What is Plastic Injection Molding ?
Plastic Injection Molding is a most common manufacturing process to produce plastic parts for nearly every products.
The injection molding process needs to use injection machine, plastic resin, and injection mold tooling. It requires a ram or screw-type plunger to force molten plastic material into a mould tool where the molten material cools and solidifies to obtain the desired shape or parts.(products).
Since injection molding has good repeatability and stability, it is widely used in the world to manufacture volume products. Needless to say, how important the injection molding process is!
Advantages of Plastic Injection Molding:
Complex Part structure available
Compared to other manufacturing process, plastic injection molding has big advantage on complex part geometry, it means you can design a part with as possible as the product functions. This is what any other molding process or machining process can never beat it.
High Efficiency, Easy Mass Production
As we know, there is a mold tool on injection machining while injection molding process. It takes seconds or minute to get a plastic parts. The manufacturing cycle is quite short compared to CNC machining or other molding process. Therefore it’s easy when you are going to produce it in large volume.
Flexible Color and Material
It’s easy to custom make a plastic parts with different color and material, it is very convenient for a new product development that requires lots of adjustment.
Easy for Engineering Modification
As we know, any of product development might change a lot so as to optimize a better products. If you design a plastic parts, you can make engineering change easily and conveniently.
Up to now, plastic injection molding is the most cost-effective way to obtain products. Because the cost of resin and manufacturing process is much lower than metal machining or other molding process. This is why nearly every products contains plastic parts.
Six Typical Injection Molding Process
Step 1. Mold Installation
When a mold trial or an injection molding production, we need to locate the plastic injection mold from mold tooling shop (warehouse) onto a plastic injection machine. Since the mold tool is so heavy, we have to use a crane to move and lift it. By four big screws to fasten the mold on injection machine tightly, it is very important process, because the fastening force will directly affect the clamping force.
Step 2. Material Baking
As you know, the injection molding process is that the molten plastic material flows into the mold tool. To perform this process, we must pre-bake the material to reach a certain of temperature. If we don’t do this process, the molded parts might come with defects or not full filled.
Step 3. Mold Clamping
When it’s time to start injection molding process, we should keep the two half mold(cavity side and core side) closed tightly on injection machine. If we don’t clamp the mold with proper force, there will result in a flash defect for injected parts. However different size of mold requires different clamping force. Normally, the bigger mold would need larger tonnage of injection machine.
Step 4. Material Injection
Injection is also called material filling. The plastic material is forced to fill into a mold cavity quickly by a high pressure device( the barrel of screw). It’s one of the most important factor that affect the quality of final molded parts. A short injection might result in unfulfilled parts or over injected parts( overflow or flash). However the injection volume is controllable by injection power, filling volume, injection time, and injection pressure
Step 5. Parts Cooling and Solidifying
After injection process, the molten plastic material with high temperature stays inside of mold cavity. Since every mold is designed with cooling system( water or oil), which helps the filled plastic to cool down and solidify. And finally it will form an desired part. When there is uneven thickness of wall, there is likely to exist some shrinkage defect on the parts. Of course, the shrinkage issue could have many potential reasons. But cooling system is one of the most important factor for parts shrinkage.
Step 6. Ejection Out
It’s time to take out the injection parts from a mold tool, but we have a special device for this process(ejection pin or ejection plate). The injection machine will collaborate with mold tool to push out the parts and fall down from mold tool. Up to now, we can say the whole injection molding process is finished.
The 5 Key Factors in Injection Molding Process
Injection molding is a complex process. The quality of injection molding parts could be affected by many different aspects, which can mainly summarized as five typical factors that are pressure, speed, position, temperature, and time. We would like to introduce them briefly as below:
The injection pressure is originally given by the hydraulic system of injection machine. During the injection molding process, this injection pressure is passed by the screw and nozzle of injection machine until mold cavity finally. It could affect the product appearance(like shrinkage, injection mark, air mark) and dimension a lot if we use different injection pressure(higher or lower).
