Prototype Injection Molding

What is Plastic Injection Molding?

Plastic injection molding is the process used to create the plastic parts that are used in most of the products you use every day. Plastics are used in a wide range of applications, from simple buttons to complex implantable medical devices visible under x-ray and everything in between. This is why plastic resin manufacturers offer over 60,000 grades of plastic. Plastics are well suited for manufacturing due to their relatively low cost per part, part consistency, and ability to fabricate large numbers of parts quickly.

The basic principle of injection molding hasn’t changed substantially since it was first patented in 1872. Over the last 150 years, innovations have involved the addition of electronic control and automation of the molding process. Plastic injection molding consists of two essential parts: an injection unit and a clamping unit. The injection unit is where the plastic resin is added to the machine, and heat is applied to change the plastic resin to a flowable liquid state. The clamping unit contains the mold.

The mold is a machined assembly that contains the part detail to be molded. Once the mold is split into two sections, it allows the clamping unit to open and close the mold as needed. In order to mold a part, the clamping unit closes the mold, and the injection unit pushes the liquid plastic into the mold. The plastic quickly cools. The clamping unit opens the mold, and the part is ejected. The process then repeats.

Prototype Injection Molding Process

When developing a new product, it is typically necessary to fabricate a prototype injection mold to perform functional testing of the product. There are a couple of key reasons engineers utilize a prototype injection mold rather than a production mold. A production mold typically takes 2-3 months to fabricate. It would be a very long time to wait for parts. Prototype molds can be fabricated in as little as one week, allowing engineers to proceed more quickly with testing.

Typically, engineers discover during testing that their part design needs changes in order to meet product requirements. In that case, a mold iteration is needed. Prototype molds are faster and easier to iterate due to the softer metals that are used. Iterations are typically completed within a few days. Production molds are made out of hard steel. While mold iterations are possible in production molds, they tend to affect the mold’s life negatively. Ideally, the part design is finalized when the production mold is fabricated, and no iterations are needed. This allows the production mold to meet the full life cycle and optimize the investment.

What Types of Materials are Used?

Materials that are compatible with injection molding are known as thermoplastics. Within thermoplastics, there are approximately 70 different types. Commonly known types include polycarbonate, polypropylene, nylon, etc. Each thermoplastic type has a set of unique characteristics and usage scenarios that distinguishes itself from other types. Material selection for a part can be challenging because the engineer must be familiar with the 70 different types and know which type to select for the particular application.

For example, polycarbonate would be an ideal type to select if a part requires transparency and high strength. Once the type is selected, the next step would be to select a particular grade of material. Websites such as and are helpful plastic resin databases that make it possible to select a particular grade.

Continuing to use the polycarbonate example previously discussed, we find that there are over 6000 grades of polycarbonate available. Selection of a particular grade requires a review of the part design and its product requirements such as mechanical, cost, tolerances, environmental conditions, assembly, sterilization, regulatory, aesthetics, and any special requirements.

For example, this polycarbonate part may be a relatively thin wall part requiring a high melt flow grade. It could also be a medical part requiring gamma sterilization, requiring an additive to prevent color shift. These requirements can be searched on and to compile a list of the grades that meet those requirements. The final step would be to contact the distributors of those material grades for pricing and availability, then place an order for the selected grade of polycarbonate.

  • Mechanical
  • Target Icon
  • Tolerances
  • Environmental
  • Assembly
  • Sterilization
  • Regulatory
  • Cost
  • Aesthetics
  • Special Requirements

Arburg Molding Machines

Protoshop uses only Arburg injection molding machines, a high-quality brand built in Lossburg, Germany commonly used in production injection molding.  Our goal is to replicate production molding as closely as possible and using a production quality injection molding machine is critical.  Testing and development using a high quality prototype part that closely replicates production makes transfer to manufacturing an exercise rather than a step involving risk.

Arburger Allrounder 370E 600-170 EDrive Injection Molding Machine
Complex Part Design Image - Complex Geometry

Complex Part Geometry is Our Specialty

We specialize in challenging molding applications.  We have successfully designed and fabricated thousands of prototype molds and don’t reject complex part geometry like other prototype molders.  If a part is moldable, we can do it.


We have extensive experience with both insert and overmolding. In addition, we have vertical injection
molding machines to best facilitate insert and overmolding applications.

Insert molding

Insert molding involves a component, typically metal such as a blade or needle, which is inserted into the mold then plastic is molded around the metal.

