Injection Molding

Injection molding is one of the core businesses of  Topworks Plastic Molding, and Injection molding department mainly works on injection molding and secondary operation.

It aims to improve production efficiency, product quality, and control production costs.

It owns exquisite molding technology, the leading mold maintenance technology, nitrogen molding technology and high-end automatic production.

The company introduces 30 Japanese Toshiba and Haitian injection molding machine, tonnage: 40T to 2800T, to ensure the quality of products and capacity of the production.

  • Different types of injection molding machines cater to different customer  requirements (40-850T);
  • Rich experience in injection molding, be good at OA, digital appliances, home appliances and industrial products manufacture;
  • Various types of Injection molding, like horizontal injection molding, vertical injection molding, and overmolding;
  • The standard operation of production ensures the stability of product quality;
  • Continuous optimization of product production cycle, improve production capacity;
  • Understand the various needs of customers, and products exceed their expectations.
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Secondary Operation:

  • Spray: PU paint, rubber paint, UV paint, high gloss paint, leather, metal paint, etc..;
  • Printing: screen printing;
  • Laser carving, plating, heat transfer, silk transfer;
  • Hot melt, ultrasonic welding.

The plastic material we usually use:

  • ABS(with glass fill)
  • Polystyrene
  • Polypropylene(with glass fill)
  • Acrylic
  • Acetal
  • Nylon(with glass fill)
  • PBT(with glass fill)
  • PC(with glass fill)
  • PC/ABS

What information is needed for injection molding?

Injection molding is considered a “primary” manufacturing process. The following outline those items needed for calculating actual injection molding manufacturing costs

  1. Material costs
  2. Raw material
  3. Recycled material
  4. Scrap allowance
  5. Estimated regrind buildup
  6. Labour charges (if not included in standard machine rate)
  7. Direct and indirect labour
  8. Straight time versus overtime
  9. Machine rate (hourly)
  10. Setup charges
  11. Scrap allowance and downtime
  12. Number of cavities in mold
  13. Minimum number of cavities allowed
  14. Cycle time per shot, in seconds
  15. Variable overhead (utilities, maintenance, quality control, etc.)
  16. Fixed overhead (rent, depreciation, salaries, etc.)
  17. Tooling charges (if amortised over product volumes)
  18. Initial mold costs
  19. Maintenance costs
  20. Volume for amortization calculations

What behind the plastic injection molding price

The price of a plastic part will depend on factors such as the price of raw materials, labor,packaging and setup costs.

it’s not difficult to determine the cost of these factors because they are known and can be calculated before the project begins

However there are two factors that make pricing a plastic part difficult and that cause the greatest difference in pricing between competitors.

They are estimated press cycle time and estimated part weight.

These two factors are difficult to calculate because they are unknowns.

A company will only have to estimate cycle time and part weight until a sample part is made.

Companies estimate the part price by using the experiences they have had making similar parts.After the sample part is made, a company can give an accurate price for the part.

Let’s go through an example using a blueprint or part drawing, the engineer will calculate the part volume then by
using the specific gravity of the specified plastic they will determine the part weight.

The cost per pound to purchase.

The raw material is applied to the part weight and the cost of plastic ,and part is calculated then the engineer estimates the cycle time in the molding cycle using his or her experience applying the number of cavities in the mold and the cost of labor and press per hour, the labor charge is calculated.

Each order will have a minimum set up charge spread over the quoted quantities.Other additions such as secondary
operations packaging and sales commissions may be added to the price.

The price for a part would be represented in different quantity breaks because of the set-up cost and sometimes because of the change in the cost of raw material given larger quantities.

if you would like to compare pricing from several moulders ,be sure to ask the moulders to disclose the cost per pound of raw material, number of cavities in the mold ,estimated cycle times and estimated part weight.

You can create a spreadsheet to compare all the information a molder that will not provide details in their quotations
should raise questions in your mind.

Keep in mind that the low bidder may not always be the best, be sure to ask for a price guarantee.

injection moulding process

During the injection moulding procedure, a comparatively cold mould, molded as the negative of the piece  developed,is filled up being forced by the shot of an established volume of heated and plasticized compound.

The shot material solidifies, and after some time cooling the mould is opened and the item taken away.
The subsequent demonstrates the primary stages of the approach as performed on a lateral plunger kind of injection moulding machine.

The procedure of the compound from granulate to the accomplished item as specified in these charts is as follows:

The granulate is loaded into the hopper of the measuring or dosing device above the injection unit.

From the hopper, a predetermined quantity of material falls in front of the plunger.

This charge of material is pushed forward by the plunger so that it is deposited,first on to the injection plunger and then, after withdrawal of the latter, in front of it.

As the injection plunger moves forward, it forces the material through the plunger guide into the heating cylinder In this cylinder which is heated on its outer surface by electric resistance-type heater bands, the material is brought to a state of a thermoplastic melt.

A spreader inside the cylinder aids Even heating of the material as it passes through the cylinder.

This spreader, or torpedo as it is also known, forces the material against the hot inner wall of the cylinder.

As successive strokes of the injection plunger force the material through the heating cylinder, it’s consistency changes until, at the front part of the cylinder, it is in the state of a homogeneous melt ready for injection.

The melt is forced into the mould through the injection nozzle.

This operation is also referred to as shooting \ while the speed of injection, which is regulated to suit the material being used and the component, is known as the shot rate

The mould, which is clamped or bolted in position in the injection machine consists mainly of halves; one member mounted on the stationary platen and one on the moving platen.

From the injection nozzle, the material passes through the sprue bush in the nozzle or fixed side of the mould. It is then distributed to the cavities through interconnecting channels or ‘runners.

