Impact of injection moulding process on product quality

Time:2019-10-23 18:16:40 / Popularity: / Source:

Injection moulding process parameters mainly include baking temperature and time, injection amount, metering stroke (pre-plastic stroke), residual material amount, anti-delay amount, screw rotation speed, back pressure, injection speed and pressure, holding pressure and time, barrel temperature, mold temperature, cooling time, opening and closing speed and pressure, ejection speed, gas-assisted blowing pressure and time.
injection moulding process 

Baking temperature and time

Because most plastic materials are hygroscopic, a small amount of water will be inhaled when placed in air. When moisture content in rubber is higher than a certain degree, there will be defects in quality of injection molding, such as silver thread, air bubbles, brittle fracture, mechanical properties and other defects. Therefore, it is necessary to dry rubber before injection molding. General material supplier will give recommended values of temperature and time of baking. However, drying time should not be too long, otherwise plasticity of plastic will deteriorate, which will lead to embrittlement of material. Some water-absorbent materials such as PA, PBT, PET, PEI and PSU are recommended to be dried by a dehumidifying dryer. Some materials with low hygroscopicity such as PP, PE, PVC, POM, etc., can be used without drying if sealed bag is unopened or stored in a dry environment.
Plastic material Permissible water content (%) Drying temperature℃ Drying time H Drying time limit H
PC 0.02 90~120 2~4 48
PA 0.04~0.08 85~120 2~4  
PC/ABS 0.04 80~110 2~4 8
ABS 0.1 70~90 2~4 8
PMMA   80~90 2~4  
PET 0.02 120~165 4  
PBT 0.03 150 2~4  
PPS 0.02 120~150 4~8  
PSU 0.05 130~150 4  
PPSU   150~170 4~8  
PEEK   150~180 2~3  
PEI 0.02 150 4  
TPU   100 3  
POM does not usually require drying if stored in a dry environment
PE does not usually require drying if stored in a dry environment
PP does not usually require drying if stored in a dry environment
PS If it is stored in a dry environment, usually does not require drying
PVC does not usually need to be dried if stored in a dry environment
Figure 1 Drying temperature and schedule

Injection volume

It reflects processing capacity of plastic injection molding machine and marks maximum weight of plastic products that can be produced. Formula for calculating injection amount is as follows:
injection moulding process 
injection moulding process 
Figure 2 Screw size structure diagram
Generally, when selecting an plastic injection molding machine, it must be considered that required amount of shot glue needs to be within 30% to 80% of maximum theoretical shot amount of machine.
injection moulding process 
Figure 3 Relationship between injection volume and screw injection

Metering stroke (pre-plasticized stroke)

After each injection instruction is terminated, screw is at the forefront of barrel. When pre-plastic command is issued, screw starts to rotate, material is delivered to screw head, screw retreats under back pressure of rubber until it touches limit switch. This process is called metering process or pre-plasticizing process, and distance that screw retreats is called metering stroke or pre-plasticized stroke. Therefore, volume occupied by rubber in the head of screw is metering volume formed by screw retreating, its metering stroke is injection stroke. Repeatability of metering stroke affects fluctuation of injection volume.

Remaining amount

After screw injection is completed, melt of screw head cannot be completely injected, and some need to be retained to form a residual amount. In this way, on one hand, mechanical collision accident of screw head and nozzle can be prevented; on other hand, repetitive precision of injection quantity can be controlled by residual material mat to achieve purpose of stabilizing quality of injection molded product (when residual amount is too small, then purpose of buffering is not achieved, and excess will be accumulated too much). General residual material amount is set to 5~2.5mm alarm.

Anti-delay (retraction)

Anti-delay means that after screw metering (pre-plasticizing) is in place, it is linearly retracted for a distance, so that internal pressure of melt in metering chamber is lowered to prevent melt from flowing out from metering chamber (through nozzle or gap). Another purpose of anti-casting is to reduce pressure of nozzle runner system and reduce internal stress; and it is easy to pull out rod when mold is opened. Setting of anti-delay depends on viscosity of plastic and condition of product. Excessive anti-delay will cause melt in metering chamber to be mixed with air bubbles, which seriously affects quality of product, and material with high viscosity may not have anti-delay. Amount of anti-delay is generally set to 1~2% of screw retraction stroke.

Screw speed

Screw speed affects thermal history and shearing effect of injection and plasticization of injection material in screw, which is an important parameter that affects plasticizing ability, plasticizing quality and molding cycle. As screw speed increases, plasticizing ability increases, uniformity of melt temperature and melt temperature increases.
Setting of screw speed is determined according to diameter of screw. Each plastic material will have maximum OD (outer diameter) line speed value, which is expressed in common units m/s. Range of conversion to screw speed is generally 30~120RPM. Maximum value is shown in Figure 31 below. For different plastic materials, material supplier will recommend screw speed setting in number of specifications. Generally for small screws, because depth of groove is relatively shallow, rubber absorbs heat quickly, which is enough to promote softening of rubber in compression section. In addition, friction heat betweenl screw and barrel is small, so a higher rotation speed can be used; On the contrary, it is not easy to use high rotation speed to avoid uneven plasticization, resulting in excessive frictional heat.
For heat-sensitive plastics (such as PVC, POM, etc.), low screw speed should be used to prevent material decomposition; for plastics with high melt viscosity (such as PC, PSF, PPO, etc.), low screw speed should also be used.

