Short Run Production vs. Long Run Production: What’s Right for Me?

Custom injection molding is one of the most versatile processes for creating plastic parts. From designing to producing, there are many decisions that play into the manufacturing process. 

So, how do you know if short-run production or long-run production is best for your part?

Short-Run Molding

Short-run molding typically falls into two categories. 

  1. Initial product development. After the prototype is done, fully tested and approved, the next step is building a mold and running some parts. This is short-run molding. You don’t want to commit to 100,000 pieces until you know how well it will sell. During this stage, parts will generally cost more because of the time it takes to set the mold and get the process up and running. On a longer run, those costs are absorbed over more pieces – which makes long-run piece price cheaper. 
  2. The other category is “Service” or “Replacement” parts molding These are terms used to describe short-run molding in the appliance and automotive industry. When they make refrigerators, for example, they make 300,000 a year – and will need 300,000 of the various parts used to make that refrigerator. Once they stop making that model, they may only need 300 pieces a year for replacement or warranty work.

Pioneer Plastics specializes in all of the above and is here to guide and talk you through these options and pricing so that you can make good educated decisions based on the known facts.

Long Run Molding

Long run molding – High volume production molding is achieved with multi-cavity steel molds. These molds will run anywhere from 20,000 to millions of parts per year. Upfront tooling cost will be higher because they are bigger multi-cavity molds, but they drive down part/unit cost due to pieces per hour, less time to produce and less labor. Payback on tooling cost is achieved through volume and our goal is to help our customers get that payback as soon as possible.

Consider the Complexity

If your part is small or large complex designs may sometimes require more advanced mold techniques, meaning cams or lifts to produce this part. This will take more time and labor and will lead to an increase in mold cost. 

If the part requires additional services or post operations like decorating ultrasonic welding or assembly, it will increase production time and cost. Labeling, over-molding, and heat staking are examples of additional services that your part may require to achieve your finished goods.

Get Started

Pioneer Plastics will educate you on all available options so that you know exactly what to expect at every step of the process. To get started on custom injection molding services, contact us today

Reshoring Injection Molding Services: What You Need To Know

Want to bring your custom injection molding back to the United States? Read on to learn more about bringing your supply chain back to the US.

Questions to Ask:

Do I own the molds? Check your agreement with your supplier. If not, a new mold will need to be built or that mold purchased from an existing supplier.

Is it a mold or insert that is part of a MUD base? Understanding this helps the new molder know what to expect.

Are drawings and or part samples available? You must have these to get an accurate quote.

Is material and packaging known? Material and packaging requirements are a key part of understanding what it takes to get you a quality part.

Is the size of the mold known? Some molders can’t run certain sized molds. Don’t waste your time with a molder that can’t accommodate your needs.

Are you able to get enough supplies before moving the tools? Transfer time or time to build a new tool will leave you without a way to get parts. Planning for this is important.

Reasons to Reshore

Global events. Think Coronavirus. Companies that purchase materials from China or other foreign countries have no access to their needed supplies in times of crisis.  

Offshore manufacturers may not assist with design. Often, the design is the most critical aspect of the process. If the design needs to be modified at any point, it’s much easier to work with a domestic manufacturer to make adjustments. Communication barriers like differences in time zones, languages, and software systems often exist with offshore manufacturers. There are far fewer communication barriers with domestic manufacturers. 

Local manufacturers will keep your designs protected. Since trademarks are territorial, a U.S. trademark registration will not protect your design in a foreign country. Trademarks must be filed in each country where you want your design to be protected. If you need your design to be protected in multiple countries, the International Bureau of the World Property Intellectual Organization (WIPO) will allow you to file an international trademark application for up to 92 countries concurrently. Applying for international trademarks may add extra time and costs to the offshoring process, which are factors that you wouldn’t run into with domestic manufacturers.

The quality standards of overseas manufacturers may not be the same as local quality standards. American manufacturers are held to strict laws and regulations regarding safety and quality. In addition to safety and quality, full regulatory compliance in the U.S. includes the makeup of resins and colorants (whereas, in international countries, those may not be included in regulatory compliance). These laws, regulations, and standards ensure the safety of employees and the safety of the product itself, so the customer can be sure that the product isn’t toxic or dangerous. 

