The Importance of Tool Material Choice

The Importance of Tool Material Choice

In injection molding, there are two critical “material” choices that need to be made: which plastic material to make the product out of, and which tool material to use when building the part’s mold. Each choice has many options, but to add to the complexity, the resin selected can impose significant constraints on which tool material should be chosen.

In this article, we’ll briefly discuss the important considerations that must be weighed when choosing the material for your mold. We’re not going to give a comprehensive review of all of the alloy choices available on the market, but instead we’ll give an overview of the main metallic categories.

What’s Important for This Part?

Before deciding on a tool material that best suits your needs, you must first determine what your specific needs really are. Your design and production team must be on the same page for each of these basic questions:

How long does this tool need to last? Put another way, how many parts does the tool need to produce over its lifetime? Are there generational changes expected that would warrant a new tool versus changes to the existing mold or cavities?

What’s the required surface finish? Does the part need to be textured or are you trying to make parts with high optical quality that must be flawless? If it’s the latter, your options narrow considerably since not all tool metal can provide or maintain a very smooth, high-gloss surface finish.

What’s the cycle time you’re shooting for? Of course, this is directly tied to the EAU (estimated annual usage), number of cavities per mold, the injection pressure of the machine you’ll be using, etc. Tool material can have a huge impact on cycle time because the available choices vary widely in thermal conductivity.

What tooling materials are compatible with your plastic? A metal that offers the best corrosion resistance is best used with a corrosive resin like PVC. Another tool material can offer better resistance to erosion from something like a glass-filled nylon. These are key reasons why the resin should always be chosen before the mold material. A contract manufacturer with experience and a highly skilled team of engineers should be able to help you determine what resin is the most appropriate for your project so that you can then decide on the tool’s material.

What’s the proper balance of up-front costs vs. maintenance costs? If your expected production volume is only a few hundred or a thousand parts, long-term maintenance might not be an issue. Cheaper tools are often the ones with the shortest lifecycle, so in cases where limited quantities are all that’s needed, this might be a trade-off worth making. If you plan on making millions of parts from a single mold however, a higher upfront cost might be the better choice. The initial expense of investing in a steel mold should hold up well without repair or extra maintenance procedures, and can help bring in an ROI over the life of the tool.

Common Material Choices

Now, let’s discuss the characteristics of a few common tool materials.

Hardened steel: Without a doubt, this is the costliest option, but it also provides the longest tool life. Its wear resistance makes it a better choice for a project that needs to use an abrasive glass-filled resin. H-13 is an example grade, with a Rockwell Hardness from 48 to 52 and available with a surface finish up to SPI A-1. Compared to other alloys however, hardened steel has a relatively low thermal conductivity, which means you might need to have extra attention focused on cooling (possibly requiring additional coolant channels or even beryllium copper inserts).

Hard Stainless Steel: Stainless is much more corrosion resistant than other steel, which is good if you’re running water through cooling channels or using a corrosive plastic like PVC. Its thermal conductivity isn’t great, but it can provide a high quality surface finish for the part.

Pre-hardened steel: Offers less wear resistance than “post” hardened steel (discussed above), but is used for lower volumes or large parts due to its lower cost.

Aluminum: Aluminum molds cost much less than the others, but if properly maintained, it can produce hundreds, even thousands of parts with no apparent issues. Aluminum also has a much higher thermal conductivity compared to the various types of steel, which results in a faster cool down and solidification of the resin, offering shorter cycle times. On the other hand, aluminum is much softer than steel making it much more susceptible to damage which can lead to unexpected, expensive repair costs, and the costs incurred by excessive downtime. Keep in mind, extra maintenance procedures will have to be performed on any tool made out of aluminum, so expect there to be a cost tradeoff involved here.

Beryllium Copper alloy: Due to its very high thermal conductivity, this alloy is used in areas of the mold that need fast heat removal. Using beryllium copper inserts in a steel mold provides one of the main benefits of aluminum, but with the longevity of steel.

Generally speaking, the harder steel provides a better surface finish over a longer period of time. Although for some applications, smoother might not equal better. If your part is a handle for example, you may actually want a textured surface so that the user gets a better grip. It’s also generally true that the harder steel has lower thermal conductivity, lower maintenance costs, and higher initial tool cost.

