Injection processing products Crack Cause and Solution

Cracking, including filamentous workpiece surface cracks, crazing, top white, cracking and because parts stick mold, stick-runner or trauma caused by the crisis, according to a release time diversity cracking cracking cracking and applications. The main causes of the following aspects:

1. Processing:

(1) The working pressure is too large, too fast, the more filling material, injection, dwell time is too long, too much stress can cause cracking.

(2) adjusting the speed and pressure of the mold to prevent the rapid releasing strong drawn products cause cracking.

(3) due to increase mold temperature, make parts easy to stripping, appropriate to reduce the material temperature to prevent decomposition.

(4) prophylaxis of welding marks, plastic degradation caused by low mechanical strength and cracking.

(5) appropriate use of mold release agents, pay attention to the mold surface adhesion often eliminate aerosol and other substances.

(6) The article residual stress, can be carried out immediately after forming annealing to eliminate stress and reduce the formation of cracks.

2. Molds:

(1) To the top of the balance, such as the number of mandrel, the cross-sectional area should be sufficient, stripping slope should be enough, the cavity surface must be smooth enough to prevent the external force so that it leads to the top of the residual stress concentration and cracking.

(2) parts structure can not be too thin, the transition part arc transition should be adopted to avoid sharp corners, chamfers cause stress concentration.

(3) minimize the use of metal inserts to prevent different inserts and parts increased stress caused by shrinkage.

(4) deep bottom parts of the release should set the appropriate air intake tract, preventing the formation of vacuum.

(5) the sprue gate large enough to feed when not enough time to solidify the release, so easy to mold.

(6) the sprue bushing and nozzle engagement should be prevented from leaving chilled drag material stuck to the fixed mold parts.

3. Material aspects:

(1) renewable materials content is too high, resulting in low strength parts.

(2) the humidity is too large, resulting in the emergence of the top of the steam cracking of some plastics and chemical reactions, reducing the strength.

(3) the material itself is not suitable for processing environment or poor quality, contamination can cause cracking.

 4. Machine: the plastics injectionmolding machine capacity should be appropriate, too small plastics insufficient and not fully blended brittle, degrades too large.

Injection products processing product shrinkage depressions cause analysis and solution

Injection molding process, shrinkage Depression is relatively common phenomenon. The main reason for this situation are:

1. Machine aspects:

(1) the nozzle hole caused by too much financial and material reflux contraction, the resistance is too small when a large quantity insufficient contraction.

(2) insufficient clamping force will cause flash contraction, clamping system should check whether there is a problem.

(3) should be used in plastics plastics lack large amount of the machine, check the screw and barrel wear.

2. Molds:

(1) design parts to make uniform thickness to ensure consistent shrink.

(2) mold cooling, heating system to ensure a consistent temperature of various parts.

(3) To ensure the smooth gating system, the resistance can not be too large, such as sprue, runner, gate size should be appropriate, the finish should be enough, the transition zone to transition circle.

(4) thin pieces should increase temperature to ensure smooth flowing material, for thick parts mold temperature should be lowered.

(5) to open the gate to be symmetrical, try to set up in parts walled parts, should increase the volume of cold slug well.

3. Plastic: the crystalline plastic than non-crystalline plastic shrink calendar harm, to appropriately increase the processing quantity, or addition change agents in plastics, to accelerate crystallization, reduce shrinkage depression.

4. Processing:

(1) barrel temperature, volume changes, especially in the first furnace temperature of illiquid plastic should increase the temperature to ensure smooth.

(2) injection pressure, speed, back pressure is too low, injection time is too short, so that the quantity or insufficient density and shrinkage pressure, speed, back pressure is too big, too long and contraction caused by flash.

(3) feeding quantity that is consumed cushion injection pressure is too large, too small, insufficient quantity.

(4) that do not require precision parts, injection dwell in is completed, the outer part of the basic condensation hardening sandwich still soft and can top out of the workpiece, as soon as possible out of the mold and allowed to cool slowly in air or hot water You can make depression gentle contraction without affecting the use and less conspicuous.

How to solve the plastic injection molding process in a blooming problem

Plastic during the injection molding, in-house products sometimes bloomings appear, and if plastics are transparent member, it will seriously affect the quality of plastic products, resulting in product failure, so why have plastic blooming it? When the product bloomings, plastic products factory is generally how to solve these problems?

