Understanding the SPI Guidelines – The Essentials of Mold Polish

Table of Contents

1. Introduction

If you work in the plastics industry, you may be familiar with the 1937-founded Society of the Plastics Industries, Inc. However, they have been doing business as the Plastics Industry Association (PLASTICS) since 2010. This professional organisation represents a number of stakeholders in the U.S. plastics industry supply chain, including suppliers of raw materials, manufacturers of machinery and equipment, recyclers, and brand owners.

Let’s now explore the significance of the SPI guidelines and how they might help you.

What the SPI guidelines are and why they’re important

a. Technical Standards: Respecting technical standards is essential in the plastics industry. SPI, now known as PLASTICS, has contributed significantly to the development of these standards. These regulations encompass the needed materials, the testing methods, and the anticipated performance of the final product. Following these guidelines guarantees your items’ compatibility and quality while also ensuring consistency. The market becomes more competitive as a result.

SPI Guidelines

b. Best Practices: SPI offers sector-specific best practises to boost operational effectiveness even further and acquire a competitive edge. These suggestions are specific to your requirements, whether you work in plastic injection moulding, extrusion, blow moulding, or thermoforming. By putting these best practises into practise, you may save costs and improve the quality of your output, which will help your company succeed.

c. Safety and Compliance: Safety should always be a top priority in the plastics industry, and the SPI guidelines reflect this. They emphasize the importance of adhering to safety procedures and legal requirements. By following these recommendations, you not only reduce the risk of litigation and financial loss but also foster a safe workplace. This commitment to safety helps prevent accidents and ensures that your business complies with the necessary legal standards.

d. Initiatives for Sustainability: Sustainable living is more crucial than ever in the modern world. To encourage sustainable practices in the plastics business, SPI is committed to doing so. Recycling, waste reduction, resource efficiency, and the use of ecologically friendly products are the main goals of their activities. These suggestions can help you satisfy client demands, take care of environmental issues, and link your company with sustainability objectives.

e. Advocacy and Representation: Finally, SPI actively promotes the plastics industry. In discussions about laws, rules, and policies, they speak for the interests of their members. Their assistance assists organisations like yours to adhere to regulations and stay abreast of market changes. You can directly influence the development of the sector by participating in its future planning.

You may get technical standards, best practises, safety and compliance procedures, sustainability initiatives, and advocacy from the SPI guidelines, which are currently under the Plastics Industry Association (PLASTICS). By adhering to these rules, you may improve your products‘ quality, increase your competitiveness, encourage a secure workplace, support sustainability initiatives, and actively participate in determining the direction of the plastics sector.

1.1 Brief overview of the process of the mold polishing

SPI Guidelines

When producing accurate and high-quality products, numerous industries may use moulds, and mould polishing is an essential stage in the mould manufacturing process. You can better appreciate the complex stages required to create a pristine mould surface if you comprehend the mould polishing procedure.

  • Prior to starting the polishing process, it’s crucial to do a comprehensive inspection of the mould. Take your time and carefully inspect the surface, looking for flaws such as blemishes, pits, or uneven patches. After evaluating the mould, you can formulate a strategy to deal with each problem and decide how much polishing is necessary.
  • It is now time to polish the surface after determining the mold’s condition. Start by thoroughly cleaning the mould to remove any impurities like dust, grease, or residue. Additionally, you might need to use abrasive equipment or techniques in a pre-polishing stage to get rid of bigger flaws or rough places.
  • You can now start polishing the mould after carefully preparing its surface. You’ll use different abrasive substances, such as polishing chemicals, abrasives, or diamond paste, to attain the desired smoothness and reflectivity. You’ll use specialised equipment, like as polishing stones, buffs, or rotary tools, to apply these substances to the mould surface. It is important to gradually smooth the surface while removing flaws.
  • Maintaining the geometry of the mould and keeping it in its original shape and dimensions is essential during the polishing process. In order to guarantee precision and uniformity, skilled polishers frequently use methods like hand polishing or fine machine polishing. In order to produce the intended result, this iterative process may call for several iterations of polishing, checking, and refining.
  • It’s time for the last examination once you’ve polished the item to the desired level. The quality and readiness for production of the mould are ensured during this step. To assess the mold’s performance, use specialised measuring equipment, do visual inspections, or even run sample productions.

Using abrasive materials and specialised tools, mould polishing is a painstaking procedure that calls for thorough analysis, surface preparation, and skillful polishing. These processes will help you create a surface that is immaculate and of the highest quality, guaranteeing that the moulds produce accurate and flawless end products.

