In a follow up to our previous blog post, Top 10 Tips For Designing Injection Molded Plastic Parts, I have listed 10 more advanced tips for designing plastic injection molding parts that can be important considerations for your next injection molded part design.
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1. Tool-bound features are your friend! On injection molded parts, groups of features that are formed by the same piece of the mold (cavity, core, slide, etc) can be held to a tighter relative position tolerance than a group of features that are formed by different parts of the mold. For example, a boss formed by the cavity cannot be held to as tight of a positional tolerance relative to a hole formed by the core as it could be to a hole formed by the cavity. Use this to your advantage when designing sets of features that need to have high relative tolerances such as sets of gear posts.
2. Pass-cores simplify molds. Pass-cores are parts of the mold where one side of the mold passes through a piece of the part and shuts off in the opposing side leaving a hole in your part. This allows you to create overhangs for features like snaps, or holes in a higher draft side surface while still having a low-cost open shut mold. Some designs get very creative with pass cores and can accomplish complex features such as bores for shafts that are orientated orthogonal to the tool action direction without needing slides.
A part that extensively utilizes pass-cores to create overhangs while remaining moldable with a 2-piece mold
3. Molded-in texture improves cosmetics. Visible surfaces can be molded with an inherent texture. The advantage of textured parts includes not showing fingerprints, hiding part blemishes like sink, and being able to contrast molded-in text and designs with opposing polished regions. However, surfaces with molded-in texture generally require more draft angle than a polished surface. There are quite a few molding standards for textures including SPI (US) and VDI (German), some more detailed reference material can be found here: https://upmold.com/vdi--vs-spi-finish-surface-roughness-comparison/
4. Ultrasonic welding creates sealed enclosures. Ultrasonic welding of plastic assemblies includes pressing two compatible parts into contact with each other while supported in a tooled fixture. The parts generally need to be designed for ultrasonic welding, ideally including having a singular welding plane and energy directors on one part to cause the weld. One part is then excited with ultrasonic vibrations to locally fuse parts where the energy directors contact the opposing part. The use of ultrasonic welding can create very strongly bonded assemblies that can be weatherproof for a low cost. Be aware that amorphous resins, like ABS, will generally bond better than semi-crystalline, like Poly Propylene (PP), and that the two resin types have different optimal energy director geometry for different weld requirements. It is best to consult with your ultrasonic welding vendor to optimize your weld geometry for your specific application.
5. Prototype with 3D printed parts for fitment. 3D printers can create very useful prototypes of parts designed for molding. Stereolithography (SLA) can produce very fine features and can have a smooth surface finish that is great for simulating part fitment and function. Multi Jet Fusion (MJF) can create parts that have nearly the same strength and flexibility properties a molded nylon part.
6. Prototyped parts perform differently than molded parts. 3D printed parts will have different wear, friction and tolerance properties. Additionally, they will lack features found only in molded parts, such as knit lines that can be a problematic zone in a molded part caused when the flow of plastic around a hole rejoins on the opposite side and solidifies with a lower strength than the base flow. Due to these differences, testing of designs with 3D printed parts needs to be done with consideration that the test data may not represent how a molded part would have performed.
7. Use Family Molds to save money. Family molds are when two or more different parts are shot in the same mold. Family molds are great if you are molding a few parts of similar volume out of the same material and want to reduce your cost on molds, especially in low volume situations. However be especially careful that a good mold-flow analysis is performed to make sure that all the parts fill properly.
8. Work with a molder to select a resin. Every resin has so many varieties and potential additives for changing things like UV exposure resistance, UL fire rating, impact properties, colors, etc. It is almost always best to give a list of requirements to the molder on the resin and have them find something that meets the product requirements.
9. Semi-crystalline resins shrink more. When designing high precision parts, keep in mind that semi-crystalline resins like Nylon and POM generally shrink and warp more than amorphous resins like ABS and Polycarbonate and it will be harder to achieve equivalent tolerances. In general, semi-crystalline resins will have better chemical resistances and work better as bearing materials, but due to their achievable tolerances in molding, you may need to use an amorphous resin when you are near the limits of achievable tolerances of the injection molding process.
