Molding process of plastic raw materials is mainly plasticization, mold filling, cooling and shaping to become a finished product. It is a process of heating and then cooling, and it is also a process of changing plastic from particles to different shapes. Following will explain processing process from perspective of each different stage:

 

Most of PP processing depends on screw to drive fluidity, so design of screw has a great influence, size of caliber affects output, compression ratio affects pressure value, also affects output and effect of finished product, which also includes a variety of materials (color masterbatch, additives and modifiers) mixing effect.
Flow of raw materials mainly depends on heater, but friction of raw materials will also generate frictional heat energy to accelerate fluidity, so small compression ratio of screw drives flow to be small, and speed must be increased, resulting in more frictional heat energy than screw with a large compression ratio.
Therefore, it is often said that there is no master in plastic processing, and person who understands performance of machine is master. Heating of raw materials is not only done by heater, but also frictional heat and residence time must be included. So this is a practical problem, experience helps production problem solving and efficiency. If screw needs to have a particularly good mixing effect, sometimes two-stage different screws or twin-shaft screws are designed and each section is divided into different types of screws to achieve various mixing effects.

Device heater (Heater) allows raw material particles to gradually melt into a fluid flow. It mainly adjusts temperature suitable for different raw materials. Increasing temperature will tend to speed up flow of raw materials, which can increase efficiency but not necessarily guarantee yield. A proper balance must be achieved.
In addition, good effect and high thermal cracking characteristics of PP are the best way to let raw materials flow smoothly to die head during production, so as to avoid phenomenon of insufficient filling or backflow. Backflow means that flow of raw materials is faster than output rate. It is one of methods available for processing to increase average flow efficiency equal to increase of MFR, but it also causes abnormal distribution of MFR, which may lead to increased instability and increased defective rate.
However, PP finished products are not products with high dimensional precision due to their application, so impact is not great.

 

Plastic reshaping depends on mold or die head. Injection molding product is three-dimensional, mold is more complicated and shrinkage rate must be considered. Others are flat, strip-shaped, and needle-shaped continuous product die heads. If it is a special shape, it is classified as special shapes, attention needs to be paid to cooling and shaping immediately.
Most of plastic machines are designed like injection syringes, and extrusion force driven by screw will cause huge pressure at small outlet, improving production efficiency. When die head is designed as a flat surface, how to distribute raw materials evenly on the entire surface, design of hanger die head is very important. Pay attention to extrusion opportunity to increase stable raw material supply of gill pump.

In addition to pouring raw materials into sprue gate, injection mold also has a cooling water channel to cool raw materials. Extrusion molding relies on cooling channel inside roller to achieve cooling effect. In addition, there are also air knives, cooling water is directly sprayed on blowing bag, hollow air blowing and other cooling methods.

Reprocessing and stretching of finished product will enhance effect. For example, speed of front and rear rollers of packing belt is different, which will cause stretching effect.
Molecular weight distribution will also affect elongation effect during high-speed production. All extruded products, including fibers, have varying elongation. Vacuum and pressure forming can also be regarded as another form of elongation.

Any raw material has problem of shrinkage. Shrinkage is caused by thermal expansion and contraction and internal stress caused by crystal formation. Generally speaking, thermal expansion and contraction are easier to overcome, processing can be done by extending cooling time and keeping pressure continuously. Crystalline raw materials tend to have greater shrinkage differences than non-crystalline materials.
In terms of PP, it is about 16/1000, but ABS is only about 4/1000. Difference is very large. This part needs to be overcome on mold, or additives to reduce shrinkage are often added to overcome it, and LDPE is often added to extruded flat plate to improve problem of necking.

 

Automobile industry (mainly using PP containing metal additives: fenders, ventilation pipes, fans, etc.), appliances (dishwasher door gasket, dryer ventilation pipe, washing machine frame and cover, refrigerator door gasket, etc.) consumer products (lawn and garden equipment such as lawnmowers and sprinklers, etc.).

 

Fault analysis and troubleshooting methods:
(1) Improper control of process conditions. Should be adjusted appropriately.
(2) Injection capacity of injection molding machine is less than weight of plastic part. It should be replaced with a larger size injection molding machine.
(3) Runner and gate cross section is too small. It should be increased appropriately.
(4) Flow distance of molten material in mold cavity is too long or there are thin-walled parts. Cold slug holes should be set.
(5) Mold exhaust is poor, and residual air in cavity leads to insufficient injection. Exhaust system of mold should be improved.
(6) Fluidity of raw material is too poor. A resin with better fluidity should be used instead.
(7) Temperature of barrel is too low, injection pressure is insufficient or injection time of feed is too short, which will also cause under-injection. Control amount of relevant process parameters should be increased accordingly.

