What are the Common Problems and Solutions in Injection Molding?

Foreword:Injection molding and plastic injection molding are the same process technology. The injection molding process includes many elements that are closely related, such as:Mold design, mold manufacturing, raw material properties and raw material pretreatment methods, mold technology and operation of injection molding machine. Masu. It is related to the environmental conditions of processing, the cooling time of the product and the post-processing techniques. Complex processing flow.

Under the constraints of so many composite factors, the occurrence of defects in injection molded products is inevitable. Therefore, it is necessary to explore the essential mechanisms of defects and predict the location and type of defects that may occur in products, which can help design and improve injection molding processes. It is particularly important to summarize the rules of defect generation and formulate more reasonable process operating conditions.The following describes common injection molding defects and their corresponding solutions.

Warping

Why Do Injection Molded Parts Warp?

Warpage is a plastic part that is not formed according to the designed shape, but has surface distortion. The warpage of the plastic part is caused by the uneven shrinkage of the formed plastic part.

If the entire plastic part has a uniform shrinkage rate, the deformation of the plastic part will not warp, but will only reduce the size; however, due to many factors such as molecular chain/fiber orientation, mold cooling, plastic part design, mold design and forming conditions, etc. It is a very complicated task to achieve low shrinkage or uniform shrinkage due to the interactive influence of the shrinkage.

The main causes are:

Injection molding machine: material tube temperature is too low; nozzle temperature is too low, melt temperature is too low or/and injection pressure is too high, improper holding pressure or holding time; improper dwell time: improper cycle time Not enough slow charging.

Operator: Not placed according to regulations.

Plastic: Poor fluidity.

Mold: The temperature difference between the male and female molds is large; the mold temperature is too low, the difference between the thickness and thickness of the mold cavity is too large, the number or position of the gates is inappropriate, the gates, runners or/and gates are too small or/and too long, Uneven ejection.

What are the Specific Causes and Solutions for Warping?

Insufficient/uneven cooling: Fully cool in the mold cavity and wait until it is completely hardened before ejecting. You can also lower the mold temperature to extend the cooling time.

Caused by ejector pins: grinding the core, reducing the demoulding resistance, or increasing the draft angle, adding ejector pins in areas that are difficult to eject, etc., or changing the ejection method.

Caused by molding strain: increasing the mold temperature, increasing the melt temperature, reducing the injection pressure, improving the flow conditions of the gating system, etc. can reduce the difference in shrinkage rate in the direction. The most important thing to note is that the wall thickness of the product should be consistent.

Crystalline plastic: The correction method used is to make a certain temperature difference between the dynamic and static molds. The deformation can be corrected by taking a temperature that causes strain on the other side of the warp. Sometimes this temperature difference is as high as more than 20°C, but it must be distributed very evenly.

Method to correct product warpage: Place the product to be corrected on the correction tool, and add a heavy object to the warped area, but the weight of the heavy object and the position where you stand must be determined.

Sink Marks

What are the Causes of Sink Marks?

Sink marks: Sink marks are local collapses on the surface of the product, also called shrinkage, dents, pits, flattening, shrinkage pits, settlement spots, etc. It refers to the phenomenon that the surface of the product appears to be concave at the wall thickness, often in the reinforcement. , countersunk holes or internal grids. If shrinkage fails, a cavity will form.

The main causes of sink marks are as follows:

Material problem: material shrinkage rate is too large.

Mold problems: product settings are unreasonable, wall thickness is too large or uniform, gate is too small or flow channel is too narrow and shallow, premature cooling during melt filling process, uneven mold cooling.

Molding process: Insufficient injection rate and insufficient feeding, too fast injection speed, insufficient injection time and holding time, and the mouth is not solidified at the end of holding pressure.

What are the Solutions to Sink Marks?

Materials: Before plasticizing, dry the raw materials according to the data provided by the raw material supplier, and replace with molten plastic with smaller shrinkage rates.nucleating agents are added to crystalline plastics to speed up crystallization.

Mold design: increase the size of the main channel, runner and gate, check whether there is sufficient exhaust position, and use gas-assisted injection molding at the ribs.

Molding process: Choose the appropriate injection molding machine and mold. When switching materials, completely clean the old material from the barrel. Increase the injection speed to fill the product and eliminate most of the shrinkage.

