Top 20 Things To Look For In A Plastic Injection Molding ...

Part 4 of 4

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This is the final installment in our four-part series on the qualities to look for in a plastic injection molding supplier. We hope you find it helpful in your research.

16. Business Knowledge

In addition to competency in manufacturing and in plastic injection molding, your supplier should know how to run a successful business. If they do, it should contribute toward their overall performance, which means higher quality parts, timely deliveries, helpful customer service, and competitive pricing. The perspective that comes from business and industry insight and understanding should result in a certain level of professionalism that makes them better and more of a valuable resource for you and for your company.

17. Guidance

Your injection molder should be an expert in their field and, as such, should provide you with the guidance necessary to make informed decisions regarding your parts and program. This can include optimizing part designs for moldability, manufacturability and performance; researching and recommending materials; and identifying potential cost saving initiatives. Additionally, as mentioned earlier, do they guide you through the problem-solving process when issues arise, and are they proactive in suggesting improvements before problems occur? Their knowledge, experience and expertise are a large part of what makes them a valuable resource to you and, consequently, should not be overlooked or undervalued.

18. Financial

Obviously, the process of finding the right injection molder will include a financial analysis. However, the bottom line often isn&#;t. There are many intangibles to factor in, and you need to decide how much you value each.

Additionally, make sure you are comparing apples to apples when looking at competing suppliers and proposals. For example, if tooling is being built, do the molds have the same number of cavities, the same warranties, the same expected lifespan, etc.? Is the molder providing you with different options for tooling and production and with quantity price breaks? Also, beware of any extra costs and fees, like mold qualification charges, process validation charges, mold setup fees, material and color change fees, outbound handling fees, etc. This is not to suggest those expenses are illegitimate, just make sure you factor them into your analysis.

19. Trust

As we all know, the foundation of any successful relationship includes trust. And, although this is related to other items on this list (e.g., dependability and the human element), we felt it was important enough to mention on its own. That&#;s because the value of truly believing your supplier will take care of you and your program cannot be overstated.

Do you trust the molder to look out for your best interests in a selfless manner, or do you think that, given the chance, they will take advantage of a situation to your detriment? There potentially will be opportunities for the supplier&#;s management and personnel to make decisions that will affect the quality and pricing of your parts. Likewise, knowing they will collaborate, communicate and cooperate with you means they truly are a business partner, which frees you up to focus on other suppliers of yours who aren&#;t.

20. Value

Not too long ago, I had an engineer tell me that the final decision on a project was not his and that, &#;Obviously, they [management] will go with whoever has the lowest price.&#; Unfortunately, this is an all-to-common and quite limited mindset, especially when considering the context.

When you are undertaking a project and engaging a company to provide you with custom manufacturing that is going to involve a significant financial investment and to require ongoing dependability over an extended period of time which could have a serious impact (positively or negatively) on your business&#;s sales, profitability and reputation, simply looking for and choosing the cheapest option is dangerously imprudent, incurs unnecessary risk, potentially hinders company growth, and generally invites problems.

Accordingly, while off-the-shelf parts may be just a commodity, the search for a supplier of custom injection molded parts should be focused on finding someone who provides the greatest comprehensive value. This must include analyzing all aspects of the molder as a supplier and as a business partner and evaluating how well they fit your wants and expectations, your company&#;s needs, and the unique requirements of the program.

Injection moulding is a versatile and efficient manufacturing process widely used across various industries to produce a vast array of components. This method allows for the repetitive production of identical parts in large volumes. However, designing parts for injection moulding requires a careful approach to ensure the quality and functionality of the end-product. This article will introduce you to ten key factors that must be considered when designing parts for injection moulding.

1.          Raw Material Selection

Various plastic compounds possess distinct characteristics, making it essential to carefully choose the appropriate material during the initial stages of the design process. When selecting raw materials for plastic moulded parts, several factors need to be taken into account. Among them are:

