Plastic injection molding is the most common manufacturing process to obtain plastic products, it’s widely used worldwide for many industries. Just look around of you, from automotive, medical, home appliance, to airplane. Nearly every product needs plastic injection molded parts.
Plastic injection molding process requires to use injection machine, mold tool, and raw material. As every injection molded product made with a mold tool, it runs repeatedly and quickly for a cycle. This is why plastic injection molding process can be used to produce plastic products with large scale of volume efficiently.
Prior to plastic injection molding process, we must get the plastic parts carefully designed to adapt for injection molding, which needs to have a precision mold tool first, then load it onto injection molding machine for injection molding production.
It takes around 15~60 seconds to finish a single plastic injection molding cycle, which means we can produce a plastic part in 15~60 seconds by plastic injection molding process. It’s much quicker than other manufacturing methods, like CNC machining, die casting, conventional milling, etc.
On the other hand, if there is a large scale of manufacturing volume needed, we can design injection mold tool with multiple cavities, or have more mould tool for production. In this way, we can manufacture large volume orders with very short-time.
Compared to CNC machining and die casting, plastic injection molded part is much cheaper, as its lower material cost and higher manufacturing efficiency.
Even through there would be some mold tool cost that we might increase our total cost, however the plastic injection molding process is still the most economical manufacturing method. Relative to say, a same product of injection molding is about 1/5~1/8 cheaper than die casting process, and 1/10~1/20 cheaper than CNC machining. This is why plastic injection molding is our first choice of making diverse of products.
Since plastic injection molded part made from mold tool, which offer an accurate dimension and consistence. For different requirement, a highly accurate parts could be tolerance of 0.01~0.02mm, 1/10 of a piece of human’s hair. Those parts are mainly used in airplane, meter, and military industries. For other lower quality parts, a +/-0.05mm of tolerance can be achieved as needed for printer parts, clock parts, computer parts, mobile phone parts, etc. Except for some material that has serious shrinkage issue(like PA, POM, PP), a +/-0.1mm of tolerance can be obtained for nearly most of plastic injection molded parts.
It means we can almost make the same accurate parts with plastic injection molding process while not increasing manufacturing cost as CNC machining or other manufacturing process.
Unlike CNC machining or other conventional manufacturing process, the plastic injection molding process offers a wide design option of complex structure, which allows screw, clipper, window, web, grid, rib, slot, hole, pillar…as complex as you can design, the mold maker can always find the way to make plastic injection molded part.
As we know, a plastic injection molded parts can be custom-made as different color as you want. Because we can change the color powder easily and mix it into plastic material. Meanwhile we can do different surface treatment like mold texture or part printing to obtain different appearance or touch feeling. It provide customers different options as we needed.
During a product design and development process, there could be some imperfects that we need to improve. A plastic injection molded parts offers flexible design change or engineering modification via modifying the mold tool. After the mold modification, the molded parts could quickly be used for new generation of production.
Injection molding machine, mould tool, and raw material are the three basic factors of plastic injection molding process. During the process, mold tool is mounted on the machine, and plastic raw material is melted and injected into mold cavity, then it cools and solidify to become desired injection molded parts. Needless to say, how important a plastic injection molding machine is, which also being said, it’s impossible to proceed a plastic injection moulding process with an injection machine.
There are two different types of plastic injection molding machine, which are horizontal and vertical machine. But most of plastic parts are made by horizontal injection machine.
An injection machine mainly includes four main functions: material feeding and molten, clamping, injection, and control panel. For different clamping force of mold tool, the tonnage of injection machine can range from 5 T~5000T, but we need to choose a proper tonnage of machine for production according different mold size.
The following video introduces how a plastic injection machine works:
Injection mould is a durable, repeatable, accurate manufacturing tool that used on injection moulding process. It’s widely used in many industries to produce plastic in large volume.
Injection mold is normally made from steel(some of molds made from aluminum or copper). It consists of two halves of plates that calls mould cavity and mould core, which forms a plastic part profile or structure. For higher complex mold, it requires to design some sliders or lifters to achieve some complex part structure that can’t be formed by just mold cavity and mold.
