Polypropylene is one of the most used varieties of general-purpose plastics. It has advantages of low density, good rigidity, flexural resistance, chemical resistance, and good insulation. Its shortcomings are poor low-temperature impact performance, easy aging, and large molding shrinkage.
Modification can improve low-temperature impact performance, molding shrinkage and thermal aging properties of polypropylene. Use of polypropylene is greatly expanded, replacing traditional engineering plastics in many occasions. Polypropylene has sufficient raw materials and low prices. In recent years, polypropylene modification has taken the first place in plastic modification industry and has become main variety of modified plastics, which has attracted more and more attention.
Shrinkage control of polypropylene modified material is an important aspect of polypropylene modification. Good shrinkage control is of great significance to promotion and use of polypropylene modified materials, it is also an important aspect of ensuring product quality. Especially use of modified polypropylene to replace traditional engineering plastics, shrinkage is very important.
Polypropylene modification has mature technology in China, and there are many research reports on physical and chemical properties of polypropylene modification, but there are few special reports on shrinkage rate. Based on many years of practical experience, I have made some discussions on shrinkage control of polypropylene modified materials.

Polypropylene (PP) Liaoyang Petrochemical Fiber Corporation;
High-density polyethylene (HDPE) Liaoyang Petrochemical Fiber Corporation
POE DuPont, USA;
EPDM Dutch DSM company;
SBS Yueyang Petrochemical Plant
Fiberglass Shanghai Yaohua;
Calcium carbonate Yingkou Dashiqiao;
Talcum powder Haicheng Jinxin
Mica powder Hebei;
Low-density polyethylene (LDPE) Yanshan Petrochemical

Extruder TM40MVC/D-40 Italy MARIS;
Injection molding machine TP120T Beijing Xinguan Machinery Manufacturing Co., Ltd.
Melt index meter μPXRZ-400C Jilin University Science and Education Instrument Factory;
Calipers;
Testing method: ASTM D955

Raw material mixing----extrusion granulation----injection proofing (placed for 24h)----shrinkage detection (ambient temperature is 23℃)
Injection conditions: temperature 170℃---190℃ pressure 80

Shrinkage molding of polypropylene is a major disadvantage of polypropylene itself, which is mainly due to high crystallinity of polypropylene. Specific gravity of crystallized polypropylene increases and volume decreases. When crystallinity is 0% and 100%, specific gravity is 0.851 and 0.936, respectively. Therefore, molding shrinkage of pure PP is generally between 1.7 and 2.2. Controlling molding shrinkage rate of polypropylene is mainly to control crystallinity of its raw material during molding: the smaller crystallinity, the smaller molding shrinkage rate; conversely, the higher crystallinity, the greater molding shrinkage rate.
In polypropylene modified plastics, addition of various modifiers destroys original crystallinity of polypropylene to varying degrees, thereby changing original molding shrinkage rate of polypropylene.

As rubber content increases, molding shrinkage rate shows a downward trend. This is mainly due to fact that addition of rubber destroys crystallinity of polypropylene itself, which leads to a decrease in molding shrinkage rate, three elastomers POE, EPDM, and SBS have different effects on molding shrinkage rate.

Mineral additives used for polypropylene mainly include calcium carbonate, talc powder, mica powder and so on. Influence of various mineral additives on molding shrinkage rate of polypropylene. It can be seen that influence of mineral additives on molding shrinkage rate of PP modified materials is more obvious. Influence of mineral additives on shrinkage rate of polypropylene modified materials mainly has three aspects: one is that mineral additives themselves do not shrink, its addition reduces shrinkage rate of polypropylene modified materials from overall ratio; second, addition of mineral additives will inevitably affect crystallinity of polypropylene, thereby affecting shrinkage rate; third, after addition of fine mineral agents, it acts as a nucleating agent, changing structure of polypropylene, preventing formation of large spherulites, and also affecting molding shrinkage rate of polypropylene.

Glass fiber has the greatest impact on molding shrinkage of polypropylene modified materials. When content of glass fiber reaches more than 30%, molding shrinkage rate of polypropylene modified material decreases from 1.8 to 0.5, surface-treated glass fiber has a greater impact on molding shrinkage rate than untreated glass fiber. Addition of glass fiber destroys crystallinity of polypropylene and affects shrinkage rate. More importantly, glass fiber restricts crystal shrinkage of polypropylene.

Addition of polyethylene also affects molding shrinkage of polypropylene modified materials. Although polyethylene is also a kind of plastic with high crystallinity, molding shrinkage rate is also very large, but after adding polypropylene to each other, crystallinity of each other is destroyed to different degrees, so that overall molding shrinkage rate is reduced.

Molding shrinkage rate of polypropylene is affected by its crystallinity, and crystallinity is affected by its own molecular weight. When MFI increases, molecular weight decreases, crystallization speed increases, and molding shrinkage rate increases. Influence of polypropylene MFI on mold shrinkage.

