Difference of pallet mold between blow moulding and injection moulding

Time:2019-04-10 09:14:34 / Popularity: / Source:

Today we are talking about difference between blow moulding pallet and injection molded plastic pallets. Obviously we can literally see that difference between blow molded plastic pallets and injection molded plastic pallets lies in difference in production process. However, due to different production processes, final blow molded plastic pallets and injection molded plastic pallets are different in performance. Let's first understand pallet mold difference between blow moulding process and injection moulding process.
Blow moulding pallet
Injection molded plastic pallets
  • Difference between blow moulding process and injection moulding process:
1. Process is different, blow moulding mean injection + blowing; injection moulding mean injection + pressure; rotomolding mean extrusion + pressure; blow moulding must have head left by extraction of trachea, injection moulding must have a gate section, rotomolding must be cut without shaving
2. Generally speaking, injection moulding is a solid core, blow moulding and rotational molding are hollow cores.
Surface of injection molded parts is bright, but surface of blow moulding and rotomolding is uneven. Compared to rotomolding, blow moulding has a blowing port. This is a general comparison. I wonder if you can understand it!!!
3. Plastic shrinkage rate and its influencing factors
Characteristic of thermoplastics is that they expand after heating, shrink after cooling, and of course volume will shrink after pressurization. In injection moulding process, molten plastic is first injected into mold cavity. After filling is completed, molten material is cooled and solidified, shrinkage occurs when injection molded parts is taken out from pallet mold. This shrinking is called forming shrinkage. When injection molded parts are taken out from pallet mold until they are stable for a certain period of time, size will still change slightly. One change is to continue shrinking, this shrinkage is called post-shrinkage. Another variation is that some hygroscopic plastics swell due to moisture absorption. For example, when moisture content of nylon 610 is 3%, dimensional increase is 2%; when water content of glass fiber reinforced nylon 66 is 40%, dimensional increase is 0.3%. But main role is forming shrinkage.
At present, methods for determining shrinkage ratio (forming shrinkage + post-shrinkage) of various plastics are generally recommended in German national standard DIN 16901. That is, when mold cavity size is 23℃ ± 0.1℃ and placed after molding for 24 hours, difference between corresponding injection molded parts measured at a temperature of 23℃ and a relative humidity of 50 ± 5% is calculated. Shrinkage ratio S is represented by following formula: S = {(D - M) / D} × 100% ; (1), while: S - shrinkage ratio; D - mold size; M - plastic part size. If mold cavity is calculated according to known plastic part size and material shrinkage rate, it is D=M/(1+S). In order to simplify calculation in mold design, mold size is generally obtained by following formula: D=M+MS; (2), If a more accurate calculation is required, following formula is applied: D=M+MS+MS2; (3) However, when determining shrinkage rate, since actual shrinkage rate is affected by many factors, only approximate value can be used. Therefore, calculation of cavity size by formula (2) also basically satisfies requirements. When manufacturing mold, cavity is machined according to lower deviation, core is processed according to upper deviation, so that it can be properly trimmed if necessary. Main reason why it is difficult to accurately determine shrinkage rate is that shrinkage rate of various plastics is not a fixed value but a range. Because shrinkage rates of same materials produced by different molding factory are not same, even same batch of same material produced by a molding factory has different shrinkage rates. Therefore, each molding factory can only provide users with a range of shrinkage of plastics produced by its’ own plant. Secondly, actual shrinkage during forming process is also affected by factors such as shape of molded plastic pallets, mold structure and forming conditions.
In China, production of molded plastic pallets mainly consists of two processes: injection moulding and hollow blow moulding. Due to difference in production process and difference in production equipment, price of molded plastic pallets, suitability of turnover work, and service life of molded plastic pallets are different even if molded plastic pallets of these two processes are of same specification and same load for pallet mould maker.
  • Hollow blow moulding method:
Manufacturing process of hollow blow pallet mold makes up for shortage of weld molded plastic pallets due to large size and specifications. In domestic blow molded plastic pallets, large size specifications are more common. Moreover, production process of hollow blow moulding is relatively advanced, raw materials used are less than raw materials of injection molded plastic pallets of same specification, and will have advantageous advantages in future development of plastic pallets for pallet mould maker. However, due to limitations of manufacturing process, blow molded plastic pallets structure is mostly a two-way fork, which affects flexibility of use. However, with continuous advancement of production process, a series of plastic pallet with single-sided blow moulding structures have emerged, which can realize use of four-way forks for convenient turnover.
