TPE material injection molding is prone to problems and solutions!
TPE injection molding process
refers to the processing of the materials to synthesize polymers. Appearance is affected by many factors, in its production process to strictly control the production process, to ensure that each link will not fail, especially in the temperature control to pay extra attention. In this article, we will introduce the problems that easily occur in TPE material injection molding and the solutions.
I. Sticky phenomenon
Compared with resin, the surface of soft TPE injection molding products
is more likely to produce a sticky phenomenon. This can be done by lowering the molding temperature. In this case, we can reduce the molding temperature, low shear, and purify the inside of the hydraulic cylinder of the molding machine with nitrogen to solve the sticky phenomenon on the surface of the product. In addition, when the molding machine is suspended, the adhesive is left in the mold cavity of the molding machine for a long time in a molten state, and sometimes it will become sticky due to heat aging. In addition, products used at high temperatures are prone to sticky phenomena. Therefore, a reasonable choice of stabilizer. Softening agent type selection and dosage.
TPE injection molding products
are also prone to aging, usually through the combination of heat resistance. Weatherability and other stabilizers, by adding ultraviolet absorbers. The light stabilizer method, to a certain extent, can inhibit the aging phenomenon. In some cases, aging as scorching occurs in the part not filled to the end and not filling the mold cavity. This can be solved by improving the exhaust method.
III. Concave hole. Different color
The phenomenon of concave holes often occurs in TPE injection molded products
. For such products, it is necessary to adopt a model design with a glue injection port or a flow channel around it. To solve the problem of the uneven color of molded products, it is necessary to increase the backpressure of the screw and strengthen the mixing when filling.
Sometimes the resin overflows from the mold cavity, resulting in business. In this case, the filling amount should be reduced first. Lowering the holding pressure shortens the holding time and pays attention to repair.
V. Poor mold release
Poor mold release means that the molded products are difficult to remove from the mold or are completely deformed during the removal process. Materials with adhesion are prone to this problem, but it can be improved by adding a release agent to the material or by applying a release agent to the mold before molding. Insufficient cooling of the molded product (insufficient curing) can also cause this problem, so adequate cooling of the molded product is essential. In addition, poor mold design can be the cause of difficult mold release, especially in the injection port. Especially in the parts of the mold that are easy to stick to, such as the injection port, the inlet channel, etc., increase the extraction angle of the injection port. Widen the plastic inlet channel is very effective.
VI. Flowing traces
The phenomenon of streaks of different glosses on the surface of molded products. Generally speaking, in resin injection molding, there are.
(1) Narrowly spaced record streaks;
(2) Wide streaks of the same phase on the top and bottom of the molded part;
(3) Narrower spacing streaks on the surface of the molded product with ectopic phases.
Solution: These methods can be used to solve the problem, such as adding pure monomer resin. Increase the injection speed. Mold temperature. Increase the injection port. Increase resin temperature and injection rate. Increase the molding temperature. Mold temperature or reduce the injection speed, etc. Increasing the injection rate and mold temperature is effective.
VII. Uneven color
When using TPE pellets and dry blends as pigment master mixes for coloring, it is easy to have the uneven color of the molded products. As a countermeasure, it is effective to increase the backpressure of the screw and strengthen the mixing of the filler.
VIII. Whitening phenomenon
When TPE is used in a general environment, the problem of whitening rarely occurs, but in high temperatures. Humidity. However, in the case of high temperature, humidity, and long-term outdoor use, it is necessary to add heat-resistant stabilizers and weather-resistant stabilizers to improve durability. It is necessary to add heat-resistant stabilizers and weather-resistant stabilizers to improve durability. Especially under high temperatures, because it is very easy to cause migration, the choice of stabilizer is also very important.
Form and calculation of shrinkage in thermoplastic molding As mentioned above, the factors affecting shrinkage in thermoplastic molding are as follows.
Plastic species thermoplastic molding process because there is also the crystallization of the volume change in the shape, internal stress, frozen in the plastic parts of the residual stress, molecular orientation and other factors, so compared with thermosetting plastics, the shrinkage rate is larger, the shrinkage rate range is wide. In addition, the shrinkage after molding is obvious. The shrinkage rate after annealing or moisture treatment is generally larger than that of thermosetting plastics.
The mold design is based on the shrinkage range of various plastics, the wall thickness of the plastic part, the shape of the plastic part, and the size of the feed opening. The shrinkage rate of each part of the plastic part is determined empirically according to the shrinkage range of various plastics, wall thickness, shape, inlet size, and distribution, and then the cavity size is calculated. For high-precision plastic parts and difficult to grasp shrinkage rate, it is generally appropriate to use the following methods to design the mold.
①Take a smaller shrinkage rate for the outer diameter of the plastic part and a larger shrinkage rate for the inner diameter to leave room for correction after the trial mold.
②Test mold to determine the form of the casting system. Size and molding conditions.
③To, post-process the plastic parts by post-processing to determine the size change (measurement must be after 24 hours after demolding).