These generalizations gloss over many of the complexities of how your tool material choice can impact your cost per part, cooling and overall production costs. That is why it’s critical to have a knowledgeable and experienced contract manufacturing partner to walk you through every trade-off and technical detail of your project, making your design a physical reality.

Providence knows all facets of injection molding, including mold material selection.

From design to delivery, Providence is ready to take on your next contract manufacturing project. What can we do for you?

The Importance of Gates

The Importance of Gates

Gates can be a standard part of the injection molding process. They are normally formed as molten plastic flows from the machine’s nozzle tip into a sprue and runner system, then into the part’s cavity. Gates are the last section of the runner system and attach it to the part. Therefore, everything from the gate up (gate, runner, and sprue) becomes scrap. (**note: with a hot runner system and special tool design, there is no resulting sprue, runner or gate scrap)

Gates are a critical aspect of mold design. Their type, location and number can impact everything from the cycle time, to the cosmetics of a part, and even possibly its structural integrity.

Gate Trimming

There are many types of gates, but you can think of them as falling into two main categories: manually trimmed gates and automatically trimmed gates. We said earlier that everything from the gate up is scrap, and doesn’t belong to the actual part. That scrap needs be separated from the part before it can actually be, well, scrapped. There are two ways to do this: automatically by the injection molding machine when the mold separates and the part is ejected, or manually by an operator who uses either a tool or fixture to cut off the gate and runner system.

Automatically trimmed gates have the obvious advantages of lower cost (since there’s no secondary operation), a shorter cycle time and greater consistency. Despite these benefits, there are times when manual trimming must be used: when the part can’t withstand the forces of automatic shearing, or when the gate is too big to be automatically removed.

There are far too many gate types to list here, so we’ll discuss only a few of the most common, and why/when they are used.

Let’s start small. Sub gates (short for submarine gates) are very small and thus always automatically trimmed by shearing when the mold opens at the end of the cycle. Sub gates offer mold designers much more flexibility than other gate types when it comes to gate location, since sub gates don’t have to be at the parting line (the seam on the part where the two halves of the mold join together). Their other major advantage is the fact that the mark left when the gate is trimmed off, is minimal and thus easier to hide. Submarine gates got their name due to gates being located just below the parting line, and are often used in two-plate mold designs.

Like sub gates, pinpoint gates (also called pin gates) are small and are automatically trimmed. Unlike sub gates, however, pinpoint gates are used in 3-plate molds since the runner exists in its own plate. To completely pack the cavity, multiple pin gates are used in different locations and the resin that flows through the mold is divided among them. This “divide and conquer” approach of pinpoint gates can eliminate long resin flow paths and ensure that thecavity is symmetrically filled. Pinpoint gates do have a drawback though: their runners tend to be big, which creates a large amount of scrap that can be costly.

Now let’s turn to two manually trimmed gate types. Edge gates are probably the most common gate type used, and as their name suggests, are located on the edge of the part. Edge gates leave their trim mark at the parting line and are easy to remove because the cross-sectional area of these gates is thin. If you’re using a viscous resin however, that small area can be a problem since the cavity will take longer to fill. (This type of drawback can also affect pin and sub gated parts.)

Lastly, fan gates create a stable, uniform flow into wide parts and therefore reduce deformation and flow lines. The molten resin “fans out” much like how a river spreads into a delta. Since the cross-sectional area of a fan gate is large, the resulting mark after it is trimmed is also large.

The Importance of Gate Location

Here are some important tips on gate location, regardless of what type of gate is used:

  • Put gates on a non-functional and/or non-cosmetic area of the part
  • Put gates at the thickest cross section of the part, as this will minimize sinks and voids
  • Place gates in areas that will make them easy to remove, whether manually or automatically
  • Don’t put gates near features that will act like flow obstructions, like cores and pins

Careful selection of the location and number of your gates can improve resin flow, which in turn can prevent flow marks and weld lines. Gate location also impacts not only cosmetics, but the strength of your finished product.

Design Tradeoffs and Gate Selection

From the above discussion of the four gate types, you may have noticed a pattern: small gates leave small trim marks, while bigger gates leave larger trim marks. Here’s another: larger gates allow higher resin flow rates with a given amount of pressure than smaller gates. To increase flow rates (and thus decrease cycle times) when using small gates, you’ll have to increase the injection pressure if possible. Remember, your contract manufacturer may be limited by the machines they have, so increasing the pressure may not always be an option.