During the injection molding process, the molten material is ejected into mold cavity where cools and solidifies to obtain desired parts. Then the mold tool opens, and the parts will be ejected with a mold mechanism(called ejection system)
The plastic injection molding is the most common manufacturing method to produce plastic products in large volume, it offers high efficiency, low cost, accurate dimension, and complex part structure.
The plastic injection molding system consists of raw material, injection mold tool, and injection molding machine. During the process, the pre-dried raw material is prepared in the hopper of injection machine, it is melted and plasticized under a high temperature of injection heating nozzle. Then those plasticized raw material will be shot onto the mold cavity where raw material will solidify and cool down to form desired plastic parts, finally the molded parts will be ejected from mold tool.
The whole injection molding cycle is very short, which typically takes only 10 seconds and 2 minutes, and we would like to summarize the injection molding stages as below:
In order to obtain a better quality of plastic injection molded products, it’s necessary to get plastic resin dried before injection molding process. As all of resin pellets normally exist a certain percentage of moisture, which could result in lowering down material physical properties, such as reducing flow rate, tensile, and impact strengths.
For different brand of plastic resin and different supplier, the plastic material has different percentage of moisture, therefore it takes different drying time ranging from 1~5 hours. After material drying, it will greatly avoid most of injection molding defects, such as flowing mark, sink mark, welding line, etc.
Injection mold is an essential manufacturing tool to manufacture plastic products, before a plastic injection molding production, we should get a mold tool designed and manufactured.
Once we are ready for plastic injection molding process, we need to get mold tool loaded onto injection molding machine. making sure the two halves of the mold tool must be steadily attached.
For different size of mold tool, it requires different injection machine that offers proper clamping force, because if an improper clamping force used on a mold tool, it could lead to injection defects that can never resolved by injection parameter adjustment.
Meanwhile, a operational trial is necessary to test if the mold tool can open and close smoothly with enough movement stock.
To get the injection molding facility prepared, it’s time to upload those pre-dried plastic pellets into the hopper of injection machine. This action could happen by either manual or automation.
After material is loaded into hopper, the material will be transferred by a nozzle screw forward to injection mold tool, during this stage, the plastic pellet will b melted and plasticized so that it can be injected into mold tool.
During the injection stage, the melted material is shot into mold cavity quickly under a high pressure from injection nozzle. However different size and different part weight requires different injection time. Normally the heavy parts, the longer injection time needed, the complex part structure, the longer injection time needed. However the injection time could also determined by injection pressure, injection speed, and sprue size.
After the molten material injected into the mold tool, it will cool down immediately as the molten material contacts the cool mold steel where the cooling system is designed inside. The cooling system could be cold water or oil, which offers a recyclable cooling channel between mold tool and its outlet.
Normally it takes few seconds to cool molten material, it will solidify in the mold cavity to form desired plastic product.
It’s very important to have a reasonable cooling system, if not, it could lead to shrinkage issue or deformation issue while injection molding process.
After the ejected material is cooled and solidified, it is the time to get the molded product released by either manual or mechanism structure of mold tool.
At this stage, the two halves of mold tool are opened, a mechanism plate is used to push the part out of the mold. But for some soft parts, we will have to take out of the part from mold.
As soon as the part is released, the mold tool close again for next injection molding cycle.
Descriptions:Incomplete filling is also called short shot, which means the injection molded part is not completely filled with molten plastic resin. And you can physically see the part structure is missing or not full compared to your design drawing.
Causes: 1). Lower injection speed or pressure could lead to incomplete filling; 2). The temperature of injection machine is lower than resin molten point. 3). The runner of mold tool is not big enough for material to travel from injection nozzle to mold cavity.
How to Identify: You need to compare the injection molded parts with your final 3D drawing, and you will easily find some of feature or shape might be missed from your design drawing.
How to Avoid: 1). Use experienced injection molding technician to set up injection molding parameter. 2). Design and build reasonable injection channel. 3). Pre-dry the raw material resin sufficiently.
Descriptions: Flash refers to the extra material that overflows from the edge of injection part, then you will see additional burrs on the part.
Causes: 1). The two halves of mold tool did not clamp tightly 2). The injection mold has worn out 2). The injection speed and pressure is too big.
