What Is Plastic Injection Molding

Plastic injection molding is a popular and economical industrial process for manufacturing a wide range of plastic consumer products. These include knobs, washers, valves, pulleys, gears, power tools, bottle caps, toys, car panels, computer monitors, keyboards, furniture and so on. These items are mass-produced using molding machines of various sizes and various specific requirements.

The process of injection molding was invented by John Wesley Hyatt. In 1868, he produced billiard balls by injecting celluloid into a mold. Later he built a plunger type injection molding machine to facilitate production. Another inventor, James Hendry, improved on Hyatt’s invention and built a screw injection molding machine in 1946.

Both plunger type and screw injection type molding machines are used in the modern plastics industry. The difference between the two is in the way the plastic is transferred to the mold. Screw injection types are more convenient and are more widely used.

In the plastic molding process, plastic resins are used in pellet or granular form. Choice of plastic type depends on the kind of product being made, its requirements and the overall budget. Of the hundreds of available plastics, only a few are safe for consumer use. Some of the ones used in the plastic injection molding process include polystyrene, polycarbonate, polypropylene, polyethylene, polyamide, polyvinyl chloride, Teflon®, Delrin®, and acrylic.

Plastics are non-degradable and therefore not environment friendly. The plastic injection molding process, however, plays a positive role in reducing waste. Plastics used in the process can be reused as often as required.

The plastic pellets or granules are poured into the feed hopper of an injection molding machine. The feed hopper is a large container that opens into a heating cylinder. In the heating cylinder, the pellets are subjected to intense heat until the plastic melts.

A plunger or a screw then pushes the molten plastic forward through a nozzle into a split-die mold. The mold is the reverse of the part to be made and can have one cavity or several to make more parts at the same time. Molds can be made of durable and expensive steel, or the more affordable, less-durable aluminum or beryllium-alloy metal.

As with selecting the right plastic type, selecting the mold metal depends upon product, cost and durability factors. Manufacturers often use a less expensive metal mold for prototype molding, and get the more long-wearing, precision-machined molds when the production demand increases.

The molten plastic cools in the mold, hardens and takes on the shape of the mold. The mold then opens and the part is ejected out. It is then either ready for packing or is prepared for any other secondary operations. The whole plastic injection molding process is completed within a few minutes and can be carried out automatically.

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Basic knowledge of plastic mold 2

injection molding is the most commonly used plastic processing method. The method is applicable to all parts of thermoplastics and thermosetting plastics, plastic products obtained large quantities unmatched by other molding methods, injection molding process as one of the main tools for plastic injection molds, precision in quality, manufacturing cycle and injection molding process in the level of production efficiency, etc., directly affect the product quality, yield, cost and product updates, but also determines the competition in the market responsiveness and speed.

injection mold by a number of steel plates with the various component parts, and basically divided into:
A molding device (die, punch)
B positioning device (guide post, guide sleeve)
C fixtures (the word board, code-mode pits)
D cooling system (carrying water hole)
E constant temperature system (heat pipe, heater)
F flow system (pump nozzle holes and flow channel grooves, flow hole)
G top of the system (thimble, top stick)

5, the system according to the different type of gating system can mold into three categories:
(1) large outlet die: the flow channel and gate at the parting line, with products in the mold with the mold release time to design the most simple, easy processing, low cost, so the more people using a large nozzle system operation.

(2) fine nozzle mold: flow channel and the gate is not the parting line, usually directly on the product, so much to design a set of outlet parting line, the design is more complex, more difficult process, generally used depending on the product requirements fine nozzle system.

(3) hot runner mold: This mold structure and fine nozzle substantially the same, the biggest difference is that the flow in one or more of a constant temperature of the hot runner plate and heat pump mouth, no cold material ejection, and the water flow port directly on the product, so the flow does not require stripping, this system is also known as non-outlet system can save raw materials for more expensive raw materials, products require a higher situation, design and processing difficulties, mold costs are higher.