Gate design is a multifaceted engineering challenge; part appearance, cycle time and tooling constraints must be balanced against your design requirements and estimated annual usage (EAU). This challenge however, can be made easier when you have access to expert input from a knowledgeable contract manufacturer like Providence Enterprise.


Using Core-outs to Reduce the Weight of Plastic Parts

Using Core-outs to Reduce the Weight of Plastic Parts

What’s a Core-out?

Core-outs are sections of a plastic part where a thick section of solid material has been replaced by a square, rectangle, or hexagonal section. This effectively replaces that solid material with empty voids surrounded by walls and ribs of uniform thickness. The core-outs themselves are usually placed on non-cosmetic surfaces, with the open areas facing down (like the underside of a handle) so that you don’t inadvertently create a depression for dirt, dust, fluids or other unwanted debris to buildup.

Carefully removing sections of a solid object is not an idea specific to the injection molding of plastic parts. For example, many 3D printers (and their associated modeling software) have options for “sparse fill”: using a honeycomb or other structure with plenty of hollow space in places that would normally have solid material. In fused deposition modeling (FDM) 3D printers—where objects are built up layer by layer through extruding molten plastic through a small nozzle—this drastically shortens the amount of time needed to print an object and reduces the amount of filament consumed during the print.

Sparse fill in 3D printing however, is different from core-outs in injection molded parts in one critical aspect: volumes which are “sparse filled” are fully enclosed and thus those internal voids are not visible from the outside nor subject to having outside material enter them. Core-outs always have one open side, a necessity due to the fact that the part needs to be releasable from the mold. As we discuss later on, this feature of core-outs has practical implications.

This same idea is behind the steel I-beam’s “I” shape, where the chief advantage is much lower weight compared to a solid beam of steel, while preserving almost all the beam’s strength. Like sparse fill in 3D printing, the I-beam’s shape allows a fixed amount of steel to make more I-beams compared to a solid steel beam, due to the subtracted volume.

Why Put in Core-Outs?

The main advantage of putting core-outs in injection molding parts is lower part weight, just like in those I-beams. Secondary advantages include reduced cycle time and reduced part price. The price reduction on each part means that in high-volume production runs, the extra time (and thus cost) needed to design core-outs into a part is more than returned, especially if that work is already mostly done for you by your contract manufacturer as part of a DFM analysis.

Core-outs can also be used to avoid sink marks (also called shrink marks) which can arise in areas of a part which are very thick. Although usually a minor cosmetic issue if they occur on a customer-visible surface, sink marks can also be a functional problem if the surface they’re on needs to be precisely flush (e.g. some liquid might pool in the depression instead of running off).

There are other ways to address these above issues without core-outs, such as splitting the part into two or more separate pieces, switching to a different material which is more forgiving with thick walls, using gas-assist molding, or even just adjusting the injection molding condition. The problem with almost all of these is that they’re more costly and time-consuming than hollowing out a few areas using core-outs.

When to put in Core-outs?

Core-outs are added to a part during the design phase, usually with input from the injection molding contract manufacturer, as we mentioned earlier. Although your contract manufacturer may offer good suggestions for the location and dimensions of possible core-outs, it’s ultimately up to you to make sure your part still functions correctly and has the necessary strength and rigidity. Also, remember that your injection molding contract manufacturer is basing their recommendations on the only context that really matters to them: the individual part. The location and dimensions of possible core-outs must not interfere with the form, fit or function of that part once it’s assembled into the final product and used by the customer or its intended users. In medical and food preparation settings, objects with exposed cavities where food or other contaminants can gather can become a health and safety risk. In those special cases, one of the above alternatives to core-outs (such as switching the material to Nylon which can tolerate thicker walls without sink marks) might be your best bet.

Coring-out sections of your injection molded part can in some cases reduce its weight, cycle time, and cost without degrading its form, fit or function. To realize all these benefits however, you must keep the full picture in mind as you work closely with your contract manufacturer on core-outs and any other design changes intended to make your part more manufacturable.

