What is rapid tooling prototyping?
The whole purpose of rapid tooling prototyping is to create tools from which you can generate multiple parts as quickly as possible. For the majority of times, this refers to plastic molded parts where production intent parts are required quickly and in lower volumes than a full production run quantity.
Overview of rapid prototype tooling
Rapid tooling prototyping is a manufacturing technique that is all about creating the tooling to make parts or products that can be produced from lower-cost more ‘temporary’ tooling that does not require large capital expenditure or long lead times (such as hard tooling used in final mass production of plastic injection molded parts).
For prototypes that are close to production sign-off, this is important as final testing can be carried out on production intent parts manufactured in the correct material. There are different rapid tool systems available, each with its own merits and benefits that will best suit the needs of the prototype test requirements. This could range from final fit, form, and function test through to initial product launch, it all depends on the quantity required as the different techniques are capable of producing different quantities of parts.
The table below shows a simplified comparison of the different rapid tooling techniques and their associated attributes and includes 3D printing, CNC machining, and conventional hard tooling for the sake of comparison, too:
You can also see that how the cost of creating small plastic parts is affected by the type of tooling used and the number of pieces created in this chart from the excellent book, Product Realization: Going from One to a Million by Anna C. Thornton:
As you can see, the rapid prototyping tooling options provide a lower cost for small numbers of pieces, but when we go into mass production injection molding, especially hard tooling, offers an unrivaled cost per part.
Rapid tooling vs conventional tooling
Conventional production injection mold tools are constructed from hardened tool steel and can take weeks if not months to complete, depending upon the size and complexity of the part to be molded. These production tools are also costly to produce due to the processing and machining time of hardened steel as well as the extending time finishing and polishing.
Conventional mold tools are designed and built to last and produce hundreds of thousands of parts, if not millions of parts, depending upon the polymer being injected. They can also produce more than one part at a time with multiple cavities such as screw tops or other smaller parts. This is why they are constructed from hardened steel and the costs are high.
Rapid tooling on the other hand is made from either softer steel or an alternative material such as aluminum. These rapid tooling prototyping systems are mostly used for prototype parts where speed is of the essence and high quantities are not required.
The ability to create low-cost tooling allows you to produce production parts for testing and validation after the initial prototype tests have been carried out. After this pre-production testing period, you should be in a position to invest in long term production tooling.
Rapid tooling techniques also allow for a product launch with a reduced capital expenditure but more importantly, a reduced time to market. This reduced time to market can create early revenue streams in your business as well as critical positioning and competitor advantage in the market.
Limitations for each of the different rapid tooling technologies
Silicone Rubber Tooling
- With some types of polyurethane, the cure time is longer and requires heating, this extended time in the mold starts to dry and crack the cavity surface which has a detrimental effect on the surface finish.
- The majority of parts are produced from polyurethane compounds that replicate the final production part polymer instead of using the actual polymer intended for production.
- With a low-temperature threshold, a silicone rubber mold can only a very limited number of polymers can be used, and these must low-temperature polymers and quick solidifying polymers reducing the time in the mold.
- Accuracy depends upon the master pattern that has been created and from what process it has been created from.
- Easy to rip and tear while removing the part from the mold.
- More costly to maintain and repair than steel mold tools.
- Cavities are easy to scratch and mark when abrasive polymers are being injected which transfer directly onto the next parts to be molded.
- Limited options when it comes to surface finish detail and texture due to the poor density and hardness characteristics.
- Earlier signs of wear and cavity deterioration thus producing non-compliant parts.
- Poor wear properties limit the polymers that should be injected into aluminum tooling, avoid corrosive polymers such as PVC and polymers with reinforced fibers.
- Due to less tool clamping pressure, parts can exhibit more flash which requires greater post-mold handwork, which adds cost to the parts.
Soft Steel P20 Tooling
- The more corrosive polymers such as PVC will attack the surface of the cavities as P20 steel is not resistant to corrosive chemicals.
- Out of the technologies discussed here, P20 soft steel alloy takes the longest to machine and produce as it is the hardest material.
Direct & Indirect Rapid Tooling Technologies
There are two distinct types of rapid tooling technologies, direct and indirect. Direct rapid tooling is where the part design is cut directly into the mold cavity material, whereas indirect rapid tooling requires soft materials such as epoxy or silicone to create the tool from a master. We are going to look at some of the different technologies available for rapid tooling here.
Silicone rubber tooling
Silicone rapid tooling is an indirect rapid tooling technique that requires a master pattern to be created for the final part. This master pattern needs to represent the final design intent as closely as possible. This can be achieved through rapid prototyping methods such as SLA or SLS or even CNC machining or direct metal laser sintering (DMLS).
Once the master pattern has been produced, the silicon rubber mold can be created. This process consists of the following steps:
- Create the mold box – the size of the mold box should not be much bigger than the master pattern itself. Ensure the box has around one-inch space around the pattern on all sides. The box itself can be constructed from plywood or polycarbonate sheet.