 1. The vacuum blooming, this blooming is mainly caused due to plastic shrinkage, plastic wall thickness at the time of cooling, since the cooling speed of the surface than the intermediate cooling faster, which would lead to lack of adhesive at the center of the plastic article , creating a vacuum-like. Solve such vacuum bloomings by reducing the injection speed, increase the injection pressure, increased feeding, high melt viscosity grade materials to improve.

 2. As the rubber compound ingredients to hit the gas. Such bloomings can be solved in two ways, 1. lengthen drying material time, of plastic materials sufficiently dry. 2. Reduce the temperature of rubber, plastic decomposition avoid more gas.

 3. Since the gas illiquid plastic produced, these bloomings we can be resolved by increasing the resin and plastic mold temperature, increase the injection rate.

 These are the injection molding process, because plastic bloomings generated and solutions.

Injection products causes blooming

Plastic receive gas at a filling process disturbances often forming blooming within blooming or silver markings or small thick-walled articles on product surface. The main sources of gas contained in the raw material gas is the degradation of moisture or volatile substances or excess lubricant, it may be too high temperature plastic material heating time degradation occur to produce long.

 A Equipment: nozzle hole is too small, the material drooling at the nozzle or drawing, barrel or nozzle obstructions or burrs generated when the high-speed stream through the friction heat make the material decomposition.

Two molds: (1) due to defects in design, such as: poor gate location, gate is too small, multi-gate parts asymmetric arrangement gates, runners small, mold cooling system is unreasonable to make the mold temperature caused by too different melt flow discontinuity in the mold cavity, blocking the air passage. (2) mold parting surface lacked sufficient necessary or exhaust vent bore tunnel, congestion, poor location, and no inserts, machining gap exhaust thimble and the like, resulting in a cavity in the plastic when air can not enter At the same time leave. (3) the difference between the mold surface roughness, friction large, resulting in local hot spots, through the decomposition of the plastic.

Three technological aspects

(1) The material temperature is too high, resulting in decomposition. Barrel heating temperature is too high or disorders, should piecewise reduce cylinder temperatures. Feed zone temperature is too high, a portion of the plastic melt prematurely full of grooves, the air can not be discharged from the charging port.

(2) injection pressure is small, dwell time is short, so that the melt and the cavity surface is not dense paste.

(3) injection speed is too fast, the molten plastic is decomposed by a large shearing effect, generation of decomposition gas; injection speed is too slow, can not be filled cavity surface density caused by lack of products produced crazing.

(4) insufficient quantity, feeding cushion is too large, the material temperature is too low or too low mold temperature will affect the melt flow and molding pressure blooming.

(5) multi-stage injection reduce crazing: injection speed filling runner gate → → Slow fill bolus injection → low pressure slowly fill the mold, the inner mold and remove and clean gas energy in each segment.

(6) when the pre-plastic screw back pressure is too low, the speed is too high, the screw is retracted too fast, easy air together with the material to the front end of the barrel.

Four raw materials

(1) heterogeneous mixed plastic materials incorporating a lot of powder or pellets, easily entrained air melted, sometimes crazing. When contaminated raw or scrap materials containing hazardous materials readily decomposed by heat.

(2) renewable materials pellets loose structure, large air pores in the reservoir; renewable excessive number of recycled materials or materials with a high proportion of new (generally less than 20%)

(3) the raw material or feedstock comprising a volatile solvent such as dyeing assistant auxiliary liquid white oil, silicone oil lubricant, a plasticizer, dibutyl and stabilizers, antistatic agents, etc. The amount of excess or uneven mixing, to product set state into the cavity formed crazes.

(4) plastic handle without drying or moisture absorption from the atmosphere. Respond sufficiently dried raw material and use the drying hopper.

(5) Some brands of plastic, itself can not withstand higher temperatures or longer heating time. Especially when containing traces of moisture, catalytic cracking reactions may occur. For this type of plastic you want to consider adding external lubricant such as stearic acid and its salts (10kg material can be added to each 50g), to minimize the processing temperature.