Providence Enterprise, with its large in-house engineering staff, is able to assist you with core-outs or any other manufacturing challenges.


Importance of DFM Analysis For Injection Molding

The Importance of DFM Analysis For Injection Molding

An ounce of prevention is worth a pound of cure; or prevents hundreds of scrapped parts at least. Of all the design requirements, meetings, specifications and documentation involved when it comes to injection molded plastic parts, one of the most critical elements of the design process is often overlooked: a formal Design for Manufacturing (DFM) analysis.

When OEMs and contract manufactures skip this step, an opportunity is lost to combine engineering and manufacturing expertise to detect manufacturing issues and correct them before significant costs are incurred. Besides avoiding expensive tooling modifications down the road, input received from formal DFM analysis can also decrease part weight and cycle time, as well as improve the structural integrity of the part—all with only slight modifications to the original design.

Formal DFM analysis is typically done during the initial tooling design phase. This allows the OEM and contract manufacturer to document and define the various tolerances, design specifications and quality checks together, thus avoiding costly mistakes because no money has, at this point, been committed to creating the molds. This can also help prevent situations from occurring where the OEM’s requirements may become unrealistic for the contract manufacturer to meet or possibly contradictory to the processes involved in manufacturing the part altogether. The feedback received helps to modify the original design slightly, within the constraints of performance, cost, safety and dimensional requirements. In fact, here’s a partial list of items typically covered by a DFM analysis:

  • Draft
  • Wall and Rib Thickness
  • Warp Analysis
  • Weld Lines
  • Print Review
  • Surface Finish
  • Ejector Pin Marks
  • Gate Locations
  • Raw Materials Selection

Both parties agreeing to a design that can be properly manufactured allows the final design to be completed quickly. This shortens the time to market and also prevents cascading delays further in the product launch schedule. Significant money is also saved by not having to modify or outright replace tooling or scrapping sample parts which have already been delivered to the OEM.

With all these clear benefits, why are formal DFM analyses often skipped?

One reason is the fact that many injection molding contract manufacturers don’t have the in-house engineering talent or necessary tools (modeling/simulation/CAD software) required to do a thorough DFM analysis. This can be especially true if you’re using a smaller, local contract manufacturer who is spread thin over several other firms in your same area.

Another reason is that when the tools are first designed, the designers are often thinking only about the easiest way to make the tool. An eye toward smoother manufacturing and attention to features like the runner location, gate size, cooling channels, or air vents is then often absent. The challenges and costs of making the tools are substantial, but the cost incurred by neglecting the actual use of the tool (to make high volumes of plastic parts dependably) is always much higher.

Other reasons have to do with OEM mindset. The OEM may claim to not have the time for what they think is an unnecessary step. Ironically, that same OEM who first claimed they didn’t have the time to do it right, will then later have the time to do it over, at a much higher cost. Other times, the OEM really is pressed for time because some other step in the product launch process has finished behind schedule and steps like DFM’s are cut to make up for lost time.

Formal DFM analyses may also be skipped because the OEM may be trying to operate too linearly, insisting that Engineering’s responsibility ends when the design is “tossed over the fence” to Production. To the contrary, a more iterative design approach actually ensures a smoother transition from engineering prototype to production part.

Another common issue, particularly prevalent in startups, is that an un-manufacturable or excessively expensive design is okay to move forward with because “we’ll fix that later”. That “later” notion then recedes as organizational inertia and other priorities, push aside fixing those problems that could have easily and cheaply been fixed with a “get it right the first time” mindset . . .and of course a DFM analysis.

The tool design and designing for manufacturing shouldn’t be two separate tasks. This means that the toolmakers and molders must both agree on the tooling design. It may take a little extra effort and time up front, but down the road it will save a lot of time and money.

You Have Options…

An unstated fear lurking behind all of these issues is likely that if the OEM actually tries to incorporate the findings of a formal DFM analysis, they’ll find that some other requirement will then become impossible to meet, and the whole design will collapse like a house of cards.

There are experienced injection molding companies who are experts at meeting the performance, safety, cost, and dimensional requirements of their customers while at the same time ensuring that the part can be made quickly, reliably and in high volume. You may have to search outside your local area in order to find contract manufacturers with the production capabilities to meet your demand and the engineering expertise required for meeting your tolerances with zero defects or delays, but such a manufacturing partner is well worth that extra effort.

