Why is Precision Injection Molding Difficult?

Precision injection molding is a way to make plastic parts. It uses special molds and machines that need to be very accurate.

In this blog, you will learn about:

• What precision injection molding means
• Its main features
• What affects the process
• How to pick the right plastic materials
• The types of precision molds used
• The qualities of precision injection machines

This method helps create parts that fit well and work properly. Understanding these basics can help you see why accuracy matters in making plastic products.

Precision injection molding is a way of making plastic parts that are very accurate in size and shape.

You might already know about injection molding. It is a process where melted plastic is injected into a mold to form a specific shape. What makes precision injection molding special is that it can create parts that are almost exactly the same every time you make them. This level of accuracy is hard to get with regular machines and normal processes.

This method has grown a lot because of new plastic materials, called polymers. These polymers allow plastic parts to replace some metal parts that need to be very precise. You will often find precision injection molded parts in things like instruments and electronic devices.

When you hear about precision injection molded products, there are two important things to consider:

• Product size repeatability: every part is almost the same size.
• Product quality repeatability: every part meets high-quality standards.

These factors help make sure the parts work well in their final products.

(Source: Adapted from general industry knowledge on injection molding processes.)

When you design and make parts using injection molding, it’s important to get the size and fit just right.

If the measurements are off, your parts might not work properly or could break. This is even more important for complicated parts made with plastic injection molding.

Tolerances are the small limits of difference allowed in the part’s size.

Here are some common tolerance ranges for injection molding:

• Typical tolerance: plus or minus 0.1 millimeters (mm)
•  Very tight tolerance: plus or minus 0.025 millimeters (mm)

A good rule to follow is that the mold’s tolerance should be less than one-third of the final part’s tolerance.

This means the mold has to be even more precise than the part you want to create.

(Source: Injection Molding Handbook, 4th Edition, by Dominick V. Rosato)

Choosing the right plastic for precision injection molding can be challenging.

One big issue is shrinkage. Shrinkage means the plastic gets smaller as it cools and hardens. Different plastics shrink at different rates. This happens because of the types and amounts of polymers and additives used.

Polymers are long chains of molecules that make up plastics. Additives are extra ingredients added to change the plastic’s properties.

Even if you use the same plastic with the same ingredients, your final parts can still vary in shape and size. Factors like the plastic’s manufacturer, the season, and the weather can cause these differences.

For precision molding, you need plastics that hold their shape and size well. This helps ensure your products come out the right size every time.

When you pick plastics for precision injection molding, look for these qualities:

• Low shrinkage rate (so your parts keep their size)
• High mechanical strength (so the parts are strong)
• Good dimensional stability (means the parts don’t change shape over time)
• Good wear resistance (so your parts last longer)
• Ability to work in different environments

Thinking about these will help you choose the best plastic for your project.

If you want to learn more, materials science books and industry guides on injection molding can give you detailed information.

POM is a strong plastic that can be made even stronger by adding carbon fibers (CF) or glass fibers (GF). This material holds up well against slow changes in shape over time (called creep resistance). It also resists damage from repeated use (fatigue resistance) and can handle different weather conditions.

POM does not catch fire easily and has good dielectric properties, which means it does not conduct electricity well. It also releases lubricants easily, which helps parts move smoothly.

PA66 is a type of nylon. When you add glass fibers to it, it becomes very tough and can resist impacts and wear. It flows well when melted, allowing you to make thin parts—just 0.4 mm thick.

Glass fiber reinforced PA66 also stands up to heat, with a melting point around 250°C. One drawback is that it absorbs moisture, so you usually need to dry it after molding.

PBT is a kind of polyester plastic. It is known for its short molding time, which helps speed up the manufacturing process. The molding time for PBT is about the same as POM, PA66, and PA6.

D.PC and GFPC are materials reinforced with fibers. They offer several useful features, including:

• Good wear resistance
• Increased stiffness (rigidity)
• Stable size and shape under different conditions (dimensional stability)
• Resistance to weather
• Flame retardancy (they don’t catch fire easily)
• Easy to shape and form

These materials are used in many applications where strength and durability matter.

If you want to learn more about these materials, you can check technical resources like material datasheets or engineering handbooks. These details help engineers choose the right material for the job.

The mold needs to be very accurate.

Mold accuracy depends a lot on how well the mold is designed, processed, and assembled. If you don’t set the right technical requirements during the design phase, or if the mold’s structure isn’t good, even the best processing and assembly won’t guarantee accuracy.

To keep the precision of your injection mold high, pay attention to these four points:

• The design accuracy and technical needs of the parts should match the product’s accuracy.
• The mold cavity’s accuracy and the parting surface’s accuracy need to match the product’s accuracy.
• The mold cavity’s dimensional tolerance should be less than one-third of the product’s tolerance, depending on the product.
• Parting surface accuracy (called parting surface parallelism) helps keep the mold cavity accurate. For small molds, this should be about 0.005 mm.