• Temperature: in what temperature range the part will be used, or what temperatures it will be exposed to during its life cycle? Will it be subject to thermal shocks? This is also the place to choose between Thermoset and Thermoplastic material. Both are polymers, but they behave differently when exposed to heat. Under heat thermoplastics melt, while thermoset plastics retain their form. Generally, Thermoset plastics can withstand higher temperatures without losing their shape, making them more durable.
• Mechanical constraints: what pressures will be applied on the part during the manufacturing process, it assembly or intended use? Some technical materials provide higher durability and resistance, especially when they contain reinforcement agents such as glass-fibres.
• Dimensions stability and precision: some materials, under certain environmental conditions (e.g. humidity, temperature, etc.), are more stable than others, and therefore are better suited for applications that require tight tolerance.
• Appearance: The final visual aspect of a moulded part can be influenced by the choice of the plastic material. For example, transparent parts can be made only by using amorphous materials.
• Environmental interactions: will the part be exposed to salts (e.g. marine applications), solvents, acids or other abusive subsentence throughout its life cycle? Will it be used for invasive medical procedures or be in direct contact with food? In such cases medical and/or food grade materials shall be considered.
• Cost: what is the target part price? Is it intended to be used in a cost-sensitive market, or a niche market with higher cost margins? The price per kilogram of some technical materials can be several times that of other, more basic ones.

The considerations mentioned above primarily pertain to the implementation or end-product. However, it is essential for the injection moulder to be involved in the decision-making process and for all parties to reach a mutual agreement. There are several compelling reasons for this. For instance, the designer may opt for a particular high-performance material, but the moulder may reveal that it is not well-suited for the intended application due to its viscosity, high glass content, or crystallinity. Alternatively, a resin may be chosen for its specific physical or chemical resistance properties, but it may pose challenges in terms of moulding or maintaining certain tolerances. Furthermore, moulders often stock certain materials or have easier access to specific grades which they purchase in large quantities. In such cases, the moulder can obtain these materials at a lower cost and subsequently offer a more competitive price to the customer&#;s benefit.

 

2.          Tolerances

Designing injection moulded parts frequently involves implementing precise tolerances to guarantee the effective functioning of the product, such as its fit with another component or its handling by an automated assembly machine. While it may be relatively simple to achieve this in a computer-aided design (CAD) system, it becomes considerably more challenging to replicate in the physical world. Consequently, one of the primary obstacles in designing injection moulded components is ensuring sufficient clearance for tolerance variations. Although it may initially seem prudent to define all dimensions with tight tolerances, the reality is far more intricate. Tolerance variation is influenced by various factors, including:

• Raw material: dimensions stability is influenced by the composition of the injected material and the shrinkage that happens when it cools down, both in terms of rate and direction. For example, amorphous materials usually have more precise tolerances compared to their semi-crystalline counterparts.
• Mould design: taking into consideration tolerance variations during the mould design phase allows to solve potential issues later on. For examples, achieving certain values may require certain design features to be intentionally designed with &#;Steel Safe&#; in mind. In this case features will be designed with enough clearance to allow the tool maker to easily machine away steel in the mould and tighten up the clearances after initial test shots are moulded. While this may involve an additional step, it is preferred over the alternative of adding steel to the mould by welding, which can compromise tooling quality, incur higher cost and make production longer. In addition, an experienced moulder can propose various solutions for maintaining tight tolerance, such as post-production machining, fixtures, and strategic gate placement.
• Mold making: another reason to consider tolerances early in the design phase is that different tolerance levels may affect the mould making process itself. Achieving very tight tolerances may require high-performance machining equipment that a tool maker might not have. Even if the tool maker does possess such machines, the machining cost is likely to be higher, increasing the finished part selling price.
• Process control: Ensuring the accuracy and reliability of measurements is crucial when defining dimensions and tolerances on a drawing. Without the ability to measure them accurately and repeatedly, these specifications lose their significance. Therefore, it is essential for the part designer to collaborate with the moulder to confirm that all dimensions and tolerances depicted on the part drawings are both measurable and achievable.
• Intended application: The tolerance level of certain parts may be affected by what occurs with the moulded component after the injection process, as well as its intended use. This is particularly true for parts that require assembly or are handled by automated machines, where precise tolerance criteria are essential.

Mechanical designers can be regarded as the primary creators of the components they design. However, it is essential for them to collaborate with moulders to identify the optimal tolerance level. This "sweet spot" strikes a balance between the necessary level for proper part fit and function, manufacturing limitations, and production costs. Tight tolerances typically result in longer and costlier production processes. Therefore, it is recommended to assess and confirm their absolute necessity to prevent unnecessary expenses and delays in production.

3.          Sink-marks & Ghost Effect

Designers of injection moulded parts often face the challenge of preventing sink marks and ghost effect. These marks not only detract from the visual appeal of the part, but they can also affect its dimensions and strength. Sink marks and "ghost" effects can occur for various reasons, some related to the moulding process itself (such as incorrect temperatures or pack/hold time), and others resulting from improper part design. When designing parts for injection moulding, it is crucial to pay special attention to thicker sections like walls, ribs, and bosses. It is also important to minimise thickness variations in the part.

