Plastic injection molding (also spelled as plastic injection moulding in the U.K.) is a very popular manufacturing process. It is typically a fairly high-volume process. Here are some of the basics:
1. Basic Process:
To injection mold plastics, you basically melt the plastic resin and then inject the plastic material into a custom mold to create the custom size of your part. An injection molding machine will clamp the custom die, injecting molten resin into the die via a sprue and then runners to go to the cavities, wait for the part to cool, and then open the die and eject the finished part.
2. Part Cost:
Efficiency can be gained by producing multiple plastic injection molded parts via one shot in different cavities. Having more cavities, speeds up and hence reduces the cost of the plastic mold injection portion of the overall process. Also, the recycling of plastic scrap (known as regrind) reduces cost. Regrind is not always acceptable for every application; you should discuss with your plastic injection molding manufacturer whether your parts need virgin pellets or whether regrind is acceptable. Relative to metal parts, injection molded plastic parts are a much more cost-effective alternative.
3. Plastic Injection Mold
(also spelled as plastic injection mould in the U.K., and sometimes mistakenly called an injection plastic mold):
This is the custom mold into which molten plastic resin is injected and then later cooled and solidified into the final injection molded plastic part. A plastic injection mold manufacturer will take a variety of considerations into effect, including the volume so that they can optimize the mold for the volume (e.g. by changing the number of cavities in the mold).
4. Injection Mold Cost:
While the material and process costs for plastic injection molding manufacturing are often quite low, the mold costs are often quite high. The die costs for injection molding are higher than many other forms of tooling, so the annual volumes should justify the cost of an injection mold.
For low production volumes, assuming the resin or part tolerances or complexity do not require a steel mold, then an aluminum mold can prove to be an effective alternative to a steel mold given aluminum injection molds are cheaper. Aluminum molds also have better heat dissipation than steel molds, which means that heating times are reduced, cooling times are reduced, and the warp or deformation in the parts are potentially reduced relative to a steel mold.
Overall cycle times are therefore often better with aluminum molds compared to steel molds, which can reduce part costs. Given the properties of aluminum, aluminum molds are also quicker and easier to manufacture, maintain, and repair than steel molds.
Proper mold design is critical to reduce mold costs, and a good injection mold design shop will usually make recommendations for the mold or modifications to the part itself which will significantly reduce mold or part costs. A tooling engineer will typically perform mold flow analysis as well to ensure there will be fewer setbacks once you hit production.
5. Materials Suitable for Injection Molding:
Plastic injection molding companies work with a variety of plastic materials – and sometimes rubber materials as well. Pellets of thermoplastic or thermoset resins are used as raw materials. Specific examples of resin include:
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- Acrylonitrile Butadiene Styrene (ABS)
- Nylon (Nylon 6 or Nylon 66), also known as polyamide (PA6 or PA66)
- Low-Density Polyethylene (LDPE) or High-Density Polyethylene (HDPE)
- Polycarbonate (PC)
- Polypropylene (PP)
- Polyethylene Terephthalate (PET), also known as polyester resin
- Acetal (POM), also known as polyoxymethylene
- Polystyrene (PS) or High-Impact Polystyrene (HIPS)
- Acrylic (PMMA)
- Polyvinyl Chloride (PVC)
- Thermoplastic Polyurethane (TPU)
- Thermoplastic Elastomer (TPE)
- Polyetheretherketone (PEEK)
- Various hybrid resins (e.g. PC-ABS)
6. Applicable Industries for Plastic Injection Molding:
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- Medical
- Aerospace
- Consumer Products
- General Industrial
- Food & Beverage
- Automotive
- Various Other Industries
7. Example Injection Molded Parts:
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- Packaging
- Automotive OEM Parts
- Medical Parts
8. Sustainability:
While many companies choose to go the route of plastic injection molding parts given the low cost, plastics are notorious for raising environmental concerns. For example, the Great Pacific Garbage Patch is one notorious example of plastic waste and debris which is harmful to the environment.
National Geographic provides more detail on this waste but, given the push for being friendlier to the environment, many plastic injection molded parts are increasingly made from regrind (i.e. recycled plastic). There also is not a lot of plastic waste from the process when regrind is used. Also, there are various new biodegradable plastics that are less harmful to the environment when discarded.
Lastly, injection molding machines are using less and less energy as new versions of the equipment come to market.
9. Major drawbacks of injection molding:
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- High Tooling Cost: as a result, there is a high initial part cost for prototypes (very low Part cost later on). Also as a result, it is difficult to change part design given mold costs are very high.
- High Lead Time for Molds to be Manufactured
10. Variations on Injection Molding:
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- Multi-Shot Molding: sometimes there is a requirement for different colors on the same part – or different materials – which could require multi-shot molding or 2K molding.
- Gas-Assisted Injection Molding: this is a process to create hollow injection molded parts – potentially for weight considerations.
- Thin Wall Molding: used for thin parts
- Many Other Parts: given the flexibility of the injection molding process, it can create many other types of parts.
11. Other Completely Different, Popular Plastic Molding Processes:
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- Rotational Molding (Roto Molding): often used for large, hollow parts (e.g. tanks)
- Extrusion Molding: useful when the part has the same cross-sectional profile throughout (e.g. hoses).
- Compression Molding: similar to injection molding but can be lower cost for smaller production runs; often used for fairly flat parts or as a replacement for metal parts.
- Blow Molding: used for thin, hollow parts such as drums, tanks, or bottles.
- Thermoforming: used for fairly flat parts such as trays.
- Comments: Look for a manufacturer with a wide range of capabilities to suggest the appropriate molding process, as an injection molder will often simply suggest injection molding irrespective of the ideal manufacturing process. Injection molding often has higher tooling costs than the aforementioned processes, so it is worth considering alternative processes.
12. Plastic Injection Molding vs. Metal Injection Molding:
Plastic injection molders are sometimes confused with metal injection molders. Metal injection molding is a process involving a binder and sintering to create final parts from metal powders. Plastic injection molding companies and metal injection molders often manufacture parts for completely different industries.
Also, it isn’t common that the same manufacturer does both plastic and metal injection molding. Injection molding of plastics is a heat-sensitive process and metal injection molding is a sintering process that requires a lot of heat. Injection molded plastics would be sensitive to heat and deform.
Pangea Technologies offers a plastic injection molding service for its customers. Please reach out to Pangea Technologies for a quick quote or free half-hour design advice. We are happy to provide a quote for injection molded plastic parts or a plastic injection mold. We differ from typical injection molded plastic manufacturers in that we are also able to quote other custom parts.
We can also help select the most cost-effective and high-quality process for your parts given your specifications and design needs.
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