts that are difficult to be glued is higher, temperature of front mold is slightly higher than that of rear mold. When temperature of each part is set, temperature fluctuation is required to be small, so auxiliary equipment such as thermostat and chiller are often used to adjust mold temperature.

3. Location parameters:

3.1 Screw position (S):

Segment conversion position of injection speed and pressure of screw is called screw position.
Specific segments are as follows: S0 S1, S2, S3, SS. Among them, S0 and SS are equal to amount of melt required for one injection, SS cannot be less than 10mm (generally between 15-20mm); S0, S1, S2, S3, and SS are screw position segments, according to flow of melt. Position of mold cavity is specifically set. S0, S1, S2, S3, and SS are injection sections. Among them, S3 and SS are pressure holding sections.

3.2 Glue pumping position (SUCK BACK):

When screw stops rotating after returning material, screw has a backward pumping action, which is called glue pumping, and distance of pumping is glue pumping distance. Generally, it is below 5mm. Purpose of evacuation is to prevent melt glue from being cast on nozzle; glue should be properly pumped. Too much evacuation will cause defects such as air flowers and bubbles in finished product.
How to set reasonable injection process parameters 

3.3 Mold opening position:

Distance between back mold surface and front mold surface is called mold opening distance. Its size is enough to take out glue piece smoothly.
It is advisable to extend cycle time if it is too large.

3.4 Thimble position:

It is distance that mold ejector pin is ejected from back mold surface. Push product away from back mold surface.
And it is appropriate to win it smoothly. Be careful not to let thimble to the end, and there must be enough margin to avoid slingshot of mold thimble plate being topped off.

4. Pressure parameters:

4.1 Injection pressure (IP):

Driving force given by screw to melt is called injection pressure. According to each segment of screw position, different propulsion forces of screw can be set to melt glue. Setting of propulsion force of each section mainly depends on position where molten glue flows in mold cavity. When shape of flowing cavity is complex and glue level is thin, resistance to molten glue is large, and a larger amount of propulsion is required. When shape of flowing position is simple and resistance to melt is small, a small propulsion force can be set to reduce loss of injection machine.

4.2 Holding pressure (HP):

When molten glue fills mold cavity, in order to compensate for space formed by mold cavity and compact rubber material due to cooling and shrinkage of rubber material, screw also needs to give molten glue a certain propulsive force, which is pressure holding force.
Here position movement of screw is: S3 SS. Holding pressure is expressed by HP. Generally, medium pressure is used for large plastic parts, and low pressure is used for small plastic parts. (Generally HP is less than IP).

4.3 Back pressure (BACK PRESS):

When glue injection and pressure holding are completed, screw starts to rotate, so that glue in screw groove and hopper is pressed into front end (measuring chamber) of barrel through screw groove. At this time, molten glue has a reaction force on screw to force screw to retreat, which is called back feeding.
In order to increase density of melt glue at the front end of barrel (measurement chamber) and adjust speed of screw retreat, an adjustable thrust must be added to screw, which is called back pressure. Adjusting back pressure can adjust mixing degree of toner and plastic material, which affects plastic effect.
Appropriate back pressure can reduce defects such as color mixing, bubbles, and uneven gloss of plastic parts, but back pressure should not be too large. Too much back pressure will cause melted glue to decompose, causing defects such as discoloration and black lines of plastic parts. In addition, increasing back pressure will inevitably prolong production cycle and aggravate loss of injection machine, generally about 10kg/cm2.


Also known as low-voltage protection, it is a protection device for injection machine to mold. From protection position of mold to moment when front and rear mold surfaces are attached, during this period, force of clamping mechanism to push rear mold of mold is relatively low. Mold will automatically open to stop mold closing action, so that if there is foreign matter between front and rear molds when mold is closed, mold can be protected.
Low pressure of clamping is generally larger for mold with row position than mold without row position, value is: 10-20kg/cm2.


Also known as clamping pressure, when mold is closed to make front and rear mold surfaces fit together, clamping force automatically changes from low pressure to high pressure. Clamping pressure should not be too high, as it will crush die surface; when adjusting, make sure that there is a certain pressure on the front and rear die surfaces, generally 80-100kg/cm2. (Generally, the whole process of mold clamping: slow speed, high speed, low pressure and low speed, high pressure mold clamping).

4.6 Thimble pressure:

Ejection force exerted by injection machine on the back of ejector plate of mold should be of a size that can eject plastic parts.

5. Speed parameters:

5.1 Injection speed (V):

When injection machine is shooting glue, screw pushes moving speed of molten glue. Injection speed is mainly affected by injection pressure, resistance of mold cavity to melt, and viscosity of melt itself. When injection pressure is greater than cavity resistance and melt viscosity, set injection speed can be fully utilized.
For example: S0, S1 are V1, at this time, melt fills cavity, requiring low speed and medium pressure; S1, S2 are V2, at this time, melt fills cavity, requiring high speed and high pressure; S2, S3 are V3, melt glue fills periphery of plastic parts, requires medium speed and low pressure, injection speed slowly decreases with increase of filling resistance of mold cavity until it is zero. Setting of injection speed of each section depends on shape of melt flowing through cavity.

5.2 Screw speed (R):

Rotational speed of screw feeding barrel metering chamber is called screw rotational speed. It affects retreating speed of screw. When back pressure is set, the higher screw speed is, the greater retreating speed will be. Adjusting screw speed can adjust plasticizing effect of rubber compound, improve uneven color tone and color mixing of product. However, if screw speed is too high, material will be decomposed due to excessive shearing, and at the same time, air will be mixed into barrel, which will cause bubbles in product.
PC, PE, PVC, POM, PMMA and other heat-sensitive plastics with high viscosity are not suitable for high screw speed. Screw speed is represented by R1 and R2. Generally, R1 uses a medium speed and R2 uses a low speed, which has a protective effect on injection machine.

5.3 Glue pumping speed (SB.SPEED):

Backward speed of screw when it is evacuated is called glue pumping speed. Generally, medium and low speeds are appropriate.

5.4 Mold opening and locking speed:

Mold opening speed is expressed by: MO1, MO2, MO3. Generally, slow speed is used when front and rear mold surfaces are separated, so molds with different templates have different settings.
General settings for two-plate mode: slow, fast, slow; general settings for three-plate mode: medium, slow, slow. Clamping speed is represented by: MC1, MC2, MC3, generally, slow speed is used when front and rear die surfaces are in contact, so two-plate mold setting: medium, fast, and slow; three-plate mold setting: medium, slow, slow.

5.5 Ejector speed (EJ SPEED):

Speed at which ejector ejects plastic part is called ejector speed. Settings of plastic parts of different structures are different, and medium speed is generally used.

6. Time parameter (t):

6.1 Game time:

Time required for different compounds is different.

6.2 Injection time (INJ-HOLD TIME):

Time required for screw to move from S0 to S3 must be set consistent with movement of screw position.

6.3 Holding time (HT):

Time for screw to start feeding from S3 is generally 1-2 seconds. It should not be too long. If it is too long, it will waste time.

6.4 Cooling time (COOLING TIME):

Time from when screw starts to feed back to when mold is ready to open is cooling time. Cooling time cannot be less than return time.

6.5 Cycle time (CYCLE TIME):

Time it takes for a injection machine from start of a brew to start of next brew. Requirement is that the shorter the better, on the premise of producing qualified plastic parts.

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