Common defects/problems in plastic injection molds and their potential solutions (Chapter 2/8):

4. Short shots:

Short shots in injection molding refer to a defect where the injected plastic material fails to completely fill the mold cavity, resulting in an incomplete or short part. This can happen due to a variety of reasons, such as insufficient clamping force, low injection pressure, low melt temperature, or improper mold design. Short shots can lead to dimensional inaccuracy, reduced part strength, and overall product quality issues. Addressing the root cause of short shots, such as adjusting the injection parameters or modifying the mold design, is crucial to ensuring consistent and high-quality injection-molded parts.

Solution: 

Increase the injection pressure and/or melt temperature to ensure the plastic material can fully fill the mold cavity. 

Optimize the mold design by reducing flow restrictions and improving the gating system. 

Adjust the clamping force to maintain proper mold closure during the injection process. 

Monitor and maintain the injection molding machine to ensure consistent performance. 

Implementing these solutions can help eliminate short shots and produce consistent, high-quality injection-molded parts.

5. Warping:

Warping in injection molding refers to the undesirable deformation or distortion of a molded part after it has been removed from the mold. This can occur due to uneven cooling, internal stresses, or other factors during the molding process. Warping can cause the part to become misshapen, affecting its fit, function, and appearance. Proper mold design, material selection, and process control are essential to minimize warping and ensure the production of high-quality, dimensionally stable parts.

Solution: 

Optimize mold design, use uniform wall thickness, proper gating, and cooling system placement to ensure even cooling and reduced internal stresses.

Select appropriate materials, choose materials with low shrinkage and good dimensional stability, such as filled thermoplastics or engineered resins.

Adjust process parameters, optimize injection speed, holding pressure, and cooling time to reduce residual stresses within the part.

Implement post-mold treatments, use annealing, stress-relieving, or other post-processing techniques to reduce warping in challenging parts.

6. Flash:

Flash in the context of injection molding, "flash" refers to a thin, unwanted layer of material that extends beyond the intended mold cavity. This occurs when the molten plastic is forced into gaps or crevices between the mold halves, resulting in excess material that solidifies and creates a thin, flaky edge on the molded part. Flash can be an indication of improper mold clamping, excessive injection pressure, or wear and tear on the mold itself. Removing flash requires additional post-processing steps and can impact the quality and appearance of the final product.

Solution:

Optimize mold design: Ensure the mold cavity is well-designed with proper clearances and tight tolerances to minimize gaps where flash can occur.

Adjust process parameters: Reduce injection pressure, optimize melt temperature, and optimize clamping force to prevent excessive plastic flow into unintended areas.

Maintain mold integrity: Regularly inspect and maintain the mold to address any wear or damage that could lead to increased flash formation.

Implement post-processing: Trim or remove any flash that does occur, either manually or through automated processes like deflashing or trimming.

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Common defects/problems in plastic injection molds and their potential solutions (Chapter 1/8):

 1. Blisters: 

In injection molding, blisters refer to small, bubble-like defects that can form on the surface of a molded part. These blisters are typically caused by trapped air or gas within the plastic material during the injection and cooling process. When the trapped air expands, it creates these unsightly blemishes on the part's surface. Blisters can be a result of improper venting, excessive melt temperature, or other process issues. Addressing the root cause, such as adjusting the mold design, temperature settings, or injection parameters, can help prevent the formation of blisters and improve the overall quality of the molded parts.

Solution:

Reduce moisture content in the raw material by proper drying and storage, as trapped moisture can turn to steam during the injection process and cause blistering.

 

Adjust the injection parameters, such as injection speed, melt temperature, and holding pressure, to ensure complete and uniform filling of the mold cavity and prevent air entrapment that can lead to blistering.

2. Sink marks: 

Sink marks in injection molding are small, visible depressions or indentations that appear on the surface of a molded part. These defects occur when the plastic material in the mold cavity shrinks during the cooling process, but the surface of the part is unable to fully follow the shrinkage. This can happen due to factors such as uneven wall thickness, improper venting, or excessive cooling in certain areas. Sink marks can negatively impact the appearance and functionality of the molded part, so addressing their root causes is important in maintaining high-quality injection molding production.

Solution:

Optimize the part design by adding ribs, gussets, or other features to provide structural support and reduce the tendency for sink marks in thick or uneven sections.

