Many of the shapes that we take for granted in our lives from the fenders on our cars to the tiny gifts that fall out of a Christmas cracker are the result of molding.
According to Ricky Wilson, engineering capability lead for tooling at the UK’s National Composites Centre (NCC): “Molds are crucial. There are not many composite parts that don’t use a mold.”
Molds dictate the shape of the required component and vary widely depending on what they are going to be used for. For example, high-end, nickel-iron alloy tooling is commonly used in the aerospace industry, while boat and wind turbine blade manufacturers typically use glass fibre reinforced plastic (GFRP) molds.
Typically, most molds are made of steel, the grade of which depends on the number of parts to be produced and the material to be injected into the mold.
This is because the injection molding process involves the mold being subject to thousands of pounds of pressure every cycle and it needs to be able to withstand repeated use without deforming. So, in the automotive industry, where thousands of parts are needed, a high grade of steel is required. Similarly, a glass fiber filled material causes a lot of wear when injected, so again high-grade steel molds would be the choice.
Another factor concerns the complexity of the part to be produced. The more complex the part, the more complex and costly the mold needs to be.
Andrew Pokelwaldt, director, certifications, American Composites Manufacturers Association, said: “The type of mold required is very much geared to the production process involved.
“Metal molds are used for high volume manufacturing such as auto parts. Composite molds tend to involve lower volumes or one-off parts for example in boat construction. How you make them depends on the volume of the job.”
Metal molding
According to China-based prototyping, rapid tooling and low-volume manufacturing company Star Rapid, factors influenced by the type of mold include production lead times, cycle times, finished part quality and cost.
For its plastic injection molding, Star Rapid uses a range of materials to construct molds. These include Aluminum 7075, a high-strength aluminum alloy with a large percentage of zinc. Its advantages are that it can be heated and cooled quickly, saving time on production runs. But it is not durable and can’t be used for corrosive materials or high clamping pressures so its use is confined to low volume productions of resins such as polyethylene or nylon.
At the other end of the scale, Star Rapid uses S136, a high-purity stainless steel that comes in various degrees of hardness. It is the most corrosion resist and can be polished to the highest degree making it ideal for in transparent plastics such as polycarbonates and acrylics.
Also, the NCC describes Invar – a steel/nickel combination – as the “benchmark” for mold production because of its very low expansion under high temperatures making it ideal for the aerospace industry.
As a rule, molds made of high-grade steel will last for hundreds of thousands of builds and aluminium will last for tens of thousands of builds. A basic polyurethane blended mold will only be good for five builds. Composite molds are used for low-volume production of items such as wind turbines blades and parts used in Formula One racing.
Prior to starting the mold-making process, it’s imperative to consider the expected production method for the final application. The method you use influences the mold you will build, and, conversely, the mold affects the production method.
Designing a mold starts with a Design for Manufacturability bringing together the customer and toolmaker/molder to work on specific requirements. The engineering team’s experience along with simulation tools such as Moldflow also enhance this process.
David Moir, technical manager at Star Rapid, said: “The complexity of a mold is directly linked to the complexity of the part it needs to produce.
“Also, quality very much begins on the drawing board. Without correctly addressing design fundamentals such as draft angles, wall thickness, rib and boss design, a designer is building in flaws and defects from the beginning.”
As each material has distinct characteristics, it is also important that material selection is made early on so the design can develop around the unique properties of the chosen material.
Moir added: “On the shop floor, highly skilled toolmakers who take immense pride in their work ensure molds are produced accurately and according to the approved design. Online inspection is carried throughout the mold build process. Once the mold is in the press, quality and process control measures ensure repeatable results.”
Surface finish
Other factors involved in producing a mold include surface requirements such as the level of gloss, speed to build and overall cost.
Return on capital depends on how quickly a mold can make high-quality parts that require minimal additional input to produce the part.
Stuart Johnston, director of UK plastic injection moulding company Rutland Plastics, said: “The simpler the mold tool the lower the cost. If possible, design parts without holes in the side walls, undercuts and other complex features.
“If the part does need holes in the side, then the decision on whether to mold these in or produce them using a secondary operation will depend on anticipated quantities of the part – the higher the number of parts required the more economical it becomes to have them molded in.”
The art of mold making stretches back centuries but, like many forms of engineering, today there are challenges with recruitment.
“A lot of younger generation workers understand the digital part rather than the physical engineering. While machines can only be as good as the someone programming the function, engineers are still needed,” Pokelwaldt added.
“Composites is a growing industry but we just don’t have as many people with the skills and experience in general across the United States and Europe. Much of this work has been sent to Asia. Many workers are retiring and fewer places are offering apprenticeships.”
Rutland Plastics describes education in the workplace is “key.” The company has hosted ‘year in industry placement students’ from Loughborough University since the 1990s, many of whom have been employed after graduation in the design department.
Johnston said: “Having a student working with us brings benefits, not just for the students but for the business as well. We also have apprentices throughout the company who, through cross departmental working, learn about design, toolmaking, the manufacturing process and quality control.”
Ultimately, mold making continues to be an evolving science with data to produce them increasingly stored digitally. 3D printing has also advanced mold making techniques for both composites and metals.
“With basic 3D printing you don’t get a very high-quality surface finish but now there are advanced machines that produce a very high quality mold that can also machine the part,” Wilson explained.
With these advanced machines, the deposition rate is no longer grams of materials per minute, it’s pounds per minute. A “roughed in” mold can be printed in a couple of hours and converted into a gel coated, high gloss surface within one to three days, depending on size.
Wilson added: “The main driver with this is the reduction in lead time. Using Invar, a tool can take up to a year to design and produce. With 3D printing it could be a matter of months which is a crucial advantage in motorsport for example. It could be a game changer but there can’t be any compromise on quality.
“The ultimate goal will be to have no molds at all, just 3D printing where modifications can be made easily. But we are a long way off that now.”
