Polypropylene Injection Molding Problems and Solutions

some times when we try to inject plastic part, problems always occur, beginning with shot mold, flashing, and some times jetting.

data below i take from Exxon Mobil Chemical, will explain various Injection Molding Problems and how to solve that properly.

it's begin with

• sink mark
  
• void
  
• shrinkage
  
• weld line
  
• flash
  
• burning
  
• warp
  
• brittle part
  
• sticking in mold
  
• gate bush and various others
  

below if you prefer read it online

Injection Molding Problems and Solutions - Upload a Document to Scribd

but if you prefer download, link below maybe more useful to read and download
http://www.scribd.com/doc/6200927/Injection-Molding-Problems-and-Solutions

this paper is very good tutorial with simple and practice solution for those each problems

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Calculation and Designing Support Pin or Leader Pin

as we know in previous posting about main construction of mold, there was pin in leader component, this pin will support main construction and cavity plate (3) from deflection and guide the cavity part and core part when mold closing. here the picture of support pin or leader pin in 3 plate mold type.


Support Pin Main Part
we can divide main part of support pin into 3 type
1. Support pin it self, some mold engineer call support pin by leader pin or main support pin.
2. Bushings, how much bushing that we needed depend on type of mold base, for example for 3 plate type of mold base we need at least 2 bushings, one in stripper plate (2) and one another in cavity plate (3). bushings prevent support pin have direct contact with mold base plate, becouse mold base plate usually use more soft material and support pin is harder material, it can cause scratch if use in long time and mass production, bushings always use harder material with good tolerance to support pin. so the other advantage is mold can be opened and closed with precision. and it can use in mass production mold.
3. Collar, it to prevent plate of mold out from leader pin when puller bolt broken or others accident possibility, with collar give the mold construction more safe.
see the main part of support pin in picture below


Deflection Calculation
ok, to learn more how to calculate the deflection in support pin, when support pin receiving the weight of a mold base plate on their tips,support pins may
deflect due to a large amount of bending moment, to choose right diameter and how much the bending first we must know the force that affect support pin, see picture below


yes at 3 plate type minimum load is from weight of stripper plate and cavity plate, with simple calculation of deflection (although isn't precision) like formula below



to calculate more complex and precision add the force from support pin weight, collars weight, and 3 dimensional calculation methods.
below is some list and use full engineering link to calculate the deflection, radius gyration list, derivation of deflection formula and others
http://en.wikipedia.org/wiki/List_of_area_moments_of_inertia
http://www.doitpoms.ac.uk/tlplib/beam_bending/printall.php

then by calculation we can get the fine radius, after calculation I, maximum deflection is about 0.05 mm, more than 50 micro millimeter, use more large radius of support pin. or place more precision guide, like block guide, linear guide and others between cavity plate and core plate, it should be more precision when mold in closed position.

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Considering Factor that affecting gate size

when we design a gate for mold injection, we need a proper size of gate, to large size of gate will make unbalanced flow and defect in product. to small size of gate make cycle time high, unfil l of product and sink mark is appear easily in product.

although there was various factor that affecting gate size, here i list some common factor that affecting gate size.

Mass of Part
of course large mass of part need more large gate size, i still doesn't have list a good relation graph or table which various material and mass of part. Herbert Ress in Mold Engineering 2nd edition page 159 listed some suggested dimension for gate cross section but it just very general list without material and other kind of gate.

Size of Part
size of part also have large effect when we design a proper gate,longer flow length, larger cavity surface in injection mold of course need larger size of gate.

Temperature of Melt
when we use possible maximum temperature then cavity still cannot be filled, use more smaller gate, because with smaller gate more heat from heater can be transfered in resin, so shear stress of resin will decrease and viscosity also decrease, then resin will flow easily with small value of viscosity.

Nozzle Tip Position
don't use smaller length between nozzle tip and gate, because it will make freeze premature, if the construction of mold does not enable to make longer length, you can prevent that by making the gate more smaller.

Viscosity of Resin
viscosity of resin and shear stress of resin have large effect in proper gate designing. if we buy a resin, shear stress and viscosity data also available, use those data to calculate proper gate size. we can calculate average plastic flow by dividing shear volume with injection time. always use average plastic flow speed more than speed of sound, more than 340 m/s, if your value below the speed of sound it large possibilities that your product will shot (unfinish fill of product).

although other factor beside i write above there are some large factor like cooling speed, cooling temperature and injection time also have valuable effect when designing the gate, the main point was i write all above. from those main point i think we should design a proper gate in mold fast and effective.