Even though some structural parts in the mold don’t directly shape the product, they still influence the mold cavity’s accuracy. So, these parts need to be made accurately too.

Check how well the moving and fixed molds fit together.

Most injection molds use a guide pillar guide mechanism to align the moving and fixed parts of the mold. But this usually leaves a small gap between the guide pin and the guide hole. This gap can cause the mold to be misaligned when installed on the injection molding machine. Misalignment makes it hard to produce precise products.

High temperatures during injection molding can cause the moving and fixed parts to expand differently. This thermal expansion can cause further misalignment and reduce product accuracy.

In precision molds, misalignment should be kept to a minimum. You can use cone positioning mechanisms or cylindrical guide pin positioning mechanisms combined with guide pillar guides to improve alignment.

The mold must be stiff enough.

If a mold bends too much when pressure is applied, its accuracy drops. That will reduce the product’s quality too.

For one-piece molds, the shape, size, and material must make it stiff enough. For molds with inserts, stiffness depends mostly on the frame that holds the inserts.

The frame should be made of 40Cr alloy structural steel. This material is heat treated to reach a hardness of about HRC30. (HRC is a scale for measuring how hard a material is.)

The movable parts inside molds also need to move accurately.

Injection molds often have movable cores or inserts. If these parts don’t return to the exact position after each movement, the accuracy of the whole mold and the product will suffer.

To fix this issue, special motion positioning structures are used. These help make sure moving parts always line up correctly after moving.

By focusing on these points, you can make sure your precision injection mold works well and produces accurate products.

Source:  Information adapted from injection molding engineering principles and best practices.

Precision injection molding machines are used to make plastic products with very exact shapes and sizes.

When we talk about how precise these machines are, there are two main things to look at:

• Size deviation: This means how much the size of the product changes each time it is made.
• Weight deviation: This is how much the product’s weight changes each time.

Size deviation is harder to compare because products come in many sizes and thicknesses. Weight deviation gives a better overall idea of how precise the machine is.

Here’s a quick guide to weight deviation in injection molding machines:

• Ordinary machines have about 1% weight deviation.
• Better machines have around 0.8%.
• Precision machines have less than 0.5%.
• Ultra-precision machines have less than 0.3%.

Precision machines usually make products with a dimensional accuracy between 0.01 and 0.001 millimeters. Dimensional accuracy means how close the product’s size is to the exact measurements needed.

(Source: Industry standards on injection molding precision)

When you need very precise and repeatable results in injection molding, it is important to use multi-level injection feedback control. This means controlling several things at once:

• Position of the injection
• Speed of the injection
•  Pressure during holding
• Back pressure
• Screw speed

The displacement sensor you use should be accurate to 0.1 mm. This helps control key parts of the process like the metering stroke (how much material is measured), injection stroke (how far the material is pushed), and the thickness of the leftover material. This precision improves the quality of every molded part.

Temperature control is also very important. You need to keep the barrel and nozzle temperatures stable. Avoid big overshoots or swings in temperature. Using PID control can keep the temperature within about half a degree Celsius. PID stands for proportional, integral, derivative — a type of control system that helps keep things steady.

The quality of plasticization matters too. “Plasticization” means melting and preparing the plastic resin before injection. If the plastic melts unevenly, the finished product won’t be good. You need special screws and plasticizing technology to make sure the plastic melts evenly.

Besides that, the temperature of the barrel and screw should be controlled tightly. PID control can keep accuracy within 1 degree Celsius. Sometimes, a method called fuzzy control works even better for precision molding.

You also need to keep the working oil at a steady temperature around 50 to 55°C. If the oil temperature changes, the injection pressure will change too. A heating and cooling system helps maintain this stable oil temperature.

Holding pressure affects the final product a lot. This is the pressure applied after injection to make sure the mold is completely filled. It helps reduce part deformation and improves accuracy. The two key things that affect quality are:

• The screw position at the end of the cycle
• The stability of the holding pressure

Mold temperature also changes how thick your parts are. If the cooling time stays the same, a lower mold cavity temperature makes thicker parts. For example, with materials like POM and PA:

• At 50°C, product thickness might be 50-100 micrometers
•  At 80°C, thickness goes down to 20-40 micrometers
• At 100°C, thickness can be just 10 micrometers

Room temperature also impacts how precisely the parts fit together.

Sources:
1. Injection Molding Handbook, Rosato and Rosato, 2000
2. Moldflow Design Guide, Autodesk, 2010

Precision injection machines work under very high pressure. Because of this, the mold clamping system needs to be very strong and sturdy.