2. What are the SPI Guidelines?

2.1 Definition of SPI (Society of the Plastic Industry) guidelines

The Society of the Plastic Industry (SPI) guidelines are a series of standards and recommendations created by the SPI, an industry trade organisation, to offer a framework for the classification and identification of plastic products. You can navigate the world of plastic materials and help maintain effective communication and consistency in the sector by being aware of these rules.

A coding scheme called the SPI resin identification code is part of the SPI specifications. Frequently seen on plastic objects, this code is represented by a number enclosed in an arrow triangle. Each number identifies a distinct kind of plastic resin that was utilised in the creation of the product. The polymer PET (polyethylene terephthalate), which is frequently used to make water bottles and food containers, is symbolised by the number “1” as an example.

The Resin Identification Code (RIC) system, a well-known SPI recommendation, serves as one notable example. To identify the precise type of plastic resin used in items for recycling reasons, this system uses a numbering system from 1 to 7, inserted inside a recycling symbol. This makes sorting and recycling procedures more effective.

Companies can assure legal compliance, increase operational effectiveness, raise product quality, and show a commitment to sustainability by following SPI standards. Following these recommendations not only helps individual organizations but also advances and helps the These rules are crucial because they make it simple to identify and classify plastic waste for recycling. Consumers, recyclers, and manufacturers can rapidly determine the type of plastic and sort it correctly by utilising the SPI resin identification code. This encourages effective recycling procedures and contributes to trash reduction.

The SPI standards also aid in disseminating knowledge about the traits and attributes of various plastic kinds. Each resin designated by the SPI code has distinctive qualities like heat resistance, toughness, or flexibility. Understanding these characteristics can assist manufacturers in choosing the best plastic for their unique applications, assuring the products’ optimal performance and lifetime.

Additionally, the rules help to standardise communication within the plastics sector. Manufacturers, suppliers, and customers can quickly refer to plastic kinds in conversations, contracts, or product specifications by utilising the SPI resin identification code. This encourages clarity and prevents misconceptions or confusion when referring to certain plastics.

The SPI guidelines may also contain other instructions for labelling, identifying, or managing plastic products in addition to the resin identification code. By promoting safety, quality, and consistency across the business, these recommendations want to make sure that goods adhere to the relevant norms and laws.

2.2 The purpose of these guidelines and why they are crucial in the molding industry.

  1. Standardization: SPI guidelines create uniform procedures, requirements, and norms of conduct for the molding sector. They guarantee consistency, compatibility, and interoperability among many stakeholders, including manufacturers, suppliers, and customers, by offering a common framework.
  1. Assurance of Quality: The recommendations support the use of best practices and high standards in the molding procedure. They ensure that molded products meet or exceed the predetermined quality requirements. They address topics including material selection, tooling design, processing parameters, and part inspection. In addition to lowering defects and raising overall product reliability, this helps to increase customer happiness.
  1. Safety and Compliance: SPI suggestions are made for ensuring safety during molding processes. They talk about maintaining equipment, handling materials properly, and following safety rules. Following these recommendations helps to decrease workplace risks, accidents, and assure adherence to applicable health and safety laws.
  1. Efficiency and Cost-Effectiveness: The ideas and insights offered by the guidelines help to optimize molding procedures, boost output, and cut expenses. They include topics like reducing cycle times, material use, waste management, and energy efficiency. These recommendations can help manufacturers increase operational effectiveness, realize cost savings, and acquire a competitive edge.
  1. Sustainability: SPI recommendations support the molding industry’s sustainability initiatives. They contain suggestions for eco-friendly products, recycling techniques, trash minimization, and resource preservation. Molding firms can lessen their negative effects on the environment, satisfy consumer demand for sustainable products, and promote a more circular economy by following these recommendations.
  1. Collaboration and Industry Reputation: Adhering to SPI recommendations shows a commitment to industry best practices and ethical standards. A company becomes a more desirable partner for partnerships and business prospects as a result of improving its standing and credibility within the molding sector.

The SPI rules have a significant function in the moulding sector. They contribute in fostering uniformity, safety and compliance, material selection, recycling activities, and communication. You can make sure that your moulding operations comply with industry best practises and support a more effective and environmentally responsible plastic production process by being familiar with and following these rules.