Five product design: the thickness is too thick, the cooling rate different list. Mold manufacturing should increase at an appropriate sprue, runner and gate size.

Plastic parts common inspection standards

1 include size, appearance, function detects all plastic parts for IQC incoming inspection, but also with suitable QC, QA quality control in the production process and the finished product acceptance inspection.

2 Sample mold after completion designer should sample letters like, at least two check samples, a seal in the Association of Manufacturers, a company IQC sealed in place, as an acceptable incoming physical standards, the quality of the dispute occurred as a basis for judgment. Designers should from time to time to seal the sample for inspection to confirm whether to continue as a reference sample test, sample problems if found, should not be the basis for judging the quality of the designer should re-sign board, old samples Obsolete. Quality samples preserved sample of department shall be responsible for and found a decline in the quality of the sample cause problems, should immediately contact the designer. For various reasons the mold modification, the designer should re-sign board in the revised stability in order to replace the old sample. 3 category definitions

3.1 plastic parts Categories

A, internal use plastic parts: assembly inside the product, under normal conditions of use, the user will not see.

B, External plastic parts: after assembly show on the outside, the user can directly see or be seen after open.

C, transparent plastic pieces: a variety of transparent material injection molded plastic parts.

3.2 defect zoning

The main surface: Under normal conditions of use can directly see the surface.

Secondary side: under normal conditions of use can not be directly visible surfaces.

4 Dimensional inspection unless otherwise noted, according to the technical requirements of plastic parts incoming inspection, if not for dimensional inspection, subject to the check sample, need to check the size, must be indicated in the design drawings. No Note dimensional tolerances according to the aforementioned technical conditions for inspection.

5 flame retardant test by drawing to UL94 standard. Reference test method GB / T 2408-1996 <plastic burning behavior of horizontal and vertical>

6 visual inspection for internal use plastic parts: in normal light, a distance of 500mm, to make judgments within 5 seconds. External plastic parts: in normal light, a distance of 500mm, to make judgments within 5 seconds.

Transparent Plastic parts: in normal light, a distance of 300mm, to make judgments within 5 seconds. (Normal light is defined as the natural light or illumination in approximate natural light 300-600LX, such as 40W fluorescent lamp, 500mm distance)

Visual inspection items:

(1) Color unevenness such defects, whether found in the major or minor side surface, are not acceptable.

(2) Modification to the check samples or drawings as the standard sample deformation no more than is required to explain the special survey on the design drawings.

(3) the crack surface of the top white, Obscure (spot), black carbonized material shortage of such defects, whether found in the major or minor side surface, are not acceptable.

(4) burr burr in principle to accept the incoming band, but without compromising the appearance, function, assembly, acceptance through subsequent processing of incoming, incoming appearance quality not lower than samples.

(5) surface has shrunk, then waterlines, Moire, wire lines to the sign as a standard sample, such defects with less than samples. (6) Regardless of surface stains appear in the major or minor side surface, are not acceptable.

(7) Design Center signed color to the color of the standard sample, detected with colorimeter.

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Plastic Processing warpage causes and solutions

I. warpage refers to the shape of the injection molded article deviates from the shape of the mold cavity, it is one of the defects of plastic products common. Warpage occurs because many parameters alone are often powerless to solve. Combined with relevant information and practical experience, the following factors affecting the injection-molded articles warpage brief analysis.

Second, the structure of the impact of the injection mold warpage of. In molds, plastic deformation of the main factors affecting the gating system, cooling system and ejector system.

1. Gating system

Injection mold gate location, gate number and form will affect the plastic in the mold cavity filling state, causing plastic deformation. The longer the distance the flow, the greater the internal stress between the frozen layer and the central layer consists of flow-induced flow and feeding; the other hand, the shorter flow distance, the shorter the article flow from the gate to the end of the flow of time, freeze time filling layer thickness decreases, reduce internal stress, warpage will therefore greatly reduced. Some flat shaping parts, use only a center gate, due to shrinkage in the diameter direction is larger than the shrinkage in the circumferential direction, molded plastic parts will produce distortion; if the switch multiple-point gate or a film type gate, can effectively prevent warpage. When molded using point gate, also due to the plastic shrink anisotropy, gate location, number of all have a great influence on the degree of deformation of plastic parts. In addition, the use of multiple gates also enable the plastic flow ratio (L / t) shortened so that the density of the melt in the cavity become more uniform, more uniform shrinkage. Also, the entire plastic parts can be filled in less injection pressure. While smaller injection pressure can be reduced tendency of molecular orientation of the plastic, reducing its