You won’t be able to guarantee that international manufacturers are following rigorous standards like those set by the International Organization for Standardization (ISO), because no company is required to adhere to ISO standards. These ISO standard certifications are set to drive waste and costs down while driving product quality up. If offshore manufacturers are not ISO certified, it can result in poorer quality of materials, operations, and overall products. 

Longer supply chains equal slower turnaround time. An increase in distance means an increase in shipping time, shipping costs, and production time (due to time zone and communication barriers). Further distances can also mean unreliable delivery times because it can take weeks or even months to travel the distance. Offshore manufacturers may also be harder to reach- communicably and physically. 

Cutting down on transit makes domestic manufacturing a much eco-friendlier option. Your company will produce a much smaller carbon footprint by eliminating ocean shipping. According to the International Council on Clean Transportation, ships are responsible for 3% of global CO2 and greenhouse gas emissions (which equates to approximately 1 billion tonnes per year). Reliance on other countries can be detrimental to your business when importing and exporting capabilities are halted in the face of tariffs, tolls, pandemics, and government partnerships (or lack thereof). Reshoring your manufacturing services will protect your business from being hurt by these global situations. 

What to Do When Reshoring

If you want to use your existing molds, you’ll need to pull them from your current supplier and ship them to your new supplier. You’ll also want to send them the CAD drawings for the product if you have them. In the case that you are unable to have your molds shipped to the U.S., your new supplier will be able to use your CAD drawings to create new molds. Keeping your existing molds and designs will eliminate the costs of creating new ones – which can often be the most costly part of the process

Learn more about the reshoring initiative and use the total cost of ownership estimator to help account for all relevant factors when evaluating your sourcing of offshore manufacturers. 

At Pioneer Plastics, we can distribute the finalized part directly to you or your customers. This allows for faster turnaround time by eliminating the lengthy overseas shipping process. If you want to check on your project at any point in the process, someone from our services team will be available to answer your questions. We are centrally located in the United States, just a few hours from Indianapolis, St. Louis, Louisville, and Nashville. This location has excellent access to all major transportation networks, which allows us to ship your products quickly and efficiently. 

To get started on custom injection molding services at Pioneer Plastics in the USA, contact us today

Advantages of Custom Injection Molding

Injection molding is one of the most versatile processes for creating a custom plastic part. There are many benefits to the process. 

Strength & Stability

The injection molding process offers numerous ways to ensure parts are strong and durable. Wall thickness, ribs (for support), and material selection are just a few options.

Thicker walls are more expensive due to longer cycle times and more material but can provide a very durable part.

Ribs in key areas can give extra durability where needed and not have the cost issues of thicker walls.

Parts made from engineering-grade resins are able to withstand extreme temperatures, friction, and corrosion. This makes them more durable than commodity grade resins, which are typically used in consumer products like packaging. 

Environmentally Friendliness

Parts that are injection molded tend to last longer due to their quality and strength which reduces the number of parts that get thrown away over time. 

The injection molding process doesn’t waste materials. When a broken or unusable part is produced, it can be remelted and reused. 

Complex Capabilities 

A variety of sizes, colors, shapes, and finishes can be created through custom injection molding

Techniques like insert molding are used to inject resin over another material. An example of this is threaded fasteners that have metal in the center and a plastic shell. Overmolding is most commonly used to enhance the appearance or function of a part, like adding a softer layer of resin to the handle of a part. 

It can be used for assembly and decoration, as well. If pieces need to be labeled or decorated, in-mold labeling may be used. Connectivity techniques like heat staking and ultrasonic welding can be used to join two or more parts together.  


It is important to pick a partner that handles the entire process – from design to actual injection molding. This is much more cost-effective because the process is handled by one manufacturer from design to distribution. 

Injection molding can handle large scale production without compromising quality or adding manufacturing costs. If you’ll be producing a large volume of your part in every run, injection molding will be a cost-effective option. 

Flexibility in Design

All of the needs mentioned above can be solved with a good design. Pioneer Plastics has over 200 years of injection molding engineering experience in our design team – we know what makes a good part based on what your needs are. 

