Design-to-Cost (DTC) in Plastic Part Design - Example housing for electronic device

Hello and welcome to a new post. In today's post we discuss the Design-to-Cost (DTC) approach in plastic part design.

Example housing for electronic device

A company is developing a new plastic housing for an electronic device. The target cost for the housing is €1.00 per unit, including material, manufacturing, and finishing.

Figure 1: 5 steps of a Design-to-cost approach for a plastic part design

Step 1: Set Cost Target

The project team establishes that the plastic housing must not exceed €1.00 per unit to remain competitive in the market.

Step 2: Analyze Cost Drivers

Material selection: Polycarbonate (PC) is initially considered, but its cost is relatively high.

Wall thickness: Thicker walls increase material usage and cycle time.

Part complexity: Complex geometries require more expensive tooling and longer molding cycles.

Surface finish: High-gloss or textured finishes may require additional processing.

Step 3: Generate Design Alternatives

Material: Evaluate switching from PC to a less expensive material, such as polypropylene (PP) or ABS, if performance requirements allow.

Wall thickness: Reduce wall thickness from 2.5 mm to 2.0 mm, maintaining structural integrity through ribbing and optimized geometry.

Geometry: Simplify the design by minimizing undercuts and eliminating unnecessary features, allowing the use of a simpler mold.

Surface finish: Specify a standard mold finish instead of a high-gloss or textured finish to reduce costs.

Step 4: Cost Estimation and Iteration

The team estimates the cost of each design alternative using supplier quotes and manufacturing simulations.

For example, switching to ABS and reducing wall thickness lowers material and cycle time costs, bringing the estimated cost to €0.95 per unit.

Step 5: Finalize Design

The design that meets both functional requirements and the cost target is selected.

The team documents the design choices and cost rationale for future reference.

Conclusions 

By applying the Design-to-Cost method, the team systematically reviewed material, geometry, and process options to ensure the plastic part meets its cost target without compromising essential performance.

Thanks for reading & #findoutaboutplastics!

Greetings, 

Herwig Juster

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Literature: 

[1] https://www.megatron.de/en/category/plastic-housing.html

Plastic Multipoint Design Data: Creep Strain of Amorphous and Semi-Crystalline Polymers

Hello and welcome to a new blog post. Today I present to you another important multipoint and long-term data set for polymer material selection and part design: tensile creep modulus. 

The creep strength and toughness of High Performance Plastics we discuss here. 

Mechanism of creep

Creep, also known as cold flow, is the deformation under a static load over time and helps to gain insights over the product lifetime. Understanding the creep behavior is one puzzle key during polymer material selection. Creep resistance materials are needed for applications such as structural components, joints, fittings and hydrostatic pressure vessels.  In general we can distinguish between primary, secondary, and tertiary creep.  When you conduct a creep test it is important to keep the applied stress on the material at a constant level. This allows in turn to plot  the lifespan of your product.

Primary, Secondary, and Tertiary Creep

Exposure to heat or load will result in a strain reaction of the polymer. It is the stretching and straining of the entanglements with the macro molecular network. The polymer passes relatively rapid through this first phase. 

The primary creep rate decreases and we see a steady state phase of the creep. This is also known as strain hardening. Polymers undergo a linear progression during this second phase. The secondary creep phase is much longer than the first and is relevant for estimation of the part failure time. 

The last phase is again a rapid phase. Microstructural changes such as internal cracks will lead to a fast failure of the part. 

Creep behavior of amorphous  and semi-crystalline polymers

Main creep mechanism of amorphous polymers is molecular de-tangling, slipping, and rearrangement. On the other hand, the creep of semi-crystalline polymers is restricted by the crystalline regions. 

Table 1 shows the creep behavior of amorphous and semi-crystalline polymers at a stress level of 8 to 9 MPa at 31°C.

Table 1: Creep behavior of amorphous and semi-crystalline polymers at a stress level of 8 to 9 MPa at 31°C.

More multipoint design data you can find here

Thanks for reading and #findoutaboutplastics

Greetings, 

Herwig 

📚My Polymer Material Selection Book - Available Here

📊 Discover your Polymer Material Selection score by taking quick test here

🎥Join my YouTube channel community 

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Literature:

[1] DuPont -  Design Guide

[2] https://www.xcentricmold.com/the-impact-of-creep-in-plastics/

[3] https://www.researchgate.net/publication/317206602_Long-term_Loading_-_Tensile_Creep_Modulus_-_data/download

Summary of Testing Standards for Polymer Material Selection

Hello and welcome to a new post.  Looking at a technical data sheet of an engineering polymer, we can find the values and also the standard how this value was estimated. 