Influence of various modifiers on molding shrinkage rate of polypropylene modified materials is shown in Table 1. It can be seen from table that various modifiers have different effects on molding shrinkage of polypropylene modified materials, and glass fiber has the greatest impact.
Molding shrinkage rate of polypropylene modified material is different due to different modifiers added. In experiment, we found that when several modifiers are used together, effect is different, and there are many influence factors. Such as degree of dispersion of modifier in polypropylene, particle size of mineral modifier itself, and surface treatment of various mineral additives, all have an impact on mold shrinkage. In addition, molding process of injection molding machine has a great influence on molding shrinkage during molding process. For example, changes in injection temperature and injection pressure have an effect on mold shrinkage.

3.1 Molding shrinkage rate of polypropylene modified material is smaller than that of pure polypropylene, which is basically between 40% and 70% of pure polypropylene. Various modifiers have different effects on molding shrinkage of polypropylene modified materials. Order of influence is roughly as follows: glass fiber>mineral>elastomer>polyethylene.
3.2 Changing crystallinity of polypropylene is key to controlling molding shrinkage of polypropylene modified materials.
3.3 Control of molding shrinkage is a complicated process. It is necessary to ensure product quality and control shrinkage. Several aspects need to be considered at the same time.
Shrinkage of plastic parts (surface shrinkage and internal shrinkage) are all defects caused by insufficient melt replenishment when thicker parts are cooled. We often encounter situations where shrinkage problem cannot be solved by increasing pressure, increasing water inlet, and prolonging injection time.
Among commonly used raw materials, due to fast cooling rate, shrinkage problem of PC materials is the most difficult to solve, shrinkage and shrinkage problems of PP materials are also more difficult to deal with.
Therefore, when encountering serious shrinkage problems of thick and large parts, some unconventional injection molding techniques need to be adopted, otherwise it will be difficult to solve problem. In practical production, we have explored a set of more effective techniques to deal with this difficult problem of injection molding.
First of all, under premise of ensuring that injection molded parts are not deformed, method of shortening cooling time as much as possible is adopted to make injection molded parts come out of mold early at high temperatures. At this time, temperature of outer layer of injection molded part is still high, and skin is not too hardened, so temperature difference between inside and outside is relatively small, which is conducive to overall shrinkage, thereby reducing concentrated shrinkage inside injection molded part. Since overall shrinkage of injection molded part is constant, the more overall shrinkage, the smaller concentrated shrinkage, degree of internal shrinkage and surface shrinkage can therefore be reduced.
Next, if it is to solve shrinkage problem, it will be discussed below. As for improvement of surface shrinkage problem, method of solving shrinkage cavity after injection molded part is out of mold at high temperature is different.
Shrinkage problem occurs because mold surface heats up and cooling capacity decreases. Surface of injection molded part that has just been solidified is still soft (unlike PC parts that are harder after being ejected from mold, which is easy to produce shrinkage holes), internal shrinkage cavity that has not been completely eliminated is due to formation of a vacuum, which causes surface of injection molded part to be compressed inward under pressure of atmospheric pressure. At the same time, shrinkage force is added to cause shrinkage problem. And the slower surface hardening speed, the easier it is to produce shrinkage, such as PP material, and vice versa.
Therefore, after injection molded part is ejected from mold early, it must be properly cooled to keep surface of injection molded part to a certain degree of hardness, so that it is not easy to produce shrinkage. However, if shrinkage problem is more serious and moderate cooling cannot be eliminated, it is necessary to take method of freezing water chilling to quickly harden surface of injection molded part to prevent shrinkage, but internal shrinkage holes will still exist. For materials with a softer surface like PP, due to effect of vacuum and shrinking force, injection molded parts may still have shrinkage, degree of shrinkage has been greatly reduced.
While taking above measures, if method of extending injection time instead of cooling time is used, improvement of surface shrinkage and even internal shrinkage will be better.
When solving shrinkage problem, because mold temperature is too low, it will increase degree of shrinkage. Therefore, it is best to cool mold with machine water. Do not use frozen water. If necessary, mold temperature should be increased. When temperature rises to 100 degrees, improvement effect of shrinkage will be better. But if it is to solve shrinkage problem, mold temperature cannot be raised, but needs to be lowered.
Finally, sometimes above methods may not completely solve problem, but it has been greatly improved. If problem of surface shrinkage must be completely solved, adding an appropriate amount of anti-shrinking agent is also a last resort. Of course, transparent parts cannot do this.
If there are still sink marks on the surface of thick-walled parts, or plastic parts such as offset walls are encountered, introduction of gas-assisted injection molding will be resolved.
Gas-assisted injection molding is a novel plastic molding technology that introduces high-pressure gas into thick-walled part of part to produce a hollow section inside injection part, complete filling process, realize gas pressure retention, and eliminate product sink marks.
Traditional injection molding process cannot combine thick and thin walls to form, parts have high residual stress, are easy to warp and deform, sometimes have sink marks on the surface. Newly developed gas-assisted technology successfully produces thick-walled and partial-walled products by hollowing out thick-walled interior, products have excellent appearance and surface properties, low internal stress, light weight and high strength.
Structure and mold design of gas-assisted products have been successfully developed, including gating system, air intake mode and air channel distribution design technology, gas-assisted injection molding process design technology, gas-assisted injection molding process design technology, gas-assisted injection molding process computer simulation technology, gas-assisted injection molding product defect diagnosis and elimination technology, gas-assisted process special material technology.
Televisions, home appliances, automobiles, furniture, daily necessities, office supplies, toys, etc. have opened up new application areas for plastic molding. Gas-assisted injection molding technology is particularly suitable for pipe-shaped products, thick walls, partial walls (parts composed of sections with different thicknesses) and large flat structural parts.
Gas auxiliary equipment: including nitrogen generation and booster system, pressure control unit and air intake components. Gas-assisted process can be fully integrated with traditional injection process (injection molding machine).
Reduce weight of products (saving materials) by 40%, shorten molding cycle, save time by 30%, eliminate sink marks, improve yield; reduce injection pressure by 60%, and small tonnage injection molding machines can be used to produce large parts, reducing operating costs ; mold life is extended, manufacturing costs are reduced, and more stable structures such as thick roots, thick ribs, and connecting plates can also be used, which increases freedom of mold design.