The mold has two parts called templates: one fixed and one that moves. These parts must stay very parallel to each other, with a difference of only 0.05 to 0.08 millimeters. This helps keep the mold aligned properly.

You also need to control the mold clamping force carefully. This force is how hard the mold parts are pressed together. If the force is too strong, the mold can bend or deform. This bending changes the size of the parts made, which is called dimensional tolerance. Dimensional tolerance means how much a part can vary in size from what you expect.

The mold needs to open and close quickly. Usually, the speed is about 60 millimeters per second.

Inside the machine, there are parts that help melt and push the plastic. These parts include:

• Screw: moves and mixes the plastic
• Screw head: part at the end of the screw
• Check ring: prevents melted plastic from going backward
• Barrel: the tube surrounding the screw

These parts are designed to melt the plastic well, mix it evenly, and inject it efficiently.

The screw is powered by a motor that provides driving torque. Torque is a force that causes rotation. The motor should offer high torque and allow for variable speeds, so you can adjust it as needed.

No matter what type of precision injection machine you use, it should keep the size and quality of the parts consistent every time.

Sources:
1. Injection Molding Guide, Plastics Technology
2. “Understanding Injection Molding” by Rahul Johar, Manufacturing Today

Precision injection molded products are designed with several important ideas in mind. These include dimensional tolerance, geometric accuracy, and surface finish.

Dimensional tolerance means how much a part’s size can vary from what was planned.

Geometric accuracy refers to how well the shape matches the design.

Surface finish is about how smooth or textured the surface is.

Besides these, you also need to think about repeatability. This means the product should maintain stable accuracy over time—whether it’s for days, months, or years—and in different environments.

The accuracy depends on a few factors:

• The properties of the materials used
• How precise the mold is
• And molding shrinkage

Molding shrinkage is the amount the material shrinks while cooling and setting. This shrinkage can affect how accurate the final product is.

There are four main types of shrinkage you should know about:

• Thermal shrinkage: Shrinking due to temperature changes.
• Phase change shrinkage: Shrinkage when the material changes from liquid to solid.
• Orientation shrinkage:  Shrinking caused by how the molecules line up.
• Compression shrinkage:  Shrinkage caused by pressure during molding.

Understanding these factors can help you better control the quality of injection molded products.

(Source: Adapted from general principles of injection molding found in manufacturing engineering literature.)

When you work with molding materials and molds, you need to understand a key property called thermophysical property. This means how materials respond to heat.

Here’s what happens:

• If the mold’s temperature is high, the product inside gets hotter too.
• When the product heats up more, it tends to shrink more as it cools.
• Shrinkage is the amount the product gets smaller after molding.
• Because of this, you should avoid setting the mold temperature too high, especially for precision injection molding.

Keeping the mold temperature balanced helps you get products that fit better and have less shrinkage.

(Source: Fundamentals of Polymer Processing, by Stanley Middleman)

Phase change shrinkage happens when a polymer changes from a soft, disorganized state to a more ordered, solid form. This change is called crystallization. As the polymer crystals form and line up, the material takes up less space. This reduction in space is what causes phase change shrinkage.

When you mold a plastic product, the temperature of the mold affects shrinkage too. If the mold temperature is high:

1. The polymer forms more crystals (this is called higher crystallinity).
2. More crystals usually mean the product will shrink more because the material packs tighter.
3. However, more crystals also make the product denser.
4. Denser material doesn’t expand or shrink as much when it heats or cools.

This is due to a lower linear expansion coefficient. (This is a number that shows how much something changes size with temperature.)

So, the actual amount your product shrinks depends on two things working against each other:

• How many crystals form (which tends to increase shrinkage)
• How dense the crystal structure is (which tends to reduce shrinkage)

Both of these effects balance out to decide the final shrinkage you will see in your molded product.

Source: This explanation is based on principles of polymer science described in materials engineering texts such as “Introduction to Polymers” by Robert J. Young and Peter A. Lovell.

When plastic is injected into a mold, its long molecules, called macromolecules, get stretched in the direction the plastic flows. As the plastic cools, these molecules tend to curl back up. This causes the plastic to shrink in the direction it was stretched.

How much the molecules line up, or orient, depends on a few things:

• Injection pressure (how hard the plastic is pushed in)
• Injection speed (how fast the plastic is pushed in)
• Resin temperature (the heat of the plastic material)
• Mold temperature (how warm the mold is)

Among these factors, injection speed has the biggest effect on how much the molecules orient.

Molecular orientation affects the final shape and strength of the plastic part. So, controlling these factors helps you get the results you want.

(Source: Principles of Polymer Processing, by Zehev Tadmor and Costas G. Gogos)

Plastics can be compressed, which means their volume changes when you apply pressure.