Fault analysis and troubleshooting methods:
(1) Insufficient clamping force. It should be replaced with a larger injection molding machine.
(2) Pin holes or guide pins of mold are severely worn. Repairs should be done by machining.
(3) There are foreign matter and impurities on clamping surface of mold. Should be cleared.
(4) Mold temperature or injection pressure is too high. should be reduced appropriately.

Fault analysis and troubleshooting methods:
(1) When mold runner and gate size of thick-walled plastic parts are small, it is easy to produce surface pores. Runner and gate size should be enlarged appropriately.
(2) Wall of plastic part is too thick. Wall thickness should be minimized in design.
(3) Too high molding temperature or too low injection pressure will cause pores on the surface of plastic parts. Molding temperature should be appropriately lowered and injection pressure should be increased.

Fault analysis and troubleshooting methods:
(1) Melting material and mold temperature are too low. Barrel and mold temperature should be appropriately high.
(2) Injection speed is too slow. Injection speed should be appropriately accelerated.
(3) Nozzle aperture is too small. A nozzle with a larger aperture should be used instead.
(4) There is no cold slug hole in mold. Cold feed holes should be added.

Fault analysis and troubleshooting methods:
(1) Moisture and volatile matter content in molding raw material is too high. Raw materials should be pre-dried.
(2) Mold exhaust is poor. Vent holes should be added to improve vent performance of mold.
(3) Nozzle is in poor contact with mold. Position and geometric size of the two should be adjusted.
(4) When silver wire always appears in a certain position, it should be checked whether there is any surface scar on the surface of corresponding cavity. If there is a double reflection of surface scars, machining methods should be used to remove surface scars of cavity.
(5) When different types of resins are mixed, silver streaks will be produced. Mixing of dissimilar resins should be avoided.

Fault analysis and troubleshooting methods:
(1) Melting material and mold temperature are too low. Barrel and mold temperature should be increased.
(2) Gate position setting is unreasonable. Gate location should be changed.
(3) Content of volatile matter in raw material is too high or mold exhaust is poor. Volatile substances in raw materials should be removed and exhaust system of mold should be improved.
(4) Injection speed is too slow. It should be accelerated appropriately.
(5) There is no cold slug hole in mold. Cold feed holes should be added.
(6) There are foreign matter impurities on the surface of cavity. Cleaning should be done.
(7) Gating system design is unreasonable. Mold filling performance of gating system should be improved to make melt flow smoothly in cavity.

Fault analysis and troubleshooting methods:
(1) Specifications of injection molding machine are too large. Injection molding machine with smaller specifications should be replaced.
(2) Fluidity of resin is poor. An appropriate amount of external lubricant should be used.
(3) Injection pressure is too high. Should be reduced appropriately.
(4) Poor exhaust of mold. Exhaust system of mold should be improved, air holes should be increased or mosaic structure should be adopted, and clamping force should be appropriately reduced.
(5) Gate position setting is unreasonable. Position of gate should be changed to make molten material in cavity flow evenly. ,

Fault analysis and troubleshooting methods:
(1) Gate and runner size are too small. It should be increased appropriately.
(2) Injection pressure is too low. should be raised appropriately.
(3) Moisture content in raw material is too high. Raw materials should be pre-dried.
(4) Wall thickness of plastic parts varies too much. Physical structure of plastic parts should be designed reasonably to avoid rapid changes in wall thickness.

Fault analysis and troubleshooting methods:
(1) Melting material and mold temperature are too low. Barrel and mold temperature should be increased.
(2) Structural design of gating system of mold is unreasonable. Mold runner and gate structure should be improved so that molten material does not generate turbulent flow when filling mold.
(3) Cooling time is too short. Cooling time should be extended appropriately.
(4) Design of ejector device for demoulding is unreasonable. It is best to use a pneumatic demoulding device.
(5) Injection speed and pressure are too high. should be reduced appropriately.

Fault analysis and troubleshooting methods:
(1) Mold temperature is too high or cooling is insufficient. Mold temperature should be appropriately reduced or cooling time should be extended. For slender plastic parts, method of cooling after mold is fixed can be adopted.
(2) Uneven cooling. Cooling system of mold should be improved to ensure that plastic parts are cooled evenly.
(3) Gate selection is unreasonable. A reasonable gate form should be selected according to specific situation. In general, multi-point gates can be used.
(4) Core of mold is eccentric. Checks and corrections should be made.