Adjust the injection parameters, such as holding pressure and time, to ensure complete packing of the mold cavity and adequate material flow to compensate for volumetric shrinkage in problematic areas.

3. Weld lines: 

Weld lines in injection molding are the visible lines or marks that appear on the surface of a molded part where two or more flow fronts from the injection nozzle meet and merge. These lines can be caused by factors like improper mold design, uneven melt flow, or incorrect injection parameters. Weld lines can affect the part's appearance, strength, and overall quality, so they are an important consideration in the injection molding process. Minimizing or eliminating weld lines is often a key goal in part design and process optimization.

Solution:

Optimize the mold design by adjusting gate locations, adding flow leaders or melt guides, and ensuring uniform melt flow throughout the cavity.

Adjust the injection parameters such as melt temperature, injection speed, and holding pressure to improve melt viscosity and promote better fusion at the weld line.

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What is blow molding?

Blow molding: It is a method of clamping molten thermoplastic raw materials extruded from an extruder into a mold, and then blowing air into the raw materials. The molten raw materials expand under the action of air pressure, adhere to the wall of the mold cavity, and finally cool and solidify into the desired product shape.

Blow molding can be divided into two types: thin film blow molding and hollow blow molding:

1. Film blow molding

Thin film blow molding is the process of extruding molten plastic into a cylindrical thin tube from the annular gap of the extruder head mold. At the same time, compressed air is blown into the inner cavity of the thin tube from the center hole of the head, and the thin tube is blown and expanded into a larger diameter tubular thin film (commonly known as a bubble tube). After cooling, it is rolled up.

2. Hollow blow molding

Hollow blow molding is a secondary molding technology that uses gas pressure to blow and expand the rubber like blank closed in the mold cavity into hollow products. It is a method for producing hollow plastic products. Hollow blow molding has different manufacturing methods according to the blank, including extrusion blow molding, injection blow molding, and stretch blow molding.

1) Extrusion blow molding: Extrusion blow molding is the process of using an extruder to extrude a tubular billet, clamp it in the mold cavity while it is hot, seal it, and then blow compressed air into the inner cavity of the billet to form it.

2) Injection blow molding: The blank used is obtained by injection molding. The blank is left on the core mold of the mold, closed with a blow molding mold, and compressed air is introduced from the core mold to blow and cool the blank. After demolding, the product is obtained.

Advantages:

The product has uniform wall thickness, small weight tolerance, less post-processing, and small waste corners; Suitable for producing small precision products in large quantities.

3) Stretch blow molding: Place the billet that has been heated to the stretching temperature in the blow molding mold, use a stretching rod for longitudinal stretching, and use compressed air blown in for transverse stretching and blowing to obtain the product.

Application:

1. Film blow molding is mainly used to manufacture plastic films;

2. Hollow blow molding is mainly used to make hollow plastic products (bottles, packaging barrels, spray cans, fuel tanks, cans, toys, etc.).

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Compression injection molding

Compression injection molding is a relatively traditional injection molding method

Principle:

First, inject the molten material into the mold cavity, and when the molten material enters the mold lung. The mold opens slightly under its pressure blade; After the molten material fills the mold cavity, use a high-pressure locking mold to obtain the required product. The second step is compression molding. Due to the fact that the molten material enters the mold cavity when the mold has been slightly opened, the required allowable mold labor is relatively small.

During the molding process, the screw no longer injects material into the mold cavity, but relies on high-pressure locking of the mold to force Lf against the plastic, resulting in a smaller product orientation and lower internal stress. This method is particularly suitable for products with high transparency requirements that are formed and have a small volume;

Advantages:

It can increase the flow length ratio of injection molded parts; Adopting smaller locking force and injection pressure; Reduce internal stress of materials; And improve processing productivity.

Injection compression molding is suitable for products made of various thermoplastic engineering plastics; Such as large-sized curved parts, thin-walled, miniaturized parts, optical lenses, and parts with good impact resistance requirements

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Low pressure injection molding

Low pressure molding is a process that injects packaging materials into molds at very low injection pressures and rapidly solidifies them into shape.

Low pressure molding process is a packaging process that injects packaging materials into molds at a very low injection pressure (2-20 bar) and rapidly solidifies (5-50 seconds) to achieve insulation, temperature resistance, impact resistance, vibration reduction, moisture resistance, waterproof, dustproof, and chemical corrosion resistance.

