mdi design guidelines

7/28/2019 MDI Design Guidelines

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Design Guidelines

MDI HEADQUARTERS

10045 102nd TerraceSebastian, FL 32958phone: 772.228.7371fax: [email protected]/12/10


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Working Together

When a designer speci es foam for a product or component, its because no other material can duplicate

the required characteristics. However, most product designers and developers do not have the time to

become an injection-molded foam expert.

The purpose of this guide is to provide a better understanding of the injection-molding process, material

and technical considerations involved in the design and manufacture of PolyCell parts. By understand -

ing these considerations, you can better control costs while improving the performance of your product.

The following is an example of what steps are required in the product development stage.

1. Customer submits 3D le of potential part to MDI Products at our U.S. Of ce.

2. MDI reviews feasibility of initial part design with our Asia factory and gives necessary product

design changes required for manufacturing

3. Ballpark part price and tooling costs are quoted

4. Customer makes required product revisions

5. Formal part and tooling quote is submitted to customer

6. PO for tooling and sampling received

7. 1st production mold is made

8. Sample parts and FAI Report are produced for customer approval

9. Customer review samples

10. Quali cation Testing (i.e. PPAP)11. Complete additional molds

12. Production begins

Rules of Thumb Let us know your critical factors ASAP. Examples to be considered: Weight, Dimensions,

Temperature Ranges, U/V, Cosmetic/Aesthetics, Price, Compression Set, Tolerances,

Quali cation Testing Requirements, Parting Line Requirements, etc.


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What is PolyCell ?

Just call it miracle foam. It makes softness possible.

MDI Products PolyCell formulas are polyole n elastomers. The material is closed cell, self skinned andextremely durable at making products better cushioned, more aesthetically pleasing, and enhanced with

unique features and details.

PolyCell is closed cell. That means its waterproof and resists mold and mildew. It oats,

cant corrode, and holds up to the elements. It wont absorb water or come apart, clump

together or lose its shape. It combats water degradation and is impervious to

bacteria and blood borne pathogens.

Polycell is chemically resistant. You can clean it with most hospital and

household cleaners.

PolyCell is cross linked. Its strong and durable.

PolyCell has high tear and tensile strength. You cant easily

rip it and it wont crack or peel.

PolyCell can also be customized to provide parts withexceptional molded logos and text readability.

PolyCell can match

PMS colors and add

interesting texture.

PolyCell is available

in varying degrees

of hardness.


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Injection-Molding Process Overview

The rst step to understand -

ing designing parts for MDIs

PolyCell material is tounderstand the fundamentals

of the injection-molded foam

process.

The machines used in this

injection molding process are

vertical multi-station presses.

This process is a multiple mold

process.

Unlike plastic injection, a single mold process, the

mold cavities are designed smaller than the nal

part size. The parts expand rather than shrink. The

amount of expansion is dependant on the material

softness you require.

The material remains in the heated mold during the

cross-linking or cooking process. The cooking time

is dependant on the thickness of the part. The mold

opens very quickly when the cooking is complete. The part expands and explosively self ejects from the

mold cavity. The part is removed from the mold station and set to cool.

Generally, cooling xtures are required to assist the part to conform to the correct size/dimensions.

The cooling xtures are included in the tooling quote.

The gate and any ash are manually trimmed and the part is ready for packing or any

secondary operations.


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Plastic injection molding uses mechanical ejection. Ejector pins and stripper plates are used to remove the

molded part from the mold. This is NOT possible with the PolyCell injection molding process. Therefore,

the parts must be designed to self-eject during the expansion process.

Self-ejection is one of the most critical design criteria and also the most dif cult concept. If a part does not

self-eject, it gets trapped in the mold.

Draft/ Corners and EdgesDepending on your part design, you have to think about your part having a minimum of 5 of draft or a

radius to even consider this process.

Depth of Cavity

The shorter the depth of the cavity in the mold the better the self-ejection. The location of the parting line

can help minimize cavity depth.