It is helpful in the material screening phase during polymer selection for your application to have a feeling which tests can be done and what standards are linked to them. 

In general there are two standardization companies: the American Society for Testing and Materials (ASTM) and  the International Organization for Standardization (ISO). Both are well known organizations in the plastics industry. 

Apart from plastic tests, they also develop all kinds of standards for different industries.

ASTM vs ISO - Are they the same? 

Results of ASTM and ISO are similar and there are one-to-one correlations of some ASTM and ISO standards. 

ASTM and ISO differ in measurement procedures and conditions leading to slightly different results.

Example tensile modulus

The plastic's tensile modulus can be measured according to ASTM D638 or ISO 527-1. Looking at the results, they are similar however not the same. 

There are cases where they are the same, however they are rare cases.

Overview of plastics testing standards: mechanical, thermal, and electrical. 

Figure 1 shows the summary of the mechanical standards, Figure 2 of the thermal standards, and Figure 2 of the electrical standards. 

Figure 1: Overview Standards for Mechanical Tests

Figure 2: Overview Standards for Thermal Tests

Figure 3: Overview Standards for Electrical Tests

There are more standards which can be accessed over the ASTM and ISO homepages. 

Also I made a video where I compare the ASTM/ISO data with real world application requirements: 

Thanks for reading and #findoutaboutplastics

Greetings

Herwig 

📚My Polymer Material Selection Book - Available Here

📊 Discover your Polymer Material Selection score by taking quick test here

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Dimensional Stability of Polymer Based Parts after Processing: 3 Considerations

 Hello and welcome back to a new blog post. Today we discuss three considerations for optimal dimensional stability of plastics parts after processing.

Polymer based parts have a dimensional stability which is not equal to that of metals. It can vary with several factors which we discuss in the following in more detail. If it is a critical part, this needs to be considered during the polymer material selection.

Definition dimensional stability

In short, dimensional stability means that the required dimensions are kept after processing and when the application is in use. Three considerations help to keep the dimensional stability of your part: moisture, mechanics, and thermal stability (Figure 1). 

Figure 1: Plastic part design - three considerations help to keep the dimensional stability of your part.

Consideration 1: Residual moisture and moisture uptake during use

General rule of thumb is that when materials are exposed to moisture, dimensional changes are likely to occur. In case your application has tight tolerance requirements, polymers with low moisture absorption should be taken. For example, an aliphatic Polyamide was specified for an application with tight tolerances. Due to the moisture uptake, part performance decreased and a replacement material is needed. In such a case, a semi-aromatic Polyarylamide (PARA) can be an alternative, since it has the lowest moisture uptake of Polyamides. There are also other polymers such as PEI, PPS, and PEEK, which have excellent mechanical, and moisture performance. PPS, PPA, and PEI can be used for applications, which are exposed to high temperature and moisture during the use of the application (water pumps in cars for example). Also during processing, keeping a maximum allowed moisture level is essential to not harm the polymer during processing. In this post, different maximum moisture levels after resin drying to ensure proper processing are shown.

Consideration 2: Mechanical strength

In case of structural applications, loading strength of the selected polymer is important and can influence the dimensional stability. For complete evaluation, short-term property data such as tensile and compression strength, together with long-term data such as tensile creep should be considered. Examples of high performance polymers which show high dimensional stability are PPS, PAI, and PEEK.

Consideration 3: Thermal stability

Temperature load can have a severe impact on the plastic part dimensions. Therefore, it is critical to evaluate the maximum use temperature and the continuous use temperature, together with the environment (air, water-glycol) of your application. For evaluation of the temperature impact, dynamic mechanical analysis (DMA) data, as well as head deflection data (HDT) of the selected polymers are helpful.

Overall, there are some factors, which influence the polymer part performance. In this post, I show you additional factors to consider for your plastic part design.

Thanks for reading and #findoutaboutplastics

Greetings, 

Herwig 

📚My Polymer Material Selection Book - Available Here

📊 Discover your Polymer Material Selection score by taking quick test here

🎥Join my YouTube channel community 

🔎Join my inner circle (monthly newsletter)

Literature: 

[1] https://apex-intl.com/2017/02/24/engineering-plastics-understanding-dimensional-stability-in-material-selection/