Whether you’re making millions of disposable medical devices or hundreds of toy parts, a formal DFM with Providence Enterprise can help you make better parts while at same time saving money, by knocking down the artificial wall between engineering and production.


Choosing the Correct Tool Cavitation

Choosing the Correct Tool Cavitation

Injection molding is a great way to produce a large number of plastic components at high speed and a reasonable cost. What OEMs often overlook however, is one key question which could make this great method even better:

How many cavities should my tool have to make the most parts, at the best quality and with the highest return on investment?

Answering this question can be the OEM’s key to getting the highest ROI for tooling and production costs, and can also benefit the contract manufacturer for scheduling efficiencies. Deciding how many cavities per tool is all about making tradeoffs between the piece price, production lead times, and initial tooling costs in order to reach the lowest total production cost for producing high quality parts. Like many other optimization challenges in manufacturing, it requires close collaboration between the engineering and management teams of both the contract manufacturer and the OEM.

An Opportunity Too Often Missed

Unfortunately, this optimization step is often skipped because OEMs (and their injection molding contract manufacturing partners) don’t see eye to eye on the large benefit of increasing the number of cavities per mold.

Instead, what typically happens is that the OEM sticks with the defaults of a one cavity tool, and the specific tonnage machine the contract manufacturer suggests. From these defaults and the specs of the part itself, the parts per hour (PPH) is determined.

The OEM then projects the number of parts per year they need (formally referred to as “estimated annual usage” or EAU) and divides this EAU by the PPH to make sure that there are enough hours to meet its annual forecasted demand. If not, then increasing the cavities per mold is the obvious place to boost the PPH. It’s more cost effective than creating two or more separate molds that would run in multiple injection molding machines simultaneously. Besides, the contract manufacturer may not have open machine capacity to run multiple molds at the same time.

This standard approach works great in a lot of scenarios, but a lot of OEMs don’t realize that even if a single cavity mold can meet the EAU requirement, they can still greatly benefit from increasing the cavitation. This is true even if those additional cavities require moving production to a slightly larger tonnage machine.

Let’s use a hypothetical example to illustrate why. Suppose you are trying to make a part that has

an EAU of 20,000 pieces and you’re using a single cavity tool in a 150 ton press. The cycle time to complete each one of these parts is 30 seconds, thus resulting in a PPH of 120.

If you were to change that tool to a 2 cavity mold, it could produce 240 PPH and you would see a reduction in piece price, mainly due to the machine time required to make each individual part being cut in half. This dramatic reduction in machine time and cost, more than makes up for the slightly increased hourly rate that might be incurred moving to a larger machine (such as a 200 ton or 225 ton), and the small increase in tool cost.

The Benefits of Increasing Cavitation

This example illustrates exactly why increasing the cavities per mold very often produces a net win for customers: large increases in the PPH can be achieved with minimal increases in cost, resulting in a significant overall reduction in the final price per part. This is, after all, the key reason why OEMs choose injection molding for manufacturing their plastic components. Utilizing multi-cavity molds helps to further squeeze maximum return on production investments.

Another benefit to the increased PPH and reduced piece price via multi-cavity tools is the decreased cost incurred by deliberately overshooting an EAU (especially if it was too conservative a forecast to begin with). Creating some excess “buffer” stock may actually save some OEMs money since it’s often less expensive to make and store large amounts of fast moving inventory than to run short and incur losses due to additional mold setups and extra machine time. In effect, using multi-cavity tools can increase revenue during unexpected spikes in demand, provided a bottleneck doesn’t appear further down the production pipeline. (This theory also assumes that the product is in high demand and is in the early growth stages of its product life cycle.)

A variation of a multi-cavity tool is called a “family mold”: a tool with many cavities, each of which is for a different part. All parts are made from the same material during the same cycle, and family molds are great for similar size assembly components like electronics enclosures, provided that the same injection molding parameters will work for each individual part (cycle time, pressure, cooling rate, etc . . .).

Increasing the number of cavities in your injection molding tool can boost your profit by producing more parts for an overall lower cost when considered over the entire length of the project. In order to reap those benefits, you and your contract manufacturer must recognize this opportunity very early on during the design phase of the tooling. Providence Enterprise is a contract manufacturer who sees the opportunities for its customers in every aspect of tooling design and implementation.