Before the mold is created, we can create a prototype of the part using our 3D-printer. This allows you to experiment with the product and make any necessary adjustments before the mold is built. 

Pioneer Plastics is a turnkey injection molder and produces everything in the United States – no international trade/shipping concerns. We are equipped to take your part from concept to consumer. Contact us today to get started on your custom part. 

Additional Services Utilized Along with Custom Injection Molding

Having additional services for injection molding allows you to customize your plastic part in many different ways. If you want to achieve characteristics like specific colors or textures, one of these processes may be used. 


In-Mold Labeling

The term “in-mold labeling” is directly derived from the technique, where a preprinted polypropylene (PP) label is placed in a mold. The mold closes and then the molten plastic is injected into the mold. It fuses with the label, and while curing, takes the shape of the mold. This results in the label and packaging/part becoming one. This in-line process eliminates any post decorating operations. 

These types of labels also offer high-resolution images. This also allows for decoration on all sides of a container with one single label. In-mold labels resist humidity and big changes in temperature, making them the best solution to decorate plastic containers for frozen and refrigerated products. In-mold labels are also scratch-resistant, will not crack, and are not susceptible to wrinkles.

In-mold labeling saves the environment because the packaging and label consist of the same material and can be fully recycled. It can be used in other molding processes, like blow molding and thermoforming.

Heat Transfer Decal

Heat transfer requires heat, pressure and dwell (time) to apply an image to a part. Heat transfers are pre-printed images using silkscreened, gravure, flexography, or digital printing methods on a release paper, film or foil. This is where the design or logo adheres to the part during manufacturing. Heat transfers are utilized on products that may undergo heavy wear and tear. 

Heat transferring offers more consistency because design placement will be the same on every single unit on the assembly line. The major advantage of heat transfer decorating is that it is a dry process. Another advantage is that the decorated part is ready to be handled or packaged directly after printing. 

Pad Printing

Pad printing is a printing process that can transfer a 2-D image onto a 3-D object. It allows you to add detailed images with high print quality to virtually any shape part. The unique properties of the silicone pad enable it to pick the image up from a flat plane and transfer it to a variety of surfaces, such as flat, cylindrical, spherical, compound angles, textures, concave, or convex surfaces. A perfect example of this is a logo on a golf ball.

Hot Stamping

Hot stamping is much like heat transfer in that it requires heat, pressure, and dwell (time) to apply an image to a part. It is good for the application of a one-color image. The foil rolls between the plastic part and the heated rubber pad. The pad applies pressure to the foil, adhering it to the part. These foils can be used to create colorful, textured, or metallic finishes on your finished parts.


Ultrasonic Welding

In this method, high-frequency sound waves are utilized in ultrasonic welding to melt the parts to become welded. No bolts, protrusions, soldering materials, or adhesives are used in sonic welding. The huge advantage of this method is that the temperature stays well below the melting point of the involved materials, thus preventing any unwanted properties or warping which may arise from high-temperature exposure of the materials.

Heat Staking

Heat staking is a method of joining two or more parts where at least one or both of the parts are made of plastic. The bond is made by partially de-forming (heating) the plastic part to fix the other. Heat staking is the most efficient way to bond metal to plastic and is commonly used in high volume/low-cost applications. Heat staking creates a solid bond with no additional hardware or adhesives.


Over Molding

Over molding is used to create one unified plastic part by adding an additional layer of resin to the existing part. It can add a soft, ergonomic layer of material over a hard surface. Over molding allows you to create features that one single piece wouldn’t be able to achieve, like adding color or enhancing the finish. Learn more about over molding here

Gas Assist

This is a low-pressure injection molding process. During injection, pressurized nitrogen gas is shot behind the molten plastic to help fill the mold. The gas forces the plastic against the walls creating hollow sections in thick areas, keeping the material from shrinking as it cools and creates smooth finishes for complex parts. This process can shorten the cycle time and save on material. 

As a turnkey injection molder, Pioneer Plastics has the capability to take your custom plastic part from concept to customer. To find out more about custom injection molding services, contact us and get started on your plastic part today.