How to Identify: By visually checking out the part edge, we will find flash(if it has).
How to Avoid: 1). Refine mold tooling 2). Adjust mold clamping force 3). Adjust injection speed and pressure.
Descriptions: Warping is also called deformation, which refers to the flatness of a molded part is too big, For example, a 200 mm length of part, the flatness should be no more than 0.15 mm or less, but a warping part might be 0.3 mm or more, and it might appear like an arc.
Causes: 1). Unreasonable plastic part design. 2). Unreasonable mold design. 3). Improper plastic material.
How to Identify: Put the part onto a measuring platform or us a steel rule to gauge the part shape.
How to Avoid: 1) Design the part with enhancement rib 2). Increase the wall thickness 3). Add venting system on mold tool 4). Adjust proper plastic resin
Descriptions: Sink marks are some recessed surface or holes that exist on the surface of plastic molded part.
Causes: Since plastic parts made from molten plastic resin, which is injected into mold tool where the material cools and solidifies to obtain desired part. When the material cools and solidifies, it will release a certain percentage of moisture, a overall shrinkage must exist on the part. But we will consider resin shrinkage rate to compensate the certain percentage of shrinkage while mold tool design(for example, ABS shrinkage rage is about 0.5%). If an uneven thickness of wall, it will result in serious sink marks or sink hole.
How to Identify:We can visually find the sink marks or holes when we check out the appearance of a molded part.
How to Avoid: 1). We should always design a plastic part with even thickness of wall. 2). Design the part wall with proper thickness( should be range from 1.2 mm~2.5 mm for most of part). 3). Avoid to use high shrink resin for structure product(like PP).
Descriptions:Flowing mark means the visible material flowing line that exists on outer surface of a part, it usually starts from part edge or hole feature.
Causes: Since molten material is injected into mold tool, then it will quickly cool and solidifies to become desired plastic product. However, if injection speed is too slow, a partial material has solidified while other material is still filling the mold, and the latter material will plasticize the solidified material again, at this time, two different material will cause the wave pattern to appear on part surface.
How to Identify:We can visually find the flowing marks when we check out the appearance of a molded part.
How to Avoid: 1) Relocate or adjust gate shape, or add more injection gators 2). Increase injection speed and pressure 3). Increase material temperature when material pre-dry process.
Descriptions: Welding line is a light line exists on plastic part surface, particularly appears aside of round or square features.
Causes: When injection molding process, the molten material flows and travel inside mold cavity, which requires to maintain a certain temperature for plasticization material to fill mold cavity and obtain different part shape. If two or more flowing material meet on round features or square feature from different direction, they should keep almost the same temperature to meet together, otherwise it will occur a visible welding line. the wave pattern to appear on part surface.
How to Identify: Put the molded part on natural light or electronic light, you will easily find the welding line.
How to Avoid: 1) Try to adjust injection speed and pressure 2). Relocated injection gate
Descriptions: Burn marks are the black colored marks existing on plastic part surface.
Causes: Overheated material from mold cavity or barrel of injection machine, or high injection speed and pressure.
How to Identify: The burn mark is in black, it’s visible by visual inspection.
How to Avoid: 1) Increase venting system 2). Set down injection speed and mold temperature 3) Avoid over-clamp by loosing mold clamping as long as there is no flash issue 4). Check if the barrel temperature is proper.
Descriptions: White ejection mark refers to some broken or deformed ejection pin marks exist on part surface.
Causes: As we know, a plastic part is ejected out by either ejection pins or ejection plate. If the ejection pins are different height or ejection stoke is too long, it will easily break or deform the part. The white ejection marks are the evident that part is broken or deformed.
How to Identify:We can find out the white ejection mark by visual inspection.
How to Avoid: 1) Adjust ejection pin length or pin diameter. 2). Relocate ejection pins or adjust number of ejection pins.
Descriptions: Scratch is an evident that part surface is damaged during part release from mold tool.
Causes: Improper draft angle result plastic part to be squeezed. And part surface could not release from mold tool smoothly.
How to Identify: We can find out the scratch by visual inspection.
How to Avoid: 1) Increase draft angle 2). Check out if ejection process is balanced.
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