Plastic Injection Molding Process Guide

plastic injection molding process guideThis plastic injection molding process guide is intended to provide useful information about injection molding considerations, processes, the primary work leading up to getting a mold on a press, and the many caveats that go into producing a finished molded part. We hope you find this guide useful. It is intended for potential customers, engineers, designers, and outsourcing purchasing professionals.

What is plastic injection molding?
Simply stated, the plastic injection molding process injects heated plastic substrates into a metal-tooled cavity of the desired shape and it is allowed to dry, cure, and set.

But not so quick—let’s do a deeper dive into the step-by-step processes and the caveats evolved throughout the entire process to manufacture a finished plastic injection-molded medical device.

The plastic injection molding manufacturing process is ideal for consistently producing finished components or parts in a cost-effective manner. The up-front tooling process can be rather costly, so it is really only suitable for medium- to large-volume manufactured parts. This manufacturing process produces very little material waste, and the leftover thermoplastics can be recycled.

Scientific injection molding (SIM) is a data-driven, scientific approach. Cycle times are minimized, machine efficiencies are improved, and productivity is increased. Scientific injection molding is a strong tool supporting the objectives and practices of a lean manufacturing culture. SIM reduces or eliminates many of the forms of waste by providing a robust, predictable molding process.

Plastic injection molding guide process:
First, based on your CAD drawings and 3D printed prototype, a decision needs to be made on which injection molding process is best.

Commonly used plastic manufacturing techniques
The most commonly used plastic manufacturing techniques are injection molding, injection blow molding, two-shot molding, insert molding, rotational molding, compression molding, extrusion molding, and thermoforming. The requirements and specifications of your finished product will determine which injection molding process is best for your finished part.

What is plastic injection molding?
Injection molding is ideal for many small precision medical and large automotive part applications. This is the most commonly used plastic manufacturing process, and it offers a great deal of flexibility in material choices that can change the appearance of the final product.

What is plastic injection blow molding?
Injection blow molding is used in manufacturing parts that need to be hollow and maintain a uniform wall thickness. This process utilizes the blowing of air into the thermoplastic cavities until the desired shape is formed. Bottles are a common product manufactured by this method.

What is plastic two-shot molding?
Two-shot molding has a few names, it is also known as 2-shot, dual-shot, and double-shot molding. This process combines two parts into a single molded part by combining two machining processes. It offers flexibility for combining different resins materials together.

What is plastic insert molding?
This is a single-step process where two parts are formed together. This process is best suited for parts that need to form together tightly.

Plastic injection molding guide considerations:
Key considerations for the manufacturing process selection and thermoplastics material choice:

• Overall design
• Functionality
• Shelf life of the product
• Cost
• Tensile strength
• Shock absorbance
• Impact tolerance
• Scratch resistance
• Heat resistance
• Performance
• Stability
• Chemical resistance
• Final product weight
• UV protection


Plastic injection molding guide Thermoplastic Material Selection
Which medical-grade thermoplastic should be used for manufacturing the medical product?

• Polyethylene
• Polypropylene
• Polystyrene
• Polymethyl methacrylate
• Polyvinyl chloride
• Polyamides (nylons)
• Acrylonitrile butadiene styrene (ABS)
• Polycarbonate

Our blog Common Thermoplastics in Injection Molding ( provides an in-depth review of the different thermoplastics used in medical products, their key benefits, and their tensile strength in psi. Given the precision required in most medical devices, it is generally best to avoid resins that have asymmetrical shrinkage.  

Injection Molding Tooling
Mold tooling is essential in producing an accurate and cost-effective manufactured product.

These key tool-making considerations need to be made before the injection molding process can begin:

• Cooling compensation
• Tool mold size and material selection
• Designing, building, and debugging the mold
• Gate placement
• Polishing process, mold fitting, and assembly
• Mold fitting and trial prototyping

How the injection molding press works
The Injection molding manufacturing process
Injection molding is a very complex process. The actual machine has three major parts: the injection unit, the mold section, and the clamp. Thermoplastic pellets are fed into the hopper of the unit and then pushed into the barrel. A turning screw pushes the thermoplastic pellets forward. Heater bands surround the injection unit, melting the thermoplastic pellets. The screw continues to turn, pushing the molten into the open cavity area of the mold. It opens and the part is ejected. The mold closes and the process repeats.

Plastic injection molding illustration

Injection molding monitoring is an ongoing process
The machine operator and engineer continuously monitor the following predetermined parameters to deliver a quality product:

• Injection plasticizing pressure
• Mold, nozzle, air, melt, and cooling temperature
• Filling time
• Packaging and holding time

Why BMP Medical should be your next plastics manufacturing partner
Located in the United States, BMP Medical is near Leominster, Massachusetts, where Pioneer Plastic City was founded, and the region has many thriving plastic manufacturing companies.

BMP Medical provides plastic contract manufacturing services to leading medical device and diagnostic OEMs in need of injection molding, injection blow, two-shot, and insert molding. BMP Medical’s 80,000 sq. ft. facility is located in Sterling, Massachusetts. We foster strong trust-driven relationships to develop inventive solutions tailored to customer and end-user needs.

From concept through commercialization, BMP offers in-depth problem-solving backed by deep industry knowledge and operational excellence.

BMP Medical is a global contract manufacturer servicing the Medical Device and Medical Diagnostic markets in all areas, including, but not limited to, IVD, IV access, sample-preparation devices, and lateral-flow technologies. Headquartered in Sterling, Massachusetts, BMP Medical can service our partner requirements on a worldwide basis.