Improve the exhaust system, lower the melt temperature, and improve injection molding. pressure or injection speed, increase the back pressure, and after the screw injection is completed, there will be a certain buffer at the front end to facilitate material replenishment.

Part design: Avoid excessive thickness differences in the parts, which may cause shrinkage marks due to large differences in shrinkage.

Jet Pattern (Jet Pattern)

What are the Causes of Jet Streaks?

Jet flow marks: When the molten material flows through a narrow area such as a nozzle, runner or gate at high speed, it suddenly enters an open, relatively wide area. The melt curves along the flow direction like a snake and cools rapidly after contacting the mold surface. If this part of the material cannot be well integrated with the resin that subsequently enters the cavity, jet lines will be formed on the product.

The main causes of jet streaks are as follows:

Main reason: Because the resin injection speed from the gate is too high. If the injection rate is improper, after the plastic enters the gate, it does not encounter an obstacle before contacting the mold cavity. It flies a long distance and cools rapidly to form jet lines.

Mold design:The location and type of the gate are unreasonable, the original size is too small; the size of the runner is too small; the cross-sectional area from the gate to the cavity suddenly increases, and the fluidity is unstable and prone to jet flow.

Molding process: injection speed is too fast·injection pressure is too high·melt temperature and mold temperature are too low.

Material: Brittle materials will aggravate the spray pattern; if the wall thickness of the product is too different, the flow instability caused by the rapid flow of the melt from the thin part to the thick part may cause spray.

What are the Solutions to Jet Streaks?

Mold design: Add a counter-needle resistance in front of the gate to prevent material flow from spraying and expand the cross-section of the gate.Lowering the injection speed is an optional measure.

Molding process: Increasing the mold temperature can also slow down the cooling rate of the resin in contact with the cavity surface, which also has a good effect on preventing the formation of surface hardened skin in the early stages of filling.Reduce the fluidity of the material and lower the melt temperature.

Material: Reduce the fluidity of the material.

Mildew

What Causes Mold?

Fog: The surface of the product is white. From a visual point of view, it appears white, so it is not obvious on white products. The black highlights are the most obvious. For PP material products, the surrounding surface shows large and small white markings. Cloud-like symptoms are classified here as white fog.

Causes of mildew: During the transformation process of the material from glassy state to highly elastic state to viscous flow state, the gaps between the plastics are compressed into the molten material to generate gas.

If the gas cannot escape outside the barrel during plasticization, the existing gas will be trapped within the molten material and injected into the mold cavity. Smooth ventilation within the mold depends on proper ventilation, which can cause clouding, air stagnation, and burns.

How to Deal with Mold?

Mold: Exhaust gas cannot be completely solved by mold. Characteristics of PP materials include a wide temperature range and excellent fluidity. Proper venting is helpful because if the mold vent is too large or in the wrong location, it will not vent properly and may cause flash defects.

Process: Lowering the screw speed will make the temperature of the molten material more uniform and reduce air inclusions. If the screw rotates too fast, the risk of air entrainment increases and the molten material becomes more sensitive to the shear forces exerted by the screw. Frictional heat makes the temperature of the molten material uneven, causing it to decompose and produce gas.

Increasing mold temperature will result in higher pressure, which will slow down filling and save more time. If the mold temperature is too low, mold can easily form on the surface of the product.

Thimble Marks

What are the Specific Causes of Thimble Marks?

Ejection pin marks: generally refer to the top white or top convexity seen on the appearance of the finished product, as well as the dark marks or shadow marks shown by different glosses that can be seen directly opposite the ejector pin position (no top convexity or depression).

The main causes of thimble marks are as follows:

Irrationality of product design: product styling design.

Unreasonable mold design: pouring system, gate design, ejection part design, mold waterway system, mold exhaust system.

Unreasonable machine parameters: injection parameters, holding pressure parameters, mold temperature parameters, material temperature parameters, ejection parameters, and clamping force parameters.

Unreasonable influence of raw materials: processing parameters of raw materials.

What are the Specific Problems and Solutions to Thimble Marks?

Product design: Generally, the thickness of products with simple structure should be guaranteed to be 2.5mm; products with complex structure and many cavity ribs should be guaranteed to have thickness of 2.8mm.