What Drives the Cost of Custom Injection Molding?

No two custom injection molding projects are the same, so working with someone that understands all the aspects of part design and manufacturing is important.  


You might want to test your concept with a prototype to make sure that it fits and functions well. The cost of a prototype depends on two things: the completeness of the design and the size of the part being designed. 

If a design is flawless, it will likely be easy to get a prototype 3D-printed. On the other hand, if a lot of design work is needed before it can be printed, designing could be the most expensive part of the process. Here are a few scenarios that illustrate this:

  • For a small part (1” square) with a good design, the prototype can be done for under $100.
  • A concept for that same part but with no design could cost around $2,000 for design work and under $100 for the prototype. 
  • For a large part (24” square) with a good design, the prototype could cost around $2,000.
  • A concept for that same part but with no design could get up to $10,000 for design work and $2,000 for the prototype. 

Mold & Size 

When the part is ready for production, the mold needs to be built. One of the biggest drivers of cost is the size of the part. If a part is bigger, it’s going to require more material, tooling, and time to build the mold

In looking at parts with simple designs, a small part with simple design could have a mold cost between $10,000 – $20,000. A small part with no design work could have the same mold cost with $2,000 added for design. 

For large parts with a simple design, the mold could cost between $30,000 – $100,000. If the part is large but has no design work, it will have an additional $10,000 added for design. 


Similarly, the more complex the part is, the more time it takes to make the mold – and, well, time is money. Some parts may require cams, lifts, or core pins, which are additions to the mold that help achieve certain features a part may require. If the part has a higher volume, the mold would require more cavitation (parts molded per cycle) which adds cost to the mold and parts handling equipment (conveyors, chutes, bowl feeders, robots, etc.).

In addition, if the demand for a part is very high (hundreds of thousands of pieces), a multi-cavity tool may be required which also increases the cost. 

In looking at parts with complex designs, a small part with complex design could have a mold cost between $20,000 – $40,000. A small part with no design work could have the same mold cost with $2,000 added for design. 


With over 100 different varieties of resin, it can be difficult to determine which one is best for your part. Once we understand the characteristics that the part requires and the end use of the part, we can help you narrow down choices and choose the ideal option. Some examples of these characteristics are:

  • Requires FDA approval
  • Resistant to impact
  • Chemical resistant
  • Transparent
  • Durable against high temperatures or low temperatures

Commodity grade resins are typically less expensive than engineering grade resins. However, engineering grade resins will likely be able to withstand extreme temperatures and friction better than commodity resins. Understanding the purpose and function of the part will ultimately determine the best material for it. 

Additional Services

There are services that you might want in addition to the creation of the mold and the part. 

  • In-mold decorating allows for colorization, stylization, and other effects to be added during the molding process. 
  • Pad printing allows for two-dimensional images to adhere to three-dimensional plastic products. 
  • Plastic packaging can be designed and created specifically for your product. 
  • Assembly can be conducted for complex parts. 

Turnkey Injection Molding

Turnkey injection molders, like Pioneer Plastics, are full-service manufacturers who build the mold and handle the injection molding process. Keeping the part in-house from beginning to end makes the process more time-efficient and cost-efficient. 

Being skilled in all aspects of the process allows us to help you with design, selecting materials, and any questions you may have. Contact us to get started on your custom part. 

Injection Molding With High Heat Resins

Temperature plays a huge role in the custom injection molding process– even helping to determine the type of resin the part is made from. 

Resins that are specifically made for high temperatures contain properties that set them apart from other resins, making them ideal for parts that need to withstand extreme heat. 

High Heat

Most plastics used in the market are only suitable for use at temperatures below 275°F.  Resins that are resistant to high heat can withstand temperatures ranging from 275°F to 500°F without losing its shape and/or properties.


Plastics that are resistant to high temperatures are a light, versatile alternative to metal, ceramics, and older-generation polymers. 

Thermoplastics are most commonly used in consumer products (like milk jugs) that won’t need to withstand very high temperatures. Thermoplastics are also great for prototyping because the material can be reused for another prototype or for final production. Thermosets are most commonly used for parts that need to withstand extremely high temperatures. 