Hot runner systems, also known as the hot runner system, mainly by the hot sprue bush, hot runner plate, temperature control electric box composition. The most common hot runner hot runner system has a single point and multi-point thermal gate two forms. Single hot runner gate is a single set of heat directly to the molten plastic injected into the cavity, it applies to a single gate single cavity plastic mold; more hot runner hot runner plate through the molten material to each branch sub-set of the hot runner and then go to the cavity, it is more suitable for feeding a single cavity or multi-cavity mold.

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Basic Knowledge of Plastic Mold

Our everyday life and production used a variety of tools and products to the large to the machine base, the body shell, an embryo as small screws, buttons and various household appliances in the shell, all have a close relationship with the mold. The shape of the mold determines the shape of these products, the quality and precision mold processing will determine the quality of these products. Material for various products, appearance, specifications and use of different mold into the casting mold, forging mold, die casting, stamping die and other non-plastic mold and plastic mold.

In recent years, with the rapid development of plastics and general industrial and engineering plastics in the intensity and precision, continuous improvement, plastic products, also expanding range of applications, such as: household electrical appliances, instruments, construction equipment, automotive, Japan with hardware and other fields, the proportion of plastic products is rapidly increasing. A well-designed plastic parts can often replace the more traditional metal parts. Plastic household products, industrial products and the trend is increasing.

2, the general definition of the mold: in industrial production, mounted by a variety of presses and press the special tools, through pressure or non-metallic materials, the metal shape of the required parts or products, this special tool referred to as mold.

3, the injection molding process Description: Mold is a tool for the production of plastic products. It consists of several groups of components parts, the combination of the successful mold cavity. Injection, the injection mold clamping in injection molding machine, molten plastic is injected into the mold cavity forming and cooling in the cavity shape, and then separated from the upper and lower mold through the top of the system will be products from the top of the left mold cavity, the mold is closed again, the last the next injection, the injection molding process is conducted in cycles.

4, the general categories of mold: Mold can be divided into plastic and non plastic mold:
(1) Non-plastic molds are: die casting, forging die, die stamping, die casting molds.
A. Casting mold – faucets, cast iron platform
B. Forging Die – Auto Body
C. Stamping die – computer panel
D. Die casting mold – super alloys, cylinder

(2) Plastic mold production technology and production based on the different products are divided into:
A. Injection Molding – TV cabinet, keyboard buttons (the most common)
B. Blow mold – drink bottles
C. Compression Molding – bakelite switch, science bowl dish
D. Transfer Molding – IC products
E. Extrusion die – glue tubes, plastic bags
F. Hot Forming Die – transparent shell molding packaging
G. Rotary molding – soft toy doll

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History of Plastic Injection Molding

The injection molding has seen steady growth since its beginnings in the late 1800’s. The technique has evolved from the production of combs and buttons to major consumer, industrial, medical, and aerospace products.

In 1868, perhaps in response to a request by billiard ball maker Phelan and Collander, John Wesley Hyatt invented a way to make billiard balls by injecting celluloid into a mold. By 1872, John and his brother Isaiah Hyatt patented the injection molding machine. The machine was primitive yet it was quite suitable for their purposes. It contained a basic plunger to inject the plastic into a mold through a heated cylinder.

Revolutionizing the plastics industry in 1946, James Hendry built the first screw injection molding machine with an auger design to replace Hyatt’s plunger. The auger is placed inside the cylinder and mixes the injection material before pushing forward and injecting the material into the mold. Today, almost all injection molding machines use this same technique.

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The Plastic Injection Molding Process

Because of its high viscosity, in order for plastic injection molding to be successful melted polymer must be injected into a hollow mold with a large force.

Preparing the Mold
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Feeding the polymer resin (pellets) down to the auger (screw) is a large open-bottomed container. An electric (or hydraulic) motor is responsible for turning the auger inside a heated cylinder which feeds the pellets up through the grooves of the auger. A gate before the injection mold cavity restricts the flow of the melt into the mold and limits backflow. The pressure created by pushing the forward through the grooves up to the gate also produces heat on the inside of the cylinder which helps to melt the polymer and prepare it for injection into the mold.