Fault analysis and troubleshooting methods:
(1) Injection speed and pressure are too high. should be reduced appropriately.
(2) Surface finish of mold cavity is too poor. Surface finish should be improved by grinding and electroplating.
(3) Improper control of mold temperature and cooling conditions. When plastic part sticks to mold core, mold temperature should be increased and cooling time should be shortened; if plastic part is stuck to cavity surface, mold temperature should be lowered and cooling time should be prolonged.
(4) Eection area of demoulding mechanism is too small. Ejection area should be increased.

Fault analysis and troubleshooting methods:
(1) Insufficient pressure holding. Injection time of feed should be extended appropriately.
(2) Insufficient injection pressure. should be raised appropriately.
(3) Mold temperature is too high. should be reduced appropriately.
(4) Gate cross-sectional area is too small. It should be increased appropriately.
(5) Pocessing temperature is too low. Barrel temperature should be increased appropriately.

Fault analysis and troubleshooting methods:
(1) Insufficient pressure holding. Injection time of feed should be extended appropriately.
(2) Mold temperature is too low and barrel temperature is too high. Mold temperature should be increased appropriately and barrel temperature should be lowered.
(3) Insufficient injection pressure. should be raised appropriately.
(4) Fluidity of raw material is too good. A resin with a lower melt index should be used instead.

 

Long glass fiber reinforced polypropylene (PP) parts are typically made from injection molded long glass fiber pellets. A novel one-step process compounds polypropylene and glass fibers together to produce injection molded parts directly. The two methods have their own characteristics, and which method to adopt should be determined according to characteristics of component production.
In automotive engineering, dashboards, front-end parts and underbody elements are increasingly made of glass-fiber-reinforced polypropylene. Polypropylene has characteristics of low density, low material cost, and easy recycling. Therefore, it is gradually replacing engineering plastics and metals in above application fields. However, polypropylene can only meet mechanical specifications if long glass fiber reinforcement enhances elastic modulus and impact strength.
Components are made of injection molded or compression molded glass fiber reinforced PP. In compression molding process, starting material is usually a semi-finished sheet made of glass mat thermoplastic (GMT) reinforced PP. Due to long and isotropic nature of its fibers, traditional GMT compression molding usually produces parts with excellent mechanical properties. However, GMT production process is very complicated. Therefore, cost of semi-finished products is relatively high.
With the latest technological developments, it is now possible to compound PP and glass fibers in-line followed by direct compression molding. With development of technology, compression molding has many disadvantages compared with injection molding. In many cases, components must be reworked. Openings in compression molded parts can usually only be formed in a downstream stamping process. This creates waste and increases overall costs.

A good bond between fibers and substrate is critical for mechanical properties of part. GMT provides higher strength and impact strength than direct processing molding compounds and long glass fiber pellet excellent bonding of fibers, fiber filaments and uniform distribution of fila resulting needle felt structure offers several advantages.
However, this advantage is lost if flow path is too long during compression molding compared to molding compounds that are injected directly or through long-fiber pellets. Since injection molding creates fiber orientation in the part, lack of needling performance can be partially offset if resulting stresses are properly engineered.
Processing method is now concluded based on damage of fiber structure in composite material. Fiber structure damage includes fiber breakage, fiber debonding, fiber pull-out and other forms. To fully utilize fiber strength, it must be ensured that fiber length is longer than so-called critical fiber length.
For fiber/substrate composites made of PP and glass, corresponding literature values for critical fiber length (LC) range from 1.3 mm to 3.1 mm. With special coupling agents, values up to 0.9 mm can be produced.
Quality of fibrous substrate coupling can be inferred from ratio between actual fiber length and critical fiber length. If actual fiber length of part is greater than critical fiber length, fibers are prone to breakage. If critical fiber length is below, fiber pullout may occur.
This primarily refers to breakage at fiber/substrate interface, which typically occurs in chopped fiber compounds where fiber length is typically 0.2 mm to 0.6 mm.
Strictly speaking, length of remaining reinforcing fibers in fiber is independent of design. For component design, mechanical properties such as strength, rigidity, and impact strength are more important. Although these properties are part of function of fiber length, relationship is very complicated.
Therefore, fiber length analysis alone can only achieve so far, but is indeed a very useful parameter for obtaining trend information.