The Importance of Uniform Wall Thickness in Plastics

The Importance of Uniform Wall Thickness in Plastics

Keeping the walls of your injection molded plastic part the same thickness is important for maintaining good cosmetics and structural integrity. In this article, we’ll explain in-depth why uniform wall thickness is so important.

Preventing Sink Marks

The number one reason to maintain uniform wall thickness is to prevent sink marks (also called “shrink marks”). Sink marks usually manifest as small depressions on a part in areas which should uniformly be flat. They are often caused by non-uniform wall thicknesses creating irregular cooling in the center of the wall (since heat from the center takes longer to escape through thicker sections of plastic). This uneven cooling in turn creates unintentional shrinkage, because not all areas of the part solidify at the same time. When the plastic in the center of the walls shrinks and solidifies, it pulls on the neighboring areas which if solidified already, cause them to cave in.

The process is similar to how sinkholes growing underground can cause a depression at the surface. Just like sinkholes, sink marks may or may not be a problem depending on where they form. A sinkhole under your house is a disaster, but one in an uninhabited field isn’t. Likewise, shrink marks on non-cosmetic surfaces, especially if they are too small to undermine the structural strength of a part, can often be accepted. This same process, however, can lead to warping, twisting and cracking in extreme cases.

It’s important to keep in mind that uniform wall thickness doesn’t always prevent shrink marks. If there are areas of your part that are too thick, shrink marks will appear, even if that area is uniformly thick. “Too thick”, just like so many other things when it comes to injection molding, is dependent on the specific plastic chosen. In addition to asking your contract manufacturer, there are many tables online which list recommended wall thicknesses for each resin.


There are few special cases when it’s advisable to deviate from this general rule. One such case is where two walls (ribs included) meet in a “T”. It’s a good rule of thumb to make one of these walls around 60% of the wall thickness of the other. This reduces the volume of plastic at the junction, which will act like a small thick section of plastic, and thus exhibit the non-uniform shrinking process described above. Keep in mind, this ratio may need to be fine-tuned for your particular material, mold and/or mechanical requirements.

Another exception to the uniform wall thickness rule occurs when the change in thickness along the length of a wall is gradual, not abrupt.

Finding the Optimal Wall Thickness During Design

Determining the best wall thickness for each part is a balancing act. It must have great mechanical strength so that the part can support the intended load of any additional parts in an assembly. A good design must also prevent oddities like strength-reducing bubbles that could occur inside the wall if the part is too thick and a material that doesn’t have the proper flow properties is chosen. Any experienced design engineer with a background in plastics should be able to assist a customer with part design to avoid costly mistakes later in manufacturing.

Other Tools in Your Arsenal

It’s worth discussing here, that uniform wall thickness isn’t the only tool at your disposal for preventing shrink marks and warping. It is, however, usually the easiest fix to implement if you catch it early in the design phase or later during your design for manufacturing (DFM) review (you are doing those, right?).

Other ways to prevent shrink marks include modifying the placement of the gates as well as increasing their size. Coring out excessively thick sections also works and has the added benefit of reducing part weight and piece price (due to decreased cycle time and decreased amount of plastic used).

Shrinkage can also be reduced by increasing the hold time and injection pressure. However, increasing hold time of course increases total cycle time per part which in turn increases the price per part. Increasing the injection pressure may require moving the tool to a larger tonnage injection molding machine, which can also slightly increase cost per part. In practice, it’s usually best to go this route only after the other methods have been exhausted.

Then there are gussets. Think of gussets as the flying buttresses of injection molding. Like their medieval counterpart, these are support structures which stick out from the sections they are supporting, a bit like external ribs. Gussets can prevent warping by adding rigidity to long, thin unsupported sections of the part.

Maintaining uniform wall thickness is an effective way to ensure your finished part has both the cosmetic appearance and mechanical strength necessary for your application. Achieving this can be an engineering challenge in its own right as you confront the tradeoffs between the optical and mechanical properties of your chosen material and design requirements. This is why you need a contract manufacturer who can bring more injection molding skills and experience to the table to get your design right the first time.