Collaborating closely with the moulder can greatly reduce the risk of sink marks and "ghost" effects. An experienced moulder can offer valuable solutions, such as removing features inside the part, minimising draft, or optimising rib heights.

However, it's important to note that cosmetic surface defects can still occur due to a wide range of factors, including gate type and location, tool quality, wall thickness, raw material type, additives, surface finish, part colour, lighting conditions, and viewing angle. Therefore, it is essential to establish acceptable criteria for surface quality in collaboration with your moulder.

 

4.          GATE TYPE & LOCATION

One of the most critical points to consider when designing parts for injection moulding is the gate type and location. These factors have direct impact on arguably every aspect of an injection moulded part. They affect appearance, warpage, tolerances, surface finish, wall thickness and other physical properties. Addressing them in the part design phase is important, because in some cases, the type and location of a gate may require adding certain features to the part geometry which were not originally planned for.

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Although some part designers utilise software simulators to evaluate the results with different gate types and locations (e.g. Moldflow analysis), it is highly recommended that the designer works in close collaboration with the mould maker and the moulder throughout the design cycle. This helps to minimise potential quality issues and ensures that the gate will not adversely affect the part performance or fit. An expect moulder can propose different types of gates (e.g. fan, capillary, valve) and explain the trade-offs of selecting each one.

 It is worth mentioning that gates usually leave visual marks on the part after injection. If done properly, these marks can be minimised, but they will be visible, nonetheless. Therefore, the part designer shall communicate to the moulder all the aesthetic and functional requirements related to the concerned part, and preferably, define areas on the part where no gate marks are allowed.

5.          Draft angles

Mechanical designers typically have a preference for designing plastic moulded parts with minimal or no draft angle. While it is technically possible to create moulds for such designs using EDM or CNC machining, it is not advisable to produce moulded plastic parts with vertical walls. The reason for this is that when the part contracts during the cooling process, it is likely to become stuck in the mould, especially on the core side. Although it is theoretically possible to eject the part by applying force, doing so runs the risk of damaging the ejector pins or even the mould itself. Even if the part is successfully ejected, it is likely to suffer from visual defects, deformation, and other quality issues.

To prevent the aforementioned problems, moulders typically insist on incorporating draft angles for certain sections of the part. The specific degree of the draft angle can vary greatly, ranging from 0.5° to as much as 10°. The determination of the optimal draft angles depends on various factors including surface finishing, texture, mould steel, raw material composition, and more. It is highly recommended that designers collaborate with both the mould maker and the moulder to determine the most suitable draft angles for each part design.

 

6.          Walls thickness 

The optimisation of wall thickness for moulded plastic parts is crucial in order to avoid potential quality issues. Factors such as the part design and the injected material should be taken into consideration when determining the appropriate wall thickness. Thinner walls can reduce cycle time and lower costs, but if the walls become too thin, problems may arise. It is also important to maintain consistency in wall thickness throughout the part. If one section of the part is thicker than another, it can result in warpage or sink marks due to variations in cooling and shrinkage rates between the two areas.

 If different wall thicknesses are unavoidable in the design of a part, it is essential to properly control and manage the transition between the different thicknesses. An experienced moulder can provide guidance on the maximum rate of thickness change and the best way to implement it in the part.

 

7.          Parting line

The point at which mould plates come together, known as the parting line, has implications not only for the visual aspect of the moulded product but also for the positioning of other features. Typically, the parting line will run perpendicular to the direction in which the mould plates open (known as vertical parting). However, certain part designs may necessitate more complex parting strategies, such as curved, bevelled, or multi-step parting, where the mould opens in different directions at different times.

Hence, it is crucial to determine an appropriate parting strategy during the design phase of the part. An experienced moulder can assist in selecting the best approach, taking into account factors such as cavity pressure, shot balance, draft angles, material shrinkage, and more.

