Design for Manufacturing

Design for manufacturing integrated course

Course details

Materials selection criteria and methods (3 credits) held by professor Luca Andena on Monday from 11:15 to 13:15 in B2 2.12 class.

Design for manufacturing (3 credits) held by professor Antonio Armillotta on Monday from 9:15 to 11:15 in B2 2.12 class.

Exam rules

Exam dates: There are five exam dates during each academic year: two in winter (Jan/Feb, after the 1st semester during which the course is held), two more in summer (Jun/Jul, after the 2nd semester) and a last one in September, before the beginning of the following year.

Exam test: The exam consists of two written tests for the two modules of the course: Materials (prof. Andena) and Manufacturing (prof. Armillotta). The tests last 1.5 hours each, and are held consecutively on each exam date. Use of any kind of written material (textbooks, lecture notes, handwritten notes) or electronic device (except for a pocket calculator, which may be needed for the exercises) is strictly forbidden during the tests with the only exception of language dictionaries (not electronic).

Enrollment: In order to take the tests, the student will have to register for the exam date through the online services on the Polimi web site (this can usually be done until a few days before the exam date). Please register for the exam only if you really plan to take it and, should you change your mind, please cancel the registration; this will help in the organization of the exam session.

Evaluation: Each test will have its own separate mark. If both marks are sufficient, the student will pass the exam with a combined mark calculated as the average of the two. It is not mandatory to take the two tests on the same exam date. The teachers will keep records of the marks for each student and they will remain valid for all the exam dates of the same academic year; their validity will expire at the end of the academic year, with no exceptions.

Refusal: The student can refuse the combined grade only through the online services. By no means will a student be allowed to refuse a partial grade through direct email communication to a teacher. After a refusal, the student can choose to repeat one or both tests (or no one, should she/he change her/his mind). However, by doing so, she/he will cancel the previous mark obtained (even if later on she/he fails to get a better mark or even sufficiency); cancellation occurs when the student physically attends the test.

Contact information

Professor Antonio Armillotta

Dipartimento di Meccanica

Tel.: 02-2399.8296

E-mail: antonio.armillotta@polimi.it

Office hours: Tuesday 9.00-12.00 (appointment by email)

1DFM_01 Introduction

During this course, we’ll discuss manufacturing technology which means learning how to create a more efficient product. This is a connection between two words “manufacturing”, which means making things, and “technology” which represents the application of science to make things work better.

Through this programme, we’ll talk about the connection between a product (for example in this phase I could say that I want it to have low cost and no defects), the process (so I have to know how it will be made) and the system (and by what resources).

During the manufacturing class, in the Product Design undergraduate programme, students' studies were mostly focused on plastics processing, metal manufacturing and a part of joining and assembly. In particular, they focused their attention on operations, equipment, tooling, applications, defects and general design rules. The objectives of the manufacturing class of the undergraduate and master's degrees are clearly different.

• Undergraduate: they try to set up the basic knowledge of manufacturing processes;
• Master: they want to investigate design choices and reviews directly on the product.

In both courses, there is a part about design for manufacturing and assembly. The schedule of this programme will be based on a specific calendar depending on the three main topics that we want to explore: process selection, assembly design and part design.

Within the course, we’ll discuss manufacturing defects such as flash, cold running, cold shuts, porosity, warpage or cracks. We’ll also see manufacturing resources such as: materials (direct, indirect), labor (direct when it is done directly on the product as it is for machine operators or assembly workers, indirect for example a supervisor), equipment, tooling etc.

One of the main questions is: how to reduce costs? How do we design inexpensive products? It is a popular opinion that inexpensive resources lead to low costs as expensive resources lead to high costs. Actually, it does not work like this, it is the opposite. This phenomenon is due to mass production: large investment (equipment, tooling) requires short cycle time which means a very low unit cost (over large production volumes). Obviously, we must think about this phenomenon when compared to hand-crafting.

Processes for plastics and metals

• Injection molding (complex parts)
• Compression molding (big size parts)
• Reaction molding (big size parts)
• Transfer molding (small part with details)
• Extrusion (constant section)
• Blow molding (small objects/high scale)
• Rotational molding (big objects/low scale)
• Die casting
• Permanent mold casting
• Sand casting (one piece)
• Investment casting
• Forging
• Sheet metal stamping (high scale)
• Machining (motors/engines)
• Tube forming

2DFM_02 Process selection, design requirements

In order to select the right industrial process for a product, we should consider two tasks:

• Choose manufacturing processes for individual parts;
• Choose joining and assembly techniques.

What we’ll do now is analyze existing products and try to reason about their process selection. Our method consists in observing and retracing the reasoning that led to a certain process selection and design choice. Furthermore, we should question ourselves if there are some general criteria for process selection.