When you press on a molded plastic part at normal temperatures:

• Its volume gets smaller
• Its density goes up (density means how much mass is packed into a certain space)
• Its coefficient of expansion decreases (this is how much it expands or shrinks with temperature changes)
• Its shrinkage rate lowers (shrinkage is how much the plastic gets smaller after cooling)

Because plastics can be compressed, they also have something called elastic recovery. This means they can bounce back a bit after being squeezed. Elastic recovery helps reduce how much the plastic part shrinks.

The amount a plastic part shrinks depends on:

• How you mold it (like temperature, pressure, and cooling speed)
• How the part is used later on

Understanding these factors helps you get better results when molding plastic products.

(Source: Callister, W. D. Jr., & Rethwisch, D. G. (2014). *Materials Science and Engineering: An Introduction*. Wiley.)

The key parts of precision injection molding are high injection pressure, fast injection speed, and exact temperature control.

Different plastics, shapes, and sizes need different pressures to mold correctly. For precision injection molding, the pressure usually needs to be between 180 and 250 MPa. (MPa stands for megapascals, a unit that measures pressure.) Normal injection molding usually uses 40 to 200 MPa. Sometimes, the pressure can go as high as 450 MPa for special cases.

How fast the plastic is injected matters a lot. Faster injection means you can make more products in less time. It also helps make the parts more accurate in size.

To get better accuracy, especially for complicated shapes, a method called variable-speed injection is used. This means the injection speed changes during the process based on the part’s shape and the plastic’s properties.

Temperature is very important for making good products. It affects how the plastic flows and how the final part looks and behaves.

If the temperature isn’t controlled well, the plastic might not flow evenly. This can cause the part to shrink or be shaped wrong.

In precision injection molding, temperature control must be very exact. Here’s what you need to know:

• The barrel (the part where plastic is heated), the nozzle (where melted plastic comes out), and the mold (the shape used) must be kept at very steady temperatures.
• A tool called a PID controller is often used to keep these temperatures steady. It keeps temperature changes within 1°C.
• Normal injection molding machines may have temperature swings from 20°C to 300°C, which is less precise.
• You also need to watch the room temperature around the machine because it can affect the part’s size once it cools and is removed from the mold.

By controlling pressure, speed, and temperature carefully, you can get precise, high-quality plastic parts.

Sources:
1. Injection molding pressure ranges: *Plastics Technology*, 2023.
2. PID temperature control basics: *Engineering Toolbox*, 2024.

When you design precise plastic parts, you need to think about the plastic’s shrinkage rate.

Shrinkage rate means how much the plastic shrinks as it cools after molding. This can affect the final size of your product.

Shrinkage rates can change a lot during the molding process. Different plastics have different kinds of polymers (long chain molecules) and additives (extra ingredients). These differences change how the plastic flows and forms inside the mold.

You also need to consider other factors that affect shrinkage:

• Manufacturer variations
• Seasonal changes in the factory
• Environmental conditions like temperature and humidity

These factors can affect the shape and size stability of the parts you produce.

The shape and size of your plastic parts play a big role in accuracy. More complex shapes and sizes usually mean the parts will be less precise.

The mold is the hollow form where the plastic is shaped. The number of cavities (spaces where the plastic forms) and the size of production batches impact part accuracy.

• Single-cavity molds (one part shaped at a time) have higher accuracy.
• Multi-cavity molds (multiple parts shaped at once) usually have less accuracy.
• Small batch production tends to be more accurate than large batch production.

Your product’s accuracy can never be better than the mold cavity itself. The mold cavity is the space inside the mold that shapes your part.

Usually, the accuracy of injection molded parts is one or two tolerance levels lower than the mold’s accuracy. (Tolerance level means how much variation from the ideal size is acceptable.)

Today, most molds are made using high-speed milling, grinding, polishing, or electrical discharge machining. These methods help achieve very high precision in mold cavities.

To get precise plastic parts, you need a high-quality injection molding machine. These machines must:

• Have enough injection power
• Maintain high control accuracy
• Respond quickly through their hydraulic system
• Have a strong mold clamping system

This ensures the parts are consistent and meet tight specifications.

Sources:
1. “Injection Molding Handbook,” by Tim A. Osswald and Lih-Sheng Turng, 2008.
2. Manufacturing process principles, MIT OpenCourseWare.

Precision injection molding can be tricky. That’s because many factors affect how well it works.

Here are some key things you need to think about:

• Tight tolerances: This means the parts must be made very accurately, with little room for error.
• Material selection: Choosing the right material is important for the part’s quality and durability.
• Precision injection molds: These are the tools that shape the plastic. They must be made carefully.
• Precision injection machines: The machines need to work precisely to produce good parts.
• Shrinkage issues: When plastic cools, it can shrink. This needs to be controlled to keep the parts the right size.

Understanding these factors can help you get better results with precision injection molding.