2. Cooling system

In the injection process, the cooling rate is non-uniform plastic parts will be uneven shrinkage forming plastic parts, such differences lead to shrinkage leaving a bending moment produced warping plastic parts. If the mold cavity during injection molding flat shaping parts (such as mobile phone battery shell) is used, the core temperature difference is too large, as close to the cold mold cavity surface melt quickly cooled down, and close to the hot surface of the mold cavity the material layer will continue to shrink, uneven shrinkage will warp the plastic parts. Therefore, it should be noted injection mold cooling cavity, core temperature tends to balance the temperature difference between the two is not too large (in this case, consider using two mold temperature). In addition to considering the appearance of the temperature within the plastic tends to balance In addition, the plastic parts should be considered consistent with the temperature of each side, that you want to keep the cooling mold cavity, core temperature is uniform throughout, so that the entire plastic cooling rate balanced, so that the contraction become more uniform throughout to effectively prevent the generation of distortion. It is therefore crucial cooling water holes disposed on the mold. After the wall cavity surface to determine the distance, as far as possible so that the distance between the small cooling water holes, walls, in order to ensure uniform temperature. Meanwhile, since the temperature of the cooling medium increases with the length of the cooling water rises, so that the mold cavity, core temperature difference along the waterways. Therefore, the required length of each cooling circuit is less than 2 m waterways. In large mold cooling circuit should be set several import a loop located near the exit of another loop. For long shaped Part should be used straight-through waterways. (And we die mostly based S-type loop ---- is not conducive to cycling, but also extend the period. Top of the system design has a direct impact plastic deformation. If the top of the system layout imbalance will result in ejection force of unbalanced leaving the plastic deformation. Therefore, the ejection system design should strive to be balanced with the release resistance. In addition, the cross-sectional area of ​​the ejection lever can not be too small to prevent the plastic force per unit area is too large (especially in When the temperature is too high under the premise of the release) leaving the plastic deformation. jack should be arranged as close to a large release resistance area. Without prejudice to the quality of plastic (including requirements, dimensional accuracy and appearance) should When the jack is provided in order to reduce as much as possible the overall plastic deformation (in other rams for the top block is the truth). with plastic (such as TPU) to produce thin-walled plastic parts deep cavity, since the release resistance is large, The material has a softer, if at all using a single mechanical push-up, will produce plastic deformation, even top wear caused by folding or produce scrap plastic parts, such as switching to a multi-element joint or gas (liquid) pressure and mechanical Top of the combination would be better (will be used later).

Third, the impact of plastics stage WARPAGE

Plasticizing stage of the glass pellets into the viscous flow state of melt processes (talked about changes in raw material plasticized Tristate training). In this process, the temperature of the polymer in the axial, radial (screw relative terms) the temperature difference will produce plastic stress; In addition, the injection pressure of the injection machine, speed and other parameters will greatly affect the degree of molecular orientation during filling, thereby causing warpage.

Fourth, the plastic filling and cooling stages influence on warpage molten state under the action of the injection pressure, filled into the mold cavity and the cavity is cooled, solidified. This process is the key injection molding. In this process, temperature, pressure, speed of the three mutual coupling effects, the impact on the quality and productivity of plastic parts are great. Higher pressures and flow rates will produce a high shear rate, thereby causing difference in parallel to the flow direction and perpendicular to the flow direction of molecular orientation, while producing "freezing effect." "Freezing effect" will produce frozen stress, internal stress formed plastic parts. Effect of temperature on warpage is reflected in the following aspects:

(1) plastic parts, lower surface temperature difference causes thermal stress and deformation;

(2) Plastic temperature difference between the different regions will cause uneven shrinkage between different regions;

(3) different temperature conditions will affect the shrinkage of plastic parts.

