Compression Moulding and 3D printing

In this first article from the moulding series, we’ll get an introduction to compression moulding as an essential manufacturing process. Then, we’ll check the importance 3D printing has for decreasing tooling costs for better productivity.

What is Compression Moulding

Compression moulding consists of placing material inside a cavity, where by applying pressure with a moving part, the product is cured. The process begins with preparing the material, commonly known as a charge. Before starting the process, parameters like material quantity, charge shape, and preheating processes must be optimally defined (It all depends on the specific workflow).

Once in the cavity, manufacturers must consider other parameters like pressure, temperature rate, and curing time. Since the charge gets compressed, cavities usually contain overflow grooves. During the compression phase, excess material should be squished into those grooves to form easily removable flashes.

Now we’ll take a closer look into some aspects of compression moulding.

Materials

It supports a wide range of thermoplastic, thermoset and composite polymers. The difference between thermoplastic and thermosets is their grade of reversibility after processing. While the first one retains its properties and can be reused, thermosets will change permanently. Additionally, compression moulding is an ideal process for combining fibers with liquid resins like epoxy to form strong composite parts (Fiber Reinforced Polymer).

Products with a wide range of properties, from flexible and rubbery-like silicone to hard and rigid like melamine, can be achieved with this technology. Compared to injection moulding, where high viscosities are an issue for resin flow, compression moulding works just fine with putty-like masses, powders, preforms, and liquid resins. Some common materials are the following:

• Silicone
• Melamine
• Epoxy
• Polytetrafluoroethylene (PTFE)
• High-density Polyethylene (HDPE)
• Polyurethane (PU)
• Polyether ether ketone (PEEK)
• Diallyl Phthalate (DAP)
• Polyamide-imides (PAIs)
• Polyphenylene sulfide (PPS)
• Phenolic resins (PF)

Costs and efficiency

When referring to compression moulding, hydraulic power machines is what come to mind in an industrial context. But, in fact, this process can be further simplified into homemade moulds pressurized with just weights, clamps or vices. To explore its full capabilities, we’ll be comparing it with other methods on an industrial level. Beginning with one of its drawbacks, we’ll focus on its cycle speed. Comparing it to injection moulding, which has mass production capabilities, compression moulding tends to be slower, it all depends on the size and complexity of the product. While a compression moulding cycle tends to take several minutes, an injection moulding cycle may only require some seconds.  

Although automatable, this process usually relies on manual work. For each cycle, an operator must prepare the material, charge it, remove it, then discard the flashes, everything while having to constantly oversee the curing process. Beyond, longer and interrupted cycles, its important to take into account manual labour as an additional cost. Taking from this drawback, one of the best practices for mould design is to add multiple cavities for smaller parts to maximize production per cycle. And here’s the main advantage, the tooling costs. Making the mould itself, installing the whole process and maintaining it, is much more affordable than with injection moulding thanks to its low-pressure tooling costs.   

Quality

As stated before, this process can generate parts with vast possibilities of outcomes for thermoplastics, thermosets, and composites. Moreover, these results have great quality, as they get the characteristic homogeneity, strength and durability of moulded parts. This is not the exception when it comes to parting lines in manufacturing processes. However, compression moulding will always ensure the absence of knit lines, which is a common issue that affects strength and surface finish for injected material. And as for composites, this process is the definitive choice for joining fibres and resins into though panels.

Design Flexibility

To get the disadvantages out of the way first, this manufacturing method has many limits when it comes to geometries. Cavities have a depth limit for the charge material to flow properly. Features like sharp edges and complex pockets are inconceivable. But on the other hand, compression moulding works great for big parts like vehicle hoods, and basic everyday life parts like buttons. With that said, geometries are simple, but the quality high.

As for versatility for the process itself, you’ll only need to generate enough pressure and heat for a specific material to cure inside a cavity. Which, at the same time must be resistant enough to perform ideally during the process. Moulding cavities are generally made through CNC machining and die casting processes.

Applications

Gaskets and seals: This might be the most common use for compression moulding. As high-quality gaskets must be rubbery and durable parts, with simple and flat geometries, this manufacturing process is the ideal choice for these water tightening applications.

Automotive Parts: By fusing composite resins and fibers to create strong panels for hoods, bumpers, trunks, doors, and spoilers.  

Electronic and electric parts: Buttons, sockets, switches, cases, keypads, and faceplates with nice heat and electrical insulation properties.

Medical and dental: Most of these devices require safe and approved materials to apply like silicone. Thankfully, parts like diaphragms, isolation bumpers, and lip seals are possible to make.

Kitchenware: The best choice for melamine bowls, cups and plates, and silicone for non-slippery and heat resistant cooking utensils.

Below is an example of some custom resin teeth impressions which have been printed using a Formlabs Form 3 SLA printer:

Compression Moulding and 3D Printing

The main difference between moulding processes and 3D printing, from a productive point of view, is production volume vs. customization. While moulding processes can produce large quantities at much faster rates than additive manufacturing, they lack the flexibility to make prototypes for new designs or adapt to variations in design when needed. Even when compression moulding is, relatively, much more flexible in tooling cost and processes, 3D printing remains superior in that sense.

This contrast between additive manufacturing and most mass-production manufacturing processes is, in the end, a great advantage for production processes as a whole. One of the best uses 3D printing has in the manufacturing industry is that of fast and customizable tooling. Many industries save huge amounts of costs printing jigs and fixtures for both prototypes and established production lines, moulding tools are not the exception.

3D printing technologies have been developing to the point where printing fast, though and customizable compression moulds is completely possible. The following case studies are great examples:

Fast Mould Prototypes for Gaskets

Recently, engineers from the kitchenware company OXO, were developing gaskets for their cocktail cups. As a fast prototyping solution, they printed a mould through a Formlabs SLA machine. They used silicone putty as the charge and then pressurized the mould with a vice. The results led the engineers into a much better insight regarding their design process. For more details on this process check out this Formlabs whitepaper.

3D Printed Composites for Compression Moulds

Last year, Thermwoods Corporation, in collaboration with Purdue University, printed their first test tool for a compression moulding process. The test part was a thrust reverser blocker door for a jet engine, and the mould was made from Techmer PM 25% carbon fiber reinforced PESU. For more information on composite 3D printing, check out what Markforged printers can do within affordable prices!

So this was our first article on the importance compression molding has for manufacturing processes, and how 3D printing applications integrate into it for faster and more affordable results.

Solid Print3D is here to help you make the right decision with your next 3D Printer purchase. For more information, please call Solid Print3D at 01926 333 777 or email at info@solidprint3d.co.uk