Back pressure

Reaction force exerted by melt on screw during plasticizing process of screw rotation back is called back pressure or plasticizing pressure, and its size can be adjusted by overflow valve in hydraulic system. When pre-molding, screw can only retreat after melt pressure of screw head is greater than resistance of cylinder.

Injection pressure

Injection pressure refers to pressure exerted by screw on the compound in barrel. Effect of injection pressure is to overcome flow resistance of plastic melt from barrel to mold cavity, to give melt a certain filling speed, compact and feed melt. Injection pressure has a great influence on degree of orientation of plastic product, length of stream, quality of product and shrinkage rate. At the same time, injection pressure is affected by characteristics of plastic, structure of product and mold, and other process parameters.
Injection pressure determines filling rate of plastic to a certain extent and affects quality of product.

Injection speed

Injection speed refers to speed at which screw advances forward, in units of mm/s. Usually, machine adopts multi-stage injection molding, that is injection speed is set in sections to control product quality by using different glue speeds in different parts of mold cavity.

Injection time

In order to obtain a product of uniform density and dimensional stability, compound must be quickly filled into cavity in a short time. Injection rate is proportional to screw advancement speed (injection speed) and inversely proportional to injection time. At present, injection rate requirement not only requires a high value, but also requires a graded injection during injection process to achieve reliable and effective control of flow state of rubber compound filling process according to material characteristics and product characteristics used.

Holding pressure

Holding pressure refers to process of compacting and feeding melt in mold after cavity is filled. Injection pressure at this stage is called holding pressure.
In actual production, holding pressure can be set equal to injection pressure, generally slightly lower than injection pressure. When holding pressure is high, shrinkage rate of product is reduced, surface finish, density are increased, weld line strength is increased, and product size is stable. Disadvantage is that residual stress in product is large when demolding, and overflow is easy to occur.

Holding time

Holding time is compaction and feeding time of plastic in cavity, which accounts for a large proportion in whole injection time; while shape is simple, holding time can be short. Before melt freezes at gate, length of dwell time has a great influence on the quality of product. If holding time is short, density of product is low, size is small, and shrinkage is likely to occur; if holding time is long, internal stress of product is large, strength is low, and mold release is difficult. In addition, holding time is also related to material temperature, mold temperature, main channel and gate size. If process parameters are normal and gating system is designed reasonably, time when fluctuation range of shrinkage rate of product is the smallest is optimum dwell time. Factors to be considered when determining dwell time are: variety and performance of plastic; conditions of product and mold; other injection molding process conditions, such as temperature, back pressure, injection pressure, injection rate, screw speed and other process parameters.

Barrel temperature

Barrel temperature refers to heating temperature of surface of barrel. Barrel is generally heated in three stages, from low to high in order from hopper to nozzle, so that plastic material is gradually melted and plasticized. The first section is feeding section, temperature is lower, bucket seat needs to be cooled by cooling water to prevent material from “bridge” and ensure high conveying efficiency; second section is compression section, which is compression state of rubber compound. Gradually melting, temperature setting of this section is generally 20 to 25℃ higher than melting point or viscosity fluidization temperature of plastic used; third section is metering section, material is in a molten state in this section, and a metering chamber is formed after pre-plastication is terminated. Store plasticized material, temperature setting of this section is generally 20 ~ 25℃ higher than second section to ensure that material is in a molten state.

Nozzle temperature

Nozzle has effect of accelerating melt flow and maintaining melt temperature. During injection moulding process, direct contact between nozzle and mold causes nozzle temperature to drop rapidly, causing melt to condense at nozzle to block nozzle hole or casting system of mold, and condensate is injected into mold to affect surface quality and performance of product, so nozzle temperature needs to be controlled.

Mold temperature

Mold temperature refers to cavity surface temperature in contact with article. For thermoplastic plastic melts, it is a cooling process because constant temperature of mold is lower than Tg of plastic or lower than heat distortion temperature (HDT). Only in this way can plastic be shaped and facilitate demolding.
Temperature of mold depends mainly on plastic properties (whether crystallized), structural size of product, performance requirements of product, and other process parameters (such as melt temperature, injection pressure, injection rate, molding cycle, etc.).

Opening/closing speed and pressure

Mold clamping speed generally needs to set two mold opening and clamping speeds. Mold is usually quickly closed first and clamped at a low speed before front and rear molds are contacted to avoid damage to mold.
Clamping force is set: low pressure causes front and rear molds to be brought together to protect mold from low pressure; then using high pressure clamping.

Ejection force and speed

When product is released from mold, a certain external force is required to overcome adhesion of product to mold, which is called ejection force. Ejection force is too small, product can not be removed from mold; if ejection force is too large, product will be warped and deformed, even product will be damaged.
In addition, ejection speed and ejection stroke also affect ejection process. Ejection speed is fast, product is easy to warp deformation and damage; ejection stroke is short, product is not easy to take off.
injection moulding process 

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