General criteria

The following criteria are the most common and typical for process selection.

• Minimum cost with given requirements
  • Example 1: a jersey barrier in thermoplastic material made by rotational molding. This process involves a heated mold which is filled with a charge or shot weight of material. It is then slowly rotated (usually around two perpendicular axes), causing the softened material to disperse and stick to the walls of the mold forming a hollow part. In order to form an even thickness throughout the part, the mold rotates at all times during the heating phase, and then continues to rotate during the cooling phase to avoid sagging or deformation. This process is one of the main techniques used for plastics but it can be considered an inexpensive process only if we specify particular requirements and conditions.
    • Requirements:
      • Material: plastics (barriers must be light, not heavy)
      • Shape: hollow, closed (easy to fill with water or sand)
      • Size: large (around 1.5m to be seen by the drivers)
      • Quality: not critical (no assembly required) because the barrier does not have an aesthetic purpose, they just focus on the quality of surfaces
      • Production volume: moderate (depends on the number of units 1,000-10,000)
  • NB. We can observe that rotational molding, even if it is not an inexpensive process, satisfies all the previous requirements for the design of a road barrier. Thermoforming, for example, would be inappropriate because it does not lead to a hollow and closed shape as we needed for the barrier. Likewise, blow molding would not satisfy the requirements of a large size.
  • Example 2: head of golf in metal made through investment casting. It is an industrial process based on lost-wax casting, one of the oldest known metal-forming techniques. In investment casting, a wax or suitable polymer pattern is coated by dipping into the refractory material slurry. Once the refractory material coating is hardened then this dipping process is repeated several times to increase the coating thickness and its strength. Once the final coating is hardened the wax is melted out and molten metal is poured into the cavity created by the wax pattern. Once the metal solidifies within the mold, metal casting is removed by breaking the refractory mold. It is a very long, complicated and expensive process but the costs are justified by certain requirements.
    • Requirements:
      • Material: strong, corrosion-resistant (for that we could think about using materials such as titanium alloy or stainless steel metal)
      • Shape: 3D, thick, curved, fine details
      • Size: small
      • Quality: accuracy (assembly), surface finish and integrity (for functional and aesthetic reasons)
      • Production volume: high
  • NB. Obviously there are cheaper processes than investment casting that could have been used to produce these head golf but they don’t satisfy the requirements. Die casting, for example, would require aluminum, but this material isn’t strong enough. Likewise, sheet metal stamping violates the shape and CNC machining and forging + machining would not be appropriate because they do not satisfy the scale. Our objective is to find the cheapest process that satisfies all the requirements. Making a list of requirements before starting the design process is essential to find the right manufacturing process for the product.

Trade-off between requirements

In this case, the designer accepts to have less of a requirement in favor of another.Designers and companies are those who decide the requirement’s priority.

• Example 1: two carrying cases for precision instruments from two different companies made through different manufacturing processes. The dark one is made with injection molding (polypropylene) while the light one with rotational molding (polypropylene + metal). Injection molding is a method to obtain molded products by injecting plastic materials molten by heat into a mold, and then cooling and solidifying them. The method is suitable for the mass production of products with complicated shapes, and takes a large part in the area of plastic processing. The two cases have a similar shape but they are made of different materials: A (dark) has very thin walls, some details to increase the strength, one single layer and it is lightweight. On the other hand, B (light) has thicker walls, double walls to make the case stronger, it is less lightweight and it has some additional metal joints. It is clear that the two companies wanted to achieve different levels of strength: injection molding is perfect for lightweight products and high production volume (wider market), while rotational molding is perfect for strong objects (that need longer time to produce one single part) and low production volume (smaller market).
  • Injection molding: lightweight products and high production volume (low costs)
  • Rotational molding: strong objects and low production volume (high costs)
• Example 2: two motorbike frames, one made with sheet metal stamping (low carbon steel) and the other with die casting. Within the sheet metal stamping process, the metal is fed into a press, where the stamping tool, also known as a die, creates the desired shape. The die is pressed into or through the metal with extreme force. Except for some specialized processes, sheet metal stamping doesn’t use heat. On the other hand, die casting is a metal casting process that is characterized by forcing molten metal under high pressure into a mold cavity. The mold cavity is created using two hardened tool steel dies that have been machined into shape and work similarly to an injection mold during the process. Most die castings are made from non-ferrous metals (aluminum).
  • Sheet metal stamping: large size, low material cost
  • Die casting: high accuracy, high stiffness (or lightweight)

Looking for unusual solutions (not cost-driven)

Another criteria used to select the manufacturing process depends on the designer. He or she can explore and find out new processes and methods that have never been used for that particular product or material.