This process is currently mainly applied to the packaging and protection of precision and sensitive electronic components, including printed circuit boards (PCBs), automotive electronic products, mobile phone batteries, wire harnesses, waterproof connectors, sensors, microswitches, inductors, antennas, and so on.

Low pressure molding is a process method that falls between high-pressure injection molding and sealing.

Features and advantages

The traditional injection molding process has defects due to high pressure, as low-pressure molding only requires a small amount of pressure to allow the melt to flow into a small mold space, thus not damaging the fragile components that need to be packaged, greatly reducing the scrap rate.

The low-pressure forming process can effectively seal, moisture-proof, waterproof, dust-proof, and chemically resistant the encapsulated components. In addition, it can also balance performance including high and low temperature resistance, impact resistance, insulation, and flame retardancy.

Shorten product development cycle

Low pressure forming molds can use cast aluminum molds instead of steel, making them very easy to design, develop, and manufacture, which can shorten the development cycle. In addition, compared to the time-consuming two-component sealing process, the process cycle of low-pressure hot melt injection molding can be reduced to a few seconds to tens of seconds, greatly promoting production efficiency.

Save total production costs

Firstly, the equipment cost of low-pressure injection molding process is low. Traditional injection molding equipment systems generally have higher costs, including purchasing high-pressure injection molding machines, as well as having a water-cooling system and expensive steel molds. The low-pressure injection molding process equipment system is generally relatively simple, consisting only of three parts: a hot melt glue machine, a work console, and a mold.

Secondly, due to the extremely low injection pressure, aluminum molds can be used for mold design, development, and manufacturing, which can save material costs and development cycles. If low-pressure injection molding technology is used to replace traditional sealing technology, it can also save the cost of sealing the shell and post heating curing.

Finally, due to low pressure and low temperature, the scrap rate of finished products can be greatly reduced, avoiding unnecessary waste.

Therefore, choosing low-pressure injection molding technology can not only significantly improve production efficiency and reduce the defect rate of finished products, but also help production enterprises establish cost advantages overall

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Gas assisted injection molding

Gas assisted injection molding, also known as gas injection molding, is a new injection molding process. It is one of the most important developments in the injection molding industry since the introduction of reciprocating screw injection machines.

Gas assisted injection molding is an extension of injection molding, which is developed on the basis of injection molding technology and structural foam injection molding; It can also be considered as a combination of injection molding and hollow molding, in this sense, it can also be called "hollow injection molding". The principle is to use relatively low pressure gas to replace the resin in the mold cavity during the pressure holding stage of the original injection molding. 

Gas assisted injection molding can be achieved by adding a gas supply device to the existing injection molding machine. The gas supply device consists of an air pump, a high-pressure gas generation device, a gas control device, and a gas nozzle. The gas control device uses a special compressor for continuous gas supply, and is controlled by an electric control valve to maintain a constant pressure. Pressure usually has three levels. A gas control device can be equipped with multiple injection molding machines.

The gas commonly used in gas assisted injection molding is nitrogen. The gas pressure and gas purity are determined by the shape of the molding material and the product. The pressure is generally between 5~32MPa, with a maximum of 40MPa. High pressure gas is injected from the gas nozzle at a set pressure timing during each injection. One or more gas nozzles, located on the injection molding machine nozzle, mold runner or cavity.

Advantages:

Gas assisted injection molding technology has many incomparable advantages. It not only reduces the manufacturing cost of plastic products, but also improves certain properties; When the parts can meet the same usage requirements, using gas assisted injection molding can greatly save plastic raw materials, with a savings rate of up to 50%. On the one hand, the reduction in plastic raw material consumption leads to a reduction in the time of each link in the entire molding cycle

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Water-assisted injection molding

Water-assisted injection molding technology is an advanced injection molding process that injects part of the melt into the mold cavity, then injects high-pressure water into the melt through equipment to finally shape the workpiece.

Advantages:

Due to the incompressibility of water, the front end of the water forms a solid interface, squeezing the inner wall of the product into a cavity. The front end of the water also plays a role in rapid cooling. Therefore, water-assisted has many advantages that gas-assisted cannot match. Research and application show that water-assisted can generate thinner and more uniform cavity walls, and the inner wall surface of the flow channel is very smooth. Especially for thick-walled workpieces, the cooling time of water-assisted can be greatly reduced compared with gas-assisted.

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