Self-Ejection

Correct With Draft/ RadiusIncorrect - No Draft/ No Radius

Vertical walls get caught before clearing the cavityand cause the part to be permanently deformed.

Draft on walls allow the partto self-eject.


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Draft/ Corners and EdgesVertical Area

Reducing the amount of vertical area will also aid in self-ejection. This can be accomplished with addingdraft angles and large radii to the part. Draft angles also help the part self-eject. Sharp corners do not

self-eject. The larger the radius on the outside of the part, the better.

Length to width ratio, plan view

A round part is optimal for ejection but not always optimal for desired part design. If a part is very long

and not wide the length to width ratio can be too high and deter self-ejection. A ratio exceeding 4:1

should be avoided, the exception being very thin parts.

Correct Ratio 4:1

The incorrect ratio causes the part to trap and bow in the mold. This is due to the sides de-molding

quicker than the ends.

Self-Ejection

CorrectIncorrect


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Draft/ Corners and EdgesSharp edges

Sharp edges are only desirable at the parting line.

Rules of Thumb The larger the radius and/or draft the better.

No sharp edges except at the parting line.

The shorter the depth of the part the better.

The length to width ratio should be less than 4:1.

Self-Ejection


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UndercutsUndercuts are sometimes needed in your part design. The following explains the possible types

of undercuts that can be created using PolyCell.

Simple Female Undercut: This type of undercut is commonly used as a way to fasten the part

onto a substrate using mushroom head pins or suction cups.

Complete Female Undercut: When the entire part ts over the substrate a complete female un -

dercut is used. This is eliminates the need to use adhesives, a signi cant cost and labor savings.

Self-Ejection

Undercut

Substrate

Foam Part

Foam Part

Substrate


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Undercuts

Radial Undercut: A radial undercut is created when there is a hole in a cored partparallel to the parting line. The hole can be created without the need for slides as

with plastics.

Mandrel Undercut: A mandrel undercut is used in order to create a hollow part. In order to coreout the part a oating mandrel is used. The mandrel needs to oat in order for the part to eject from

the cavity.

Self-Ejection


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Undercuts

Male Undercut: In most cases male undercuts are NOT possible/advisable.The parts will trap during expansion and be permanently distorted.

Rules of Thumb An undercut on the inside of a part acts as a puller to help pull the

part out of the mold

Self-Ejection

Incorrect

Parting line

Mold

Mold


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Overall Wall ThicknessIn injection molding, a part would ideally have uniform wall thickness .The recommended minimum wall

thickness is .12 - .15. The recommended maximum wall thickness is 2 - 2.35. Wall thickness is not astand-alone speci cation and is heavily affected by the overall size of the part. For instance, due to ow

properties, a very thin part cannot also be a very big part. On the other hand, a very thick area will in -

crease the cooking time, thereby reducing the number of parts per hour and increasing the cost per part.

If the thickness is uneven, cooling cross sections can also result in distortions, such as sink marks.

Rules of Thumb Minimum Wall Thickness of .12-.15 (3mm - 4mm) - Only smaller parts/small areas of parts

Maximum Wall Thickness of 1.18-1.38 (30mm - 35mm) - Even thickness/ at parts preferred

CoringCoring is the removing of unnecessary material from the part. It is a very important detail when

it comes to part cost. The following show types of coring options.

Original Solid Part - Cross Section

Complete Coring: If the part can meet its functional criteria the best coring option is complete cor -

ing. This will minimize the mass of the part as well as reduce section thickness and cooking time.

l

Wall Thickness

Correct


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CoringPartial Coring: This design is used when the part needs to have a rm feel or resist collapsing.

Conventional Plastic Coring: This is the type of coring used on rigid plastic parts. This type of

coring does NOT work with the process. The deep narrow ribs create pockets of gas and the part will

burn. Also the shallow draft angles and sharp corners trap the part in the mold.

Rules of Thumb Uniform wall thickness is most desired.

Coring contributes to signi cant cost savings.