Insert molding requires extensive experience in effective methods of holding the insert in the mold consistently under the high heat and pressure that occurs during molding.

Blade insert molding example blue handle

Blade Insert Molding Example

A blade shown in gray that is insert molded with a blue polycarbonate slide.

Needle insert molding example blue handle

Needle Insert Molding Example

A needle is insert molded with a blue polypropylene hub


Overmolding involves first molding a substate material that is overmolded by one or more materials to form a single part. The most common application is a toothbrush. A rigid substrate is overmolded by a flexible elastomer that functions as a grip feature. Material selection is critical since it requires selecting a compatible material pair. Part design is also important to ensure that the overmold is properly captured and/or bonded to the substrate as well as ensuring all shutoffs prevent flashing of the overmold.

Overmolding example – Polycarbonate substrate is shown in orange and flexible TPE overmold shown in yellow

Overmolding Example Part

Overmolding example – Polycarbonate substrate is shown in orange and flexible TPE overmold shown in yellow

Overmolding example – A complex micro molded COC substrate is shown in orange and a flexible TPE overmold is shown in yellow.

Overmolding Example Part

Overmolding example – A complex micro molded COC substrate is shown in orange and a flexible TPE overmold is shown in yellow.

Benefits of Injection Molding

  • Surface and appearance defects
  • Non-uniform or unreasonable wall thickness
  • Incorrect gate type or location
  • Critical dimensions, warp, and material shrink

What is the Difference Between Injection Molded Prototypes and 3D Printed Prototypes?

There are significant differences between injection molded prototypes and 3D printed prototypes that determine when they are used during the development cycle. 3D printed prototypes are typically used in early development because each part is relatively expensive, but they are built quickly. This is conducive towards rapid testing and iteration of a small number of parts needed during early development that allows a majority of mechanical testing to be completed. The drawback of 3D printed prototypes is that they are only a rough approximation of the final material to be used. 

In addition, the accuracy and surface finish of 3D printed parts is typically inferior to prototype molded parts. At a point later in development, the decision is made that development using 3D printing prototypes has reached its limit, and completion of development requires fabricating prototype molds. Switching to injection molded prototypes allows the engineer to perform testing using parts that more closely replicate the final production version of the product.

Prototype Molding vs. Production Molding

Prototype molds have a wide variety of variations, depending on the method that the prototype molder uses to fabricate their molds. A low-quality prototype mold is fabricated using 3D-printed cavities. The advantages of this method are the low cost of the mold and the speed at which it can be fabricated. The drawback is that molded part accuracy tends to be poor, and these molds are not very robust, sometimes only providing less than 100 parts. Also, not all molded parts have a geometry that is compatible with 3D printed cavities.

A mid-quality prototype molder would use a soft metal such as aluminum to fabricate the mold. Part quality and accuracy are improved over 3D printed prototype molds, and hundreds of parts are typically possible with this method. A high quality prototype molder would use a combination of steel and soft metal to replicate production mold quality and accuracy more closely.

A high-quality mold can provide thousands of parts, but the mold cost and lead time typically increase as well. Protoshop’s prototype molding method takes high-quality molding a step further by adding temperature control to the mold with other proprietary features that can provide tens of thousands of parts, providing superior part quality at lower cost and lead time.

Production molds are built with hardened steel designed to provide millions of molded parts. They are several times the cost and lead time of prototype molds. While prototype molds run manually with an operator, production molds are designed to run automatically using robots and conveyors.

Why Choose Protoshop?

Engineers and machinists founded Protoshop from the medical product development industry. We have tried working with several different prototype injection molding companies and found that they all failed to meet our expectations. Too many parts that we submitted for quotation were rejected. Protoshop accepts complex parts, including insert and overmolding.

If it’s moldable, we can do it faster than anyone else. Part quality at other prototype molders was sometimes so poor that parts received were not functional. Protoshop has developed a proprietary molding method that more closely replicates production quality and provides high quality parts that will shorten your time to market.

Protoshop also offers part design and material selection assistance, helpful services not offered by most prototype molders that ensure your part designs are successful the first time. Protoshop molds are also designed to allow for the rapid mold changes needed for product development. Other prototype molders are either not set up to provide rapid mold iterations or don’t offer them at all. Finally, Protoshop molds are robust, transferrable, and able to mold tens of thousands of parts quickly to support the development and may be used for bridge tooling.