  

8.          PART strength

Multiple features can be used to increase the strength of plastic moulded parts, and they shall be carefully evaluated as part of the part design process. Follows are the main ones:

• Wall thickness: generally, thicker walls provide more strength to the part. However, the thickness must be defined by considering the risk of sink marks, warpage, deformation, shot size and other parameters to avoid quality issues.
• Ribs and Gussets: these two features can increase strength by providing mechanical support, especially in large flat areas that risk to bend while the moulded part is cooled down. However, when using ribs, special care must be given to their position and thickness. If not done properly, these features might result in sink marks and warpage.
• Bosses: some plastic parts include bosses, which are usually cylindric features aimed to accommodate a screw, a metal insert, or an assembly section of a matching part. When designing bosses, it is important to attach them to walls by ribs, or to the floor surface by gussets. It is also important to ensure their right thickness to minimise the risk of breakage or visual defects.
• Radii and fillets: a moulded part design shall avoid as much as possible sharp corners to improve the material flow and minimise weakness areas. In addition, rounded edges facilitate the part ejection from the mould.

 

9.          PART FINISHING

Inexperienced mechanical designers may mistakenly believe that part finishing is merely a superficial concern that can be addressed once the part design is complete. However, part finishing actually impacts nearly every aspect of the injection moulding process, including raw material selection, mould design and injection moulding parameters. Here are the main reasons to consider part&#;s surface finish right from the beginning of the design process:

• Colour: certain customers have specific preferences when it comes to the colour of their moulded parts, which may differ from the original natural colour of the raw material. To meet these requirements, a moulder can either use pre-tinted plastic granules or mix the natural material with a colorant. In situations like these, particularly with the latter option, the inclusion of additives can potentially modify the properties of the material and subsequently alter its behaviour during the injection process. 
• Ejection: moulded plastic components can be realised with varying degrees of surface roughness, which can be classified into four major categories: glossy (min., roughness of 0.012μm), semi-glossy (min. roughness of 0.05μm), matte (min. roughness of 0.35μm) and textured (min. roughness of 0.8μm). Generally, the smoother the surface of a part, the easier it is to remove it from the mould. This means that the choice of surface finish will impact the draft angles of the component.
• Mold material: plastic injection moulds are commonly made of either aluminium or steel materials. However, it is important to note that these metals have distinct effects on the final surface finish of the moulded part, and the choice between them should be made based on the desired outcome. For instance, when aiming for a highly glossy surface, hardened steel would yield superior results.
• Plastic raw material: the field of chemistry research and development has witnessed significant technological advancements in recent years. As a result, designers now have an abundance of options when it comes to choosing material for their moulded parts. However, different materials may yield different finishing results. Certain materials are more suitable for achieving high-gloss finishing, while others are better suited for creating textured surfaces. This wide range of options allows designers to have greater flexibility and control over the appearance of their final products.
• Injection moulding process: the surface finish of a moulded part can be affected by the different parameters of the injection moulding itself, like the injection speed, pressure, and temperature.
• Part functionality: the purpose of part finishing extends beyond its aesthetic appeal; it can also impact the functionality of the part. Textured finishing, for instance, offers improved surface for adherence of adhesives and paints. Additionally, it is less susceptible to visual imperfections such as scratches and provides better grip. The latter property proves beneficial in handles or safety applications. Conversely, highly polished surfaces find their utility in consumer products or optical components due to their pleasant aesthetic and light-transmission or reflection properties.

 

10.       POST-INJECTION PROCESSES

One of the most common mistakes made by designers of plastic moulded parts is overlooking the potential impact of post-production processes on their designs. These operations, such as printing, labelling, machining, ultrasonic welding, or adding inserts, can significantly influence the design of the part itself. Furthermore, they may require designated tooling, which can also impact the part&#;s design or require additional capital investment. Therefore, it is important to consider and discuss post-production operations with the moulder during the part design phase to ensure proper attention is given to them.

 

Conclusion

As the factors outlined above suggest, designing parts for injection moulding is a complex process that requires multidisciplinary understanding. Even highly competent mechanical designer may lack the necessary industrial experience to effectively assess the manufacturability of their designs. Therefore, it is highly advisable for part designer to consult with the mould maker and the moulder at the early stages of the design process. Such close collaboration will not only ensure the production of high quality, functional and cost-effective plastic moulded parts, but will also minimise the number of revisions. Ultimately, it will save both time and money throughout the entire process.

At TGV Group, we have been assisting hundreds of customers worldwide in the development and production of their plastic components. Our highly experienced team comprises experts in the domains of injection mould design, mould making and plastic injection moulding. This expertise allows us to add significant value throughout the project life cycle, saving our customers both time and money.

Whether you are in the conceptual stage or require assistance with ongoing production, our team is committed to providing tailored solutions to meet your specific needs. We invite you to visit our website www.tgv-international.com or contact us at to discover how you too can benefit from our expertise.

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