Certifications: ISO13485:2016; FDA registration #1220502; ISO Class 8 injection molding; ISO Class 8 assembly & packaging; 21 CFR 820 Quality System Regulation; FDA registered; MedAccred Certified

Services Offered:
• Class 7 Assembly
Clean Room Manufacturing (Class 8)
• Clean Room Packaging and Assembly (Class8)
• Custom Two-shot Molding
• Custom Assembly & Packaging
• Custom Injection Blow Molding (IBM)
• Custom Injection Molding
• Custom Insert Molding
• DNase- and RNase-free manufacturing
• FDA Registered Facility
• Pad printing
• Product design & development services
• R&D tooling
• Sterilization services

Have a question for our technical team on the plastic injection molding process?
Get to know BMP Medical today.




Plastic Products Catalog

BMP Medical is a quality manufacturer of plastic consumables and plastic products for research and medical diagnostic laboratories. Our products are manufactured to the highest quality standards. We offer a wide selection of products sold through the world’s premier laboratory distributors.

Non-Sterile and Sterile Centrifuge Bottles

  • Available with standard caps for low-speed work and sealing caps for high-speed applications
  • Caps have a built-in gripper to facilitate removal from the rotor
  • Extra thick walls for durability-resistant to most chemicals
  • Wide mouth design makes filling and decanting easier
These precision molded centrifuge bottles are available in polycarbonate or polypropylene. For low speed applications the bottles provide excellent value for large volume/large batch centrifugation procedures. For high-speed work, the easy-to-use Noryl® (Noryl® is a registered trademark of General Electric Company) sealing caps with silicone O-rings enable leak-free processing at high centrifugal forces over long durations. Bottles must be filled to 80% of capacity. Centrifuge bottles may be autoclaved at 121ºC at 15 psi for 20 minutes. Caps should be placed on the bottles without engaging the threads. Bottles are qualified to the rated g-forces using Sorvall® (Sorvall® is a registered trademark of Kendro Laboratory Products) and Fiberlite® (Fiberlite® is a registered trademark of Piramoon Technologies Inc.) high-speed rotors, tested at 20ºC, with bottles filled to 80% of capacity. Failure to use the correct rotor cushion or adapter will shorten the life of the bottle and may prevent achievement of the maximum rated RCF and/or bottle failure. Prolonged autoclaving and reuse may cause stress cracks, crazing or bottle discoloration.
centrifuge bottle
Sterile Centrifuge Bottles 
centrifuge bottle
Non-sterile Centrifuge Bottles

Cytology Funnels Category

BMP Medical Cytology Funnels for the Shandon CytoSpin® Centrifuge BMP Medical Cytology Funnels are the functional equivalent of the Thermo Shandon CytoFunnel®. Fully compatible with the Shandon CytoSpin Centrifuge, BMP Medical Cytology Funnels can be used to deposit a thin layer of cells in a clearly defined area of a microscope slide. The filter card absorbs any excess fluid. Operation is identical to those supplied by Shandon and easy-to-follow instructions are included in each pack. All products sold in case quantities only.


metal cytology funnel clip

Metal Cytology
Funnel Clips

Double Cytology Funnels

Double Cytology Funnels

Single Cytology Funnels

Single Cytology Funnels

Hemodrop Blood Dispenser

The Hemodrop is designed to reduce the risk of exposure to bloodborne pathogens in the Hematology laboratory. To prepare a manual differential smear, a drop of blood needs to be dispensed onto a glass slide from the specimen tube. Simply puncture the rubber stopper with the Hemodrop, and then invert and press gently onto the slide. A single drop of blood will be dispensed and the slide is ready. No more removing the stopper and manually putting a drop on the slide to test blood.  

Multipurpose Containers

These high-quality non-graduated lab containers are designed for the collection, transport, and storage of specimens and samples. Tight fitting lids prevent leaks and odors. Ideal for storing biologically hazardous samples for terminal autoclaving before disposal. Molded from various materials. The stackable containers are resistant to freezing and boiling.


Translucent Multipurpose


Polypropylene Beakers

Resistant to most commonly used laboratory reagents, including a variety of strong acids, bases, and organic solvents, these high quality polypropylene beakers are the premier choice for scientific laboratory applications. All beakers are transparent for clear content visibility and graduated for easily estimating volumes. Molded-in pouring spouts, and a rolled lip, for a dripless working environment. Autoclavable to 121ºC ( 248ºF).
Polypropylene Beaker

Specimen Collection

For specimen collection and transport, this system provides disposable products for the convenient collection, delivery, and testing of urine specimens. A plastic cup is provided for specimen collection and the pressure sensitive patient identification labels have ample writing space. The lightweight transport rack is small and easy to carry yet provides space for up to 20 tubes with labels and caps. The tubes are made of sturdy polystyrene and are safe for centrifuging. The flared top permits the use of a midget urinometer ensuring a tight cap fit.   
Specimen Collection Kit
Commode Specimen Collection Systems

Tissue & Ultra Tissue Grinders

We offer two lines of tissue grinders for the clinical lab and research lab. The standard product is all plastic and is designed to grind softer tissues such as liver and spleen. The Ultra tissue grinder line has a vitrified tip and is designed for tougher material such as muscle or bone. Both lines are delivered sterile sealed Tyvek® (Tyvek® is a registered trademark of DuPont) package.

tissue grinder

Tissue Grinders

ultra tissue grinder

Ultra Tissue

Urine Sedimentation System

BMP Medical, a manufacturer of diagnostic products, has a full line of urinalysis disposables. We offer the components separately, or in various kit sizes and configurations. The line is built around our 10-chambered Decislide which is made out of optically clear acrylic. It can take 10 individual samples on the same slide eliminating the need for a new slide for each sample.


Urine Sedimentation

Urine Sedimentation