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Injection moulding and vacuum forming are two of the most popular approaches used to process plastics. This article provides an overview of each method and reviews the advantages and disadvantages of each.
What is Vacuum Forming?
Vacuum forming also recognised as thermoforming is a manufacturing process whereby plastic sheets are preheated in automated vacuum forming equipment until soft and malleable. The preheated sheet then makes contact with a mould or form and a vacuum source is turned on that sucks the sheet to the mould. When cool, the sheet takes the shape of the mould. It is then trimmed with either a five axis CNC router or die cut. Should additional detail be required, positive air pressure is used to abet the forming process, a technique generally known in the industry as pressure forming.
Vacuum forming moulds can be either male or female depending on the geometry of the parts and where the weakest areas need more material thickness. Moulds can be surfaced to convey different finishes using a female mould and positive air pressure.
What is Injection Moulding?
Injection moulding is a far more complex manufacturing technique than vacuum forming and requires an intense level of engineering expertise. The process of injection moulding begins with granules of polymer which are put in a hopper and released into a heated barrel. A screw feeds the material along the heated barrel where it is plasticised. The then liquid polymer is injected into a steel or aluminium mould through a gate, being held in a press under acute pressure. Post cooling, the mould opens and the finished parts are ejected from the mould.
A great amount of engineering time and cost is needed to manufacture the mould used in the injection moulding process, making vacuum forming the favoured process for many applications where time-to-market and low cost is critical.
Each method however, has its own distinct merits and also unique drawbacks.
Advantages of Vacuum Forming
- Reasonably fast prototyping and production time frames
- Ability to create large parts (up to 48 inches x 96 inches)
- Lower start-up costs — patterns and moulds can be made economically from MDF, high density foams and epoxy
- Ideal for repeat jobs — aluminium castings can be made which have practically unlimited lifetimes
- Good price point on small and medium runs
Disadvantages of Vacuum Forming
- Consistent wall thickness is not achievable, very deep parts can be challenging
- Intricacy of parts is restricted, additional details can be added with pressure forming
- Some clear parts will exhibit mark-off (defects or dirt from mould will transfer to parts)
- Higher per-piece costs make vacuum forming non-competitive with other computerised processes where quantities are higher
- Only one material can be formed at a time
- Finishing costs can be high and labour intensive
Advantages of Injection Moulding
- Allows for high production runs
- Inserts can be used within the mould, and fillers added for strength
- Close tolerances on small intricate parts are achievable
- Ejected parts normally have a very finished look
- Low levels of waste – scrap can be reground to be reused
- Full automation is possible
- Lower unit cost compared to vacuum forming
Disadvantages of Injection Moulding
- Higher start-up costs
- Requires an intense level of engineering expertise and time
- Longer time frames due to manufacture of mould
Conclusion
To conclude, both processes have their strengths and weaknesses. In determining which method is best for your particular product Paul Norman Plastics Limited would recommend that you consider the following issues:
- Production quantity
- Design & Engineering requirements
- Time frame
- Start-up costs & Budget
For additional information on plastic injection moulding or if you have a project that you would like to discuss, please contact us at enquiries@pnplastics.co.uk or call us to speak with a highly skilled engineer on 01453 833388.