During processing of long glass fiber reinforced polypropylene, it must be ensured that the longest fibers are incorporated into part so that the best mechanical properties can be produced in composite.
However, there is currently no reliable way to avoid fiber breakage when mechanical stress is applied, resulting shortening of fibers during compounding and injection molding cannot be avoided. The most damage to fibers occurs when fiber-containing melt is injected the mold.
However, proper design can reduce magnitude of fiber shortening. At the same time, melting process also greatly affects length of fibers. In this regard, injection molding machines and injection molding compounding machines (IMCs) are very different.
When processing with an injection molding machine, initial fiber length is limited by size of pellets (typically 10 mm to 25 mm). Long glass fiber manufacturers offer jacketed pellets and pultruded pellets. In pultruded pellets, fibers are wetted by substrate in a melt bath and then assembled into small bundles.
This way substrate soaks all fibers evenly. In sheathed pellets, fibers and substrate are coextruded together. Injection molding machine must dissolve mass of fibers during melt process and then thoroughly infiltrate fibers with substrate.
During melting process, degree of fiber damage decreases with decrease of flow resistance. When cross-section of the runner is larger, damage to fiber is less. Therefore, when processing long glass fiber pellets, screw configuration and check valve should be improved accordingly.
When pellets are injection molded, fibers go through the entire melting process. Mechanical stress will be maintained on fibers for a considerable time. Plasticization begins with high forces on the fibers because substrate is not fully melted at this stage. Some fibers are sandwiched and subjected to enormous shear forces.
In contrast, injection molding compounders melt pure substrate without fibers. Fibers are added after base material has melted and are therefore subject to less mechanical stress.
This method has less damage to fibers than injection molding machine melting, and average length of fibers increases. Endless rovings (not chopped strands) can be mixed directly into melt with an injection molding compounder. Although rotation of screw breaks roving into shorter pieces, final length of fiber is relatively long.

Price of starting material is key to production of fiber-reinforced PP components. Compared with GMT semi-finished products, price of long glass fiber pellets for injection molding is lower. However, processors pay higher prices for pellets than when purchasing individual components. One of main advantages of injection molding compounders for secondary processors is that starting material prices are lower than long-fiber pellets, and material costs are a reduced percentage of part production costs.
Processing glass fiber reinforced polypropylene as pellets on an injection molding machine requires a smaller investment than on an injection molding compounder. However, it is also possible to modify or replace plasticizing unit on an existing injection molding machine to make it suitable for processing long glass fiber pellets. Even if retrofitting is not possible and new machinery must be installed, investment required for injection molding is relatively low. Adding twin-screw extruder required for an injection compounder only complicates equipment.

In addition to above-mentioned advantages regarding uniform distribution of fiber lengths in the part, injection molding compounders offer cost savings in terms of starting materials, however, this potential can only be realized with additional investments.
Therefore, choice of injection molding machine or injection compounding machine should be determined by weight and output of produced parts. Injection molding compounders offer advantages if output is high.
Reason is that cost savings in purchasing starting materials can quickly exceed basic investment in purchase of equipment, so amortization of investment is done in a short period of time. If part is small and production volume is low, it is more cost-effective to use injection molding machine for long glass fiber processing, because investment of injection molding machine is small.
Injection molding compounders increase processors' production flexibility, allowing materials to be tailored to specific requirements. Processors can modify substrate/fiber/sizing system so that fiber content of part is fully in line with requirements of each specification. When processing pellets, this optional modification is only possible under certain conditions because manufacturer only supplies pellets with a certain fiber content.
To change fiber content for conventional injection molding, processors must mix long glass fiber pellets with unreinforced polypropylene, a step that places additional demands on machinery and material supply systems.
However, since injection molding compounders offer processors a higher degree of freedom in terms of material composition, this increases product liability and processor liability. Processors now have to assume responsibility for quality assurance, in addition to guarantees previously imposed on pellet manufacturers. However, there is also a huge future opportunity here. With injection molding compounders, processors can realize substantial added value.
In summary, long glass fiber reinforced PP parts can be produced by modified injection molding machines and compounding machines, which one is better depends on production conditions. Advantage of injection molding compounding is that initial raw material investment is lower, damage to fiber is less, and longer fiber can be retained, but it is more suitable for parts with high output and high mechanical specifications. If part size is small and output is small, it is better to use a traditional injection molding machine.