3. The SPI Finish Standards

3.1 Detailed breakdown of the SPI finish grades (A, B, C, D)

For molded plastic parts, the SPI offers a widely used finish grading system. The system divides the surface finish’s appearance into four major grades: A, B, C, and D. The SPI finish grades are broken out further in the following manner:

SPI StandardFinish (Grade)Surface Finish (Type)Typical Surface Roughness
A-1GlossySuper0.012 to 0.025
A-2GlossyHigh0.025 to 0.05
A-3GlossyNormal0.05 to 0.10
B-1Semi-GlossyFine0.05 to 0.10
B-2Semi-GlossyMedium0.10 to 0.15
B-3Semi-GlossyNormal0.28 to 0.32
C-1MatteFine0.35 to 0.40
C-2MatteMedium0.45 to 0.55
C-3MatteNormal0.63 to 0.70
D-1TexturedSatin0.80 to 1.00
D-2TexturedDull1.00 to 2.80
D-3TexturedRough3.20 to 18.0

3.2 Describing how each grade affects the surface finish of the mold

The mold surface finish is crucial and based on the SPI finish grades, it may have various effects. According to the SPI ratings, here is a breakdown of the significance of mold surface finish:

Grade A:

  • Importance: The greatest standard of surface finish quality is represented by the Grade A finish. It is essential for components like consumer goods, automobile parts, or cosmetic packaging where aesthetics play a significant role in the design.
  • Benefit: The molded pieces’ visual attractiveness is improved by achieving a Grade A finish by giving them a smooth, glossy appearance. It can have a good effect on the entire customer experience, gives off a more premium appearance, and raises quality expectations.
SPI Guidelines

Grade B

  • Importance: Grade B finishes are frequently appropriate for components where a high degree of shine is not the main need or expectation. It is appropriate for uses in which utility supersedes aesthetics.
  • Benefits: While Grade B finish is less shiny than Grade A, it still has a passable appearance. It is easy to accomplish and ideal for a variety of materials when surface flaws may be overlooked or tolerated.

Grade C

  • Importance: Grade C finish is distinguished by its rough surface, which resembles fine stone or paper. It is often utilized when a particular texture is sought for utilitarian or aesthetic reasons.
  • Benefits: The Grade C finish’s textured surface can provide advantages like greater grip, less visibility of flaws, and improved design aesthetics. It is frequently used in applications where a tactile or ornamental impact is sought, like appliance handles or electronic casings.

Grade D

  • Importance: Grade D finish, the lowest quality surface finish available, is frequently utilized for utilitarian components where aesthetics is not important.
  • Benefits: Despite any evident flaws, Grade D finishes are nevertheless capable of performing their intended functions. They are appropriate for parts that will be concealed from view, included in bigger assemblies, or exposed to further post-processing procedures like painting or coating.

4. The Process of Mold Polishing According to SPI Guidelines

4.1 Step-by-step explanation of how molds are polished to meet SPI standards.

  • Assessment and preparation: Before polishing, the surface of the mold is examined to look for flaws and gauge how much polishing is necessary. To get rid of any impurities or leftovers that can obstruct the polishing procedure, the mold is then completely cleaned. Thus, polishing outcomes are optimized.
  • Rough polishing: In this process, glaring flaws on the mold surface are removed using coarse abrasives or grinding tools. It contributes to the elimination of observable flaws like scratches or tool marks, resulting in a smoother base for additional polishing.
  • Fine Polishing: To further polish the mold surface, finer abrasives are used, such as polishing stones or compounds. In order to obtain a better level of smoothness and gloss, this stage gradually reduces the size of the abrasive particle. In order to get consistent results, polishing compounds are applied using rotary polishers or other specialized instruments.
  • Grit Progression: During the polishing process, several abrasive materials’ grits are utilized, starting with coarser grits and progressively moving to finer ones. This procedure aids in removing scratches left over from earlier steps and gradually improves the surface polish to the desired SPI level.
  • Multiple Polishing Passes: To attain the desired surface finish, multiple polishing passes may be necessary. Each pass concentrates on a different component of the mold, ensuring full coverage and reliable outcomes. Professional artisans employ a variety of methods, including hand polishing and the use of specialized equipment, to properly access complex mold features.
  • Final Inspection and Cleaning: The mold goes through a final inspection to make sure it complies with SPI standards after the appropriate surface polish has been obtained. The mold is completely cleaned to remove any polishing residues and any residual flaws are fixed. This gets the mold ready for use in the manufacturing procedure.