- Master layout – position the master pattern centrally in the mold box. The pattern should also be placed upon standoffs so that it is held off the base of the mold box, recycled cured silicone from a previous project can be used for this, otherwise, pieces of wood can be used. If your pattern is an irregular shape and there are gaps between the pattern and the walls of the mold box, recycled cured silicone scraps can be used to fill in some of these spaces, this will also provide additional strength and stability to the silicone mold once it has been cured. The layout must also include a sprue and runner so the liquid silicone can be poured into the created cavity to fill every detail.
- Silicone pouring – after you have selected the desired silicone rubber for the mold and mixed it thoroughly, it is time to pour the silicone mix into the mold box, ensuring you do not move the master pattern or any filler parts you may have included.
- Curing – the silicone now needs to cure, this can be left at room temperature or in a heating chamber depending upon the specification of the silicone used as the mold.
- Demolding – removal of the master pattern is referred to as demolding and this is achieved by cutting the newly formed silicon mold in half to reveal the master pattern. The cut or parting line does not have to be symmetrical or precise, it just needs to expose the pattern so it can be removed, and once removed, the two halves need to go back together cleanly.
- Injection molding parts – the polymer used for the injected part has to have a lower melting point than that of the silicone mold. Generally, the silicone used to create the mold would have a high-temperature characteristic allowing it to handle temperatures between 250 and 300 degrees centigrade. Another criterion to take into consideration is the injection pressure. Conventional injection molding utilizes high pressure to inject a molten polymer into a steel mold. With silicone molds, high pressures can not be used as it would damage the mold, so gravity feed molten polymer is poured into the silicone mold, often with the air of a vacuum chamber. Care needs to be taken while using low pressure to fill a silicone mold so that the entire cavity is filled, otherwise the part will exhibit short shot failures. The most popular polymer used for injection with silicone molds is polyurethane which can be specified with different mechanical properties that are similar to popular engineering-grade polymers such as Nylon, and ABS.
The life of silicone rubber molds depends on many factors, the main one is being exposed to high temperatures for an extended period, therefore, the higher the melting point of the polymer being injected, the shorter the life of the mold. On average, a silicone rubber mold can produce about 20 before it starts to deteriorate, it could last longer if the part is simple in design or less if the part is more complex with undercuts and a lot of surface detail.
Aluminum Injection Mold Tooling
When we talk about rapid aluminum tooling, we are referring to injection mold tools that can be produced in a matter of weeks as opposed to months for conventional injection mold tools. This style of tooling is referred to as a direct rapid tooling technique where the part design is machined into the mold and the part produced by injecting a polymer into the core and cavity.
Not just any aluminum alloy can be used for rapid tooling, it has to have specific mechanical properties that are suited to the high pressures and tough working conditions of injection molding. The most popular aluminum alloy for rapid aluminum tooling is the 7075 aluminum alloy which has excellent high strength, high thermal properties, good toughness, and resistance to fatigue, as well as its ability to be highly polished. Other options from the 7000 series are 7020 and 7050 alloys, both have very good mechanical properties suitable for injection molding.
The use of aluminum tooling in China is very limited as most suppliers go directly to soft steel tooling as discussed below.
The main reason aluminum tools are so quick to produce is the material is easy to machine making production time so much shorter. This also has the benefit of reduced costs over conventional injection mold tools.
The tool life span of aluminum injection mold tools is generally up to 10,000 shots, depending on the complexity of the part and the polymer being injected. If the part is more complex and the polymer is reinforced, for example, the tool may only last between 2000 and 5000 shots.
Soft Steel Injection Mold Tooling
Soft steel injection mold tools are also a very popular option for a low-cost with a quick production time injection mold tool that can be used for the final prototype design verification that transitions into initial production purposes. The construction of soft steel mold tools is with pre-hardened steels which are of a lower grade than the conventional injection mold tools. This pre-hardened steel is easier to machine thus faster to produce but it does suffer from a shorter life span and lower wear resistance.
The main steel used for soft steel injection mold tools is P20 which is low carbon steel with added alloys such as chromium and nickel which give this steel its toughness and some hardness properties. The P20 steel is generally used in the carburized condition which gives it the pre-hardened condition.
A tool made from P20 steel can produce between 50,000 and 100,000 shots, depending upon the complexity of the part and the polymer being used. This makes it perfect for prototyping through to medium volume production quantities.
The use of P20 soft steel for rapid tooling is the preferred option for the vast majority of suppliers in China for producing rapid prototype tooling and low volume production tooling.
Hybrid of Aluminum and Soft Steel Injection Mold Tooling
There is a version of the aluminum injection mold tool that will provide low cost, quick production, and longer tool life, which is a hybrid of the aluminum and soft steel tool. This hybrid tool is constructed from aluminum but has P20 steel for the core and cavity. With this configuration, the hybrid tool proves all the benefits from both the rapid tooling techniques of aluminum and soft steel, so if you are looking for an injection mold tool that will bridge the gap between your prototype run, low volume production, and the need to invest in fully hardened steel production injection mold tooling, this could be the tool you need.
An Alternative to Rapid Tooling
The one alternative to investing in any of these rapid tooling technologies is to simply machine the part from a piece of material, this is best done using a CNC machine. The machined prototype can represent the final design intent including all finishing processes such as paint or other engineering surface finishing.
This construction method is referred to as rapid prototyping utilizing CNC machining.