Examples of industries that use high heat resins in their products are: 

  • Automotive – gears, hot fuel systems, fuel reservoirs, ignition modules, oil screens.
  • Aerospace – lightweight aerospace components like brackets, gaskets, guides, seals, spacers, and washers. 
  • Electrical – wiring, cabling, sleeving and electrical shielding products.
  • Medical – electronic equipment.
  • Industrial – Welding masks.
  • Restaurants – Pans, lids, and trays.

Mold Design

Custom Injection molding with high heat resins are atypical to conventional molding. From mold design to processing, each process is different for thermoplastics and thermosets. 

The mold needs to be designed for extremely high heat temperatures and clearances. They also require a special mold-temperature control unit with oil to attain adequate high mold temperatures. An experienced mold processor should handle high heat resins to ensure that the process is completed without any defects

Our Process

Pioneer Plastics has the equipment and experience to meet the heat resistance needs of your part.

If you have an idea for a custom plastic part, we can assist you every step of the way – from design to distribution. Contact us here to get a quote for custom injection molding services. 

Complex Molding Techniques For Complex Parts

Injection molding is perhaps the most versatile process for creating custom plastic parts. By utilizing the complex molds involved in this process, manufacturers can create intricate parts in a variety of sizes, colors, shapes, and finishes. 

Everything from small buttons to washing machine tubs can be created through this process.

Custom molds are made out of metal using lathes, mills, or CNC machines. This state-of-the-art machinery ensures that the mold is made to the precise measurements of the part. 

Building the Custom Mold: Exploring Specific Features

Most molds are built using a traditional straight-pull. This is when the mold can open and the part falls out without any help. Sometimes, parts that require specific features or functions may require a more complex mold or a longer ejection process. 

In order to achieve certain features, more advanced mold design is sometimes necessary. 

When undercuts on a part are necessary, cams and/or lifts can be used. These cams and lifts enable parts of the mold to be moved out of the way after the molding cycle enabling the mold to open and the part to come out of the mold.

Mold with cams
Mold with lifts

Some parts may require a small hole going through its entirety. Features like these can be solved with core pins that are affixed to the mold. This allows the melted resin to form around the pin and harden to form the opening. 

Insert molding and overmolding are typically used to achieve effects that a typical production mold cannot achieve, like enhancing the appearance, function or adding a layer to the part. 

Insert molding is a process of injecting a plastic resin over another material, typically metal. One popular example of this is threaded fasteners. Overmolding is used to add an additional layer of resin to an existing plastic part, like adding a soft layer to the handle of a tool. 

Cover panel with brass inserts

Multi-cavity molding is used for producing large quantities of parts because the mold can produce multiple parts in one single shot. These types of molds are more expensive to make but can lead to piece price savings due to a faster production time.

After the mold is built, tested, and perfected, it is transferred to the injection molding machine for production to begin. 

Injection Molding Techniques 

The initial injection molding process consists of injecting plastic (heated to a liquid state) into the mold cavity, where it cools and forms into the custom part. 

Secondary processes like assembly and decoration are used if pieces need to be connected, decorated, or labeled. Decoration includes hot stamping, labeling, and printing on the parts.

Foil hot stamping in-mold label
Drink-n-Plate in-mold label

Ultimately, the design of the part will determine the design of the mold and the techniques used to produce it. From design to distribution, Pioneer Plastics has a streamlined production process that saves time and money. 

Contact us today to get a quote for your custom plastic part. 

Injection Molding: Your Questions Answered

If you have an idea for a custom plastic part, you may have a lot of questions about the manufacturing process. Injection molding is a time-efficient and cost-efficient way to achieve the quality part you need. 

Here are answers to some of the most frequently asked questions about custom injection molding: 

How do you know if injection molding is the right process for a product?

There are several ways to manufacture a plastic part. Understanding and knowing the part design, the end function of the part, the quantity needed, and the target part cost will help determine if injection molding is right for you. 

What is the minimum number of units required for a new molding project?