Ejector marks caused by deformation of the ejector pin due to molding pressure: reduce the molding pressure (by modifying the flow channel and entry point size) or modify the ejector pin size; change the ejector pin to an ejector block structure.

Ejector (print) marks caused by the pressure of the demoulding product: When setting the ejection pin position, consider whether it is at the maximum force point and whether the ejection is balanced.

The thermal conductivity of the ejector pin and the mold steel material are different, resulting in ejector pin marks: This effect can be improved by adding 0.05~0.20mm of glue to the ejector pin position; apply bite marks or skin texture treatment on the ejector pin surface.

Ejector (imprint) marks caused by the influence of the mold water path; Under normal circumstances, stress is likely to occur when the mold temperature is low. Appropriately increasing the temperature can reduce the stress; increase the injection speed appropriately: adjust the cooling water path.

Machine and parameter selection should be optimal: injection molding machine selection; injection molding process conditions; processing temperature and injection pressure.

Stress Marks

What are the Specific Causes of Stress Marks?

Stress marks: Most appear on the edge of the product close to the parting surface. They are a collection of countless stretched and oriented molecules perpendicular to the direction of the material flow and the tiny distances between them.

There is still a polymer connecting phase in the direction of the stress trace, so the stress trace is not a crack. Under appropriate heating, it is possible to return the stretch-oriented molecules to their natural curl state and make the stress trace disappear.

Causes of stress marks: Stress marks caused by uneven glue thickness are caused by uneven material flow and cooling shrinkage; stress marks at the ejector pin are caused by the rapid increase in local temperature of the mold during molding.

What are the Solutions to Stress Marks?

In terms of product design: the overall thickness is too thin or the flow end is thick, resulting in excessive filling pressure and pressure holding pressure, which requires the addition of glue or the flow end to steal the meat.

That is to say, when the holding pressure is reduced, see if the shrinkage area can be improved. If not, you need to add glue to the larger surface; try to avoid thickness differences, and if there is, you need to make large differences; avoid making the rib of the male model too large. Resulting in rib stress marks on the master mold.

Mold design: gates are too small or too few in number and unevenly distributed; loose molding of moving parts or unreasonable waterway configuration results in high mold temperature.

In terms of molding conditions: set the holding pressure and time reasonably (lower); increase or decrease the mold temperature (increasing can improve filling and reduce holding pressure; lowering can make the textured surface shiny, surface and stress marks.

Close to, generally speaking, lowering the mold temperature is the majority choice); increasing the speed is beneficial to reducing the residual orientation stress (it is slightly beneficial to the deformation of thin-walled products, but has basically no effect on stress marks).

Detention

Why Does Detention Occur?

Stagnation: When the molten material enters the filling mold cavity from the gate, the flow rate of the melt slows down or stagnates in some directions, which is stagnation. Stagnation is the highest form of unbalanced melt flow and can cause many defects.

Reasons for retention: In actual production, mold cavities may be thick or thin, straight or curved, with cold surfaces and hot surfaces. This results in different pressure gradients and different temperature gradients when the molten material flows, so the material flows in some places.

As long as it slows down, the skin of the expanding flow will quickly solidify under the action of thermal diffusion, and the flowing core will soon become thinner, causing the flow to become even slower, and stagnant flow will form.

How to Improve Retention?

Changing the mold structure is the most fundamental way to improve the stagnation effect. Improving the mold design requires rationalizing the product structure, avoiding sudden changes in wall thickness, and making the melt resistance basically uniform throughout the cavity.

Changing parameters can also alleviate the retention effect, mainly by increasing the material temperature and increasing the firing rate, but the temperature sensitivity must be distinguished.

Changing the mold temperature has little effect on improving the stagnation effect.

Changing materials is not a good method and is not very effective.

Yin and Yang Side

Why is There a Yin and Yang Side?

Yin and yang surfaces: After the plastic mold is opened and shrunk, the product is severely stressed, has yin and yang surfaces, and has an uneven appearance. The textured surface products are black and shiny, and the high-gloss products have inconsistent gloss, which greatly reduces the appearance of the product.

The reason for the formation of Yin and Yang surfaces: When plastic products are cooled from solution to solid state, as the temperature increases.