Normally we use high pressure for big size, complex part structure, and thin wall of part. On opposite, for a thick wall of part, we will have to use low pressure.
It means the holding time during which the melted material is filled into mold tool cavity under a certain pressure while injection molding process. The function of holding pressure is to maintain a pressure to compensate for resin shrinkage when it cools and solidifies, Therefore keeping enough holding pressure and holding time will significantly influence the injection molded parts quality.
When a injection process finished, the injection screw would have to retract, in this time, there must arise a pressure(we call it back pressure). In theory, back pressure should never exceed 20% of the injection molding machine pressure. Most of time, we try to use as lower back pressure as possible.
In order to withstand the injection pressure, there must apply for a certain clamping pressure during injection molding process. The clamping pressure provided by a mechanical clamping device that facilitate on injection machine, which is adjustable for high, middle, or low pressure.
We need to choose appropriate clamping pressure according to different mold size, injection type, size, and injection plastic resin. If a mold clamping pressure is too low, there will result in flash or unfulfilled parts. On opposite, if a over-clamped pressure, it could damage a mold tool.
Even though there is no fixed rule to define a exact speed for every part. Since there are lots of different plastic resin, injection machines, and part structure. however, relatively to say, a thin wall of part requires to use high speed injection process, but a thick wall of part, we will have to use lower speed. So an experienced technician is necessary when we define an injection molding parameter.
Per our experience, we use 0.1-10 for buffer flow, 11-30 for slow speed, 31-60 for middle speed, and 61-99 for high speed.
Injection speed refers to the time of molten resin accumulation from nozzle to mold cavity, we need set up injection speed according to appropriate part material and structure.
Normally we use a lower injection speed for those resin with good flowability, like PP, LDPE, TUPE, TPR, TPU, PVC, etc. and for resin with medium flowability with middle thickness of wall(1.5mm or above),we will use medium injection speed (like ABS, HIPS, GPPS, POM, PC+ABS, HDPE). However for material with worse flowability(PC, PC+GF, PA+GF,PBT+GF,LCP), we will have to use high speed injection process.
Material Molten Speed
A proper material molten control is as important as other injection parameter. We can’t ignore it when injection molding process. Material molten speed must follow up the injection cycle.
A quick molten resin will result in over-burned material that could change part color a lot, but a slow molten speed would lead to prematurely molten part, which looks like small injection dot.
Mold Open Speed
Because there are lots of different mold structure, we should adjust appropriate mold open speed to adapt the best injection molding process. For instance, a multi-plates mold(three plates mold or more), a complex hydraulic cylinder sliders, and screwing sliders molds should be opened slower. But for some simple structure mold without slider can be exerted quicker.
It depends on mold structure and part structure to determine how quickly we should set up the ejection system. In theory, we should leave the ejection speed as slower as possible as long as the plastic parts is not damaged or deformed when ejection process. In this way, we can save the life time of ejection system
When setting up an injection molding parameter, we need to set a right injection position based on part weight and structure. Ensuring our part is fully-filled. Yet it doesn’t come with flash or other defects.
Mold Open Position
Mold open position is mainly depends on mold structure. If an incorrect mold open position will damage either injection molding machine or mold tool.
Ejection Pin Position
In theory, the length of ejection out is two times of mold core height. However most of time, we did not follow this rule. Per our experience, the height of ejection could be adjusted as soon as the parts is ejected out while not damaging plastic part.
The Barrel Temperature of Injection Machine
The barrel temperature will directly or indirectly affect resin flowability, color, shrinkage, deformation, etc.
Different resin has requires different injection temperature, however we have a basic temperature data for common plastic material.
Here below is a barrel temperature list:
High Impact ABS: 230°C~260°C
Low Impact ABS: 190°C~230°C
High Density PVC: 160-200、
Low Density PVC140°C~180°C
High Density PE:240°C~300°C
Low Density PE: 180°C~230°C
High Density TPE: 170°C~200°C
Low Density TPE: 140°C~180°C
High Density TPU: 160°C~200°C
Low Density TPU: 120°C~160°C
The mold temperature setting is mainly based on resin flowability. For example, PC and PA+GF have a relatively worse flowability, we need to use a high speed injection with high mold temperature. However for clear PC part, we just need a lower mold temperature for injection molding.