Wall Thickness

l l

l

Correct

Incorrect


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HolesWhen designing a hole in a part, there are a few design requirements to consider. Although holes ARE

possible, the following conditions can be issues:

Diameter

Depth of the Hole(s)

Direction of the Hole(s)

Multiple Holes

Distance Between Holes

Rules of Thumb Hole(s) should be vertical with the parting line for the best result.

The recommended minimum diameter of a hole is relative to the depth of the hole.

Wall Thickness


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TextureThe choice of a surface nish is normally based on cosmetic considerations. A textured nish can en -

hance the appearance of a part. Texturing a parts appearance will add richness, therefore making the partmore marketable, and giving it higher value and quality. Multiple textures can be used on one part for

visual contrast.

Rules of Thumb Aggressiveness of texture has no limit on the top or bottom of the part.

We may limit the aggressiveness of texture on the sidewalls of the part, due to the

fact that it can prevent the part from self-ejecting.

Color Matching Polycell is able to match almost any

PMS color, A color matching fee may apply.

Cubic Dip GraphicsThe Cubic Dip process will put a highly graphic

surface on your product. It is accomplished by

printing the design on a water-soluble lm. The

lm is then oated on water, causing the lm to

dissolve leaving the graphic behind. Your part is then

dipped into the water, causing the graphic to adhere to your part permanently.

Size Limitations:No minimum size. Maximum size is 15.75 x 15.75 (400mm x 400mm.)

Standard/Custom Designs:Standard designs or create your own.

Rules of Thumb Give us almost any PMS color and we can match it.

If you want a graphic surface on your product, talk to us about it early in the process.

Aesthetics


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Logos/ Fine DetailsLogos and ne details are possible. Placement is critical considering the need for the part to self eject.

Raised vs. recessed is also something to consider.

Rules of Thumb For the same reason as texture, logos are not advisable of the sidewalls of the part.

It is best for logos/ ne details to be raised rather than recessed.

Gates/ Parting LinesThe gate in this process is signi cant, so part design needs to consider this. It is critical that you call out

areas where gates can not be tolerated so that potential problems can be eliminated in the mold design

phase.

Sometimes the presence of a parting line is objectionable to the designer for functional or aestheticreasons. Part design needs to consider the location of the parting line.

Aesthetics


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Over MoldingOver molding is NOT possible with this process. The cavity size is much smaller than the nished part. A

full size mating part will not t into the cavity.

Shrink-to-FitHowever, our parts can shrink-to- t over a secondary piece. Once your product initially ejects out of

the mold the secondary piece can be easily inserted. During the cooling phase, the part shrinks to t. An

example of this would be a tire shrinking to t onto a rim.

UndercutsThe use of undercuts can be used to assemble two parts. The mating part can be a variety of materials including another component made from PolyCell, metal, plastic, etc. This is also

a good way to have two-color parts.

Assembly of Parts


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SewingPolycell can be sewn onto material or hook and loop fasteners. The thickness of the ange should be

considered with part design using a minimum of 3-5 mm.

Hook and Loop Fastener Sewn onto PolyCell to Attach Secondary Piece

AdhesivesOn the one hand, the material is resistant to most chemicals (including glues), which makes it highly

cleanable for sanitary applications. On the other hand, you may have a need to bond your molded foam

part to another material: plastic, aluminum, steel, etc. Depending on the foam/substrate combination, we

can glue the foam part together with a secondary substrate.

Rules of Thumb Polycelltm parts can be sewn to many materials as long as the sewing is the right thickness.

Parts can be bonded to substrate surfaces with a variety of glues. Tell us early so we can

begin to look for the right adhesive.

Assembly of Parts


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This is a multiple mold process.

MDI Products and our Asia team handle the designing of all molds.

The recommended mold con guration depends on the size and complexity of the part, anticipated pro -

duction volumes, and molding process requirements.

The number of cavities in the mold is primarily determined by the size and complexity of the part. The

number of cavities in the mold has a direct relationship to the productivity and cost of manufacturing the

part.

The endless combination of variables related to part function and production requirements

makes every new part a unique challenge.

There is no rapid prototyping for this process. The initial step is to create the rst production mold. This

allows validation of the part design and function.