• Example 1: Philippe Stack’s chair. It is made with different processes, but we’ll focus on its polypropylene body made with blow molding. Blow molding is a manufacturing process for forming hollow plastic parts. It is also used for forming glass bottles or other hollow shapes. The blow molding process begins with softening plastic by heating a preform or parison. The parison is a tube-like piece of plastic with a hole in one end through which compressed air can enter. The plastic workpiece is then clamped into a mold and air is blown into it. The air pressure inflates the plastic which conforms to the mold. Once the plastic has cooled and hardened the mold opens and the part is ejected. This process is quite unusual for a chair but he was probably inspired by something different such as a container since blow molding is used for bottles etc.
• Other examples: warm radiator made with hydroforming, extrusion or tube cutting and welding.

When to select the process

Now that we know the general criteria used to select the right manufacturing process, we should understand what is the right time to make this decision. What is the right time to select the process?

• Conceptual design: during this conceptual phase, the designer decides the main shape, size and interface of the product. This could happen through sketches and fast renderings.
• Embodiment design: this phase is also called “preliminary design”. The designer designs the parts and components of the product and the way through which the parts are connected and interface with each other. The designer may use digital tools such as 2D drawings etc.
• Detail design: this phase is characterized by the selection of some standard parts of the product from catalogs. Starting from this part of the process the engineer will take over the designer in order to test the product from a structural point of view.

In which one of these phases a designer would select the process? Sometimes it happens that the designer chooses the process during the conceptual phase considering processes as additional specifications. Most of the time the designer chooses processes along with other part attributes (like shape, size, material etc.) during the embodiment phase. It would be probably too late for a designer to choose processes from final part drawings. Students and non-expert designers usually make the mistake of deciding the process during the detail design phase. And who chooses the process? The designer or the engineer? It is the designer that decides the process.

What we need to know

• Design requirements (conditions the product must satisfy)
• Process alternatives (basic and critical knowledge on manufacturing processes)
• Cost estimations of manufacturing

Design requirements

Design requirements are conditions that influence the whole process selection and they are set as initial design specifications or previous design decisions. There are three main categories:

• Production specifications: scale/production volume
• Assembly requirements: permanent or removable joints
• Part requirements: how single parts are made, geometry

It is necessary to specify the difference between requirements and specifications:

• Requirements: as we said before, they are conditions and terms imposed by the company that influence the process selection. Example: the company could ask for a light suitcase.
• Specifications: they are something more than requirements because they are linked to quantification. A specification is a requirement associated with a metric (number) and a value (unit of measure). Example: the company could ask for a suitcase lighter than 1 kg (metric/value).

1. Production specifications

These kinds of requirements are specified at the beginning of the design process. An example could be represented by quantity specifications that the company could set depending on its resources. The quantity concerns three parameters:

• Total production volume (units) because of the tooling cost
  • Example 1: Kitchen sinks made of steel. The first one is made by deep drawing and it has a rounded shape, curved surfaces. The second one is made with cutting and bending and it has a boxy shape. What can we say about the production specification? We know that for deep drawing we need dedicated tooling that costs but it requires a short cycle time. This leads to a high production volume (1000-10000 units). On the other hand, cutting and bending requires reusable tooling (less expensive) but it requires a longer cycle time. This leads to a low-medium production volume (100-1000 units). This is characteristic of every product, if it has curved surfaces it has probably high production volume, if it has a boxy shape a low-medium production volume.
• Annual production volume (units/yr), is about the speed of the process
  • Example 1: door engines (cerniere). To choose the manufacturing process the company has to decide the amount of units to produce every year. Let’s suppose that the company wants to produce 1,000,000 units in 2 years (for a total of 500,000 units per year). 220 day/yr x 8 hour/day x 60 min/hour x 60 sec/min it’s equal to 6 million seconds per year available to produce the units. This means that the process would require more or less 10 seconds to produce 1 unit (a good speed for die casting). The die casting process requires 20 seconds to produce one part so if the company invests in more than one machine they could make the wanted amount of units. (sheet metal stamping requires more or less the same time)
• Sales lifetime (yr)

2. Assembly requirements

• Permanent vs removable joints
  • Depending on the design requirements, if the product has to be as strong as possible or as easy as possible to maintain. We should care about: structural requirements, service requirements and ease of assembly/recycling.
    • Example: the difference between the bike frame which has welded joints (much stronger) and the flashlight that has screws, snaps and connection since the aim is to make the product easier to disassemble/recycle and reuse.
• Consolidation vs simplification
  • Do we want a few parts or simple parts? Consolidation promotes the production of more complex parts which are more difficult to create and manufacture.