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Pin Point Gate Construction

pin point gate is commonly used in 3 plate mold types, see explanation more about 3 plate more here, basically the pin point gate part is like picture below
































those picture is pin point gate picture, to make gate like that we can buy pin point gate bush in mold manufacture company or we make it by EDM (electronic Discharge Machine) Process, the special EDM torch for Pin Point gate also available in mold part markets.
Pin Point gate Bush
picture below shown simple pin point gate bush,




































the construction to assembly the pin point gate bush in mold is like picture below
straight, without extended bush, picture below shown, locating ring, pin point gate bush, runner lock pin, runner, product and mold base plate (top plate, stripper plate and cavity plate)



some times, we need extended pin point gate bush, why when plastic material have high shear rate, we need to assembly extended bush, so from diameter gate until runner plastic material will easy to flow the cavity.
see the construction at picture below


here some good link of standard component of gate,
http://www.misumi-europe.com/www/lang/catmold_eng/nav/006/hl/007/menu/catalogues/fn/21/content.html

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Opening Construction of 3 Plate Mold Type

previous post http://mould-technology.blogspot.com/2008/08/main-construction-pin-at-3plate-mold.html
we can see closed condition of three plate molds

picture below is shown three plate condition when open


basically there are three opening type in 3 plate molds base
1. Sprue Cut Opening
2. Runner Opening
3. Product Opening

below i try to explain in simple words
1. sprue cut opening
this opening will cut sprue from nozzle, so the feeding runner can take easily by arm robot or manually by hand, this opening use puller bolt and runner stripper plate to cut the sprue. the stripper plate will move after puller bolts pulled by cavity plate, remember that puller bolts also end mounted in cavity plate.
the opening length between 5mm until 10mm.
2. runner opening
this opening use tho open runner system, than runner will take by robot or manually,this opening will make space between stripper plate and cavity plate.
to make sure this runner opening safely, attach spring in puller bolts between stripper plate and cavity plate, or use other methods like attach spring in out of puller bolts. length of this opening is same with runner length + 20 mm.

3. product opening, as we know this opening is to open the product, make space so the product can be ejected from core.
this opening length is about length the product + length robot arm + 10 until 20 mm to make save space when product ejected.

Opening Sequence

actually to pull the cavity plate, so the cavity plate can pull the puller bolts than open the stripper plate have two types, using tension link and magnet.
OK, at this post let's learn when using tension link.
1. first Opening is Runner Opening
why ? becouse between stripper plate and cavity plate,some springs always attached at there, some mold designer attach at puller bolts, and the other attach in each side using pin and make some hole to attach the spring. but i prefer attach the spring in puller bolts, it more easy in process.(sorry at picture above i don't write some spring)
2. Second Opening is Product Opening
Parting Line between core - cavity plate always easy to open, when moving plate at the injection machine pull the bottom plate until core plate, it will make Product Opening open.
3. Sprue Cutting Opening
after moving plate pull the core plate, with tension link core plate will pull the cavity plate until cavity plate pull the puller bolt, and make some space to cut the sprue from nozzle.

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Main Construction Pin at 3Plate Mold

I take those topic, to start learning about kind of pin on 3 Plate Molds, becouse between 3plate and 2 plate mold have several different function of Pin.
Basically 3 Plate mold to support the main construction have 3 kind of Pin.Look at picture below


(click the picture to enlarge)

3 Part Pin are
1. Main Support Pin
2. Positioning Pin
3. Puller Bolts

those pin will support both opening and closing process of 3 plate type of injection Mold. the function each pin are

Main Support Pin
this pin is totally support cavity, and runner plate when molds in open,when we buy mold base plate from fabricator, they always have exact diameter of Support Pin that we needed, it was calculated, but if your part is precision ensure it by calculating the deflection of those pin caused by cavity plate and runner plate.
make sure the deflection of pin not larger than 0.1 mm, choose more large diameter of Support pin when deflection is large.

Positioning Pin
Because support pin doesn't precision when closing, we need more precision guide part, this part also named Guide Pin, becouse it will guide core and cavity matching together. use this part with the Bush, in high precision part, some times we need more than one Positioning Pin, we can also add block guide, taper block set, taper pin set and other high precision positioning tools.

Puller Bolts
puller bolts doesn't support any plate, this part actually include in opening part of three plate molds.
working together with tension link or magnet block, this part will make parting line at stripper plate and top plate, this parting line used to make sure that sprue was cut from nozzle.

Parting line Opening is about 5-10 mm, depend on large of molds.

look at picture below to learn the length of each Pin.

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