Once the design is validated, additional production molds will be completed.

Molds


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The allowances for product variations are listed as tolerances. Part dimensions speci ed may display a

variation for every different design.

The amount of variance or tolerance allowed will often dictate or limit the productivity of the process.

We prefer as much tolerance as can be allowed, due to the fact that Polycell is soft foam that expands

and contracts during the manufacturing process.

There is no comprehensive guideline that will allow design engineers to anticipate the expected toleranc -

es for every type of design. When circumstances warrant MDI Products, and our Asia team, will provide

guidance based on actual manufacturing experience.

We encourage customer inquiries at the design stage if there is a question of adequate tolerances. We

also reserve the option to request tolerance adjustments if our manufacturing experience differs from

the initial allowable variances.

Rules of Thumb Typical production tolerances yield +/- 2% of the nal dimensions.

Harder material has a lower expansion, and produces a closer tolerance. Part designs with uniform cross sections will usually expand uniformly and have

minimum model deviation

When exact t is required, PolyCell parts should be designed before the rest

of your assembly

Unlike plastics (and like leather), foam can have imperfections.

Tolerances/Quality Standards


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Maximum Size Cubic Volume: approx. 579 in 3 (i.e. 25 x 20 x 1.15 = 579 in 3) (9500cm 3)

Length: 25 to 27.5 (650 to 700mm)* * Some designs could be up to 800mm in length but must be reviewed on a cases by case basis.

Width: 20 to 23.6 (500 to 600 mm)

Part Depth: 6. (150mm)

Rules of Thumb Maximum cubic volume is based on the Polycell material chosen.

Maximum dimensions are based on the orientation of the part in the mold.

This process is not usually the most economical for extremely large or thick parts.

Minimum Size Part Size: 2 x 2 (50.8mm x 50.8mm)

Rules of Thumb This process is usually not the most economical for very small parts.

Weight Range .25 oz - 4.4lbs (7g - 2000g)

Softness Range 00 Scale: 70 95 *

A Scale: 18 70

C Scale: 40 65

* 00 Scale is preferred method

Rules of Thumb Softness is different than density. Density is based on weight and represents the weight of

the material per cubic foot. Density is not a controllable factor with the PolyCell process.

Polycells softness range:

70 on the 00 Scale is approx. 12lb/ft 3 density (191 kg/m 3)

95 on the 00 Scale is approx. 16 lb/ft 3 density (254.4 kg/m 3)

Size Parameters


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Cost Factors All of the factors listed below, contribute to part and pre-production costs and are taken into

consideration in the quoting process.

Annual Volume The higher the annual volume, the lower the cost.

Size of Part The size and complexity of the part will affect the number of cavities that can be put in a mold.

Number of Cavities The more cavities in a mold increase the rate of productivity.

Thickness of Part Since this is a cooking process, the thickness of the part seriously affects the cycle time.

(Parts that have uniform thickness of no more than .75 (19mm) will be most cost effective.)

Cubic Volume of Part This determines the amount of material usage.

Color of Part This determines the price of the material.

Mold Factors Number of Molds Required

Part/Mold Complexity

Mold Cost

Mold Life

Cost Factors


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Design Revision after rst Article Inspection Report (F.A.I.R) Additional charges will apply if design changes are required after F.A.I.R.

Part Trimming Number of places and dif culty.

Secondary Operations/ Packaging Only considered if required

Cooling Fixtures Because the part comes out of the mold very hot and very soft, a cooling xture is often

required. The cost of the cooling xtures will be included in the quote for tooling.

Quali cation Testing Additional charges will apply if a PPAP or other pre-production approval procedures are required.

Cost Factors


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Native CAD File Formats

We maintain current versions of the following CAD applications. Solidworks *

Power Shape

Rhino

Standardized File Formats

We also maintain the capability to view and import the following standardized le formats.

IGES

VRML

STEP

DXF

3DS

VDA

STL

PDF

DWG

* Preferred CAD System

CAD Data Interchange Capabilities

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