Changes will continue to shrink:At the same time, temperature and shrinkage also affect the reproduction effect of the product. Products molded from plastics with a large shrinkage rate are prone to blackening; low mold and material temperatures are prone to blackening at the moment of material/mold contact. It works, and simultaneously the injection pressure and holding pressure,Small, plastic filling density is small, which will also cause the product to turn black.

What are the Specific Causes and Solutions for Yin and Yang?

Yin and yang surfaces around the parting surface: sufficient exhaust, especially in the end flow area.

The area of the product close to the gate will be shiny, and the area far away from the gate will be dark: enlarge the gate area as much as possible to avoid causing a large shear rate in the gate area to form a local yin and yang waterfall. Under the premise of ensuring product quality, the surface adjustment machine Try to use low pressure, low speed and low material temperature to use mold exhaust to change the flow speed of the material.

The thickness of the product is uneven and the flow speed changes: throttle the thicker areas of the product to artificially change the flow direction and speed of the material.

Yin and yang surfaces appear at the corners of the product: the thickness at the corners should be uniform with the thickness of the side wall to reduce the impact.

There are ribs on the back of the product that easily form a yin and yang side: try to make as few ribs as possible on the back of the exterior surface, especially the higher ribs, to reduce the thickness of the ribs.

It is easy to produce negative and positive surfaces near the weld lines: Strengthen the exhaust at the end of the flow and the welding line should not be on the same line as much as possible. The welding line should be dispersed through throttling and exhaust.

The water channels on the exterior surface are unevenly arranged, resulting in the formation of yin and yang surfaces in the hot and cold surface areas of the product: cooling should be as uniform as possible under the influence of the structure.

Mix Colors

What are the Specific Causes of Color Mixing?

Color mixing: The ratio of melt amount to melting capacity is the main factor that easily causes color mixing of injection molded parts. Color mixing of injection molded parts is related to the screw of the injection molding machine, which is mixed with other raw materials without cleaning them. The most important thing is that the raw materials added are mixed in color, or the barrel is not clean.

What are the Specific Solutions for Color Mixing?

Under normal circumstances, when injection molded parts have mixed colors: appropriately increase the melt back pressure or increase the melt temperature by 10 to 15°C, or add some diffusion oil for color matching, increase the amount of nozzle material, improve the mixing operation, etc. If the screw speed is too fast, reduce the speed by about 20-50 rpm.

Solutions and machine adjustment techniques when color mixing is serious:

First adjust the melting speed to 100 to 150 rpm, then increase the melting back pressure until the entire melting time is close to the cooling time of the injection molded part. In this way, the heating time and stirring times of raw materials and toner can be greatly increased without affecting the production speed.

The quality of the melt will be significantly improved. At this time, the color mixing problem has been greatly improved. If necessary, You can extend the melting time a little longer, and the results will be better.

Due to the increase in shearing time and speed, the melt temperature will be increased, which may affect the color deviation, and color adjustment is required if necessary.

Mix the raw materials and toner in advance, and then pellet them with a pre-plastic pelletizing machine. After the raw materials are granulated, there will be almost no color mixing problems, but you still need to pay attention to the color change.change.

When the problem cannot be solved by using the above methods, the only way is to adjust the mold to a larger machine or a machine with better or newer quality for production. The injection molding machine is too small or too thick, which is an important reason why the color mixing problem is difficult to solve.

Conclusion

Injection molding process is a complex process involving many elements such as mold design, mold production, raw material characteristics and raw material pretreatment methods, molding process, injection molding machine operation and injection process. It is also closely related to environmental conditions. , product cooling time and post-processing process. process.

Therefore, the quality of plastic products not only depends on the accuracy of injection molding and the measurement accuracy of the injection molding machine, but also depends on the quality of the mold design and the processing accuracy level of the mold.

Usually, it is also affected by warpage, sink marks, spray marks, etc. It is affected and restricted by factors such as flow lines (jet lines), mold, ejector marks, stress marks, retention, yin and yang surfaces, mixed colors and other factors.

In order to solve these problems, corresponding measures need to be taken, such as adding cooling systems, optimizing mold structures, and selecting high-quality materials. By solving these problems, the quality and efficiency of injection mold manufacturing can be improved, thereby enhancing the competitiveness of products.