④Correct the mold according to the actual shrinkage.
⑤ Try the mold again and slightly correct the shrinkage value by changing the process conditions appropriately to meet the requirements of the plastic part.
The size of thermoplastic fluidity, generally from the molecular weight size. Melt index. Archimedes helix flow length. Performance viscosity and flow ratio (flow length / plastic wall thickness) and a series of indices to analyze. Small molecular weight, wide molecular weight distribution, poor molecular structure regularity, high melt index. The length of the spiral flow is long. If the viscosity is small and the flow ratio is large, the fluidity is good. For the same name of plastic, we must check its manual to determine whether its fluidity is suitable for injection molding. According to the requirements of mold design, the fluidity of commonly used plastics can be roughly divided into three categories.
PA, PE, PS, PP, CA, poly(4)methyl garcinia;
② Medium flowability Polystyrene series resins (such as ABS. AS). PMMA. POM. polyphenylene ether;
PC. hard PVC. polyphenylene ether. Polysulfone. Polyaryl sulfone. Fluoroplastics.
2.2 The fluidity of various plastics also varies according to each molding factor, and the main influencing factors are as follows.
Thermoplastics can be divided into two categories: crystalline plastics and non-crystalline (also known as amorphous) plastics, according to the phenomenon of crystallization when condensing.
The so-called crystallization phenomenon is the plastic from the molten state to the condensation, the molecules from independent movement, completely in a state of no order, into the molecules to stop free movement, according to a slightly fixed position, and a tendency to make the molecular arrangement into a regular model of a phenomenon.
As a criterion to distinguish the appearance of these two types of plastics depending on the transparency of the thick-walled plastic parts, the generally crystalline material is opaque or translucent (such as POM, etc.), and amorphous material is transparent (such as PMMA, etc.). However, there are exceptions, such as poly (4) methyl garoulein is crystalline plastic but has high transparency, ABS is amorphous material but is not transparent.
In the mold design and selection of injection molding machines
should pay attention to the following requirements and considerations for crystalline plastics.
①More heat is needed to raise the material temperature to the molding temperature, so use equipment with large plasticizing capacity.
②The heat released during cooling and tempering is large and should be fully cooled.
③The specific gravity difference between the molten state and the solid-state is large, the molding shrinkage is large, easy to occur shrinkage. Pore.
④Fast cooling, low crystallinity, small shrinkage, high transparency. The crystallinity is related to the wall thickness of plastic parts, the wall thickness is slow cooling, high crystallinity, high shrinkage, good physical properties. So the crystalline material should control the mold temperature as required.
⑤ Anisotropy is significant and internal stress is large. The uncrystallized molecules tend to continue to crystallize after demolding, are in a state of energy imbalance, and are prone to deformation. Warpage.
⑥Crystallization temperature range is narrow, easy to occur at the end of the unmelted material into the mold or plugging material mouth.
XII. Stress cracking and melt rupture
Some plastics are sensitive to stress, molding is prone to internal stress and brittle and easy to crack, plastic parts under the action of external forces or in the role of solvent that cracking phenomenon. For this reason, in addition to adding additives in the raw materials to improve the crack resistance, the raw materials should pay attention to drying, reasonable selection of molding conditions to reduce internal stress and increase the crack resistance. And should choose a reasonable shape of the plastic parts, should not set the inserts and other measures to minimize the stress concentration. The mold design should increase the slope of the mold release, choose a reasonable inlet and ejection mechanism, and adjust the material temperature appropriately when molding. Mold temperature. Injection pressure and cooling time, try to avoid the plastic parts being too cold and brittle when demolded after molding plastic parts should also be post-treatment to improve anti-cracking, eliminate internal stress and prohibit contact with solvents.
When a certain melt flow rate of the polymer melt, at a constant temperature through the nozzle hole when its flow rate exceeds a certain value, the surface of the melt occurs lateral cracks called melt rupture, the appearance and physical properties of plastic parts. Therefore, the selection of a high melt flow rate of polymer, etc., should increase the nozzle. sprue. Inlet cross-section, reduce the injection speed and increase the material temperature.
XIII. Moisture absorption
There are various additives in the plastic, so it has different degrees of affinity to water, so the plastic can be broadly divided into moisture absorption. Adherence to moisture and non-absorbent and not easy to adhere to the moisture of the two, the water content in the material must be controlled within the allowable range, otherwise at high temperatures. Under high pressure, the water will become gas or hydrolysis, making the resin blister. Mobility is reduced. Appearance and mechanical properties are not good. Therefore, moisture-absorbing plastics must be preheated according to the requirements of appropriate heating methods and specifications to prevent re-absorption of moisture when in use.
In the case of TPE and TPR elastomers, the above seven factors, crystallinity. Thermosensitive hydrolysis and hygroscopicity are not so obvious. The remaining factors such as shrinkage. Flowability. Stress cracking and cooling rate are the key factors that need to be considered when designing and manufacturing TPE, TPR material molds. For those who are new to TPE, TPR materials, you need to consult with your material supplier for information.