Injection of Polymer Melt Into the Mold

As the auger moves forward it injects polymer melt into the mold at high pressure (typically 10,000 – 30,000 psi), holds it, and adds more melt to ensure the contraction due to cooling and solidification does not leave gaps in the final product. Eventually the gate solidifies and isolates the mold from the injection cylinder.

Cooling the Mold

Molds are typically air or water cooled. Sometimes small holes are bored into the mold that allow a cooling liquid (such as water) to be circulated. Injection mold cooling consumes about 85% of the cycle time for the entire process.

Unloading / Demolding

After solidification, the clamp holding the two halves of the mold together closed is opened allowing the part to be removed. The injection molding process can then be repeated.

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Plastic injection mold design features of the slider

Slider is a plastic injection mold to complete the side core pulling an important part, it is mainly connected with the forming core, with the angle pin lead for core pulling.

Under normal circumstances, it is core to form with the lateral side of the slide core, called the modular side of the slider. In the side of the core is simple and easy processing of cases, can also side slider and side core made ​​of one, called the integral side of the slider.

Slant-hole slider for cooperation with the bevel pillar, in line with the same time to make single 0. 5MM the gap, so the moment there is injection mold in a small space travel, the slide core is not tic and activities before the mandatory plastic extrusion die or punch, and to lock the block from the first slider, and then the pumping cell. Slider structure, depending on the mold side core pulling force structure and size of the decision.
With generally slide the slider and guide, to enable a smooth slide-led core for core pulling and accurate, must be fixed template template or guide chute opened, the slider and the guide chute to be a good match and lead slip in the slide after the completion of the action, still remain in the guide chute, the stay in the lead trough the length of the slider block length should not be less than 2 / 3, guide chute structure there are two commonly used species, rectangular-shaped guide dovetail guide chute and chute.

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What is Injection Mold Design

In the fast paced world of plastic molding, injection mold design is one of the most interesting and challenging jobs to be found. You will draw upon all of your creative abilities as you develop the design for new products.

It might look easy because of the powerful CAD programs, but in reality, they are just tools to help you. In the field of injection mold design you often must develop new and original methods of plastic molding. This sometimes requires a lot of creativity and inventiveness. What is a typical day like for injection mold design? Most injection mold designers follow a schedule similar to the mold makers. Because their mold designs are soon going to be manufactured by the mold makers, there is a very close relationship between these two.

A mold designer spends most of his time at his computer, using powerful CAD programs such as Unigraphics, AutoCAd, SolidWorks, MasterCam and many, many others. Nowadays, the programs are incredibly fast and powerful.

Very often the mold designer will be required to communicate with the various mold makers, CNC programmers, WEDM operators, etc.. This rapport is critical for a successful career as an injection mold designer.

Usually the designer does not work quite as many hours per week as the mold maker. Often mold designers have a shop background and help out in the mold making shop as well. This is especially common if there is a slowdown in design and a lot of work in the shop. How do you become an injection mold designer? Essentially, there are two paths in the USA. One is to learn on the job and the other is to learn at a design school. Both are common and work well.

Many plastic molding designers come from a mold making background. This is especially helpful to provide a realistic approach to mold design. There is no substitute for practical experience!

Several tech schools and universities offer excellent courses on plastic injection mold design. A background in mechanics, spatial relationships, ability to visualize 3D parts, and mathematics are all essential.

Is there a future in injection mold design?

Like everything else associated with the plastics industry, the answer is yes and no. Yes, because the plastics field is growing all the time and skilled designers are in high demand and low supply.

No, because the field is so competitive on a global scale. In this electronic age the designer does not even need to be in the same country as the mold maker. I had this experience at one shop; the designer was in Canada and we were in the USA. It worked well, but required considerable phone time on the part of the project manager. Conclusion Working in injection mold design is challenging, interesting, secure, well paying and in demand. Anyone interested will find many good courses available and companies seeking qualified designers.