4.2 Importance of each step and how they contribute to the final finish.

  • Assessment and preparation: The assessment process is essential since it enables the identification of flaws and establishes the level of polishing needed. By thoroughly cleaning the mold surface, impurities are removed, resulting in the best polishing results. These actions are crucial for obtaining the correct surface finish and fulfilling SPI requirements.
  • Rough Polishing: Rough polishing is necessary to get rid of obvious flaws like scratches or tool marks. This procedure lays a smoother base for subsequent polishing. It raises the surface quality of the mold, making it simpler to meet the needed SPI standard.
  • Fine Polishing: For perfecting the mold surface, fine polishing is a crucial step. Smoothness and gloss are gradually improved with the application of finer abrasives and polishing agents. In order to get a high-quality finish that satisfies SPI standards and guarantees client satisfaction, it is imperative to follow this procedure.
  • Grit Progression: Grit advancement is crucial for gradually improving the surface finish. It is easier to eliminate prior scratches and flaws by starting with coarser grits and working your way up to finer ones. It guarantees consistency and raises the mold surface’s SPI rating.
  • Multiple Polishing Passes: To provide full coverage and reliable results, multiple polishing passes must be made. This process makes sure that the mold’s fine intricacies and all of its surface regions are properly polished. It helps to achieve the desired SPI standard and guarantees a consistent surface finish all throughout the mold.
  • Final Inspection and Cleaning: The final inspection phase is essential for confirming that the mold complies with SPI standards. It enables the detection and correction of any remaining flaws. The mold is made clean and ready for use by thoroughly cleaning it to eliminate polishing residues.

5. Benefits of Adhering to SPI Guidelines in Mold Polishing

5.1 Discussing the advantages of using SPI guidelines for mold polishing including factors such as consistency, quality and improved product lifespan.

  • Using the SPI (Society of the Plastic Industry) recommendations for mould polishing has several benefits that can considerably improve your operations, raise the calibre of your goods, and increase overall effectiveness.
  • The encouragement of uniformity is a key benefit of utilising the SPI principles. The SPI resin identification code’s standardised coding system ensures that all parties engaged in the manufacturing process, from suppliers to manufacturers, have a clear grasp of the plastic resin being utilised. Through clearer communication and the elimination of ambiguity, this consistency promotes more productive operations and supply chain collaboration.
  • The rise in product quality is another benefit. The SPI guidelines offer useful details regarding the traits and attributes of many kinds of plastic resins. You can choose materials that match your product specifications and desired performance by choosing the suitable resin based on the SPI code. This results in the creation of items of higher quality that satisfy the essential requirements and client expectations.
  • Utilising the SPI rules can also contribute to the items’ lifespan being increased. You may make sure that the components used in the mould have the essential properties to survive the intended applications by choosing the appropriate plastic resin with the aid of the SPI code. This includes characteristics like toughness, resistance to heat, chemicals, or wear. Utilising the proper materials will lengthen the useful life of your products and lessen the possibility of premature failure or degradation.
  • The SPI principles also assist industry standards and legal compliance. You may make sure that your products comply with the rules and specifications established by the relevant authorities by following the instructions and using the proper resin identification numbers. This assists you in avoiding potential legal problems, maintaining a positive reputation, and gaining the trust of clients who need items that adhere to accepted standards.

6. Case Study: SPI Guidelines in Action


One of the multinational companies manufactures various plastic components for the home products sector. They maintain over 3,500 active tools and manage approximately 30 mold changes every day. In 2013, the company approached had to solve an issue with sticking parts.


Most of the company’s molded products consist of thin wall polycarbonate, which requires small ejectors. However, before mold polishing carried out, the components stuck to the tool, broke the ejectors, and left operators probing out the plastic. There were also release issues with a tool running rougher materials. The sticking parts were causing significant production delays. The manufacturer had tried multiple solutions, but none provided a reliable release.


The company needed a surface treatment with exceptional lubricity/release.

The mold polishing carried out with help reputed mold polishing experts to achieve the required results.


This prevented the ejectors from breaking, which helped the company reduce cycle time and repair costs.

7. Conclusion

Finally, following the SPI (Society of the Plastic Industry) recommendations for mould polishing offers several benefits for your business operations and product quality. By adhering to the standard coding scheme offered by the SPI resin identification code, you guarantee uniformity along the entire supply chain and promote efficient interaction between stakeholders.

Utilising the SPI recommendations enhances product quality by offering useful knowledge about various plastic resins. You can meet requirements and surpass customer expectations by choosing the suitable resin depending on the SPI code. By utilising the proper resins with properties that survive intended uses, boosting durability, and preventing early failure, the standards also help to increase product longevity.

SPI Guidelines

The SPI guidelines also encourage adherence to industry standards and conformity with legal requirements. Adopting these principles shows your dedication to upholding industry standards, preserving your good name, and avoiding legal troubles. You may improve processes, create goods of high quality, and establish consumer trust by adhering to the SPI rules.

Adopting the SPI recommendations for mould polishing has several benefits, including consistency, increased product quality, a longer product lifecycle, and adherence to industry standards. By following these recommendations, you may improve your production procedures, produce trustworthy goods, and establish yourself as a respected participant in the market.


Gary Liao

Gary Liao

Gary Liao is the Engineering Manager of TDL Company and has more than 20 years of mold design experience.

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