Fifth, the effect of stripping stage WARPAGE

Plastic parts from the mold cavity and allowed to cool to room temperature in the process, mostly glassy polymer. Stripping force imbalance, launch mechanism movement is not smooth or stripping the top of the area is easy to make inappropriate product deformation (As already mentioned). Meanwhile, in the filling and cooling stages "frozen" stresses within the plastic parts due to loss of external constraints, it will be released in the "deformed" form, leading to warpage.

Sixth, the direct cause of the contraction injection products injection warpage warpage impact on that uneven shrinkage of plastic parts. If you do not consider the shrinkage of the filling process in the mold design stage, the geometry of the product design requirements will vary greatly, resulting in a serious distortion products will be scrapped (ie problem of shrinkage). In addition to filling stage will cause deformation, the mold temperature difference between the upper and lower wall will cause contraction of the upper and lower plastic surface differences, resulting in warpage. Warpage analysis, the contraction itself is not as important differences on shrinkage. In the injection molding process, the molten plastic in the injection mold filling stage because the polymer molecules are arranged along the flow direction of the plastic shrinkage in the flow direction is larger than the shrinkage in the vertical direction, leaving the injection produce warpage (ie each anisotropy). Generally uniform volumetric shrinkage caused only plastic parts, only uneven shrinkage will cause warpage. Differential shrinkage of crystalline plastics in the flow direction and the perpendicular direction than non-crystalline plastics large, and the shrinkage rate than non-crystalline plastics large, large shrinkage after its crystalline plastic shrinkage anisotropy effect superimposed result warpage tendency crystalline plastic parts is much greater than that of non-crystalline plastic.

Seven of the influence of the residual thermal stress warpage in injection molding process, the residual thermal stress is an important factor in causing warpage, and have a greater impact on the quality of injection products. Due to residual thermal stress on warpage is very complex, not repeat them here.

Eight, influence of warpage insert metal to put inserts plastic products, due to the shrinkage of plastic than the large metal, so easily lead to distortion (some even cracking); to reduce this case, First preheat metal (generally not less than 100 ℃), and then put into production. IX Conclusion Influence of Injection warpage many factors, structure, physical properties of plastic materials and heat molding process conditions and parameters of the mold are different degrees of influence on warpage articles. Therefore, warpage of injection molding process must consider these factors.

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Plastic parts Warpage

Free reverse: reverse the non-circular cross-section member, the original cross-sectional plane not remain flat, resulting in warpage, that member in torque effect, each point along the rod axis direction displacement cross-section. If the reverse axial displacement without any constraints freely sectional warping, called free reverse. The free torsion, warping each section of the same longitudinal fibers remain straight line and length remains the same, only the shear stress on the cross section, no longitudinal normal stress. Constraints twist: As the conditions or external force supporting manner sectional warping torsion member is constrained, called constraint reversed. When restrained torsion, bending deformation member, the section will have a vertical normal stress, called warping normal stress. It also must produce and maintain a balance of warping normal stress warping shear stress.

Warpage (warpage) is not in accordance with the shape of the design of plastic parts molding, but the occurrence of twisted surfaces, plastic parts to warp guide due to uneven shrinkage shaping member. If all the plastic parts with uniform shrinkage, plastic deformation will not warp, but will only reduce the size; however, due to molecular chain / fiber regiospecifically, mold cooling, plastic parts design, mold design and molding conditions and other factors the interaction, to be able to achieve low shrinkage or even contraction is a very complex task. The reason plastic parts due to uneven shrinkage and warping, shrinkage change include: plastic internal temperature uniform. When the plastic solidifies flesh along the direction of the pressure difference and the cooling rate differences. Plastic has not fully cooled to top out, or the top of the pin deformation, improper undercut deep, push-up, stripping slope and other factors may cause inappropriate plastic warping. Plastic flesh changes lead to differences in cooling rate. Plastic parts having a curved or asymmetric geometry. Plastic materials, no added fillers difference. The flow direction and perpendicular to the flow direction of the molecular chains / fibers with directional differences, resulting in differential shrinkage. Holding pressure difference (for example, over the gate packing, packing away from the gate but insufficient).