Shrinkage of hard plastic parts (surface shrinkage and internal shrinkage) are all defects caused by lack of sufficient space left by concentrated shrinkage when melt is cooled and contracted by melt from direction of water inlet. Therefore, factors that are not conducive to shrinkage will affect us to solve shrinkage problem.
Most people know that too high mold temperature is prone to shrinkage, and they usually like to lower mold temperature to solve problem. But sometimes if mold temperature is too low, it is not conducive to solving shrinkage problem, which is not noticed by many people.
Mold temperature is too low, melt is cooled too fast, and the thicker glue position far away from water inlet is blocked by feeding channel due to too fast cooling of middle part, melted glue cannot be fully replenished far away, resulting in shrinkage problems. It is more difficult to solve, and shrinkage problem of thick and large injection molded parts is particularly prominent.
Moreover, mold temperature is too low, it is not conducive to increase overall shrinkage of injection molded parts, so that amount of concentrated shrinkage increases, shrinkage problem becomes more serious and obvious.
Therefore, it is beneficial to remember to check mold temperature when solving more difficult shrinkage problem. Experienced technicians usually touch surface of mold cavity with their hands to see if it is too cold or too hot. Each raw material has its suitable mold temperature. For example, shrinkage hole problem of PC material, if hole will be better improved, but if mold temperature is too high, injection molded part will shrink again.

Most people also know that if melt temperature is too high, injection molded parts are prone to shrinkage. If temperature is appropriately lowered by 10-20℃, shrinkage problem will be improved.
However, if injection molded part shrinks in a thicker part, adjust melt temperature too low, for example, when it is close to lower limit of injection melt temperature, it will not help to solve shrinkage problem, and it will even be more serious. The thicker piece, the more obvious situation.
Reason is similar to that mold temperature is too low. Melt condenses too quickly, and a large temperature difference conducive to feeding cannot be formed from shrinking position to nozzle. Feeding channel at shrinking position will be sealed prematurely, solution of problem becomes more difficult. It can also be seen that the faster condensation speed of melt is not conducive to solving shrinkage problem. PC material is a material that condenses quite quickly, so its shrinkage problem can be said to be a big problem in injection molding.
In addition, too low melt temperature is also not conducive to increasing amount of overall shrinkage, resulting in an increase in amount of concentrated shrinkage, thereby exacerbating shrinkage problem.
Therefore, when adjusting machine to solve more difficult shrinkage problem, it is also extremely important to check whether melt temperature is adjusted too low. In addition to looking at thermometer, it is more intuitive to check temperature and fluidity of melt by air injection. .

To solve shrinkage problem, the first thing that comes to mind is to increase injection pressure and prolong injection time. But if injection speed has been adjusted very fast, it is not conducive to solving shrinkage problem. Therefore, when shrinkage is difficult to eliminate, it should be solved by reducing injection speed.
Reducing injection speed can make a larger temperature difference between molten glue walking in front and water inlet, which is conducive to solidification and feeding of molten glue from far to near, and it is also conducive to shrinkage position far away from nozzle to obtain a higher pressure supplement, which will greatly help solve problem.
As injection speed is reduced, temperature of melting glue in the front is lower, speed has been slowed down, injection molded parts are not easy to produce a sharp front, injection pressure and time can be increased and longer, which is more conducive to solving problem of serious shrinkage.
In addition, effect will be more obvious if last-stage end filling with slower speed, higher pressure, longer time, pressure holding method of slowing down and increasing pressure step by step are used. Therefore, when it is not possible to shoot at a slower speed from beginning, it is also a good remedy to use this method from later stage of injection.
However, it is worth reminding that filling is too slow and it will not help to solve shrinkage problem. Because when cavity is filled, melt has been completely frozen, just like temperature of melt is too low, there is no ability to feed shrinkage in distance.