Due to the high cost of the mold ($10,000 – $200,000), injection molding is typically a good choice if you need a higher volume/quantity of your part. If you only need 100 parts, injection molding may not be right for you. This depends on your budget and how critical the part design is to your project’s success.

Will a prototype be tested for durability and usability before the mold and lot are produced?

Sometimes yes, only if the customer requests it and or if it’s recommended by our engineering team. There are two types of prototypes: prototype parts made by a 3D printer (extremely small volume) and prototype molds (which will produce exactly what the production mold will produce). The prototype mold is used to produce many parts for sizing and extensive physical testing, prior to production mold build.

How do you know what type of resin is best for a product?

Since there are over 100 different types of resins, we’ll need a good understanding of your vision for the product and what it will be used for. If it needs to have physical requirements like a special color or specific heat resistance, among many other possible factors will help determine which type of resin will meet those needs. Learn more about resins here

How long does it take to build a new mold?

Depending on the complexity of the part, the number of cavities (parts in the mold), and the size of the mold, it can take anywhere from 2 to 26 weeks to build a new mold.

Will the molds for my part be maintained between production runs?

Yes, all molds at Pioneer Plastics are on a PM (preventive maintenance) schedule.

Can the injection mold be modified after the first run of production?

In some instances it can, but not always. It depends on what the change is and whether or not it interferes with the function of the mold. If the modification is major, it may simply require a new mold.

Are there any size limits on parts that Pioneer Plastics can produce?

Pioneer Plastics can produce items as small as a pen cap and as large as a lawn chair. Learn more about our injection molding machines here

Does Pioneer Plastics offer secondary services (like part assembly)?

We offer assembly, in-mold decorating, pad printing, heat transfer decal, and more. Learn about all of the additional services we offer here

For more than 35 years, we’ve been experts in the design, development, and distribution of custom plastic parts. Contact Pioneer Plastics to get a quote for custom injection molding services.

Insert Molding vs. Overmolding: What’s the Difference?

The custom injection molding process starts with a conceptualized design and ends with a uniquely manufactured product. 

If your product needs to be aesthetically pleasing or ergonomically friendly, injection molding techniques like insert molding or overmolding might be used to achieve those needs. 

It might be difficult to figure out which process is best for your custom plastic part. Here’s a break down of the insert molding and overmolding processes. 

Insert Molding

Insert molding is a process where a rigid plastic resin is injected over another material, typically metal. The metal piece is placed in a tool for the plastic to be injected around. Encapsulating the insert with plastic creates a single molded piece that’s typically stronger than one created by assembling separate pieces together. 

An example of insert molding (a white blower wheel)

This process is most commonly used for custom-machined metal parts like threaded fasteners and electrical parts. There is no chemical bonding between metal inserts and plastic, so the insert and resin components must be designed for mechanical bonding. 

Insert molding is controllable and allows better encapsulation than other methods used to mold plastic around metal parts. Molded inserts eliminate the need for a secondary insert installation process, making it more time-and-cost-efficient. 

Benefits of Insert Molding: 

  • Eliminates secondary assembly operations like gluing and fastening. 
  • Creates design functions and features that are not feasible by plastic alone. 
  • The encapsulating process can add strength and durability to parts. 


Overmolding is the process of adding an additional layer of resin to the existing plastic part. This process adds characteristics that a single piece of plastic can’t provide. 

An example of overmolding (a red caster wheel)

One common reason to use this technique is to add a soft, functional, ergonomic layer of rubber-like material (typically a thermoplastic elastomer) over a hard surface. This improves the grip of a hand-held item. 

One example of this is toothbrushes. Toothbrushes are often made from a hard plastic center with a soft plastic grip around the handle. 

Another use for over-molding is to change or enhance the appearance of a part by overmolding material of a different color or finish. 

There are two primary methods of overmolding: 

  • Two-shot molding uses a single production mold.
  • Pick-n-place molding uses two production molds where an entire batch of parts are molded. Then, they are manually placed into a second mold where the overmold resin is injected to produce the completed parts. 

Thousands of possible combinations exist for over-molded material. Resins have to be adhesive and compatible with each other in order for the process to work. 