Plastic filler material is added or not, will result in differences in contraction. When the plastic parts with a contraction difference, its flesh direction of flow generating unequal to shrink, internal stresses caused by plastic parts may cause warping. Since the reinforcing fiber heat-shrinkable plastic parts will be small and the modulus increases, so add fiber thermoplastics can inhibit the contraction, which add fiber along the direction of arrangement (usually flow direction) of the shrinkage ratio of the lateral contraction. Similarly, thermoplastics particulate filler added is much smaller than the non-additive plastic shrinkage rate. On the other hand, if no added filler material plastic parts with a high degree of molecular chain with isotropic, anisotropic contraction was, it has a relatively large shrinkage in the direction of the molecular chain arrangement. Self-reinforcing structure of the liquid crystal polymer with a tight regular arrangement, which tends to anisotropic shrink.

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Plastic Product warpage reason

Mainly due to the injection molding process to produce a polymer bond warping is due to internal displacement occurs under stress due. Deformation depends on the product stiffness and the degree of uneven shrinkage. Reasons imbalance causes contraction can be analyzed from the following four aspects:

     (1) non-uniform mold temperature distribution

 The cooling rate on both sides of the article surface in the thickness direction of the article leads to different shrinkage asymmetric, resulting in a surface of the article to the low cooling rate (higher mold temperature) of the side of the net bending moment. Corner effect which is a typical example: the product due to uneven shrinkage of the corner of a small warpage, will be extended to products and the longitudinal direction of the bottom wall and produce significant warpage in the middle of the wall.

(2) non-uniform wall thickness distribution

  Other conditions being equal, including the thickness of thin sections of the cooling rate of the same, the thickness of thin sections of frozen proportion than large walled area, the shrinkage rate lower than the walled area.

(3) The uneven pressure distribution uniform

  Filling pressure and holding pressure caused by uneven distribution, near the gate area freezes at a higher pressure, resulting in lower shrinkage, the gate area due to the low pressure generated far higher shrinkage.

      (4) the orientation effect produced uneven shrinkage

  Polymer chains oriented by stretching along the flow direction in the flow direction will result in products and differential shrinkage occurs perpendicular to the flow direction. Close to the die wall melt rapidly frozen molecules remain stretched state; melt away from the mold wall layer is frozen spacer layer, and thus a higher temperature, so that the molecular chain is composed of inner tension turn into a relaxed state, resulting in a greater orientation to the outer core almost no orientation gradient, leading to the surface and pull the core by compression state. Net shrinkage cross section is the sum of the shrinkage of frozen layers. Flow direction shrinkage generally larger. Glass fiber-containing material, because the orientation in the flow direction shrinkage of less than the shrinkage perpendicular to the flow direction, is more prone to deformation.

When the product has a deformed, from the above four aspects, basically you can find the distortion / warping reason.

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Plastic product deformation analysis method

One. Deformation causes:

1, the shape of the article, particularly molding shrinkage rates with the relationship between the residual stress caused by the thickness of the article.

2, since the residual stress generated by molding conditions

3, the residual stress generated when stripping

4, due to insufficient cooling time caused by the deformation

II. The associated knowledge

1. Reasons product deformation (warping, bending, small wrinkles) with the same cracks. I.e., residual stress within the article. Molding conditions should be set towards the elimination of internal stress setting products. That increase after the cylinder temperature, mold temperature, the flowability of the material in good condition, the injection pressure is not too high.

2, in order to reduce residual stress, there is annealed, i.e., below the thermal deformation temperature of 10 degrees, there was heated over 2 hours to eliminate the effect of internal stress. Annealing in this manner but the high cost due to the use of less common.

3, if the cooling water hole of the mold can not be uniform cooling products, will produce residual stress, which means that the cooling water hole not too light.

Third, the solution.

1, real-time: in the mold sufficiently cooled and solidified (prolonged cooling time timer) enhance barrel temperature, lower injection pressure.

2. Short-term: the mold cooling uniform.

3. Long-term: to avoid differences in thickness of the article to set the thickness of the gate in products where large (1-1), because the straight line is easy to cause warping, made a large R curve, reversible bending mold products, increase the ejection lever months the number increased stripping slope.

Fourth, the material differences:

1, crystalline materials (polyethylene, polypropylene, POM, nylon) Molding shrinkage, and prone to bias, non-crystalline materials (polystyrene, ABS) likely to cause residual stresses.

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