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How Plastic is Molded

Plastic has, quite literally, become the cornerstone of our society. We make so many things from plastic that it is hard to imagine what our lives would be like if it was never invented. With so many of our everyday products being made of plastic, it is easy to understand why plastic injection molding is such a huge industry.

Approximately 30% of all plastic products are produced using an injection molding process. Of this 30%, a large amount of these products are produced by using custom injection molding technology. Six steps are involved in the injection molding process, after the prototype has been made and approved.

The first step to the injection molding process is the clamping of the injection mold. This clamping unit is one of three standard parts of the injection machine. They are the mold, the clamping unit and the injection unit. The clamp is what actually holds the mold while the melted plastic is being injected, the mold is held under pressure while the injected plastic is cooling.

Next is the actual injection of the melted plastic. The plastic usually begins this process as pellets that are put into a large hopper. The pellets are then fed to a cylinder; here they are heated until they become molten plastic that is easily forced into the mold. The plastic stays in the mold, where it is being clamped under pressure until it cools.

The next couple of steps consist of the dwelling phase, which is basically making sure that all of the cavities of the mold are filled with the melted plastic. After the dwelling phase, the cooling process begins and continues until the plastic becomes solid inside the form. Finally, the mold is opened and the newly formed plastic part is ejected from its mold. The part is cleaned of any extra plastic from the mold.

As with any process, there are advantages and disadvantages associated with plastic injection molding. The advantages outweigh the disadvantages for most companies; they include being able to keep up high levels of production, being able to replicate a high tolerance level in the products being produced, and lower costs for labor as the bulk of the work is done by machine. Plastic injection molding also has the added benefit of lower scrap costs because the mold is so precisely made.

However, the disadvantages can be a deal breaker for smaller companies that would like to utilize plastic injection molding as a way to produce parts. These disadvantages are, that they equipment needed is expensive, therefore, increasing operating costs.

Thankfully, for these smaller companies, there are businesses that specialize in custom plastic injection molding. They will make a mock up mold to the exact specifications, run it through the complete process and present the completed piece along with an estimate to complete the job to the customer.

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Production Calculators

The production calculators below will calculate the answers to 3 of the most common scheduling questions in an injection molding plant:

  1. What is the production rate?
  2. How long will it take to complete each production run?
  3. How much plastic material will be required for each production run?

Production Rate

Use this calculator to find the number of parts produced per hour given cycle time & number of cavities in the mould.

Reduce the cycle time by just 1 second to see how many more parts can be made per hour. The more parts made per hour the lower the part cost.

Calculation For Part Count Per hour
Cycle time (seconds):
Mold cavitation:
Parts per hour:

Production Time

Use this calculator to find the time required to complete a production run given the part quantity, cycle time & number of cavities in the injection mold.

Reduce the cycle time by a small amount to see the overall reduction in the production time.  For example, if a single cavity mould is required to produce 20,000 parts with a 19 second cycle time then the number of hours required is 105.6hours. However, if the cycle time is reduced to 18 seconds then the time is reduced to 100.0 hour. That’s more than 5 hours less which not only makes the production run cheaper per part but also makes the moulding machine available much sooner for the next production run.

Calculation For Number of Hours Required to Make x Parts
Quantity of parts required:
Cycle time (seconds):
Mold cavitation:
Hours required:


Mold Design Services

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Production Calculators for
Plastic Material Requirement

One of the most useful production calculators is a material requirement calculator.

Use this calculator to not only find out quantity of material required to produce a given number of parts  but also to work out material savings when comparing a hot runner mould to a cold runner mould.  For example, a 2 cavity cold runner mould making handles for buckets – the handle weight is 20 grams and the runner weight is 8 grams per shot. So for a 50,000 production run 1200kgs of material  is required.  However, if the same 2 cavity mold is converted to a hot runner then the amount of material required is 1000kg which is 200kgs less resulting in a saving of hundreds of dollars.