When the cooling period has elapsed the mould is opened, and the components (T) are ejected from the moving mould-half by means of ejector pins which, as the mould approaches the fully open position,are operated by the ejector tail striking the stroke limiting screw.

Thus, the injection moulding cycle may be divided into five stages, as follows :

  • The measuring out, in front of the injection plunger, of a quantity of granulate equivalent to the capacity of the mould cavity or cavities.
  • The melting of this material in the heating cylinder to a consistency suitable for injection.
  • Injection of the melted material into the closed and relatively cold mould.
  • Cooling the injected material to the point of solidification.
  • Opening the mould and removing the product.

Injection Molding Component Design

injection molding part design

There are several basic concepts in component design which, if followed, will help to relieve many of the problems that beset the injection molder. Among these are:

Differences in thickness of only 15-25 percent will often lead to sink marks forming in the thicker parts.

At worst,voids could be produced and these could affect the performance.

Rigidity can often be imparted by introducing a stepped section (cf. corrugated roofing sheets), and even box sections are made more rigid while keeping their thickness even,by slightly rounding the corners.

If ribs and bosses are to be introduced, sink marks may occur and may be difficult to avoid.

A useful rule is to make ribs only two-thirds of the general wall thickness and to design bosses similarly, causing holes to penetrate one-third of the depth into the wall.

  • The wall thickness should be no greater than is necessary to give the strength and rigidity needed.

This keeps the use of material to the minimum and provides the shortest possible time cycle- Careful use of multiple gating can probably solve problems of mold filling.

  • Undercuts and re-entrant shapes cause extra costs both in mold production and in molding time (side core operation very often increases the time cycle)

They can often be avoided by the use of simple assembly methods applied to two or more components.

One of the most favored of these is the interference fit in which one component has a clip or other device which engages strongly with the other part and holds the two together (cf. the poppet bead).

Welding by friction,hot plate,or ultrasonic means can also be used, but in general,cementing with solvent or solvent-based adhesives is not recommended for the custom molder because of fire and toxicity hazards and problems of solvent recovery.

  • Avoid the use of molded-in inserts if possible.

Use instead staked or spring-clip inserts into blind holes which are easier to make by injection molding.

Insert molding is often very wasteful of time,of both operator and machine, and this is generally the most expensive part of the process.

Sharp corners, both external and internal,are natural stress-raisers but a radius of 0.06  in (1 -5 mm) will significantly relieve the situation and may increase impact strength by as much as five or ten times.

Component designers may call for something that is difficult to mold.

As the producer of a well-known brand of hand-tool manufacturers put it in an advertisement: “Occasionally, so rarely, in fact,that one feels rather guilty about mentioning it, designers have been known to create things which are just a teeny-weeny bit difficult to make.

Production men rejoice in challenges of this kind, of course,and the English language would be poorer without them. “The same could,perhaps, be said of the injection molder.

Designers may be working in a state of ignorance—ignorance of the service conditions of a component,ignorance of the service performance of a material, and ignorance of the many factors involved in proceeding from component design to complete plastic molding.

There is a need for education but, even more,there is a need for consultation and cooperation.

Regular liaison between end user,component designer,mold designer, and molding technologist will prevent many of these problems from arising.

Injection Molding Troubleshooting

troubleshooting injection molding

troubleshooting injection molding

1. Short shot: The edges of the plastic parts are smooth, but irregular and incomplete. It often occurs at 1. the farthest point of the gate;  2. thin and long ribs root.

2, Shrinkage: Due to the shrinkage of parts, the surface of the plastic parts is dent, uneven, and it shows a wavy appearance to the light, often occurs in a place 1. Where the thickness of the plastic molded parts is uneven 2. thick section of injection molding part; 3. boss and rib section.

3, Flash: plastic parts edge with an extra thin skin, it often occurs in 1. parting line; 2 movable core; 3 ejector pin position, boss position, hole position, snap position.

4. Bubbles: There is a bubble on the surface of the plastic part which color  is different from the surrounding color. It is generally :1 bubble formed by gas, air and water gas which are not discharged in time;  2 vacuum bubble caused by shrinkage. The bubbles in the transparent piece are particularly noticeable.

5. Weld line: When two or more flow front meet, a deep weld mark is formed on the surface of the plastic part. It usually occurs at the confluence of multiple molten flow fronts.

6, Burning : the surface is not flat, there are dark or black burnt marks, generally occurs in section where difficult to fill and trap the gas easily.

7, Black spots: there are black dot-like impurities on the surface of the plastic parts, caused by mixed materials.

8. Discoloration: Does not match the required color. The slight change in the color of the transparent plastic part is obvious, and the discoloration is generally caused by the wrong pigment ,the wrong mix ratio, or the wrong mold temperature.

9. Wrinkles: There are wavy lines on the surface of the plastic parts, which are caused by the cooling of the resin flowing.

10. Deformation: The plastic parts are distorted, uneven,curved,and one this occurs in the bosses, ribs,and box-shaped injection parts. PP injection molding are especially common.

11. Wrong materials: Different from the specified materials, it can be identified by checking the packaging label, the density of the beer (discharge) and the burning continuity, the color of the flame and the smoke.

12. Stick to the mold: Residual plastic part staying in cavity  make the ejected part incomplete after cooling (but different from the short-shot),or due to the lack of designing the mold ejector system, the plastic part is difficult to be ejected out of the cavity , generally it occurs in the thin wall section, the ribs, bosses and the snap.

13. Scratch:The plastic parts are rubbed against the surface of the cavity during the ejection, so that the surface of the plastic parts has scratches.

14, Over-flow: due to cavity damage , generally occurs in the active section, bosses, ejector pins and parting surface.

Click for more information as to the injection molding troublshooting.