  • Injection moulding method:
It is also production process mainly used in domestic plastic pallet mould maker. Plastic pallet produced by injection moulding is characterized by smoothness, smoothness and reasonable structure. In general, molded plastic pallets for injection molded structures are mostly one-shot injection molded. Plastic pallet of this structure has strong impact resistance, is not easily bent and deformed. In some large-sized plastic pallet mold, such as 1600*1400 size plastic pallets, due to large size span, one-shot molds are difficult to achieve, which results in a double-sided welded plastic pallet. Moreover, in some molded plastic pallets with double-sided flat structures, welding pallet often appear. Unfavorable structure of such a pallet makes it easy to open weld during long-term frequent turnover operations.
  • Characteristics of blow moulding pallet:
1. Summary of features we see:
First: Appearance
A, Manufacturing process is limited, must be a double-sided pallet;
B. Manufacturing process is limited to two-way forks;
C. Intermediate support is a columnar structure with a small aperture, internal and surface cleaning is inconvenient;
D. Load-bearing is realized by continuous folding surface instead of rib position, which limits surface bearing capacity and bearing capacity of upper frame is small;
Second: Manufacturing aspects
A, Need secondary heating;
B. High material requirements - products are formed by a variety of high-strength HMWHDPE plastic compounded materials;
C. Because air pressure in blow moulding process is same as pressure radiation, it is impossible to completely control reinforcement of corners.
Third: Transportation
A. Double-sided pallets have high transportation costs, and empty storage space is large;
B. Load-bearing capacity of upper frame is small, and it is impossible to strengthen steel pipe.
2, Advantages seen by blow pallet mould maker (from network):
A. Blow molded plastic pallets is made of high-strength high-molecular-weight polyethylene (HMWHDPE). Plastic is the best environmentally friendly product because of its good viscosity, good toughness, aging resistance, corrosion resistance and pollution-free.
(Note: HMWHDPE has a long molecular weight with high price)
B. Safety performance is good. Surface of product adopts special anti-skid pattern design, which is safe and non-slip, and can be set with special rubber sliding pad to ensure stability of pallet stacking code. It does not support combustion, conduct electricity, generate static electricity, and it is non-slip, has no burr flash, has no other auxiliary connections to ensure safety of goods and operators.
(Note: There is no obvious difference from ordinary PE surface, and chemical properties are the same.)
C. Good environmental performance, non-toxic and tasteless products, corrosion resistance of acid and alkali salts, easy to clean and disinfect, no decay, and waste pallets can be recycled and meet environmental protection requirements. Appearance is bright and colorful, and can be selected according to customer requirements.
(Note: Chemical performance is no different from ordinary PE)
D, Strong carrying capacity. Because high-strength blow molded plastic pallets is reasonably designed according to force, and actual running condition is simulated by computer engineering analysis, so that bearing capacity of product is strong, evenly distributed bearing capacity is: static load 5~6t, dynamic load 1~ 2t, overhead load 0.7~1t.
(Note: Surface is thin and cannot withstand high pressure.)
E, Long service life. Product is formed by a variety of high-strength HMWHDPE plastic compounded materials, high strength, strong impact resistance, and improved outdoor life through a variety of material formulations. It can be used for more than 5 years under normal conditions; 2 to 3 years in harsh environments. Its service life is 5~7 times of wooden pallets, 2~3 times of steel pallets; 3~4 times of injection molded plastic pallets.
(Announcement: Life promotion is clearly fraudulent)
F. Low transportation costs. High-strength blow molded plastic pallets is light in weight and can greatly reduce cost of container cargo transportation. When conducting foreign trade, it is not necessary to carry out fumigation and disinfection by health and quarantine department like a wooden pallet.
(Note: Transportation cost is lower than most wooden pallets and injection molded plastic pallets)
G, Low maintenance costs. High-strength blow molded plastic pallets do not need to be repaired and maintained, even if appearance is damaged, as long as internal columns are well bonded, it will not affect normal use.
(Note: It can only be made in one piece, and welding cannot be effectively performed due to narrow cross-section performance.)

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