Although the cost of a hot runner mould is more than a cold runner mould this cost difference must be compared against the cost of managing the regrind (paying employees to feed cold runners into a grinding machine then blending it with virgin material). On top of this, the cost of a grinding machine itself along with the electricity cost must be included.

If the quantity of parts required are large enough then the extra cost of a hot runner will be justified. Even better, hot runners will generally have shorter cycle times than cold runner moulds meaning your production time will be lower.

Calculation For Plastic Material Requirement
Preferred unit of measurement:
Quantity of parts required:
Part weight (grams):
Mold cavitation:
Runner weight per shot (zero for hotrunner):
Material requirement (kg) :


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Estimating Cycle Time

Estimating cycle time in injection molding can be difficult because every injection molding company has a different way of doing things. Different levels of operator training and machine capability play a significant role in cycle time.

Material selection and part design also have an influence.


Part Design

The wall thickness of a part will directly affect the cycle time; thicker walls create longer cycle times while thinner walls create shorter cycles that’s why thin wall molding has cycle times in the 2 to 5 seconds range.  Click here to learn more about thin wall injection molding. The fact is, the part designer needs to make the wall thickness as thin and uniform as possible while still maintaining adequate strength in line with the application.

The height of a part also effects cycle time. The higher the part the further the machine platen has to open and the more time required to eject it off the mould core.

Mold Design

The cooling system within a mould is a heat exchange system.  There must be a sufficient number of cooling channels and sufficient flow rate of cooling fluid (usually water) to be able to extract the heat energy out of the plastic part within a specified time – this is called the cooling time of the moulding process and this directly effects cycle time. Different injection mold designers will design cooling systems that have different capabilities. This means cycle times will be different when using different designers.

Another aspect of mould design that will directly effect cycle time is the ejector system.  The ejector system should be rigid enough to remove the part off the mould core without delay.  Slow moving ejectors will only add to cycle time unnecessarily.  What’s more, it is important to know when to use ejector pins and when to use a stripper plate for ejection as there is a big difference in the cost of the mould.

The cycle time calculator does not take into consideration hydraulically or pneumatically operated moving slides and cores. This method will add considerable time to mould ejection.

Also, the use of compressed air in many applications is vital to achieving quick cycle times because this allows the vacuum between the part and the core to be removed before the ejector pins (or stripper plate) take over.  Without air, ejection is slower and will likely cause quality issues such as part distortion.

Machine Capability

A moulding machine will affect the cycle time in a number of ways:

  • New technology machines are faster than old technology machines.
  • Machines that have been well maintained will be faster and more reliable than machines that have not been maintained.
  • Smaller machines generally cycle faster than larger machines.
  • Machines with toggle clamps will give shorter cycle times than machines with hydraulic clamping units because platen speed will be faster.
  • All-Electric machines have the fastest platen speeds so cycle time will be the shortest (everything else being equal)

Temperature Controller

The temperature controller (chiller) needs to be able to extract the heat energy from the cooling fluid (usually water) fast enough to maintain mould temperature for the required cycle time.

Adequate flow rate is also essential so the pump must have sufficient power. Centralized systems can be inadequate especially when additional moulding machines are installed without regard to temperature controller capacity.

Technician Capability

Ask 5 different technicians to set the same mould and you will probably get five different cycle times.Different technicians have been trained differently and have different experiences and understandings so the chance of variation is huge.

Runner System

When estimating cycle time, the type of runner system will play a role. Hot runners and insulated runners (thick runners) are faster than cold runners (used in 2 plate and 3 plate moulds). This is because there isn’t a runner to eject between each cycle and the injection time is less since the flow length is shorter.

Whats more, hot runners and insulated runners are not affected by cooling time whereas cold runners, when too large, require more cooling time than the part needs.

Cycle Time Calculator

Use the following cycle time calculator for general purpose molding to get an estimation keeping in mind that the points above can have considerable influence. This calculator is to be used as a guide and the results are not guaranteed.

It accounts for parts with wall thicknesses between 0.8mm and 5.0mm  (0.032 and 0.196 inches).

Refer to https://www.improve-your-injection-molding.com/

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