If the goal of using overmolding is to enhance grip or increase cushioning in your product, make sure your injection molding company knows those goals. Factors like cushioning, flexibility, and friction will play into the type of resins that are used in the product.

Benefits of Overmolding:

  • It can provide a soft, non-slip grip to your product. 
  • It acts as an environmental barrier to shock, vibrations, and noise.
  • It creates colorful, visually attractive surfaces. 
  • It reduces the number of secondary steps and costs associated with them, in turn reducing the complexity of assembly. 
  • It can provide adhesion between different materials and eliminate the need to assemble different materials by hand. 

Why Choose Pioneer Plastics?

We specialize in all aspects of the process- from concept to consumer. We can design, build, produce, and distribute your part. From insert molding to overmolding and so much more, we can help you produce your custom plastic part.

Contact us today to get a quote for the production of your part. 

Injection Molding Defects and How to Avoid Them

Just like the design of the part, the manufacturing process is important to the integrity and function of the part. If not executed properly, the injection molding process can cause cosmetic defects and form defects, sometimes compromising the safety and performance of the product. With that in mind, be sure that your injection molding partner takes the proper precautions with your product so that none of these types of defects occur.

Here is a list of common injection molding defects and how to avoid them:

Flow Lines

Flow lines are lines commonly caused by a variation in the cooling speed of the material as it flows through the mold. They often appear in a wavy pattern and might be a slightly different color than the rest of the piece. Flow lines don’t typically impact the integrity of the piece, but they can be unsightly and unacceptable in certain products (like high-end frames for glasses).

How to Avoid Them:

Flow lines can be avoided by increasing the injection speed and pressure to ensure the material fills the mold before cooling. You can also try rounding the corners of the mold to increase wall thickness and keep flow rate consistent.


Burns can be caused by trapped air or overheated resin in the mold during injection. Excessive heat or increased injection speeds can cause overheating, in turn causing the resin to burn. Burns can appear as black or brown colored spots on the edge or surface of the part.

How to Avoid Them:

To prevent burns, lower the melt and mold temperature to prevent overheating, reduce the injection speed to limit the risk of trapping air, and shorten the mold cycle time so that trapped air and resin doesn’t have time to overheat. 


Most commonly heard when referring to wood that has dried unevenly, warping is a deformation that appears when different components of the plastic part shrink unevenly. During the cooling process, uneven shrinking can put stress on parts causing bending or twisting. Most often, the cause of warping is the cooling process happening too quickly. 

How to Avoid It:

Warping can be prevented by ensuring the cooling process is long enough to prevent uneven stresses on the material and by lowering the temperature of the material or mold. 


Sink marks are depressions on an otherwise flat surface of a part. These can occur when the inner part of the mold shrinks (or cools too slowly), pulling the outside inward before it has a chance to completely cool. 

How to Avoid It:

To avoid sinking, increase cooling time and increase holding pressure to allow the material at the part’s surface to cool. 

Air Pockets

Air pockets are trapped air bubbles in the finished part. Air bubbles are commonly caused by inadequate pressure to force air out of the mold. Larger air pockets can weaken the part. 

How to Avoid Them:

To avoid air pockets in the mold, raise the injection pressure to force out trapped air pockets.

Weld Lines

Weld lines are a result of weak material bonding. They appear on the surfaces of parts where the material has come back together after splitting into two or more directions in the mold. If the material bonding is weak, the overall strength of the part is lowered. 

How to Avoid Them:

Weld lines can be avoided by increasing the material temperature to prevent uneven solidification and increasing injection speed and pressure to limit cooling before the mold is filled. 


Jetting occurs in molded parts when the initial jet of molten material starts solidifying before the rest of the material fills the mold. Jetting looks like squiggly lines on the surface of the part. 

How to Avoid It:

Jetting can be avoided by reducing injection pressure (to prevent rapid movement of the material into the mold) and increasing material and mold temperature (to keep the material from solidifying too early). 

While there are many potential problems that can occur, partnering with an experienced custom injection molding company like Pioneer Plastics should relieve your worries.

If you have an idea for a plastic product and would like to partner with a company that has been manufacturing them for over 35 years, contact us today!