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10. Multi-shot parts can create a lot of function beyond cosmetics. Multi-shot parts are single parts that are composed of multiple resins. Multi-shot parts can add a lot of functionality to your design such as integrating light pipes or adding softer material for grip or cushioning, or even just adding more colors to a part.
Toothbrushes are a great example of multi-shot parts including resins with very different stiffnesses
The following injection mold classifications are guidelines to be used in obtaining quotations and placing orders for uniform types of injection molds. These classifications are for injection mold specifications only and in no way guarantee workmanship or injection molding service. This guide will attempt to give approximate cycles or mold life, for each type of plastic injection molding mold classification excluding wear caused by material abrasion, poor mold maintenance, and improper injection molding technique. Maintenance is not the responsibility of the mold maker. Normal maintenance such as the replacement of broken springs, broken ejector pins, worn rings, or the rework of nicks and scratches should be borne by the molder. Mold rework costs should be closely considered when deciding which classification of mold is required. This document does not constitute a warranty or guarantee by the Society of the Plastics Industry, Inc., or its members for spi mold classification or specifications set forth herein.
The following contains a brief synopsis of the various mold classifications and detailed descriptions of each injection mold classification.
1. Customer to approve mold design prior to the start of construction.
2. All molds, with the exception of prototype molds, to have adequate channels for temperature control.
3. Wherever feasible, all details should be marked with steel type and Rockwell hardness approximately .005 deep.
4. Customer name, part number, and mold number should be steel stamped on the mold.
5. All molds should have eyebolt holes on the top side. There should be one above and one below the parting line to facilitate mold removal, if required, in halves.
Cycles: One million or more
Description: Built for extremely high production. This is the highest priced mold and is made with only the highest quality materials.
– Detailed mold design required.
– Mold base to be minimum hardness of 28 R/C.
– Molding surfaces (cavities and cores) must be hardened to a minimum of 48 R/C range. All other details, such as sub-inserts, slides, heel blocks, gibs, wedge blocks, lifters, etc. should also be of hardened tool steels.
– Ejection should be guided.
– Slides must have wear plates.
– Parting line locks are required on all molds.
– Temperature control provisions to be in cavities, cores and slide cores wherever possible.
Over the life of a mold, corrosion in the cooling channels decreases cooling efficiency thus degrading part quality and increasing cycle time. It is therefore recommended that plates or inserts containing cooling channels be of a corrosive resistant material or treated to prevent corrosion.
Cycles: Not exceeding one million
Description: Medium to high production mold, good for abrasive materials and/or parts requiring close tolerances. This is a high quality, fairly high priced mold.
– Detailed mold design required.
– Mold base to be minimum hardness of 28 R/C.
– Molding surfaces should be hardened to a 48 R/C range. All other functional details should be made and heat treated.
– Temperature control provisions to be directly in the cavities, cores, and slide cores wherever possible.
– Parting line locks are recommended for all molds.
The following items may or may not be required depending on the ultimate production quantities anticipated. It is recommended that those items desired be made a firm requirement for quoting purposes:
a. Guided Ejection
b. Slide Wear Plates
c. Corrosive Resistant Temperature Control Channels
d. Plated Cavities
*Cycles: Under 500,000
Description: Medium production mold. This is a very popular mold for low to medium production needs; most common price range.
– Detailed mold design recommended.
– Mold base must be a minimum hardness of 8 R/C.
– Cavity and cores must be 28 R/C or higher.
– All other extras are optional.
*Cycles: Under 100,000
Description: Low production mold. Used only for limited production preferably with non-abrasive materials; low to moderate price range.
– Mold design recommended.
– Mold base can be of mild steel or aluminum.
– Cavities can be of aluminum, mild steel or any other agreed upon metal.
Cycles: Not exceeding 500
Description: Prototype mold only. This mold will be constructed in the least expensive manner possible to produce a very limited quantity of prototype parts.
– Molds may be constructed from cast metal or epoxy or any other material offering sufficient strength to produce minimum prototype pieces.
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