Here blow is a reference mold temperature list for different resins:
ABS=30°C~50°C（Part is easy to deform or has high appearance requirement: 60°C~110°C）
PC=50°C~80°C( Thin part or part has high appearance requirement: 80°C
HIPS=30°C~50°C( Clear PS or part has high appearance requirement: 60°C
PMMA=60°C~80°C( Thin part or part has high appearance requirement: 80°C
PE= 10°C ~50°C
TPE, TPR, TPU=10°C ~50°C
PA, PBT=30°C ~60°C（If they are combined with GF: 70°C ~100°C）
The environment temperature seems not so important for most of plastic parts. However, if we require a tight dimensional stability, we need a stable temperature workshop. Like medical parts, airplane parts, precision equipment parts, etc. We will need to keep an injection environment temperature about 15°C ~20°C
As long as we meet product requirement, we try to use the shorter injection time. Normally a thin wall of parts requires a shorter time, on opposite, the thicker wall parts need slow injection time. If any special part structure, we may need to adjust injection time gradually until the best quality product is injected.
Holding Pressure Time
In principle, the holding pressure is used to control part shrinkage and maintain dimension stability. The molding pressure time depends on part structure and requirement. Relatively to say, we use longer holding pressure for thick wall of parts. On opposite, we will adjust it as shorter time.
As we know, all of injection molded parts will have a shrinkage issue after it is injected from machine. We need get it cooled down and solidified before they are in use. Normally it takes about 12~24 hours for complete cooling. After this time, the molded parts will keep almost the same shape and dimension.
Because of resin shrinkage matter, an injection molded part would need time to cool and solidify. Normally we can either wait for 3~5 hours or put the part inside cold water before we check the part dimension and appearance. Otherwise your measurement data will be inaccurate or has big deviation from its real data.
Common Defects of Plastic Injection Molding Products:
Flash – The additional material overflew from desired shape or dimension
Shrinkage – The material reduction portion after the plastic product cools from mold tooling. Normally there will be a shrink mark on the surface a plastic product.
Deformation – When the final plastic product come out from injection machine, the product looks not straight obviously or measured by a certain equipment.
Flow marks – An outstanding pattern existing on the surface of plastic product, it normally result by slow injection speed.
Short Shot – The molten plastic does not fully fill in the mold cavity, resulting in a portion where lacks plastic material.
Welding line – It’s also called the juncture line where normally happens on round feature that two flow fronts meet and are unable to join together during the injection molding process.
Burn Marks – It looks like rust colored, that appear on the surface of the injection molded products.
Our Automatic Injection Molding Process
Our custom plastic injection molding services:
Inno Molding is a leading plastic injection molding company in Shenzhen China, we provide precision plastic injection molding services to worldwide clients in application of automotive, medical, consumer electronics, home appliance, and other commercial products.
Our injection molding shop is equipped with 30 sets of advanced injection machines. All of injection molded parts are 100% inspected with dimensional, visual, and functional before shipping. So that our clients will get high quality products from us.
Part Weight: 0.25g~3.0KG
Injection Machines: 50T~850T
Plastic Material we work for :
PC, ABS, PMMA, PP, POM, PP, PE, PBT, PA, PSU, TPU, TPE, TPR, silicone are available(other materials are negotiable)
We are an ISO9001-2008 & ISO14001-2004 certified enterprise.Every our project starts with professional team that consists of project managers, project engineers, production manager, QC manager, QC inspector, and shipping manager. Normally we check products by dimensional measurement, cosmetic inspection, and assembly testing. Finally we document those data and make enclosure along with shipment to customer for review.
FA Insepction Report(First Article Qualification)
SPC Report(Statistical Process Control)
DFM Evaluation(Design for Manufacturing)
FMEA Analysis(Failure Modes and Effects Analysis)
Full dimension inspection for all of mold components.